Multiple Preamplifier Disorder

[anti-idiot]

Hello everybody!

I wanted to share this with you. It's a simplifyed version of the pramp I'm desingning. It consists of multiple bypassable stages, switchable mods and it was designed using 3 Marshall Amps: 1959SLP (1 stage + pre-eq gain), JCM800 2203 (2 stages + pre-eq gain) and JCM900SL-X (3 stages + pre-eq gain). Each stage consists of a Noiseless biased SRPP using FETs

There are switchable mods: Bright cap, Boost (only for modes 2 and 3), Thick (for the 3 modes) and modable Tonestack (33k-470p/56k-220p/46k-690p).

I recommend you to use @15-18v

Here's the schematic:

http://i20.photobucket.com/albums/b228/Anti-Idiot/MPD-SingleVersion.jpg

EDIT: I forgot, please excuse me for not numbering each component, i'll do it ASAP. If you need to point something out, the reference is: 1st stage, 3rd stage, 2nd stage and pre-EQ stage. Also, the LED is a multicolored LED (R/G).

This is a single version. the other version should have 3 gains and 3 tonestacks with volume per channel (15 pots), output buffer, master volume, internal switching using relays (for bypassing stages and switching the gain/tonestacks), external channel switching (using this switching) and poweramp w/presence & resonance (this one).


Please ask.

[davidallancole]

The poweramp w/presence & resonance that you link to, is this something you have come up with or is it based off of something?  The 1000uF cap looks like it could be reduced to a much lower value.

[anti-idiot]

The poweramp is from the Marshall MG/ModeFour, and there's a error. It's not 1000u, it's 1u. What do you think about the preamp?

[davidallancole]

Do you have a link to the schematic of the Marshall MG/ModeFour?

I haven't built anything with these Mu Amp building blocks yet, so I don't have much of a back ground to comment on this.

Have you built it yet?  Your gain block before the tonestack should probably be more like the one in the General Guitar Gadgets BSIAB2.  As you have it, it doesn't have a flat gain.

[anti-idiot]

Have you built it yet?  Your gain block before the tonestack should probably be more like the one in the General Guitar Gadgets BSIAB2.  As you have it, it doesn't have a flat gain.

http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm

The BSIAB uses a Source follower, mine doesn't need one (mine is the last one)

[liquids]

Some things to consider:

I've been using the ideas R.G. presented as the SRPP, and like it a lot...love the mini-booster, liking the SRPP even better.

Anyhow, I'm not sure what your build actually looks like, but your SRPPs don't match what R.G. did, comparing yours and his schematics: http://www.geofex.com/Article_Folders/modmuamp/jfetmu3.gif

You might want to get to the bottom of it, to find out if the schematic is just incorrectly drawn, or your build indeed matches your schematic (in which case you're not really using the SRPP stages, and might be improved).

I really like the core sound of the mini-booster when tweaked up, but I find R.G.'s SRPP workings a useful dare I say 'improvement.'  Though it's not a drop in replacement in circuits like the BSIAB, you may want to tinkering with some values to yield what you like tonally, again, but just having the lower impedance and more potential gain is a good thing.  I've been modding my BSIAB here and there for months and months now and love it, but tinkering with just two SRPP run into each other seem to be even better in some ways, for what I'm after.  That being said, it seems like few use it despite it's improvements...glad to see you using it.

Anyhow, it still has an issue or two to me. I've not tried it in this circuit directly, but the way he 'noiseless bias' the upper JFETs seems to only work on theory.  Take a reading at the gate of the upper JFETs and I doubt you'll find they are half the power supply.  You might want to make the 10k resistor from +V to the 1/2v junction more like 4.7 volts to get something closer to 1/2v by the time it reaches the gate.   Or just use two 1M resistor pairs for each...if you don't mind the added noise (not sure if it's noticable or not, because I haven't tried).

