Some time ago I asked for help with this common problem, consisting in a subtle but noticeable bleeding of the effected (i.e. distorted) sound through the bypassed sound (the noise level is maximum when the drive knob is at max).
I was told to check the Q1 jfet for leakage, because that transistor blocks the passage of the effected signal to the output buffer in bypass mode, so I replaced it with a new one, but the bleeding through was still there, so the problem wasn't Q1's fault (I also verified this by playing the pedal with no transistor in Q1). I later noticed that the level of the distorted signal bleeding through the clean sound doesn't change with the position of the level knob, so I could have just used this information to exclude the role of Q1...
Someone suggested to lower the two 22k resistors which are bitween the two jfets Q1 and Q2 in the schematic, but this didn't work either (I also can't understand the logic of trying this, to be honest).
Today I decided to test the circuit with the famous "audio probe" and I found out that the bleeding through must be the IC's fault (at least in my case), because the noise is already present immediately after the input buffer and even at the input jack (although reduced by the internal resistance of the input transistor in that direction), while there is none at the +4.5V and +9V points (which are the only two other possible ways for the noise to reach the uneffected audio path).
I don't know much about electronics, so this is only what I suppose, but I'm pretty sure I must replace the IC. I'll do that as soon as possible and I will tell you if I was right!
Giulio
I socketed the IC and tried different (and compatible, of course) ones, but the leakage is still there. I now think that maybe every SD-1 has got this problem, just not everyone notices that.
Any other suggestion?
Giulio
QuoteI also can't understand the logic of trying this, to be honest
It's to do with how leakage gets attenuated around that part of the circuit - it takes some visual circuit analysis to see the leakage mechanism.
Quotebleeding through must be the IC's fault (at least in my case), because the noise is already present immediately after the input buffer and even at the input jack
You have done pretty well so far. Because it's at the input jack the audio probe could be picking-up stray signals, so your result might be false. Assuming for now that it is in fact real I would suggest two options - you should try both:
1) For the sake of experiment try changing the opamp to a TL072 (no others).
2) Try adding a 10k resistor in series with C2 (18n) at the input of the opamp. You will have to lift the leg of C2 out - so be careful no to lift the tracks off (especially if you try to bend C2). After you try 10k, if the fizz is still there try a higher value like 47k, or lower like 1k. You could get away with 10k as a solution but higher values are really only useful to identify the mechanism.
Quote from: shredgdI socketed the IC and tried different (and compatible, of course) ones, but the leakage is still there. I now think that maybe every SD-1 has got this problem, just not everyone notices that.
Yes, they all have the problem. But most people don't notice it.
Sorry for taking so long to reply: I didn't get the emails to notify me of your replies, I don't know why.
I tried a TL072 as well, but it is the same. I still didn't try the resistor before the IC, but I'll keep you informed. I just wonder why tubescreamers, which have a very similar circuit, don't have this kind of problem (I did the experiment using an audio-probe with my Son Of Screamer).
I had the same problem with an old Sd1 from about 1986, removed and jumpered the pair of 22k resistors and there was some improvement.
Then fitted two new 2SK30A's to the flip-flop circuit and it was fine, no distortion in the bypassed signal at all.
I got them from Olaf Nobis at Banzai Effects, www.banzaieffects.com
Marty.
8)
I have an SD-1 (well, a bit modified...) and I'm quite sure the clean mode is.. clean. I can't hear any distorsion and, imho, I have a trained hear.
Are you sure the first bjt works correctly? I changed the input stage with a jfet, it also rises the input impedance to 1M. it's a simple mod, just desolder the original tranny and insert a bf245 (or 2n5457, j201...) fet (keep an eye on the pin out, it's different, you have to bend to legs) then change the 510k R with a 1M or higher resistor. cheap and effective, and you can exclude another potential source of problem.
bye
Quote from: nooneknowsI have an SD-1 (well, a bit modified...) and I'm quite sure the clean mode is.. clean. I can't hear any distorsion and, imho, I have a trained hear.
I never heard it either but it's there in the background if you listen VERY carefully with a loud clean amp. I have not tried the above methods to fix it yet.
Quote from: analogmikeQuote from: nooneknowsI have an SD-1 (well, a bit modified...) and I'm quite sure the clean mode is.. clean. I can't hear any distorsion and, imho, I have a trained hear.
I never heard it either but it's there in the background if you listen VERY carefully with a loud clean amp. I have not tried the above methods to fix it yet.
It's actually much more apparant when you turn the gain knob all the way up (with the pedal in bypass mode), and even more so when you use a guitar with hot pups. Every sd-1 has this problem, and boss is actually aware of it, but to be honest, its not enough of a problem to them to change anything, since most of the folks that buy them simply can't hear it.
my boss dd3 has a similar problem...... if i turn the gain up all the way on my amp, i can hear the delay signal faintly.
Finally I found the time to work on this issue again...
I didn't try to put a resistor after C2 as Boofhead suggested because I wanted to solve the problem radically... but I didn't manage to do it! :(
I really can't see where the wet signal enters the bypass path. I tried everything:
using a probe I could hear it (after the input buffer) even after I desoldered both the jfets (I did this test because, as my TS clone doesn't have this problem and it is an almost identical circuit, I thought the flip-flop bypass mechanism should be involved).
