tonight's SRPP JFET preamp gain stage

Hi everyone, as usual welcoming advice and comments about my projects,

For some reason, I feel compelled to build another solid state amp
with JFET preamp. I really do love the little 5W one I have here,
whose schematic I plan to someday post, but I want something with
plenty gain for lead and not a lot of noise or instability issues.
There are a lot of JFET circuits out there, for guitar, which
basically emulate classic common-cathode stages, and they do sound
good! But they don't have as much gain as their tube cousins, and so
you need a lot of stages, and when you get those stages together in
sufficient quantity to give you some sustain, then you have to be
ultra paranoid about feedback and motorboating running through the
supply rails. At least that was my own experience.

For that reason I've been looking into the SRPP, series-regulated-push-
pull stage as it is called, although that's a fancy name for putting a
current source instead of the plate/drain resistor. These are at
least supposed to be good at rejecting power supply noise, and
simulations I've done in LT-SPICE seem to indicate that. If they work
that way in real life, I won't have to worry about feedback and
motorboating running through the supply rails, nor will I have to be
ultra-paranoid about how I regulate the preamp power supply to keep
hum out, but I should be able to build a real high-gain amp. So good
noise rejection is something that I like. At the cost of only one
more JFET per gain stage.

I totally like having more gain, too. One way to do that is to run
the JFET with a huge drain resistor. Or, in the case of the SRPP, a
low regulated current. If the output impedance is too high, I can
always buffer it with a unity gain stage. So fine, now I'm up to 3
JFETs per gain stage. I wonder though - do JFETs get ugly when run
close to cutoff? The standard square-law current model doesn't seem
to indicate any. Is there ugliness? Guess I got to play it and see.

So tonight I set up the following circuit. Battery power only for
now, so I can't really test the power supply rejection ratio, but hey,
I simulated it first and it ran fine. Yes, I am using MPF102 parts,
and I know their reputation for flaky random parameters, but luckily
the last ones I bought from Radio Shack all matched each other really
well, and the SRPP pair are highly well matched with Idss of about 11
ma, Vp of 3.65, matched to within 3%! Which is cool because any
mismatch will limit my voltage swing, at least I think so. Although
I'm running them very, very cold in this circuit:

* img246.imageshack.us/img246/8793/srppgw8.png

So here's the circuit. Guitar goes into one side, output goes into a
power amp.

What's it sound like? There's not a lot of headroom here. The full
output swings only about 4 Volts up and down, because I'm dropping so
many volts across the bias resistors. And there is gain, tons and
tons of it, it starts clipping when my guitar volume is at about 2
even if I play gently. I can't get a clean tone out of it, it just
hits the wall waay too quickly. It doesn't clip with a buzz, it clips
with a gentle second harmonic coming in, followed by an octave fuzz
tone, followed by a bluesy squall. Very neat, I think it'd make a
nice effect, but I don't know if this makes a good generic gain stage
for preamp design yet. I don't know how I'd get a metal crunch or a
clean tone out of a preamp based on this stage. It really seems to do
what it does really well though.

Perhaps if I were to raise the idle current, drop the gain, then it
would make a good generic gain stage. What do you guys think? Have
any of you used the SRPP, what do you think of the sound?

On Thu, 8 May 2008 23:51:17 -0700 (PDT), morris.slutsky@gmail . com
wrote:

