Synthbox

[Ben N]

2 questions: Why 2 series (coupling) caps in the filter section (the 2.2ufs)? And did you find any difference between MSA18 and 2n5089, that you chose to use the MPSA18 in the input stage followed by the 2N5089s? I always thought they were pretty much interchangeable types in terms of hfe, noise, etc.

Anyway, nice work--the clip sounds great.

[liquids]

2 questions: Why 2 series (coupling) caps in the filter section (the 2.2ufs)?

Because I'm an sophomoric hack.     No, really, I am.  I'm not sure it matters.  I thought I needed to block DC out of the transistor before the filters, and then needed another cap to isolate the bias voltage on the next resistor.  I was being extra cautious, but it may not need both.  I'm not sure.  Someone like Gus, R.G. and the like may have better insight into it being 'good design' (which it probably isn't) vs totally uncessary to have two caps.

And did you find any difference between MSA18 and 2n5089, that you chose to use the MPSA18 in the input stage followed by the 2N5089s? I always thought they were pretty much interchangeable types in terms of hfe, noise, etc.
Anyway, nice work--the clip sounds great.

The difference is likely negligible for those sections (buffers), especially in this circuit.  I labeled those as MPSA18 as an ideal, for a buffer stage.  It has slightly lower noise specs than the 2N5089.  So it's 'better' as a buffer on paper, but in reality may be negligible.  My memory sucks, but I dare say MPSA18s have higher hfe, though on paper the datasheets make that unclear--the 2N5089 has a higher 'maximum' with MPSA18 having the higher minium hfe, so it's more sensible to talk to 'trends' and one actual transistor vs another, meaning they're pretty close.  I dare say the MPSA18 is higher from minimum experience, as I've not found a 2N5089 beyond 999 hfe with my DMM, but if I recall correctly, a few and/or most of the handful of MPSA18s I have read beyond 1000 (i.e. they are unreadable on my DMM). 

To be honest I no longer even recall what I actually have in there on the breadboard    Possibly 2N5089s, since I have them in abundance currently.

[earthtonesaudio]

2 questions: Why 2 series (coupling) caps in the filter section (the 2.2ufs)?

Keeps DC off the volume control, preventing scratchy noises when that control is adjusted.

[Derringer]

Concerning Vero, here's a vero I did for the Shocktave as designed by Joe ... change the values of a few components and you have the shocktave section of your synthbox ... so if anyone needs to, go ahead and use this vero for that part

http://www.diystompboxes.com/smfforum/index.php?topic=76966.0

[liquids]

Big update!

That 'noiseless biasing' arrangement for the Vref at the base in and out of the buffers also works better in theory than in practice. I'm looking to get about .7v above 1/2 the voltage at the base.  On the breadboard I had 1M from +9.4v (one spot) to base and 2.2M from base to ground to get it about there in theory.  Checking the voltage, even that wasn't working as planned.

Switching over to using the Vref on the current schematic with a 4.7k/10k arrangement, I was getting about 3-4 volts, which is not ideal.   Trying to make it a little stiffer I tried 47k/100k, but with the same voltage reading.  it too 4.7k/15k pair to get me to about 5.6 V (1/2 x 9.4=4.7) which is good.  At that point, just running 1M resistor from the 9v to base with nothing to ground was not much different, though is 'bad design' in TAoE, so I'll stick with the 4.7k 15k for now.

Here comes the embarrassing part.    I changed this all, only to notice I was getting a slightly different sound with notably less decay than I was getting before.  Since I was messing with these changes on the breadboard, parts got moved around and I re-seeded them.  The soudn was different, but I couldn't figure out what.  I even swapped all the caps in fear I tried one, with little luck.

I don't know how I came to it, but this morning after a new look, I realized that I was witing the second transistor (on my schematic) all wrong. The schematic and shocktave indicate a buffer arrangement with the 220k resistor from emitter to ground on this transistor, with a 10k reistor feeding from the emitter to the 3rd transistors base. That is how I wired it up this time, causing the slightly less ustaining sound...

I don't know how I went back to it, possibly by the same slight of eye putting the lead in the wrong hole, but it seems I originally I had the 10k feeding from the base of the 2nd transistor to the base of the third, which is 'incorrect'.  Believe it or not, this is a BETTER sound (and what you heard on the samples), though I don't know what would be happening in that stage now....   This is another indication that this could definitely be improved.  I'm back to the breadboard working on stuff.  Anyone with insight, please chime in. I'll leave the schematic as is.  It works that way, just not as sustainy and edgy.     

[isildur100]

it seems I originally I had the 10k feeding from the base of the 2nd transistor to the base of the third, which is 'incorrect'.  Believe it or not, this is a BETTER sound

Does that mean the second transistor was actually bypassed? Or is it that it made 2 signal paths going to that third transistor?

[~arph]

Been fiddling around with the circuit a bit. I took out the tracking pot.
Replaced my misinterpreted 51pF with a 47nF and now it sounds more synthy even.
I came up with a three parts mod that will give you, ring modulation and two octaves down 

Do this:

At the 10k/68nF junction after Q4 (the part that is feeding the oscillator) put another 10k into a 100k pot an then into a 10uF electro into the base of the last mpsa18.
This mod works best with the volume up 100% so one might want to move the volume pot to the end of the circuit.

