HiwattDr103 pedal clone unstable

[Sonny]

I finally like to post a project I'm running a couple of months now. It's nearly done but there's still one problem left and
I obviousy don't have any clue.

It's an IC-based emulation of a Hiwatt DR103 as a (larger) pedal. It's based on this schematic here

BAJA Hiwatt DR103 amplifier pedal 9v 120318

Here is my version of the Hiwatt DR103 normal channel preamp and amplifier output stage - I have not built it as yet (waiting on some enclosures) but it should work ok :wink: cheers bajaman The master volume can be omitted if desired - simply replace it

www.freestompboxes.org

All in all it's working very fine and sounds are fantastic. But when I increase gain and/or the amount of input signal e.g. with humbucker or an additional boost in front I got an disturbing bias-like noise (fuzzy, starving, dying battery) noise on the top of the audio signal.

I already did a bunch of tests to locate it or get rid of it:
a) Checked connections to v bias in the feedback networks
b) Took the breadboard far away from other voltage sources + drove the circuit from a fresh 9V battery.
c) Swapped the TL072 and even the TL062 , changed the 2SK117, changed diodes.
d) took an audio probe on different Points of the circuit, starving noise seemed to apear right after IC1a (in signal line)
e) Read out voltages on both JFETS and ICs. Voltages an all pins -without increased- input seemed to be quite usual, all readings between 4.15-4.95V. Increasing the input per boost decreased voltage on the output of the first position 2SK117 and the inverting input of the TL072 temporary - then increasing fast to stabilised values as the guitar swungs out. But that's the usual effect of saturation via boost, isn't it?
f) suspicious on this deviation I tweaked different resistors in feedbacknetwork of the IC1a. Increasing the Feedback resistor (3,3K) increased gain but
no change in disturbing noise. Decreasing Biasing resistor in front of IC (1 M) increased input voltage - but this doesn't lead to vanish the starving bias noise at increased input levels.

So right now I'm kind of desperate. The pedal sounds great, but for completion it
is necessary to drive it in sweet terrain via boost or fuzz.
So I would be very very thankful for suggestions or hints and of course
good questions.

Best regards.

 

[Vicious C]

I’ve actually built a few of Baja’s versions of amps, and I have to say I’ve had the same issue, and I never was able to diagnose it. I hope you get some answers! I wish I had them!

 

[Brett]

I'm no electrical engineer, but I see a few areas in the power supply scheme that I'd start with:

1. No bulk cap for +9VDC. None.
2. There's no current limiting circuitry in the supply.
3. No form of reverse polarity protection whatsoever.
4. No form of power filtering in the supply whatsoever.
5. A 1µF bulk cap for reference voltage (v bias) is pretty conservative.
6. I wouldn't put money on it, but if I had to guess, I'd assume that this circuit is probably fairly noisy and may even oscillate at extreme settings, even when working as designed.

I start by establishing a filtered, current limited +9VDC supply. Something like this:


Treat the former +9V in Baja's schematic the same as VCC in the image I posted. From there, replace the 1µF capacitor at the junction of the v bias voltage divider with something more substantial, start with a 47µF.

Not saying this WILL fix the issues you're having, but even if they don't, they'll at least improve your power supply.

 

[Sonny]

At first thanks to you for the fast reaction.
@Vicious C : Thanks for your wishes, I let you know .

@Brett
I have to admit having left out some information of the final version of the circuit.
The basic schematic is in fact the posted one from bajaman. Tone stack is slightly advanced as shown here

...but that doesn't seem to touch my problem.

But for the finally wired version I went with this veroboard

BAJA Hiwatt DR103 Amplifier Pedal

Schematic and info HERE

dirtboxlayouts.blogspot.com

So if I'm not completely wrong, my actual version has got
3. a reverse polarity protection in the form of the 1N5817 (series diode version) and
4. basic power filtering in the form of a 100uF electrolytic cap from hot to ground, but not this complex as your suggestion

This is my first circuit this complex (went with transistor based circuits until now)
and I have to admit I let myself be seduced by the apparently sucessful buildings
in the comments beyond the circuit postings. So I'm open as much as I can and free to ask

1. What exactly is a bulk cap?

To 6. In fact the circuit is on breadboard and even soldered with open shielding quiet as could be. I had some parasitic oscillation in the beginning caused by my own fault - wires of IN and OUT ran to close to eachother. AND increasing treble and/or presence increases a bit of white noise - but still works for me.

5. Thanks to your suggestions, I'll try this partly. After doing audio and voltage probes, I still guess problem has to be somewhere round IC1a OR the power supply/voltage divider.
My thesis comes from this: in voltage test the voltages on all IN and OUT pins of the 4 TL062 Stages kept unchanged at around 4.5 even with increased input signal. Only the Voltage at non-inverting IN of IC1a went crazy. AND in Audio Probe (like this one https://diy-fever.com/misc/audio-probe/) the problem first appeared AFTER IC1a. So maybe theres a bug in power supply and TL072 is the only IC which reacts weird OR theirs something wrong around that biasingnetwork.

Is it possible to be just one resistor or cap - no false value but in lack of sufficient wattage...like a breakdown of a single part?

But I'll try the supply suggestions at first, thanks a lot.

 

[m4268588]

e) Read out voltages on both JFETS and ICs. Voltages an all pins -without increased- input seemed to be quite usual, all readings between 4.15-4.95V. Increasing the input per boost decreased voltage on the output of the first position 2SK117 and the inverting input of the TL072 temporary - then increasing fast to stabilised values as the guitar swungs out. But that's the usual effect of saturation via boost, isn't it?

Try measuring Vbias instead of the signal pins.
This shouldn't fluctuate because it is a buffered, but you may find something strange.