>Hi everyone, as usual welcoming advice and comments about my projects,
>
>For some reason, I feel compelled to build another solid state amp
>with JFET preamp. I really do love the little 5W one I have here,
>whose schematic I plan to someday post, but I want something with
>plenty gain for lead and not a lot of noise or instability issues.
>There are a lot of JFET circuits out there, for guitar, which
>basically emulate classic common-cathode stages, and they do sound
>good! But they don't have as much gain as their tube cousins, and so
>you need a lot of stages, and when you get those stages together in
>sufficient quantity to give you some sustain, then you have to be
>ultra paranoid about feedback and motorboating running through the
>supply rails. At least that was my own experience.
>
>For that reason I've been looking into the SRPP, series-regulated-push-
>pull stage as it is called, although that's a fancy name for putting a
>current source instead of the plate/drain resistor. These are at
>least supposed to be good at rejecting power supply noise, and
>simulations I've done in LT-SPICE seem to indicate that. If they work
>that way in real life, I won't have to worry about feedback and
>motorboating running through the supply rails, nor will I have to be
>ultra-paranoid about how I regulate the preamp power supply to keep
>hum out, but I should be able to build a real high-gain amp. So good
>noise rejection is something that I like. At the cost of only one
>more JFET per gain stage.
>
>I totally like having more gain, too. One way to do that is to run
>the JFET with a huge drain resistor. Or, in the case of the SRPP, a
>low regulated current. If the output impedance is too high, I can
>always buffer it with a unity gain stage. So fine, now I'm up to 3
>JFETs per gain stage. I wonder though - do JFETs get ugly when run
>close to cutoff? The standard square-law current model doesn't seem
>to indicate any. Is there ugliness? Guess I got to play it and see.
>
>So tonight I set up the following circuit. Battery power only for
>now, so I can't really test the power supply rejection ratio, but hey,
>I simulated it first and it ran fine. Yes, I am using MPF102 parts,
>and I know their reputation for flaky random parameters, but luckily
>the last ones I bought from Radio Shack all matched each other really
>well, and the SRPP pair are highly well matched with Idss of about 11
>ma, Vp of 3.65, matched to within 3%! Which is cool because any
>mismatch will limit my voltage swing, at least I think so. Although
>I'm running them very, very cold in this circuit:
>
> * img246.imageshack.us/img246/8793/srppgw8.png
>
>So here's the circuit. Guitar goes into one side, output goes into a
>power amp.
>
>What's it sound like? There's not a lot of headroom here. The full
>output swings only about 4 Volts up and down, because I'm dropping so
>many volts across the bias resistors. And there is gain, tons and
>tons of it, it starts clipping when my guitar volume is at about 2
>even if I play gently. I can't get a clean tone out of it, it just
>hits the wall waay too quickly. It doesn't clip with a buzz, it clips
>with a gentle second harmonic coming in, followed by an octave fuzz
>tone, followed by a bluesy squall. Very neat, I think it'd make a
>nice effect, but I don't know if this makes a good generic gain stage
>for preamp design yet. I don't know how I'd get a metal crunch or a
>clean tone out of a preamp based on this stage. It really seems to do
>what it does really well though.
>
>Perhaps if I were to raise the idle current, drop the gain, then it
>would make a good generic gain stage. What do you guys think? Have
>any of you used the SRPP, what do you think of the sound?
>
>
Might be morphed into a SRV pedal if nothing else...

Just my two cents,

It seems that the circuit has way too much gain for any realistic
application. I think you can omit the cap bypassing the source
resistor and still have a circuit that provides enough of gain. If
not, then just leave the cap there but put some series resistance to
it. Perhaps a pot for gain control?

A major part of the clipping in your circuit comes also from the
source follower. Such circuit has 100% degenerative feedback so the
clipping onset is quite abrupt and clipping is not very soft. That=92s
not a bad thing, though. In fact, this characteristic is pretty widely
utilized in tube amps as well. The source follower also introduces
plenty of asymmetry =96 and therefore even harmonics. The SRPP by itself
would introduce less of those kinds of characteristics since it=92s a
push-pull stage.

What comes to =93metal crunch=94, that=92s pretty much a result of careful
voicing. Take for example the typical FET circuitry from SS Randall
amplifiers: Two high gain common source circuits (latter one of them
directly coupled to a source follower) are coupled in a way that
reduces bass frequencies a lot =96 I mean a lot. Treble is pretty much
reserved throughout the circuit. Cutting the bass pre-clipping
preserves much of the =93clarity=94 and =93definition=94 because clipping of=

low frequencies tends to =93mask=94 the higher frequencies that ride at
the top of the LF wave. Reducing bass pre-clipping creates a less
=93buzzy/fuzzy=94 tone. The approach is quite the opposite of what your
circuit seems to do: There=92s not much bass roll off and the gate
capacitances reduce the treble.

