Bass control on Widgy Box EQ cuts overall volume when rolling past noon?

[Andon]

Howdy, playing around with the EQ section from the Widgy Box and it's working pretty well! Well, everything except for the bass control. Here's the schematic for reference:



Full size available here: https://i.imgur.com/iJQiIx4.jpg

As it is on the schematic anything past noon significantly cuts the overall volume rather than boosting the bass, almost as if there's a bad connection. If you roll it back it will cut the bass as it should.

Removing the first 18k resistor coming in from the signal, removing the 15nf capacitor, or bridging the left and middle lugs (1 and 2) with the omitted parts seems to fix the issue, but I'm curious as to why? I've checked my breadboard over and over and when everything is wired up as per the schematic I get that same volume drop past noon on the bass control, but the aforementioned omissions and bridges work. The middle and treble frequency knobs work just fine.

Thoughts?

[Andon]

Bumping for the next day crowd. I've been using it without the aforementioned resistor or capacitor, but still can't deduce why using them would exactly half the range of the pot (and it's sort of difficult to find or guess the best terms to google to get an applicable answer).

[antonis]

http://bitsavers.trailing-edge.com/components/national/_dataBooks/1980_National_Audio_Radio_Handbook.pdf

pages 2-46 <-> 2-54

[Andon]

Thanks! Been looking for a copy of this. I've read through the cited section (similar information elsewhere, too) but I should clarify that when I say it cuts the volume I mean it completely cuts out the entire signal at/past noon - no sound. Rolling it back reduces the bass as it should, but again anything past noon just kills the signal if you leave the resistor and/or capacitor in place, and either myself or the source material are too dense to see why. Maybe I'll use a design with a designated op-amp for each control instead....

[iainpunk]

it sounds like a BIAS problem, amplifying the bias makes sound cut out sometimes. check the build again for bias continuity.

cheers, Iain

[PRR]

....I should clarify that ....

Yes.

...when I say it cuts the volume I mean it completely cuts out the entire signal at/past noon - no sound.....

Capacitor 22u NP is shorted.

Check my guess by reading DC on that opamp output. I think it is varying with Bass pot setting.

[Andon]

You were on the right track! DC is actually constant (around 4.48v+/-) with or without the 22u cap, BUT that first 22k resistor definitely appears to be the culprit, as it's swinging from 5-9v from min to max. Removing it clears it up.

I'm only using the EQ section from this schematic, which is being fed by the output of a volume/level pot from a gain section. Interestingly, if that first 22k resistor is in place then wherever I have that volume/level pot set will determine where the bass pot cuts off, so it's something to do with how much resistance there is (if the volume pot is at noon then the bass cuts signal around noon, the more it's turned up the greater the threshold).

[PRR]

... that first 22k resistor .....

?? I do not see any 22k resistor.

....I'm only using the EQ section from this schematic, which is being fed by ............

That's a significant change I do not recall being mentioned.

[Andon]

Sorry, that was a typo (had the 22u cap on my mind), I meant the first 18k resistor going into the bass pot.

....I'm only using the EQ section from this schematic, which is being fed by ............

That's a significant change I do not recall being mentioned.

First sentence of my first post, but I can see the confusion - my bad.

[antonis]

I'm only using the EQ section from this schematic, which is being fed by the output of a volume/level pot from a gain section.

Would you plz post the complete schematic..??
(including the gain section..)

[Andon]

It's just something I've got on a breadboard right now, but it's essentially a MXR Distortion + circuit going into the Widgy Box EQ with an output buffer afterward. 

EDIT: The bass still cuts off at noon even when you just plug into the EQ section and omit the gain stage before it (guitar > EQ > buffer > amp), so it's not the plug-and-play factor.

[PRR]

> bass still cuts off at noon...
>  the signal cuts off at noon.

Now we have a new schematic, different, yet same problem.

Study the DC situation. Output of U1 is at 4.5V. Diodes return to zero V so we need a cap. Input of U2 is at 4.5V, so we need a cap here.

[Andon]

Ah, this makes total sense, and it works properly now - thank you, Paul! This also fixes the issue if you're just using the EQ block standalone as well.