And Even better than that, while they are more parts, if you have the space, is 'noiseless biasing' in conjunction with an op amp for a highly STABLE Vref that sits close to 1/2V with little wavering despite load...  I don't quite understand why people don't use that more, I find it worth it to give up a little space and add a few components to gain a more stable Vref.

[anti-idiot]

Some things to consider:

I've been using the ideas R.G. presented as the SRPP, and like it a lot...love the mini-booster, liking the SRPP even better.

Anyhow, I'm not sure what your build actually looks like, but your SRPPs don't match what R.G. did, comparing yours and his schematics: http://www.geofex.com/Article_Folders/modmuamp/jfetmu3.gif

You might want to get to the bottom of it, to find out if the schematic is just incorrectly drawn, or your build indeed matches your schematic (in which case you're not really using the SRPP stages, and might be improved).

I'm doing this:

"This trick is known as 'noiseless biasing'. Note that if you have several JFET stacks as some effects pedals do, you can run a single 1/2 meg resistor to each one from the same two 10K resistors and they'll be perfectly happy. You can do this on any mu-amp setup."

I really like the core sound of the mini-booster when tweaked up, but I find R.G.'s SRPP workings a useful dare I say 'improvement.'  Though it's not a drop in replacement in circuits like the BSIAB, you may want to tinkering with some values to yield what you like tonally, again, but just having the lower impedance and more potential gain is a good thing.  I've been modding my BSIAB here and there for months and months now and love it, but tinkering with just two SRPP run into each other seem to be even better in some ways, for what I'm after.  That being said, it seems like few use it despite it's improvements...glad to see you using it.

Anyhow, it still has an issue or two to me. I've not tried it in this circuit directly, but the way he 'noiseless bias' the upper JFETs seems to only work on theory.  Take a reading at the gate of the upper JFETs and I doubt you'll find they are half the power supply.  You might want to make the 10k resistor from +V to the 1/2v junction more like 4.7 volts to get something closer to 1/2v by the time it reaches the gate.   Or just use two 1M resistor pairs for each...if you don't mind the added noise (not sure if it's noticable or not, because I haven't tried).

And Even better than that, while they are more parts, if you have the space, is 'noiseless biasing' in conjunction with an op amp for a highly STABLE Vref that sits close to 1/2V with little wavering despite load...  I don't quite understand why people don't use that more, I find it worth it to give up a little space and add a few components to gain a more stable Vref.

Well, the op-amp may be more stable, but what if I need some SAG?

BTW, what do you think of this?
http://i20.photobucket.com/albums/b228/Anti-Idiot/MarshallFETPreamp.gif

(it's not mine, a buddy gave it to me)

Here's an improved version (without the bright swtich and the tonestack monstruosity) and a few mods
http://i20.photobucket.com/albums/b228/Anti-Idiot/MPD-SimpleVersion.jpg

[Brymus]

I saw this over at SSguitar too 
Do you have any sound clips for this or is it still just on paper?

[anti-idiot]

I saw this over at SSguitar too 
Do you have any sound clips for this or is it still just on paper?

still on paper. this is the "simple" version. the "complex" version uses relays and i'm still working on it, but they're basically the same.

BTW, what happens if I apply positive voltage to both "pins" of a relay? +v to both terminals

[Brymus]

Wouldnt the voltage stay at zero across the coil without a difference in voltage or polarity?
The coil couldnt energize and create a magnet to attract the throw of the switch I'm guessing.

[anti-idiot]

Wouldnt the voltage stay at zero across the coil without a difference in voltage or polarity?
The coil couldnt energize and create a magnet to attract the throw of the switch I'm guessing.

yes, i know, what i wanted to say was that if doing the afore mentioned action would causa any damage to the relay?

[earthtonesaudio]

Some things to consider:

I've been using the ideas R.G. presented as the SRPP, and like it a lot...love the mini-booster, liking the SRPP even better.