Continuing on the Randall circuit, the FET stages have a huge output
voltage swing but they are followed by a normal shunting diode
clipper. This produces almost square-wavish overdrive, except that the
signal before this clipping stage is already softly =93pre-limited=94 by
the FETs and clipped in asymmetric manner. This causes the diode
clipper to introduce a duty cycle modulation; which in turn enhances
even harmonic content. Finally, the whole stage is fed to an ordinary
=93mid scooping=94 tone control that is voiced to reduce high frequencies
and enhance the bass frequencies. I can't remember what the scoop
frequency was but it has a big effect to overall tone. The bass
boosting pre-clipping creates the massive bottom end =93chunk=94 metal
tones usually have.

Now, that=92s just one example and its point is mainly to show that
often creating a good sounding overdrive is simply something that
cannot be done in a single circuit. Yes, SRPP stages overdrive nicely
but that=92s just one part of the whole thing. The way how you process
the clipped signal frequency-wise is the other part and it often has a
way bigger effect. You could, for example, experiment with a graphic
EQ following that gain stage and see what you can do by varying the
frequency response. Try different HF and LF roll off frequencies and
introducing scoops and boosts at various bands. You might find some
interesting settings. Sometimes doing this voicing gradually, stage-by-
stage, creates a more "organic" tone. That's what most tube amps tend
to do anyways.

Also, expecting that overdrive circuits do clean tones well is pretty
pointless. That approach is always a big compromise between various
things. Just use different channels for clean and OD.

On Fri, 9 May 2008 07:00:08 -0700 (PDT), teemukyttala@gmail . com wrote:

>Just my two cents,
>
>It seems that the circuit has way too much gain for any realistic
>application. I think you can omit the cap bypassing the source
>resistor and still have a circuit that provides enough of gain. If
>not, then just leave the cap there but put some series resistance to
>it. Perhaps a pot for gain control?
>
>A major part of the clipping in your circuit comes also from the
>source follower. Such circuit has 100% degenerative feedback so the
>clipping onset is quite abrupt and clipping is not very soft. That’s
>not a bad thing, though. In fact, this characteristic is pretty widely
>utilized in tube amps as well. The source follower also introduces
>plenty of asymmetry – and therefore even harmonics. The SRPP by itself
>would introduce less of those kinds of characteristics since it’s a
>push-pull stage.
>
>What comes to “metal crunch”, that’s pretty much a result of careful
>voicing. Take for example the typical FET circuitry from SS Randall
>amplifiers: Two high gain common source circuits (latter one of them
>directly coupled to a source follower) are coupled in a way that
>reduces bass frequencies a lot – I mean a lot. Treble is pretty much
>reserved throughout the circuit. Cutting the bass pre-clipping
>preserves much of the “clarity” and “definition” because clipping of
>low frequencies tends to “mask” the higher frequencies that ride at
>the top of the LF wave. Reducing bass pre-clipping creates a less
>“buzzy/fuzzy” tone. The approach is quite the opposite of what your
>circuit seems to do: There’s not much bass roll off and the gate
>capacitances reduce the treble.
>
>Continuing on the Randall circuit, the FET stages have a huge output
>voltage swing but they are followed by a normal shunting diode
>clipper. This produces almost square-wavish overdrive, except that the
>signal before this clipping stage is already softly “pre-limited” by
>the FETs and clipped in asymmetric manner. This causes the diode
>clipper to introduce a duty cycle modulation; which in turn enhances
>even harmonic content. Finally, the whole stage is fed to an ordinary
>“mid scooping” tone control that is voiced to reduce high frequencies
>and enhance the bass frequencies. I can't remember what the scoop
>frequency was but it has a big effect to overall tone. The bass
>boosting pre-clipping creates the massive bottom end “chunk” metal
>tones usually have.
>
>Now, that’s just one example and its point is mainly to show that
>often creating a good sounding overdrive is simply something that
>cannot be done in a single circuit. Yes, SRPP stages overdrive nicely
>but that’s just one part of the whole thing. The way how you process
>the clipped signal frequency-wise is the other part and it often has a
>way bigger effect. You could, for example, experiment with a graphic
>EQ following that gain stage and see what you can do by varying the
>frequency response. Try different HF and LF roll off frequencies and
>introducing scoops and boosts at various bands. You might find some
>interesting settings. Sometimes doing this voicing gradually, stage-by-
>stage, creates a more "organic" tone. That's what most tube amps tend
>to do anyways.
>
>Also, expecting that overdrive circuits do clean tones well is pretty
>pointless. That approach is always a big compromise between various
>things. Just use different channels for clean and OD.
I am surprised..I could actually understand this..my studies must be
paying off...but my solid state audio electronics is still
weak...but.."Onward Thru the Fog" as Oat Willie always said...