So I thought the wet signal might make its way via Vr (the 4.5v points) through the 470k resistor that connects the base of the input transistor to Vr or the 100k that connects the input of the opamp to Vr, but the audio probe has stated Vr is "clean" from any signal.
If anyone ever finds out what I couldn't, please write it in this forum!!
P.S.: by the way, MERRY CHRISTMAS!!
Did you try lowering the 22k resistors?
Quote from: Fret WireDid you try lowering the 22k resistors?
So did anyone try this or did everyone get lazy over christmas like me? :)
Quote from: 12StringStratMakerQuote from: Fret WireDid you try lowering the 22k resistors?
So did anyone try this or did everyone get lazy over christmas like me? :)
I did it, didn't make a difference at all with my setup, unfortunately.
I've had the resistor method work two out of three times. It's a first step, that's all. If the problem can be fixed at the lowest level, that's where you start. If not, it's jfet replacement time.
I had more of a problem with my own early SD-1 ( 1986 ) and had to change out the two "flip/flop" Jfets on that one. ( 2SK30A'a )
A newer one ( bought to try various mods on ) was rescued by just reducing the two 22k resistors to 1k's.
On all of the "Brand new" SD-1's that I've sold as "MOD" versions (15 !!) there has been a very small amount of "break through" and I've also reduced those resistors to cure that problem.
In those cases I went with 2k2's and it worked fine.
Marty. 8)
so has anyone found a solution to this problem?
I've tried replacing the 22k resistors with a straigh thru connection and it didn't change anything...
this pedal is only usable at a really low gain setting now and as soon as you turn up the dirt it is really noticable when playing loud...
does anyone know an actual solution??
thanks
curt
The "solution" is in the above post ........ new 2SK30A's i'm afraid !!
Marty. 8)
I re-soldered all the joints of the switching fet's for a SD-1 from a friend of mine and that solved "a" switching problem. I can't remember if it was bleed-through, because it was some time ago.....
Search GEO (RG's site) for an article called "more fun with the millenium bypass"
http://www.geofex.com/Article_Folders/Mill2extn/mil2plus.htm
It explains how to shunt the input of the FX when it's bypassed, using the millenium. You can easily adapt this concept to the boss Jfet switching. You just need an extra transistor that can be driven by one of the FF outputs, and find an appropriate place in the circuit where the shunting can be done.
If you shunt the input to the fx, there will be no fx to bleed through :) and the bypassed tone will be clean.
The Marshall Blue Breaker (at least the original) also uses input muting when bypassing (well, not exactly the input - it shunts the signal between two stages).
thanks for all the help
okay so I'm going to order 2 2SK30A's and where exactly do I solder these in??
curt
anyone??
Yesterday, after three months, I had the time/will to work on the problem again.
I did other tests focusing on the few differences between the SD-1 and the TS (as the latter is free from this problem):
- the level pot on the SD-1 is only 10k (instead of 100k on the TS), so I thought the signal could pass through it to Vr more easily and then re-enter the "clean" (bypassed) pathway somewhere, so I lifted one leg of the last resistor before the level pot: no improvement.
- continuing with the supposed Vr involvement, I lifted one leg of C4, the cap which connects to Vr after the gain stage (the TS, instead, has a resistor here, and a cap to ground): no improvement.
- I noticed the 0.047u cap (C3) in the feedback loop of the gain stage connects to Vr, instead of ground as in the TS, and it does it closely to where R4 (the 100k resistor before the gain stage) also connects to Vr, so again I thought the effected signal might make its way into the bypassed signal at this level. Therefore I lifted the leg of C3 and connected it to ground: no improvement.
- I bypassed R7 (the resistor between the gain stage and the tone stage), because the TS has 1k here, and I thought too much resistance after the gain stage might bounce the sound back to the input of the IC (fantasy started to dominate...): no improvement.
- So I even lifted one leg of R7: no improvement (indeed, moving the tone pot doesn't affect the timbre of the bleed through. At least, now it is sure what comes after the gain stage is not involved).
- With R7 still lifted I even disconnected C9 from the jfet and connected it directly to a cable to my amp. By doing so I virtually eliminated the jfet switching and the output buffer from the circuit: no improvement.
- I lowered R4 from 100k to 10k and raised C2 from 0.018u to 1u (the cap and resistor between the input buffer and the gain stage) to have the same values as in the TS: no improvement.
- I bypassed the "extra" (compared to the TS) diode in the feedback loop of the gain stage and set the drive pot to half, in order to reproduce a TS with the drive pot set to max (as the drive pot in the SD-1 is 1M instead of 500k): the bleed through is there anyway.
So I'm officially giving up, I tried anything and I can't find a solution. This is indeed a poser.
The only possible explanation is C3 is too close to C2 in the circuit and "something" happens because of this...
To summarize, I'll add my previous observations, so to have them all together in this post.
- First of all, be sure the drive pot is set to max to clearly hear the bleed through (the best way is to strike a low E or A string and look for the background sizzle)
- The problem doesn't depend on the two jfets: I removed both (and then of course I used an audio probe to pick up the signal after the input buffer) with no results, and yesterday's test confirmed that.
- Therefore, lowering or even jumpering the two 22k resistors between the jfets doesn't solve the problem.
- It doesn't depend on the IC. I tried a TL072: same thing.