[Andon]

Welp, we're back here again. I got a couple of newer PCBs in with the 4.7uF capacitor Paul mentioned and it's once again cutting volume past noon - removing R7 (the first 18k capacitor in the schematic he posted) solves the problem again, but it's frustrating because this was working fine on the breadboard. All solder points seem good.

I'm also having an issue with a blatty/gated effect when I strum harder on lower notes/chords but not on higher notes/chords. This would be a telltale sign of a biasing issue, but I checked the 2n5458 on the output buffer and even replaced it and it still persists. However, I am getting some different readings on a couple of points when I reference this thread:

https://www.diystompboxes.com/smfforum/index.php?topic=111505.0

From that thread:

Here are the voltages from my Vintage Rat reissue:

BAT: 8.96V

2n5458
G: 0V
S: 1.69V
D: 8.93V

LM308
1: 8.29V
2: 4.45V
3: 4.02V
4: 0
5: 0
6: 4.45V
7: 8.92V
8: 4.52V

Here are my readings:

9V power supply

2n5458
G: 0V
S: 1.86V
D: 9.43V

OP07
1: 9.33V
2: 4.71V
3: 2.3V
4: 0
5: 0
6: 4.72V
7: 9.45V
8: 9.33V

And the TLO71 just in case:

TLO71
1: 1.43V
2: 4.72V
3: 4.33V
4: 1.28V
5: 1.43V
6: 4.72V
7: 9.45V
8: 0

There's no way the compensating capacitor (30pf across pins 1 and 8 on the OP07) would cause pin 8 to read that high, right? The op-amps are also socketed, so I've tried different ones and the readings are all similar (+/- .02V).

The diodes are also each reading .64V, which might indicate a leaky C7 capacitor per the linked thread, but I replaced it and the values stayed the same. The voltage divider is sitting at 4.48V, and everything else appears nominal aside from what I listed, but I'm also pretty new to this so I'm probably overlooking something.

Aside from the bass control issue and the blatty/gated response on lower notes/chords everything is working as it should.

[ElectricDruid]

<removed by author for being insufficiently helpful>

[Andon]

The whole schematic is a couple of posts up, it's this one:

Link for image size.

I'm trying my best to understand it (that's why I'm asking  ) but it's hard to search for these things sometimes, and sometimes more difficult to grasp without a little help - I appreciate the patience.

[Rob Strand]

2n5458
G: 0V
S: 1.86V
D: 9.43V

You can raise the source voltage by running another 1M resistor from the JFET gate back to the 4.5V supply.   That will give you more signal swing.  Technically you need to increase the 22nF cap to 47n.    Another way is to replace R15 with 2M2 and run a 2M2 from the gate back to the 4.5V supply, then you can leave the cap at 22n.

OP07
1: 9.33V
2: 4.71V
3: 2.3V
4: 0
5: 0
6: 4.72V
7: 9.45V
8: 9.33V

pin 3 is fine

The "true" voltage on that pin is around 4.6V.  What's happening is your multimeter has a 1M ohm input impedance and that's loading down the 1M resistor R2.

TL071 voltages look OK.


EDIT:
Something which occurred to me is to remove C13, R15, Q1, R16.   Then wire a 1k resistor from pin 6 IC2 (TL071) to the + side of C14.

[Andon]

Thank you for your reply, Rob - I'm going to try a couple of these out, but just so I understand...

2n5458
G: 0V
S: 1.86V
D: 9.43V

You can raise the source voltage by running another 1M resistor from the JFET gate back to the 4.5V supply.   That will give you more signal swing.  Technically you need to increase the 22nF cap to 47n.    Another way is to replace R15 with 2M2 and run a 2M2 from the gate back to the 4.5V supply, then you can leave the cap at 22n.

Here are you first suggesting to add a 1M back to 4.5V and also change the 22nF to 47nF? I just want to be clear - I understand the second option.

OP07
1: 9.33V
2: 4.71V
3: 2.3V
4: 0
5: 0
6: 4.72V
7: 9.45V
8: 9.33V

pin 3 is fine

The "true" voltage on that pin is around 4.6V.  What's happening is your multimeter has a 1M ohm input impedance and that's loading down the 1M resistor R2.