Hi teemukyttala,

On May 9, 10:00=A0am, teemukytt...@gmail . com wrote:
> Just my two cents,
>
> It seems that the circuit has way too much gain for any realistic
> application. I think you can omit the cap bypassing the source
> resistor and still have a circuit that provides enough of gain. If
> not, then just leave the cap there but put some series resistance to
> it. Perhaps a pot for gain control?
>

I appreciate your thoughtful advice. Like I mentioned before, I
desire a gain stage with a good PSRR. Not that it matters for a
single gain stage running off battery power - it totally doesn't - but
for cascaded gain stages and an AC power supply, good PSRR is
something I really want in the future.

I agree that this circuit does sound pretty bassy, but that cap has to
stay. That cap is the reason why this circuit has good PSRR. It
unbalances things just right! It doesn't have to be a cap at all - it
could be a couple dropper diodes, a zener, a battery, a DC bias supply
with (relatively) low output impedance - a cap is just the most
convenient thing to use. It's big for PSRR - I know I don't need a 7
Hz lower passband - that's not why it's so big.

Take the schematic and pull out the bypass cap. Imagine that the
JFETs are identical and that the trimpot is set at precisely 47K -
it's really not too far from the truth here anyway. Ground the input
signal, no input at all, shorting out the gate resistor. Now you've
got 2 identical circuits in series, stacked up between V+ and ground.
Each is naturally going to drop exactly half of V+, it's a voltage
divider. PSRR =3D 3 dB, which is pretty bad. Might as well be using a
drain resistor. It's like a stretchy string, everything wants to
move, the resistors and JFETs all want to drop more of the voltage.
Now put the cap back in. Blip a positive-going pulse onto V+.
Pulling on a string, voltages rise everywhere except - - - the drain
of the lower JFET is being held down. Now Vgs of the lower JFET is
greater than Vgs of the upper JFET, turning it 'more on' than the
upper JFET, this pulls the drain of the lower JFET right back down.
Run SPICE on it, you'll see what I mean, that cap is the key. I
imagine that were that cap to get TOO big, in real life as opposed to
in simulation, that the bias of the SRPP would wander up and down at
random. But that's just my usual paranoia.

I suspect that the bassiness is only an issue because I'm driving it
from a high-impedance source. If I had a buffer stage between the
guitar and the SRPP, the gate capacitance wouldn't be such a big
deal. Or even put a passive EQ network between that buffer stage and
the SRPP, that's a good possibility. Definitely a thing to be
tweaking.

> A major part of the clipping in your circuit comes also from the
> source follower. Such circuit has 100% degenerative feedback so the
> clipping onset is quite abrupt and clipping is not very soft. That=92s
> not a bad thing, though. In fact, this characteristic is pretty widely
> utilized in tube amps as well. The source follower also introduces
> plenty of asymmetry =96 and therefore even harmonics. The SRPP by itself
> would introduce less of those kinds of characteristics since it=92s a
> push-pull stage.
>

I was trying to bias the source follower so that it wouldn't clip
unless the SRPP was. But maybe I don't have it right. I suppose it's
all good.