Giulio
If you removed both JFETs how did you get any signal at all?
Anyway, if you want to do a shunt you can run it off the flip flop that's already in there.
Come to think of it, what the gate/source voltage when the JFETs are off (switch open)? JFETs work when the gate/source voltage is 0 (low resistance). If the JFETs are simply being turned off (gate/source voltage made negative, low drain current) but not "off enough" there could be bleedthrough.
Then again that wouldn't explain why with Q1 removed you still get bleedthrough. And the TS9 doesn't have this problem. Hm. Maybe it's a PCB layout issue? What happens when you remove C2 AND Q1? Maybe the audio signal is modulating the gate/source turn on voltage? Then again the gate diode should be limiting the on voltage. Hm.
You most love your stompboxes, then they will love you...
If you love your SD-1, then you will give him a 100n film-capacitor, a Diode 1N 4148 and a FET e.g. 2SK 30A-Y or -GR and then your SD-1 will love you and everybody will be happy. :lol:
You will find a schematic how to do it here:
http://forum.musikding.de/yabbse/index.php?board=8;action=display;threadid=2224;start=0#lastPost
analogguru
Guys, this is not that hard. It's not magic, it's not mysterious.
The distorted signal leaks because the JFET in the distorted signal path is not turning off fully. Period.
This in turn is because the JFETs do not have a high enough reverse Vgs to fully turn them off.
This in turn is because the sources of the JFETs are not high enough for the ground voltage on their gates to fully turn them off for devices that are within the guaranteed ranges on the device datasheets.
The answer is to either (a) get JFETs with lower Vgsoff or (b) raise the bias voltage on the sources of the JFETs you do have or (c) both of the above.
You may, as analoguru suggests kill the gain of the distortion section while in bypass, but that's an extra credit thing. That just lowers the leakage through the distortion path so it is below the distortion threshold and although it still leaks, it leaks the same signal you're listening to, so you don't notice.
Specifically, Q1 is the problem. Its source is sitting at the bias voltage of 1/2of the battery. This may be as low as 8V easily, and its gate is pulled down by Q4's collector in the discrete flipflop. The series diode means it can't get pulled lower than a diode drop, so you have 4v-0.5V = 3.5V of back bias on Q1's gate.
The 2SK30A is listed as having a Vgsoff of -0.5V to -5.0V. If you get a -5.0V device, it will not turn fully off in the SD-1 circuit.
You can do a couple of things. One is to stick in a J201. The J201 is notable for having a Vgs0ff of -0.1 to -1.5V. The 3.5V of the existing circuit will turn this one off very well, in all cases. The J201 is often used for a replacement for the 2SK30A in some synth circuits.
The pinout of the 2SK30A is SGD with pins down, flat facing you. The J201 is pinned out DSG in the same orientation.
You can also find other JFETs, such as the 2N5292 or 2N5485, which will work almost all the time, as their maximum Vgsoff is -3.5V.
You can raise the bias voltage from half the battery to some higher voltage by changing R18 and/or R19 to get a bias voltage of maybe 5V or 5.5V. This will probably work just fine. The lowest effort version of this is to change R18 to a 22K or to parallel it with a 51K or 68K resistor.
While I have not done either of these, the schematic and Mother Nature say they have to work - unless there is something even more subtle that Mother Nature is trying to teach me, which does happen from time to time.
Quote from: R.G.
Specifically, Q1 is the problem. Its source is sitting at the bias voltage of 1/2of the battery. This may be as low as 8V easily, and its gate is pulled down by Q4's collector in the discrete flipflop. The series diode means it can't get pulled lower than a diode drop, so you have 4v-0.5V = 3.5V of back bias on Q1's gate.
Thanks for explaining that more clearly than my "hmm, this or that" musings, RG :)
what is a J201 exactly and where would I be soldering this in?
R.G.- so by theory, if you lowered R18 and R19 from 33k to 22k it would raise the bias voltage which would turn off the JFET in the distorted signal path?
Quote from: cbugattiwhat is a J201 exactly and where would I be soldering this in?
R.G.- so by theory, if you lowered R18 and R19 from 33k to 22k it would raise the bias voltage which would turn off the JFET in the distorted signal path?
A J201 is another FET, like a 2N5484.
Changing R18 and R19 (the 4.5V bias generating resistors) would make no difference. You have to lower (or eliminate) R11 and R14. If that doesn't work try a different FET (as suggested above).
QuoteChanging R18 and R19 (the 4.5V bias generating resistors) would make no difference. You have to lower (or eliminate) R11 and R14.
Can you expand on your reasoning for this statement? I may have missed some thing in examining the circuit.
If I read the schematic right, the distortion path signal is fed through Q1, the buffered clean signal through Q2. While I have not examined an SD-1, I have fixed several TW-9's that have this same behavior. The issue was always failure to turn off Q1 completely, and changing either the bias voltage to allow more Vgsoff reverse bias or the JFET to a lower Vgsoff type so that the voltage that was present would turn off the JFET always fixed the problem.
Changing R18 and/or R19 to raise the bias voltage makes more reverse Vgsoff available because it raises the voltage on the sources. You can't change the gate pulldown voltage much, as it's pulled to as near ground as the circuit can pull it.
I don't understand the reasoning that says that you must lower or eliminate R11 and R14. They're not part of the root cause of the problem.