I actually learned about this last night on another forum where PRR and someone else were helping with an issue with a RAT. I had my suspicions that something wasn't quite right with the reading when I checked R2 (1M) and R3 (1K) and they were also sitting at 2.3V even though they were coming directly from the 4.5V line. The multimeter I have right now is relatively inexpensive, but I also read that some 1M multimeters can still be accurate, or at least readable if you account for the input impedance - looks like an upgrade is in my future!

TL071 voltages look OK.

I thought so, I just posted them for the sake of disclosure in case there was something funky going on that I was missing.

EDIT:
Something which occurred to me is to remove C13, R15, Q1, R16.   Then wire a 1k resistor from pin 6 IC2 (TL071) to the + side of C14.

So here you're suggesting to remove those as the unitary gain amp and basically just use the TL071, is that correct? This is in lieu of adding another 1M to 4.5V and changing the 22nF to 47nF since you're removing them of course in this case.

And lastly, if you have a moment - how did you come up with these solutions? It's great to have some ideas to work off of, but knowing how you got to them would be a big help. I've been reading and reading so much and I understand basic, practical concepts but sometimes the particulars are missed (forest for the trees and all), and it's both awesome and confounding when someone comes here with an issue and they're met with a near-immediate solution (or two, or three).

For example, I knew there was a biasing issue somewhere (because I read about it a while back - in the debugging), but I didn't know how to fix it. To use a personal metaphor (20 years in the food service industry), it's the difference between following a new recipe to create a dish and hoping it turns out alright, and following a recipe and running into issues along the way but knowing how to correct for them to still offer something delicious. Time and experience are the obvious answers, and I'm in no way looking for shortcuts or a bridge across, but I am looking for any stepping stones to help me along my way.

[Rob Strand]

Here are you first suggesting to add a 1M back to 4.5V and also change the 22nF to 47nF? I just want to be clear - I understand the second option.

Correct.

Currently the 22n and 1M form a high-pass filter.  The -3dB cut-off frequency is f = 1/(2*pi*R*C).   When the second 1M resistor is added the capacitor will see two 1M resistors in parallel.  Rparallel = 1/(1/1M +1/1M) = 500k.    That raises the cut-off frequency so the cap needs to be increased.    The second option uses 2M2 and the two 2M2 resistors in parallel results in a parallel resistance of 1.1M so it doesn't change the cut-off.

The multimeter I have right now is relatively inexpensive, but I also read that some 1M multimeters can still be accurate, or at least readable if you account for the input impedance - looks like an upgrade is in my future!

I'll admit a 10M meter is nicer to use but on occasion you still need to account for the 10M input impedance for accurate results .  For example your 4.6V might measure 4.2V.   So it still looks off, but by less.   You can learn to spot loading issue and know when to correct the reading.

A trick to avoid meter loading problems is to first measure the point and convince yourself there is a voltage there (not open) and that the meter is roughly the expected loaded reading.   The next step is to accurately measure the 4.5V reference voltage.  Then measure the voltage *between* the 4.5V reference.  You need to place the meter's negative lead on  4.5V reference.   If all is well you should read zero.  The accurate reading is Vaccurate = "V 4.5V reference" + Vbetween_reading.   If you get close to 0V for the "between" reading you know the point you want to measure is in fact 4.5V and the first measurement was low due to meter loading.   While it takes some words to explain it is easy to do in practice, all you want is to see a zero measurement.

So here you're suggesting to remove those as the unitary gain amp and basically just use the TL071, is that correct? This is in lieu of adding another 1M to 4.5V and changing the 22nF to 47nF since you're removing them of course in this case.

That's correct it's another option.  It completely removes the biasing issue of Q1.  The TL071 has sufficient drive to drive the output on its own.  The 1K resistor on the output of the opamp is a wise addition as it prevent the opamp oscillating when driving a cable.   Sometime you add a JFET buffer for sound but in your case it's causing trouble because the tone controls boost the signal after the clipping diodes.

For example, I knew there was a biasing issue somewhere (because I read about it a while back - in the debugging), but I didn't know how to fix it. To use a personal metaphor (20 years in the food service industry), it's the difference between following a new recipe to create a dish and hoping it turns out alright, and following a recipe and running into issues along the way but knowing how to correct for them to still offer something delicious. Time and experience are the obvious answers, and I'm in no way looking for shortcuts or a bridge across, but I am looking for any stepping stones to help me along my way.