> What comes to =93metal crunch=94, that=92s pretty much a result of careful=

> voicing. Take for example the typical FET circuitry from SS Randall
> amplifiers: Two high gain common source circuits (latter one of them
> directly coupled to a source follower) are coupled in a way that
> reduces bass frequencies a lot =96 I mean a lot. Treble is pretty much
> reserved throughout the circuit. Cutting the bass pre-clipping
> preserves much of the =93clarity=94 and =93definition=94 because clipping =
of
> low frequencies tends to =93mask=94 the higher frequencies that ride at
> the top of the LF wave. Reducing bass pre-clipping creates a less
> =93buzzy/fuzzy=94 tone. The approach is quite the opposite of what your
> circuit seems to do: There=92s not much bass roll off and the gate
> capacitances reduce the treble.
>
> Continuing on the Randall circuit, the FET stages have a huge output
> voltage swing but they are followed by a normal shunting diode
> clipper. This produces almost square-wavish overdrive, except that the
> signal before this clipping stage is already softly =93pre-limited=94 by
> the FETs and clipped in asymmetric manner. This causes the diode
> clipper to introduce a duty cycle modulation; which in turn enhances
> even harmonic content. Finally, the whole stage is fed to an ordinary
> =93mid scooping=94 tone control that is voiced to reduce high frequencies
> and enhance the bass frequencies. I can't remember what the scoop
> frequency was but it has a big effect to overall tone. The bass
> boosting pre-clipping creates the massive bottom end =93chunk=94 metal
> tones usually have.
>

I haven't really gotten to see any GOOD solid-state guitar amp
schematics. Are these Randall schematics available online? There's
about a billion crappy homemade op-amp with clipper diode circuits, I
think nobody needs to see any more of those!

> Now, that=92s just one example and its point is mainly to show that
> often creating a good sounding overdrive is simply something that
> cannot be done in a single circuit. Yes, SRPP stages overdrive nicely
> but that=92s just one part of the whole thing. The way how you process
> the clipped signal frequency-wise is the other part and it often has a
> way bigger effect. You could, for example, experiment with a graphic
> EQ following that gain stage and see what you can do by varying the
> frequency response. Try different HF and LF roll off frequencies and
> introducing scoops and boosts at various bands. You might find some
> interesting settings. Sometimes doing this voicing gradually, stage-by-
> stage, creates a more "organic" tone. That's what most tube amps tend
> to do anyways.
>
> Also, expecting that overdrive circuits do clean tones well is pretty
> pointless. That approach is always a big compromise between various
> things. Just use different channels for clean and OD.

Yeah, I was sorta figuring that I'd switch gain stages in and out of
the same basic layout. That's how I did my last couple projects - one
was a clean multistage JFET amp with a switchable bipolar gain stage,
the other is a tube amp where you can switch one gain stage in and out
(to give you either 2 or 3 stages before the P.I.). But I could
definitely see doing things a more flexible way for a future project.

Thanks for your comments and advice, I do appreciate you taking the
time to let me know what's up with my circuit,

morris.slutsky@gmail . com wrote:
>
>
> I haven't really gotten to see any GOOD solid-state guitar amp
> schematics.

1980...?

MM Amp Schematics (SS Pre-amp)

These ampifiers were designed, manufactured, and sold by the Music Man
Company ... 2475-150 and 2275-150, 2475-150_&_2275-150.zip ·
2475-150_&_2275-150.pdf ...
w w w .ernieball . com /mmonline/techinfo/old_amps/

Gamma Ray Bursts U wrote:
> morris.slutsky@gmail . com wrote:
>>
>>
>> I haven't really gotten to see any GOOD solid-state guitar amp
>> schematics.
>
> 1980...?
>
> MM Amp Schematics (SS Pre-amp)
>
> These ampifiers were designed, manufactured, and sold by the Music Man
> Company ... 2475-150 and 2275-150, 2475-150_&_2275-150.zip ·
> 2475-150_&_2275-150.pdf ...
> w w w .ernieball . com /mmonline/techinfo/old_amps/
>
Not quite all-SS.