It's always possible that I missed something. Could you expand on your reasoning?
@R.G.
I´ll try to explain it to you.
To make it easier, lets assume following conditions:
1.) Output impedance of Input Buffer is <1 kOhm
2.) Off-resistance of the FET is 1 MOhm
3.) On-resistance of the FET is 0 Ohm
Now, if the Pedal is in Bypass Mode the distorted signal is still present at FET Q1. In this state Q1 is comparable to a resistor of 1M which forms a voltage-divider with R14 + the output impedance of the input stage...
For this reason, the distorted signal is only attenuated by the factor of 50.
If you remove R14 the voltage divider is formed by the 1M of the FET and (only) the outout impedance of the input buffer, so the ratio is approx 1000....a difference in damping, eh ?
analogguru
Quote from: R.G.QuoteChanging R18 and R19 (the 4.5V bias generating resistors) would make no difference. You have to lower (or eliminate) R11 and R14.
Can you expand on your reasoning for this statement? I may have missed some thing in examining the circuit.
Mmm, probably not because your theory is 1000x times better than mine. My reasoning: OK I missed something with R18 and R19. If you change both R18 and R19 to the same value (say both to 10k, or both to 100k) it won't make a difference, since you still get 4.5V. Change one or the other or both to different values (say to 3V) then it makes a difference (right?) but you can't stray too far from 4.5V or else you start to mess with the rest of the circuit. Hmm as for R11 and R14 - that's my mistake, I thought they were connected to D, not S (I always get that wrong with FET drawings that don't have the G close to the S!) So there's no drop from 4.5V across either which shouldn't affect the switching.
Am I right in thinking that you could just replace D6 and D7 with two 4148s in seriesto fix things?
Sorry for not writing before, I've been very busy.
cd wrote:QuoteIf you removed both JFETs how did you get any signal at all?
To get signal after I removed both jfets, I connected one leg of a 0.1u cap to the tip of a guitar cable (plugged into my amp) and the other leg to various testing points, for example C2 and C9. Of course I also connected the ground of the pedal to the ring of the cable jack. This method is what I previously refered to as "audio probe" (copyright R.G. Keen).
This way I could pick up the signal of my guitar right after the input buffer. The leakage of the distorted sound into the clean path was already there!
analogguru wrote:QuoteIf you love your SD-1, then you will give him a 100n film-capacitor, a Diode 1N 4148 and a FET e.g. 2SK 30A-Y or -GR and then your SD-1 will love you and everybody will be happy.
You will find a schematic how to do it here:
http://forum.musikding.de/yabbse/index.php?board=8;action=display;threadid=2224;start=0#lastPost
Very, very good idea! That will work for sure!
I'll let you know about the results (they can't be other than good) when I have the time to materially do the mod! Bravo!
R.G. wrote:QuoteGuys, this is not that hard. It's not magic, it's not mysterious.
The distorted signal leaks because the JFET in the distorted signal path is not turning off fully. Period.
Thanks for your explanation about jfet switching voltages but, please, read previous posts before answering.
Jfet switching is clearly not involved in this problem (read above), at least in the case of the SD-1.
Indeed, in my opinion, this discussion about jfet switching can be useful, but not for this particular problem.
Ciao!
Giulio
If he removed the ON JFET, and still has the noise, FETS will not help. If it does remove the noise, it will help. I'm not sure which it is. Easiest way to test is just pull out the ON JFET, and play in the OFF position. Can you try this and see what you get, Giulio?
If you mean I should try to remove Q1 only (the jfets which lets pass the distorted sound or not), that's of course the first thing I tried before pulling out the other jfet as well: yes, the leakage is always there.
Go through this thread from the beginning and read my posts to see how many tests I did, most of which firmly exclude the role of the jfet in this problem!
i know for a fact that removing r11 and r14 do not do anything....I'm going to try changing r18 to 22k and if that doesn't help than r19...should get this done tonight and will post when finished
Quote from: analogguru
I´ll try to explain it to you.
To make it easier, lets assume following conditions:
1.) Output impedance of Input Buffer is <1 kOhm
2.) Off-resistance of the FET is 1 MOhm
3.) On-resistance of the FET is 0 Ohm
Ah, thanks for the analysis.
The problem with that is that 2.) is not true. 3.) is not either, strictly, but it has no effect on the issue at hand.
The off resistance of a JFET biased off is not 1Mohm, but a few orders of magnitude higher. For the 2SK30, the datasheet says that the specified cutoff voltage will reduce the drain current to under 100nA at 10Vds. Doing the math, Rds at cutoff is 10V / 100nA, or 100Mohm. That makes the division ratio 100M/22K = 4545, or -73db if I punched the correct buttons on the calculator. While is it possible to hear sound 73db down, that's pretty good isolation for a simple switch.
And the 100na of leakage is only what the maker guarantees. The typical device is usually better. Switching JFETs specify this at 10na.
So if you are not getting at least 73 db isolation in this setup, *you are not turning the JFET off hard enough*.
We also find confirmation of this:
Quote from: cbugattii know for a fact that removing r11 and r14 do not do anything....
But it's possible, as I said, that Mother Nature is trying to teach me something different...
[quote="shredgd"Thanks for your explanation about jfet switching voltages but, please, read previous posts before answering.