You sound like a switched on guy.  I can tell you electronics *is* exactly like that.     The easy approach to electronics is to have an idea of cause and effect.   You see a problem then you need to counteract it.   It does take time to build up your library of "thought tools".  You can do a lot of calculations to calculate the expected voltages but that's a whole different level (perhaps like trying to calculate the increase decrease(!) in PH from adding lemon juice).   

So for the JFET.
- The problem in you case is the source not sitting near the middle of the supply so the output voltage it can't swing much in the negative direction.
- You want to raise the source voltage.
- The solution is to raise the gate voltage.    That's how JFETs work.  Raising the gate voltage will raise the source voltage.
- Next is how much.  You want about 4.5V on the source and your source is at 1.9V so it needs to be lifted 2.5V.
  To raise the source by 2.5V we *roughly* need to raise the gate by 2.5V.    It's not quite like that as when we raise the
  source voltage more current flows down the the 10k source resistor and that changes the voltage between the source and
  gate.
- Currently your gate is at 0V.   By connecting the gate to a DC voltage source via a resistor we form a voltage divider between
  that DC source and the gate.

  Here,
                Vout = Vin * R2 / (R1 + R2)


In your case the added resistor is R1, R2 is the existing 1M resistor R15, Vin is 4.5V.

If we add a 1M resistor we get Vout = 4.5 / 2 = 2.25V, which is close enough to the 2.5V we need.



FYI, there's a few common biasing schemes for JFETs.    Which one to choose depends on depends on the JFET.   The JFET has a parameter Vgs(off), loosely called the pinch-off voltage VP.   VP can be anything from 0.25V to 10V for different JFET models.  The voltage difference between the gate and the source is strongly affected by this parameter.   

The gate will always sit between 0V and VP below the source.

The first scheme is your original circuit, that's the simplest circuit.    However where the source ends-up is almost entirely determined by VP.      If you want the source to sit at 4.5V  you need quite a large VP JFET.   So this form of biasing limits your choice of JFET.   Only a few JFETs work optimally like this. 

The next simple scheme is like the Boss circuit.    Here the gate is raised to 4.5V.     You want the source to be at 4.5V but you know the source is going to be at a higher voltage than the gate.  In this case you want a low VP JFET so the source isn't too high.  With modern JFETs which tend to have low VP's this is quite common.



Another form is to  bias the JFET with its own resistors.   When the two gate resistors are equal this more or less operates liek the Boss circuit.     The difference is the 9V supply can be noisy whereas the 4.5V rail is usually quiet.

The advantage of this scheme is we can tune one of the resistors to tweak the gate voltage to get the right source voltage.   The extra resistor gives us the freedom to play with the voltages.



For your JFET the VP isn't quite high enough to get  good performance from the simple circuit, as you found.   However the VP is a bit too high to work properly in the Boss circuit.   The circuit I suggested lifts the gate just enough and the added resistor connects to 4.5V which is nice an clean.   So IMHO it solves the problem and has little side effect.   You probably won't see this form of circuit very often.

As for bypassing the JFET altogether.   The thinking behind this idea is simple, the opamp can drive the output and there's no real need to for the buffer.

[Andon]

Rob, thank you so much for taking the time to explain all of that for me - I really appreciate it!

The breakdown of biasing schemes for JFETs was great, and it helped to see the Boss style circuit laid out a bit more since I was familiar with it but only in passing (tried to really grasp it from the ElectroSmash DS-1 breakdown and elsewhere since I know they reliably use it in pretty much all of their pedals). I also noticed on the standard Boss buffer they also have a 1M resistor (two, in fact) going to 4.5V with a 47n cap between them - I assume this is for the aforementioned purpose of providing the output voltage with enough room to swing in either direction? Their input buffers are also pretty much identical save for a couple of values, which reinforces what I've read about "bookending" circuits (for lack of a better term) to sort of set them up, so to speak (but please correct me if I'm mistaken!)

I'm going to read over everything a few more times to try and really drill it in, and I will update this thread if I have any additional questions or success stories - thank you!