TD Madden wrote:
> Gamma Ray Bursts U wrote:
>> morris.slutsky@gmail . com wrote:
>>>
>>>
>>> I haven't really gotten to see any GOOD solid-state guitar amp
>>> schematics.
>>
>> 1980...?
>>
>> MM Amp Schematics (SS Pre-amp)
>>
>> These ampifiers were designed, manufactured, and sold by the Music Man
>> Company ... 2475-150 and 2275-150, 2475-150_&_2275-150.zip ·
>> 2475-150_&_2275-150.pdf ...
>> w w w .ernieball . com /mmonline/techinfo/old_amps/
>>
> Not quite all-SS.

That'd be the (SS Pre-amp), mentioned...

teemukyttala -

Revised for lower gain, and input hi-pass filter over 160 Hz.
Definitely cleaner, it doesn't buzz/fuzz, it overloads cleanly and
very expressively. Slightly more output headroom as well, now that
I'm not dropping quite as much supply voltage across the source
resistors. A few of these with tone shaping between them might well
be a good high-gain channel. As far as clean - well, it's still not
clean. Mostly clean if the guitar volume is 3 and I play light, but
not really. I'll probably try it again with even smaller source
resistors, go for even less gain and more headroom, see if that would
do for a clean channel. It's trimmed to give about 9V at the output
source follower, which is probably not quite right - I probably don't
even need that stage anymore, now that I've lowered the value of the
drain resistors the output impedance of the SRPP is probably plenty
low - but hey I should have thought about that earlier while actually
breadboarding.

Circuit here:

* img237.imageshack.us/img237/7928/srpprevny8.png

Definitely sounds like rock'n'roll - just running it into the 'power
amp in' of a peavey bandit 1x12", that's a very clean amp and I'm sure
it's rendering the tone honestly.

Thank you - and others - for the amp schematics. I did look at the
Randall one, it is nice, although I don't think I want any diode
clipping. Cool power amp stage, no integrated circuits in that thing
I guess.

Here=92s the one [schematic] I was talking about:

* w w w .schematicheaven . com /newamps/randall_rg100es-rg80es.pdf

Yes, I know it=92s pretty old but the overdrive channel in newer stuff
like Warhead X2 is nevertheless almost identical to this one all the
way down to point that feeds the tonestack. After that those amps get
pretty different.

There are actually quite a many Randall schematics that can be found
from Internet if you care to search for them. Most of them are for the
older amps, exception being the X2 schematics, which I believe can be
found somewhere from MTS forums.

If you like to see schematics for nicely designed SS amps then try to
hunt down stuff like newer Peavey, Traynor, Fender and Vox Valvetronix
series schematics. In older stuff, Acoustic Control and GMT / Gallien-
Krueger pretty much rule. Not all of these amps sound exceptionally
great (and that=92s a subjective debate anyway) but you can get plenty
of ideas from studying most of those circuits. There's a lot of neat
stuff that some SS amps do.

Since that stuff is solid-state its not found from virtually every
schematic site. Unfortunately I don=92t really care to keep track of
links, I just save all the interesting schematics to my hard drive/CD-
R for easy access. Therefore I can=92t really help you much in this
issue.

<morris.slutsky@gmail . com > wrote

> I appreciate your thoughtful advice. Like I mentioned before, I
> desire a gain stage with a good PSRR. Not that it matters for a
> single gain stage running off battery power - it totally doesn't - but
> for cascaded gain stages and an AC power supply, good PSRR is
> something I really want in the future.

9v battery for each stage... it doesn't get any better
than that.
__
Steve
.