Jfet switching is clearly not involved in this problem (read above), at least in the case of the SD-1.[/quote]
I'm sorry. I'm getting on in years and my mind isn't as quick as it once was. Can you point out the part I missed, please?
[quote="cd"]Change one or the other or both to different values (say to 3V) then it makes a difference (right?) but you can't stray too far from 4.5V or else you start to mess with the rest of the circuit. [/quote]
Yep, that's correct. You want to RAISE the bias voltage a bit. Get it up to 5V or 5.5V if you can by reducing R18 or increasing R19. Don't change them both to some different but equal value.
QuoteAm I right in thinking that you could just replace D6 and D7 with two 4148s in seriesto fix things?
No, that is incorrect. Doing that decreases the off-bias to the JFETs and makes it worse.
I've got some 22k resistors...so if I put one in r18 this should raise the bias voltage to around what...5..5.5V?
if this doesn't cure it completely, I should rasie R19, but I don't have any higher value resistors right now...if put a 22k in series with the 33k in R19 would this raise or lower that value?
thanks
*update*
So I put the 22k resistor in R18. No improvement. Then put two 22k resistors in series in R19. No improvement. I've got no idea. I just know that I absolutely love the sound of this mosfet/germanium modded SD-1 and I can't turn the drive up at all to get it.
Does anyone have a definite solution for fixing this problem without building a bypass box? If so, how, and can you explain it at an easy level to understand?
I don't know where I would solder a J201 in (r10/q5/c3...etc) and I have no idea where I would put the parts that "analogguru" was talking about
thanks
curt
R.G. wrote:Quote[quote="shredgd"Thanks for your explanation about jfet switching voltages but, please, read previous posts before answering.
Jfet switching is clearly not involved in this problem (read above), at least in the case of the SD-1.
I'm sorry. I'm getting on in years and my mind isn't as quick as it once was. Can you point out the part I missed, please? [/quote]
Simply, as I also said in my latest post, if I remove the JFET which normally blocks the effected sound in bypass mode (the one you are saying is not turning off hard enough), I can still hear the leakage in the background of my clean sound.
I think this is enough to exclude its malfunctioning!
Read back to know about my other experiments, which led me to the conclusion it must be a layout problem (some components too close to each other).
cbugatti wrote:QuoteDoes anyone have a definite solution for fixing this problem without building a bypass box? If so, how, and can you explain it at an easy level to understand?
I previously said I was giving up about this problem, but I decided I will still work on it. Analogguru posted what seems an already good solution, but I have another idea, now, which I'll write about only if it proves good, so just wait for now, I will soon explain you at least how to do the analogguru mod.
Giulio
Well, as I noted, it's always possible that Mother Nature is trying to teach me something new.
Doctors have a thing they tell new apprentice doctors - "When you hear hoofbeats in the hallway, look for horses, not zebras."
If you remove the single JFET that blocks the effected sound, yes, that is an even higher resistance than a reverse biased JFET.
If you remove both JFETs - which is what I understood you did - you could well get capacitive leakage of both signals by paths which would not be operational with the "dry" signal there.
Given that, I would look to power supply or ground trace coupling of the effect signal into the output buffer, not mysterious electromagnetic transmission or layout coupling, on the horses-versus-zebras theory. The output buffer, being a single bipolar transistor has 0db of power supply and ground noise rejection.
Does this thing have a decoupling cap on the power line?
If that's not it, I'd go to board contamination.
On the whole, it's really quite hard to radiate audio frequencies between PCB components in smallish, low power effects unless you're using JFET buffers.
R.G.,
that's of course what I did, and it is the reason of the tests I did (which, again, you can read about above in this thread) after I excluded a jfet malfunctioning.
I understand you don't have the pedal and you probably didn't look at the schematic, so that's why you couldn't follow my previous posts.
Anyway, just have a look at this schematic http://www.freeinfosociety.com/electronics/schematics/audio/bosssd1.html and consider:
- I pulled out the 10k resistor between the gain stage and the tone stage (to exclude the second half of the circuit and therefore exclude a leakage from what comes after the gain stage)
- I pulled out the leg of the 0.047u cap connected to Q2's drain and connected it to a cable to my amp
and I could still hear the leakage!!
- Then I connected the 0.047u cap of the feedback loop of the gain stage to ground instead of 4.5v (as I was afraid the distorted signal made its way into the clean path through it, via the 100k resistor above, in the image)
Still leaking!
So, today I did analogguru's mod (http://forum.musikding.de/attachements/Boss_SD-1_mod_c.gif) to my pedal and, as expected, it works perfectly!
As I study Medicine, I would say it's not a "causal therapy" but only a "symptomatic therapy", but at least now I can finally use my SD-1!
Thanks, analogguru!
Curt,
as you asked, here is a guide to perform the mod.
You need an SK30A jfet, a 0.1u (=100n) cap and a 1n4148 diode.
Obviously, you have to solder your new components to the top side of the board, because you won't be able to close your pedal anymore if you work on the traces side...!
Watching the front (the flat side) of the jfet, the three legs pointing downwards are named (from left to right) S, G and D (Source, Gate and Drain, respectively).
Yuo have to solder S to D4's cathode (the side with the black ring), G to your new 1n4148's anode (the side without the black ring) and D to one side of the cap.
