FET-BJT preamplifier

[DDD]

The circuit is about 3 years old, and I like it because of its good sound, low noise, low current consumption (about 210 uA), virtually no need to adjust it, etc.
I use this preamp in my overdrive stompboxes generally.
Features:
1. Pickup resonance frequency 3-position selector
2. "Fat" 3-position selector
3. Gain (mode) 3-position selector (from clean boost to high-frequency boost to hi-gain), from -7dB to +43dB
4. Gain control
5. Volume (Master) control

Schematic:


Gain vs. X1 pot, Low-gain Mode:


Gain vs. X1 pot, High-gain Mode:


DC Voltages:

[knutolai]

Looks interesting. Reminds me of the amz minibooster and R.G. SSRP. Got sounds?

[DDD]

I do not use this pre-amp as a separate gadget.
It's just an input part of some of my stompboxes, and it is intended to boost/compress pickup signal with relatively low distortion and some output dynamics.
Hence I do not have samples of just this circuit.

[bool]

Perhaps you could omit the C11. What does it do other than to look good in the circuit? R13/15 could also be made into a pot to control the clipper?

The "gain" control is sheer genius. I also have (two) standardised circuits with tweakable mu/srpp active loads, but for different purpose than yours.

[DDD]

Wow! Thank you for C11 reminder! That C11 is a remains from the previous full-BJT version, and it should be omitted surely.
Changing D1 and D2 with different color LEDs makes more impressive difference in clipping rather than playing with R13/R15 values.
To be correct, D1 and D2 work not only as a feedback elements, but also as a clippers at FET's gate. To be frank, I can hardly understand their full duties.

[midwayfair]

If you omit C11, you'll have DC appear on the output via R14. This might matter to some people who might use it alone, or if you're plugging this into FETs.

[DDD]

Both R14 terminals are grounded, so there's no any DC voltage on it.
Hence C11 is absolutely useless. Moreover, its impedance makes negative feedback frequency-dependent. I do not know if it's good or bad...
Also, I've changed D1 and D2 with RED LEDs, and now the circuit can handle up to 2 Volts of input signal without distortion (@Low gain mode, 1nF input cap).
The improved schematic is below.

[midwayfair]

Typo. R13. The one that bypasses the output cap.

[DDD]

Output cap is C10 (stops DC from Q1 emitter).
C13/R13 on the newest diagram is a HF correction. If there's enough high frequencies (HF), one can omit C13/R13 and connect C10 directly to X2 pot.

[bool]

Well clipper diodes will limit the gate voltage with something like Vin - (Vout*(R15/R13+R15)*Vpnjunction) * (ahem ... super secret expression wrt impedance ratios) so their "function" will be in all cases heavily dependant on what "goes in" and "goes out" ... imho best left to a simulator to determine and to play-back the marvelous waveforms for us peeps to admire ...

[DDD]

Well, please find below the requested wondrous waveforms 
The AC plot shows maximum gain vs. frequency. Other plots are just examples of distorted waveforms.

One can clearly see that output waveform duty cycle depends on the input signal amplitude. Also, there is a great lot of possible controls combinations that affect the distortion pattern drastically. Hence we can expect it's possible to find a "sweet spot" with necessarily dynamic tone.

[Ben N]

Very interesting, and beautifully explained and documented, Dimitriy. May I ask for some examples of what effects you have preamped with this circuit?

[bool]

Nice plots.

To clarify, I didn't "request" the waveform plots - I suggested that to understand what the clipper "does" to the input signal, it would be the easiest to just sim and plot it.

My further suggestion is to just observe (probe) the following:
-the actual input signal w.form (the other two below all voltage plots)
-the w.form at the fet gate (where the clipper "action" would be seen)
-the w.form at the D1/D2/R12/R14 junction
-the w.form of D1/D2 current (!)

That would imo clarify the clipper "action" just fine ...

To be correct, D1 and D2 work not only as a feedback elements, but also as a clippers at FET's gate. To be frank, I can hardly understand their full duties.

[DDD]

I use the subject preamp for some hi-gain overdrive stompboxes and for couple "full amplifiers" that are powered with 12VDC mains supply and have 2-3 Watts output capacity.
There are couple informative waveforms below.
The first one is at the FET gate, the lower one shows the output waveform at the same time/conditions.
Other waveforms at the FET gate are too weird and poor informative, so they are absolutely not interesting for sure.

[DDD]

There are three hi-gain sound samples below.
Sorry for the ugly tunes, I can hardly play "Mary Has A Little Lamb" 
Chain: PhotoGenic Les Paul guitar -> overdrive stompbox (the present preamp plus PP output stage parody) -> Roland Cube-60 combo (JC Clean mode) -> Samsung GT-S5282 smartphone.
Full Chords:
http://zalil.su/545509
Power chords
http://zalil.su/609367
Single notes:
http://zalil.su/618549

[LightSoundGeometry]

I want to read sin waves at home..is there a tool under 75 I can buy to do this?

[DDD]

I want to read sin waves at home..is there a tool under 75 I can buy to do this?

There's a lot of "soft scopes" floating on the NET, many of them are free.
Just make a very simple input probe ($2) for your PC sound-card, install one of the numerous scope programs - and you have a whole set of powerful research instruments at hand.

[tca]

Don't forget that the input mic clips fairly low. The input probe should have a high input impedance and some kind of amplitude limiter.

Cheers.

[DDD]

I use less sensitive line input with 1:10 or 1:100 input voltage divider with couple protective 1N4148 diodes.
It's not a problem to find thoroughly tested and discussed diagram of such a divider as well as a lot of useful recommendations on the NET.

[LightSoundGeometry]

so I need to make a probe with a usb end , then install free software?

instead of googling the wrong thing, acan you link me to a probe I can buy for this application? I might buy instead of amking one so it works properly