Triode emulater revisited, again: a mu-amp/minibooster variation

[tca]

> how does this new alternate schem compare to the vero layout above? 
The vero does not fit this last circuit.

> You've added a bypass Cap across R2. 
The cap across R2 just bypass the resistor for more gain in AC.

> Does one schem sound cleaner than the other, different frequency peaks, etc? 
This version does not compress the signal as much as the mu-valve, but I didn't play enough to give you a straight answer. It is very hard to describe the sound of these circuits using words, you really have to breadboard and listen for yourself. The same happens with Jack's minibooster, it ads something to the guitar signal and makes it more organic and fluid.

> I have an empty AMZ Mini-Booster PCB and an empty AMZ Dual Booster PCB that would work nicely, if one of these circuits warrant a complete pedal..

Cool.

Cheers.

[amptramp]

We had a discussion of this before:

http://www.diystompboxes.com/smfforum/index.php?topic=94719.msg843061;topicseen#msg843061

Most people use µ-amp amd Shunt Regulated Push-Pull (SRPP) as synonyms.  Consider a tube or depletion-mode JFET version of the SRPP.  You have an upper device, a lower device and a resistor from the drain/plate of the lower device to the source/cathode of the upper device.  If the gate/grid of the upper device is returned to its own source/cathode, it becomes a current source (high load resistance) in the JFET version or a reasonably low load resistor in the tube version.  This gives you an amplifier stage with high gain for a FET or moderate gain for a triode. This is not SRPP.

If the upper stage gate/grid is returned to the drain/anode of the lower stage on the bottom of the resistor, the behaviour of the circuit is entirely different.  The drain/plate current flows through the resistor and provides a negative bias with more current through the bottom device tending to turn off the top device.  Let's say you have JFET's or triodes with a 2000 µmho transconductance, that is, a change of 1 volt at the input provides a change of 2 mA of current.  What size of resistor would cause the current in the upper device to change in the opposite direction by the same amount as the change in current in the lower device?  We already know that a 1 volt change in the input to the lower device causes a 2 mA change in current.  This 2 mA across the series resistor must cause the same amount of voltage change in the opposite direction in the upper stage. The 2 mA change must force the voltage to change by 1 volt across the resistor, so: 1V/2mA is 500 ohms.  This shows how the resistor must be the resiprocal of the upper device transconductance in value to get the same current change in the opposite direction for upper and lower devices - in other words, balanced push-pull operation.

But this is a totem-pole circuit - the current in the upper stage minus the current in the lower stage has to be delivered to the load or the stage will simply slam up against the upper or lower power rails.  Thus, you need a finite load (which may include a resistor for the upper gate or grid if it is AC-coupled to the lower drain/plate) or a tone stack.  We have seen this already in the waveforms on the previous page and now you know why.

[tca]

But this is a totem-pole circuit...

(edit for some corrections)

Yes. The MPF102 has a typical 5000uS transcunductance (VGS=0, VDS=15V) which gives a a 5mA current rise for each V which means that the source resistor for the upper jfet should be 400 200 (edit: corrected). Half the transcunductance is 400, I'm looking for Idss/|VP|. The value of the "triode" source resistor is about .84*400 which gives something  close to 300. This justifies the choice of the resistor values for the circuit. The 10k resistor is there just to control the gain, otherwise the sound is to harsh.

Thus, you need a finite load... or a tone stack.

I've build a Fender type tone stack after it and it sounds lovely.

Thanks, again, for mention this.

[amptramp]

I get that 5 mA/V transconductance should be paired with a 200 ohm source resistor.  In fact, the transconductance varies quite a bit and can vary with bias and the idea here is to have the current rise in the lower transistor match the current drop in the upper transistor and vice versa.  Your values are close and they are not that critical.  Since it is a totem pole, current through the top minus current through the bottom should equal current through the load.  That is where the finite load comes in.  Glad to see you have a circuit you are satisfied with.

[chptunes]

Hey TCA, I built the Mu-Valve.. Sounds great.  Upon first listen, sounds a bit smoother than a Mini-Booster.  You can definately hear and feel more compression too.

Sweet circuit.. thanks.

[ct_anthony]

Hello all,

I'm a little bit late to the party (and still noobish ) but want to build this nevertheless...

I have some trouble choosing the resistors R1/R2...

Let's assume I have the 2N5457 with  the following values:

Vp = 2,34v
Idss = 0,00565A

Then I calculate Rs as ~ 348R which is close to the mentioned 330R.

Assuming V = 9v... then Vp/V is 0,26

For R1, R2  my math gives me (hopefully it is correct) something like R2 = R1 * 0,26 / (1-0,26)... but how do I chose either R1 or R2 correctly?

When I set R2 to the mentioned 2M I get something like ~ 773K for R1... is that correct? Why not choose R2 as 1M ??

Thanks and Regards,
anthony

[tca]

> When I set R2 to the mentioned 2M I get something like ~ 773K for R1... is that correct? Why not choose R2 as 1M ??

Go for it, no problem... the crucial value is the value of RS. Also try R1=R2=1M

Happy hacking!

[ct_anthony]

> When I set R2 to the mentioned 2M I get something like ~ 773K for R1... is that correct? Why not choose R2 as 1M ??

Go for it, no problem... the crucial value is the value of RS. Also try R1=R2=1M

Happy hacking!

Thanks for the fast reply! Well then i´ll give both 2M/1M and 1M/1M a try...

[ct_anthony]

Hi again!

Unfortunately I can not build until Saturday morning

But in the meantime I can plan

Is it ok to "bias" the upper JFet like in the following schematic? It would easily match Vp...



It shows values for Rs, R1, R2 which are "calculated" for the average 2N5457 from the Rog Fetzer article:

average Vp: -1.58V (spanning from -1.22 to -1.80)
average Idss: 3.27mA (spanning 1.96mA to 4.11mA)

And hopefully the adapted input does not have a negativ effect....

Regards,
Anthony

[al3xandr3]

I've breadboarded it, using original (tca) design.

Didn't had MPF102, so tried first with J201 but got distortion, then tried 2N5457 and it sounds great !

Suspect is one of those that will be most of the time on.
Need to try with other amps also ( only tried with an LM386 based amp for now)

thanks for sharing tca !