The other side of the cap must be soldered to the point where R5 and R6 "look each other" (they are connected together at that side, so it doesn't matter if you solder to R5's or R6's leg, choose one!).
Solder the cathode (again, the side with the black ring) of your new 1n4148 to the cathode of D8.
Done!
Giulio
Good work.. although I'm interested in seeing R.G.'s analysis of your analysis after he sees the schematic. He seems to know his stuff.
Am I right that this is still not a True Bypass circuit? With the changes that you've made, is there still the danger of having some signal loss when the circuit is in bypass mode?
Quote from: syzygyAm I right that this is still not a True Bypass circuit? With the changes that you've made, is there still the danger of having some signal loss when the circuit is in bypass mode?
Yup, it's a MUTE circuit. The output from the clipping circuit is cut off before it reaches the tone control.
The slickest (and best, IMHO) way to get true bypass in a BOSS case is with a latching relay. I've heard of folks mounting DPDTs and 3PDTs in the battery case but that's ugly IMHO.
SHREDGD,
That sounds like a great fix from analogguru :D
Well, no its not "true bypass" but if you read Roger Mayers explination in
the Dave Hunter book, then you wont place too much significance on TB
anymore !!!
Yards of cable, un-buffered between gtr and amp ......... :shock:
Or just use some 70's "curly" leads, that will remove all your top end 4 ya !!
Marty. 8)
OK, I'll bite. Are you saying there is something wrong with True Bypass? By using stomp pedals you're increasing the distance between instrument and amp anyway, so the way I see it, True Bypass will at least prevent you from the signal loss into effect boards of those stompboxes, assuming your wires are shielded properly, and all connections are in top shape.
Before anyone goes fruit bonkers crazy with an argument for/against true bypass, let's all take a deep breath and use the search function above :)
OK, well I'm no where near as "experienced" ( excuse the pun ! ) as Roger Mayer, so I'll quote roughly from his comments :
Manufacturers ( boutique ) seem to place importance on "true bypass" as a feature.
The best signal path from guitar to amp is a short piece of wire, usualy not possible/impractible, so you go through an fx pedal, bypassed though a piece of wire and through a 20 foot cable, in most cases you will be better off using a buffer, to eliminate noise maintain the signal etc.
Amps like to be driven from a low source impedance.
Amps seeing a long cable attached to a pickup can start oscillating.
So "buffered" outputs can be very useful here.
It depends on how many pedals are in the chain and where the buffer is in that chain.
He talks of his "vision octavia" where he has TB outputs and buffered outputs, on short cables there's no difference but with longer "real world"
cable runs, thats when buffering becomes very important.
He reckons it was a 3DPDT switch marketting con, which stems from the old days of crappy pedals/components.
Modern pedals should not have that problem and hence NOT require TB.
I've always been told that a few "Boss" buffered pedals in a chain can
help out with this problem.
So, a whole line of "TB" pedals ...... perhaps not a great idea !!!
as that can add up to a lot of "tone sucking" cable....
Hope that makes sense ?
Marty. 8)
it works....
I'm amazed...I really am...I modded my pedal months ago and found an unreal fuzzy/overdriven/mild distorted sound...used a mpf 102 transistor in d6, 2 1n34a diodes in series in d5 and a led and 1n34a in parallel in d4...along with changes in alot of other places...
now I can finally use the pedals drive knob...
thank-you everyone: analagguru, shredgr, rg, cd and everyone else who contributed to this....the problem has finally been solved...
I also tested to see if there's a loss of high's b/c of the lack of TB and I didn't notice any...
so stoked...thanks...
curt
Quote from: cdBefore anyone goes fruit bonkers crazy with an argument for/against true bypass, let's all take a deep breath and use the search function above :)
CD, you just beat me to it :D
no argument, just interesting to get "RM's" take on it, as I said he's much more qualified to have a valid opinion on this than me.
I know it depends on a lot of things, but the more pedals/cable there is
the more I can see how buffering rather than "TB" comes into its own !!
Marty.
oh yeah...I also used a J201 instead of a SK30a....seems to work flawlessly...thanks again
QuoteAs I study Medicine, I would say it's not a "causal therapy" but only a "symptomatic therapy", but at least now I can finally use my SD-1!
Thanks, analogguru!
You are absolutely right... :D
The cause may be capacitve crosstalk between pcb tracks, this would require a complete new pcb design....like if you make an organ-transplantation e.g. a new eye...for what, when its possible to wear glasses or contact lenses to achieve the same result... :roll:
analogguru
Can a MPF102 be used instead for the FET? I don't have a J201 or SK30a. Mine is having the same issue but since I have started using the ZW-44 it's no hurry on my end.
MartyMart,
Thanks for the extra info; If that is the case at those long cord lengths, it would be much better to build one True Bypass box around your whole stompbox chain to reduce the amount of wire the signal has to go through when bypassed.
Luckily for me I don't have that many pedals to lengthen my signal, so I'll be building True Bypass in every one
8)
Quote from: vanhansenCan a MPF102 be used instead for the FET? I don't have a J201 or SK30a. Mine is having the same issue but since I have started using the ZW-44 it's no hurry on my end.
hmmm...I was going to try the same thing as I only had MPF102's lying around but since I had to go get the 1n4148's I picked up something else...
the gate-source voltage max is higher at -7.5V than the J201 or SK30a...