Anyhow, I'm not sure what your build actually looks like, but your SRPPs don't match what R.G. did, comparing yours and his schematics: http://www.geofex.com/Article_Folders/modmuamp/jfetmu3.gif

You might want to get to the bottom of it, to find out if the schematic is just incorrectly drawn, or your build indeed matches your schematic (in which case you're not really using the SRPP stages, and might be improved).

I'm doing this:

"This trick is known as 'noiseless biasing'. Note that if you have several JFET stacks as some effects pedals do, you can run a single 1/2 meg resistor to each one from the same two 10K resistors and they'll be perfectly happy. You can do this on any mu-amp setup."

I really like the core sound of the mini-booster when tweaked up, but I find R.G.'s SRPP workings a useful dare I say 'improvement.'  Though it's not a drop in replacement in circuits like the BSIAB, you may want to tinkering with some values to yield what you like tonally, again, but just having the lower impedance and more potential gain is a good thing.  I've been modding my BSIAB here and there for months and months now and love it, but tinkering with just two SRPP run into each other seem to be even better in some ways, for what I'm after.  That being said, it seems like few use it despite it's improvements...glad to see you using it.

Anyhow, it still has an issue or two to me. I've not tried it in this circuit directly, but the way he 'noiseless bias' the upper JFETs seems to only work on theory.  Take a reading at the gate of the upper JFETs and I doubt you'll find they are half the power supply.  You might want to make the 10k resistor from +V to the 1/2v junction more like 4.7 volts to get something closer to 1/2v by the time it reaches the gate.   Or just use two 1M resistor pairs for each...if you don't mind the added noise (not sure if it's noticable or not, because I haven't tried).

And Even better than that, while they are more parts, if you have the space, is 'noiseless biasing' in conjunction with an op amp for a highly STABLE Vref that sits close to 1/2V with little wavering despite load...  I don't quite understand why people don't use that more, I find it worth it to give up a little space and add a few components to gain a more stable Vref.

Well, the op-amp may be more stable, but what if I need some SAG?

BTW, what do you think of this?
http://i20.photobucket.com/albums/b228/Anti-Idiot/MarshallFETPreamp.gif

(it's not mine, a buddy gave it to me)

Here's an improved version (without the bright swtich and the tonestack monstruosity) and a few mods
http://i20.photobucket.com/albums/b228/Anti-Idiot/MPD-SimpleVersion.jpg

I think what Liquids meant when he said your schematics don't match is that the SRPP takes the output from the source of the upper FET, and the cap between them goes from the upper FET's gate to the lower FET's drain.  In your schematic, those two parts connect to the "wrong" ends of the 1k resistor.  This may sound fine, but you're not getting the full benefits of the SRPP stage, in particular the low output impedance.

I think the issues Liquids has with noiseless biasing apply more to BJT-based circuits than FET-based.  There's still a minuscule voltage drop by the time the bias voltage reaches the JFET gates, but not nearly so much as with a comparable BJT stage.  Replacing Vref with an op-amp won't fix that, because the voltage drop is across the 1M resistor.  In any case, it's the voltage at the drain of the lower JFET that you care about, so you should take your measurements there rather than at the upper JFET gate.

[liquids]

I think what Liquids meant when he said your schematics don't match is that the SRPP takes the output from the source of the upper FET, and the cap between them goes from the upper FET's gate to the lower FET's drain.  In your schematic, those two parts connect to the "wrong" ends of the 1k resistor.  This may sound fine, but you're not getting the full benefits of the SRPP stage, in particular the low output impedance.

I think the issues Liquids has with noiseless biasing apply more to BJT-based circuits than FET-based.  There's still a minuscule voltage drop by the time the bias voltage reaches the JFET gates, but not nearly so much as with a comparable BJT stage.  Replacing Vref with an op-amp won't fix that, because the voltage drop is across the 1M resistor.  In any case, it's the voltage at the drain of the lower JFET that you care about, so you should take your measurements there rather than at the upper JFET gate.