One side of the pot rotation gives you the normal sound (1 oct down) the other side of the rotation gives you two octaves down.. and in between is where it gets interesting..


EDIT: I tried the transistor bypass mentioned above, but found the sound better using the schematic as is.

[bluesdevil]

Wow, I love circuits like this! Thanks for sharing.

[liquids]

Been fiddling around with the circuit a bit. I took out the tracking pot.
Replaced my misinterpreted 51pF with a 47nF and now it sounds more synthy even.
I came up with a three parts mod that will give you, ring modulation and two octaves down 

Do this:

At the 10k/68nF junction after Q4 (the part that is feeding the oscillator) put another 10k into a 100k pot an then into a 10uF electro into the base of the last mpsa18.
This mod works best with the volume up 100% so one might want to move the volume pot to the end of the circuit.

One side of the pot rotation gives you the normal sound (1 oct down) the other side of the rotation gives you two octaves down.. and in between is where it gets interesting..


EDIT: I tried the transistor bypass mentioned above, but found the sound better using the schematic as is.

I greatly appreciate your feedback! Seems like you are enjoying it as much as I am!

[liquids]

Does that mean the second transistor was actually bypassed? Or is it that it made 2 signal paths going to that third transistor?

I've done further work and experimenting,  and yes, it seems that for all intensive purposes signal was havling little to do with the 2nd transistor.  I suspected that, but wanted to make sure. In the shocktave, that stage is mostly functioning as a buffer, however it also importantly feds a base bias voltage to the transistor that followes it, which is why you cant just bypass that stage altogether, as in elminate it.  There need be compensation.

So, the main difference in sound between feeding the boost stage off that stages emitter vs or straight off the base was the voltage difference between the two. The slightly more positive voltage at the base happened to sound 'better' to my ears as is, it turns out, what I tweaked it around; with signal tapping off the emitter, it gated a bit sooner, due to a slightly lower voltage. 

Since I have a buffer stage in there already, it's probably not needed and can be worked around.  The base of the transistor providing the 'boost' could be biased on it's own, similarly, by feeding it the 2.2M/1M or something similar.  By eliminating the second transistor stage, I used a voltage divider (pot) to get down to exactly what voltage the base wants to see. it's really in optimal range in the area around 3V (with a 9.4v supply).  I was surprised, but it won't bias at all above 4v or below 2V, roughly speaking. 

Since I was back at the drawing board with a renewed interest in improving it, I'm working on the filtering amongst other things.  I may even add a buffer stage after the booster stage, with some more filtering, before the frequency-to-voltage and oscillator sections...

The circuit works with the schematic posted as-is, and still rather similarly to the sound samples, as ~arph has confirmed.  That being said, I'll definitely post here when I've updated it with further revisions.

[isildur100]

Thanks for this update!

cheers

[liquids]

The schematic has been updated.  A few tweaks in there, but I mostly just removed the second transistor stage and biased the base of the 'boost' stage directly.  I also tweaked the voltage divider to bias the base(s) of the emitter follower stages more optimally.

I worked on the filtering a bit while I was at it, eased up on the pre-boost filtering to make the trimmer's range more 'usable,' but also added a little more filtering after the booster stage. 
 
Overall, the circuit can always be tweaked and modded to your personal needs if you are so inclined, from Joe's original circuit on up.  I am always tweaking and experimenting, but I definitely think this 'corrected' version is a great build with no mods required, so to speak.  So I'm going to try to leave the schematic as is from here on out.      

I am back to working on a reasonable sized vero layout, which may be more practical now that I've eliminated the 'unnecessary' transistor stage.  I will post an update about that when I've finished and boxed one up for myself, whenever that is!   

[mantella]

Vero update please!

[liquids]

Nothing final to speak of.   For me, i'd say it would be a months time till I really got to it and finished a vero at the soonest, FYI, probably more, and it doesn't seem like anyone else is working on a vero layout so far.

I talked to John Lyons about doing a 'run' of PCBs a few weeks ago.  How much interest would there in that?  The disadvantage is it's obviously costlier than doing it on vero.

If there were enough people interested in buying a PCB for the circuit from him, the time/cost might be worth it, and it would get the circuit in hands far sooner.

Maybe PM me, or post here if you think you'd buy a PCB from John if he he designed one for us.   Otherwise, be very, very patient, grasshopper. 

[~arph]

I'll buy a PCB.

A vero for this would be very large. I started on one, but it got too big too fast.

[funkycam]

I would buy a pcb.
Possibly 2 so I could make one for my bassist buddy.
I think this circuit sounds freakin fantastic

[Naz Nomad]

The soundclips inspired me to draw up this, based on the schematic from page 1 ... at least I found a use for my insomnia, right?  



... not verified, but it looks ok AFAIK.

[isildur100]

Here is my effort at making a perf layout for this fine effect  If you see any errors, please tell me and I will update it.



cheers

[liquids]

Isildur...you listed the resistors following Q1 as 33k, but they should be 3k3 / 3.3k, FYI.

[isildur100]

Thanks, I just corrected the layout, refresh the page if you still see them as 33k.

cheers