I'd try it see what you get and if not just pick up a J201 or something similiar..
Quote from: cbugatti
thank-you everyone: analagguru, shredgr, rg, cd and everyone else who contributed to this....the problem has finally been solved...
Maybe not. There seems to be two different problems with the same symptom, bypass leakage. When you read the whole thread over again, you'll see that myself and MartyMart have fixed this symptom a couple of times by lowering/removing the 22k resistors. So maybe we have two possible problems with the SD-1 that can both show the same sympton, bypass leakage. :?:
Quote from: Fret WireWhen you read the whole thread over again, you'll see that myself and MartyMart have fixed this symptom a couple of times by lowering/removing the 22k resistors. So maybe we have two possible problems with the SD-1 that can both show the same sympton, bypass leakage. :?:
yeah that's the first thing I tried...removing the 22k's...didn't help at all though...I still don't understand the whole theory as to why there is the leakage...
I'm finally satisfied though...I've tried fixing this for at least 4 months on and off...roland should just fix it in the first place...
Hi,
I found this thread through another message board
For the BOSS SD-1 it would be clearly noted the the designers did not think the gain of the circuit would warrant a J-FET switch as used in BOSS DS-1 e.g.
At maximum settings there could be bleed through and is partly through the design of the PCB and the pedal as a whole. Easily a J-FET switch as used on the BOSS DS-1 at the input of the OD section could be used, but it would require outboard parts.
Also the ON resistans and OFF resitans of a J-FET would neither be 0 Ohms nor 1M Ohms respectively but just as RG pointed out in the order of maybe 100 Ohms ON and maybe 100M Ohms OFF.
It would be possible to use a MOS transistor to get higher OFF resistans.
Still as in any high gain circuit with a side chain of low gain such as amplification 1, would warrant silencing the high gain chain in some way.
This is the reason a Fuzz Face has the kind of bypass it has- grounding the input-if you don't input will hang as an antenna with the maximum gain of the fuzz circuit and you will hear a fuzz sound riding in bypass. Now the BOSS SD-1 is nowhere near that high gain so the economics department would say use just the minimum parts needed.......
Right so, there can also be this problem that the incoming flank triggers the switching J-Fets if amplitude of incoming flank is higher than the Ugs OFF and this can be due to a leaky J-FET device, and substitution for one with higher Ugs OFF could solve the problem.
Now it is also possible to install a J-fet switch into the feedbacknetwork as in series with the resistor going to pseudo ground and hooking up the gate of which to bypass output of the flip/flop............which would have no influence on sound; it is ofcourse also possible to short circuit the feedback loop by shorting the output to inverting input resistors baut that would leave the J-FET used exposed to the outputswing of the OP-Amp and it could 'falsely' trigger, depending on the device......and really just circuit knowledge would tell this.
Don't sweat it use the connections as suggested by Analog Guru.
No I can not see it and never have, but I can guess what it does and have employed it before I knew there even was an Analog Guru and it will work as suggested by RG and evrybody will be happy as suggested by Analo Guru.
Please respect Analog Guru's property rights to his drawings- just in case that is not enforced
Gentlemen have fun
BJ
Quote from: shredgd on April 06, 2005, 12:17:35 PM
R.G.,
that's of course what I did, and it is the reason of the tests I did (which, again, you can read about above in this thread) after I excluded a jfet malfunctioning.
I understand you don't have the pedal and you probably didn't look at the schematic, so that's why you couldn't follow my previous posts.
Anyway, just have a look at this schematic http://www.freeinfosociety.com/electronics/schematics/audio/bosssd1.html (http://www.freeinfosociety.com/electronics/schematics/audio/bosssd1.html) and consider:
- I pulled out the 10k resistor between the gain stage and the tone stage (to exclude the second half of the circuit and therefore exclude a leakage from what comes after the gain stage)
- I pulled out the leg of the 0.047u cap connected to Q2's drain and connected it to a cable to my amp
and I could still hear the leakage!!
- Then I connected the 0.047u cap of the feedback loop of the gain stage to ground instead of 4.5v (as I was afraid the distorted signal made its way into the clean path through it, via the 100k resistor above, in the image)
Still leaking!
So, today I did analogguru's mod (http://forum.musikding.de/attachements/Boss_SD-1_mod_c.gif (http://forum.musikding.de/attachements/Boss_SD-1_mod_c.gif)) to my pedal and, as expected, it works perfectly!
As I study Medicine, I would say it's not a "causal therapy" but only a "symptomatic therapy", but at least now I can finally use my SD-1!
Thanks, analogguru!
Curt,
as you asked, here is a guide to perform the mod.
You need an SK30A jfet, a 0.1u (=100n) cap and a 1n4148 diode.
Obviously, you have to solder your new components to the top side of the board, because you won't be able to close your pedal anymore if you work on the traces side...!
Watching the front (the flat side) of the jfet, the three legs pointing downwards are named (from left to right) S, G and D (Source, Gate and Drain, respectively).
Yuo have to solder S to D4's cathode (the side with the black ring), G to your new 1n4148's anode (the side without the black ring) and D to one side of the cap.
The other side of the cap must be soldered to the point where R5 and R6 "look each other" (they are connected together at that side, so it doesn't matter if you solder to R5's or R6's leg, choose one!).