Thanks Alex...I didn't know how to explain that half as well as you with twice the verbage.  

The above is also helpful to me, since my experiments, as always, are limited and indeed have been mostly with BJTs of late.  From my talks with an EE here at work, I had assumed the voltage drop and the values needed to get truly 1/2 +V at the 'base/gate' could be determined via complex calculations, but the BJT vs JFET factor wasn't mentioned for whatever reason.  To keep it simple for me, in short, we discussed that if the resistor coming from the junction was that much larger in resistance (1M+) than the value of the resistors in the divider (presumably 10k), it would 'always' be an issue and need to be compensated for....  

When I was asking questions and working on a project, I had another local guy 'demonstrate' to me how much more stable a op-amp based Vref supply was, with the value of the resistor (1M for example) having far less effect on the output voltage, and it was a great find to me....  I've been using these ideas building blocks on the breadboard, but without knowing the ins and outs...since I'm just a hack.    Your complimentary info about the variations in other situations (rather than it being one way or another, across board) is appreciated!  And thanks for always being so nice about it.  

[anti-idiot]

[facepalm] ok, ok, i got it!

i was so distracted i didn't see the mistake. I'll fix that.


damn you, 1k resistor!

[anti-idiot]

**UPDATED SCHEMATIC**

http://i20.photobucket.com/albums/b228/Anti-Idiot/MPD-SimpleVersion.jpg

thank you, liquids and ETA.

BTW, do you know the differences (soundwise) between  a mu-amp stage and a cascode?

[earthtonesaudio]

Your schematic still doesn't match the SRPP.  The caps going to the upper JFET gate should have their other end connected to the lower JFET drain.  Again, it may sound fine (better?) this way, so of course let your ears be your guide.  I'm just pointing out that it's not quite the same as an SRPP stage.

For your cascode question, the main differences are bandwidth and output impedance.  The cascode isolates the output from the input (physically and electrically) and so reduces Miller capacitance, which leads to increased bandwidth.  The cascode has high voltage gain, but a high output impedance, so loading on the output reduces the gain significantly.

The output impedance of the cascode can be improved with an extra buffer stage (or tacking on an SRPP to the top for a 3-JFET gain stage), but by this point the complexity is increased quite a lot, and the only real benefit to the cascode vs. the mu-amp or SRPP is the high bandwidth.  For audio (and especially guitar) the bandwidth from a non-cascode stage is plenty, so personally I don't see the point.

[anti-idiot]

Your schematic still doesn't match the SRPP.  The caps going to the upper JFET gate should have their other end connected to the lower JFET drain.  Again, it may sound fine (better?) this way, so of course let your ears be your guide.  I'm just pointing out that it's not quite the same as an SRPP stage.

fixed:

http://i20.photobucket.com/albums/b228/Anti-Idiot/MPD-Simple.jpg

For your cascode question, the main differences are bandwidth and output impedance.  The cascode isolates the output from the input (physically and electrically) and so reduces Miller capacitance, which leads to increased bandwidth.  The cascode has high voltage gain, but a high output impedance, so loading on the output reduces the gain significantly.

The output impedance of the cascode can be improved with an extra buffer stage (or tacking on an SRPP to the top for a 3-JFET gain stage), but by this point the complexity is increased quite a lot, and the only real benefit to the cascode vs. the mu-amp or SRPP is the high bandwidth.  For audio (and especially guitar) the bandwidth from a non-cascode stage is plenty, so personally I don't see the point.

thanks for your answer. i was reading in another forum, and i think you have lots of experience with Mu-amp/srpp stages. in which pedals did you use uAmp/SRPP?

[earthtonesaudio]

I've actually never boxed up a mu-amp or SRPP circuit into a final pedal.  So the only experience I have with them is on the breadboard.  But, I have breadboarded circuits with them many times.  I haven't used them in a cascaded distortion like the BSIAB or your circuit, mostly just when I need a gain stage with low output impedance.