Solder the cathode (again, the side with the black ring) of your new 1n4148 to the cathode of D8.
Done!
Giulio
Wow... nice one... just did analogguru's mod to my SD-1... no bleed - all clean in bypass with drive up full! thanks analogguru :icon_biggrin:
Quote from: R.G. on April 03, 2005, 12:46:17 PM
Guys, this is not that hard. It's not magic, it's not mysterious.
The distorted signal leaks because the JFET in the distorted signal path is not turning off fully. Period.
This in turn is because the JFETs do not have a high enough reverse Vgs to fully turn them off.
This in turn is because the sources of the JFETs are not high enough for the ground voltage on their gates to fully turn them off for devices that are within the guaranteed ranges on the device datasheets.
The answer is to either (a) get JFETs with lower Vgsoff or (b) raise the bias voltage on the sources of the JFETs you do have or (c) both of the above.
You may, as analoguru suggests kill the gain of the distortion section while in bypass, but that's an extra credit thing. That just lowers the leakage through the distortion path so it is below the distortion threshold and although it still leaks, it leaks the same signal you're listening to, so you don't notice.
Specifically, Q1 is the problem. Its source is sitting at the bias voltage of 1/2of the battery. This may be as low as 8V easily, and its gate is pulled down by Q4's collector in the discrete flipflop. The series diode means it can't get pulled lower than a diode drop, so you have 4v-0.5V = 3.5V of back bias on Q1's gate.
The 2SK30A is listed as having a Vgsoff of -0.5V to -5.0V. If you get a -5.0V device, it will not turn fully off in the SD-1 circuit.
You can do a couple of things. One is to stick in a J201. The J201 is notable for having a Vgs0ff of -0.1 to -1.5V. The 3.5V of the existing circuit will turn this one off very well, in all cases. The J201 is often used for a replacement for the 2SK30A in some synth circuits.
The pinout of the 2SK30A is SGD with pins down, flat facing you. The J201 is pinned out DSG in the same orientation.
You can also find other JFETs, such as the 2N5292 or 2N5485, which will work almost all the time, as their maximum Vgsoff is -3.5V.
You can raise the bias voltage from half the battery to some higher voltage by changing R18 and/or R19 to get a bias voltage of maybe 5V or 5.5V. This will probably work just fine. The lowest effort version of this is to change R18 to a 22K or to parallel it with a 51K or 68K resistor.
While I have not done either of these, the schematic and Mother Nature say they have to work - unless there is something even more subtle that Mother Nature is trying to teach me, which does happen from time to time.
Just to report in on this: I replaced the 2SK30's in my SD-1 with J201's with no noticeable result. I tried raising R18 to a 22K, resulting in a gated muffled square wave fuzzy tone. Probably useful in some context, but not very overdrive-like ;) Good thing i use it in a looping system most of the time...
Hi,
I'm sorry because I'm reviving an old but usefull thread, I think that there is a lot of useful information being said here and would be better to keep in the same thread all information regarding to this matter for people to consult it (like me).
Because our country situation and the hard-to-find things is a present element, I have to solve with the things I can find. About the analog guru fix, I cannot find either a j201 or a SK30a or any other replacement suggested here. I've found some j113 in an older DOD pedal I was going to dump, and according to the specs needed, I would like to know if these fets could work to do the bypass bleed fix with the cap and the 4148. Here are the specs I find for the j113:
Source and drain are interchangeable
Gate to source breakdown voltage of 35V
Gate to source cutoff voltage of 3V
Zero gate voltage drain current of 2mA
Power dissipation (pd) of 625mW
Operating junction temperature of -55°C to 150°C
Drain source on resistance of 100 Ohm
Other site states a more detailed info in the following PDF:
h t t p://pdf.datasheetcatalog.com/datasheet/calogic/J111.PDF (remove the space in the http)
Thanks in advance for all the help/info you can share.
Erick.
Welcome.
> our country situation
Say where you are. Someone may know where/how to buy parts.
> I've found some j113
Vgs(off) is really 0.5V-3V, and the low end is a bit low to handle guitar levels cleanly.
But try it.
Thanks for the reply PRR,
I live in Venezuela so, you can imagine...
Also thanks for the clarification, I'll try with the j113 and report back. May I suppose that the components are away from risk of damage trying with that j113, right?
Which would be the most important values that I need to check if I find some other jFETS?
With gratitude,
Erick.
Hi folks,
I'm back to report... Looking for more alternatives in the web before buying the other components I needed, I found the cctsim bleed fix which uses a 2N5457. So, I decided to search a little further and ask many friends and stores as possible here looking for the J201 alternatives (even in SMD) and now this new other FET (2N5457). Finally, one friend that works building amplifiers and that kind of stuff found me a 2N5457, so I trade with him my j113 for his hard-to-find-here FET.
At first, I tried the cctsim mod, it completely shut down the signal and it gave me a total clean sound but.. when playing too hard, the pedal behaved as some kind of compressor and cut the signal intermittently so, as it didn't worked for me I reverted the pedal back to its initial state.
Now I tried the other popular fix with the cap and the diode using this other FET (2N5457), and it worked flawlessly. Conclusion: if you can't find the J201, the 2N5457 is an alternative that works (the vgs(off) is -1.5 - 6, right?)..
Thanks for the help!
Take care,
Erick.