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.

 

[Sonny]

P.S. and in addition...

1. Input signal concrete: I know the sunday driver is a high impedance boost/buffer. Problem appeared with singlecoil in series/humbucker without any boost in front of the circuit as well. Problem appeared with another boost pedal (Tasakalis molucular) as well.
2. IC's running out of headroom: Mmh, so I really don't know what to think about or where to measure. Maybe the connection to increased gain could be a hint to stages 3 and/or 4. Their voltage seemed to be stable during increased input. I could repeat the test -but is that the right place to measure operation range (headroom or not) at all?

 

[Brett]

Have you tried running the circuit with 18V input voltage supply instead of 9V? Is there any change in the problem's behavior?

 

[Sonny]

Hi Brett,

no, never ever. I have to admit, normally I would be afraid to damage something running it doubling the voltage. I mean, wattage of the resistors are 0,25 W all the way (I'm not sure with the trimpots)...but...all the schematics and veros are declined to run it on 9V.
What do you think, how high is the risk?

There are two other questions, now you see the schematic:


1. "Hot signal" means "input impedance" or "input level"?

2. I'm still a bit confused about "running out of headroom" and maybe there might be my fault.

Until know I thought, opamps are -beside other parts- supposed to run out of headroom, especially when pushed by a strong signal via strong guitar signal and/or boost. In the circuit related discussion here ( dirtboxlayout), guys even talked about increasing gain by increasing the feedback resistor (3,3k) in the feedback network of the first IC (IC1a/IC2a in my case). But I may have understand it the wrong way.

So assumming I got things wrong (a likely case) and IC's were never meant to clip - the value of the feedback resistor just regulates the amount of the output of the IC's - and therefore the amount of signal level beating the diodes, which are the only parts supposed to clip at all.

Is that correct at all?

So problem may be, input level may be so "hot", it pushes the IC's into UNWANTED clipping. Following this understanding, this is exactly what I hear as "problem noise". And what I hear as "natural gain" may be the WANTED clipping of the diodes.

Ist that right?

But I'm still confused, if I got it right, cause there are a view questions open in this case:

• if there is nothing to drive into clipping and/or saturation in front of the "Master Volume" pot (after tone stack), why there is in the original one channel schematic a Master Volume at all?
• does that probably mean, the original circuit was never meant to be pushed by a pedal in front? I'm still confused here, cause problem noise appears at even expectable "normal" input levels like a humbucker signal

Just thoughts, but I learn a lot. Thanks to you.

 

[Brett]

What do you think, how high is the risk?

The risk is very low as long as all of your capacitors and ICs are rated above the input voltage. For example, if you are using an 18V supply voltage instead of 9V, you'd want to make sure that your caps are rated at 25V. If you're using the ICs listed in the schematic, those can handle 18V safely.

1. "Hot signal" means "input impedance" or "input level"?

Referring to input level.

Until know I thought, opamps are -beside other parts- supposed to run out of headroom

Not unless the circuit is designed to clip the opamp(s). Opamp clipping is typically undesirable and designers rely on clipping diodes in either a soft-clipping arrangement (in the feedback loop of the opamp) or hard-clipping arrangement (back to back diodes to ground) for signal clipping/compression.

I can't recall without reading back through all you've tried, but have you attempted to use an audio probe with this circuit? An audio probe will allow you to listen to the signal at various stages of its journey through the circuit. If you haven't tried this troubleshooting step yet, see below:

Build yourself an audio probe following the instructions here: https://diy-fever.com/misc/audio-probe/

If you have a looper pedal, record a couple lines and pass the output from the looper into the input jack of the pedal in question. Don't plug anything into the output jack of the pedal in question. If you don't have a looper, you can use anything that outputs sound/music that can be connected to the input of the pedal in question, using a 1/4" to 1/8" plug adapter is perfectly fine here. This is your SOURCE.

After building the audio probe in the aforementioned link, you'll plug the 1/4" jack of the audio probe into an amplification device (headphone amp, low-wattage practice amp). Connect the audio probe ground to the circuit ground of the pedal in question. From there, you'll just need to apply power and start sending sound/music to the pedal in question. You'll use the probe side of the audio probe to touch junctions inside the pedal in question to determine if audio is present at that point.

Generally, you'd want to check the off-board junctions first, as outlined above.

Using a combination of the schematic and your eyes, you can generally trace the path that the audio should flow. When you get to a point in the circuit that the source audio stops and you're sure you've followed the signal path, your problem is typically adjacent to the location you last had audio from the source.

In some cases, it may be easier to start from the end of the circuit and work your way backwards. Although it's less methodical, you can also just probe around the board (just be mindful to avoid power junctions) until you find a point where you DO have audio, then proceed from there.

 

[Sonny]

Hi Brett,

here my answers in short:
I did an audio probe already. Took probes in steps right from circuit input and further on in the signal path. I thought to recognize the "problem noise" right after the first IC (IC1a/IC2a) the first place - that's why I was focused that strong on the first signal stage (from input to IC1a/IC2a).

The info, that clipping op-amps is rather unusual is very useful to me, thanks a lot.
I already thought this way (supply voltage) a month ago - but in another kind of "thinking": I assumed there might be a voltage drop from the standard supply voltage at the IC's for some reason - and the standard voltage of 9V would be sufficient to feed the IC's. Course of the reverse current protection diode in series, voltage is already dropped to about 8.76 V.

So I'll check the rating of the parts. The two 10k + 100r in the voltage divider seem to be the only resistors directly connected to full voltage.
Is there anything to take care about the LED and LED current limiting resistor?

I don't have got an 18V power supply but an 12V - is that o.k. to test a possible influence?

Greetings!

 

[jwin615]

Also your protection diode...

 

[Sonny]

Yes, indeed. As I wrote before, a drop to 8.76 V. This didn't seem to bother the guys here (dirtboxlayout) but maybe in my case.

I forgot about protection diode and decoupling caps in the schematic anyway.
Here is the update.

 

[Sonny]

Did the test with higher voltage today. First I did the research about all the ratings of caps, JFETS and diodes.
1N5817 seemed the lowest rated, with 20V. But for I only got a 12V outlet at my cioks, this seemed to be alright.

What can I say...
In the first place this test round showed me another mistake in my test setting for I recognized having run the Lehle Sunday Driver
on too low voltage of 9V for some weeks. Fixed it and it doesn't seem too relate to my problem due I did some tests with another boost pedal before.
Sound changed with 12V, bass got a bit tighter and the overall sound a bit... less "softer" or saturated (very subtle) and I needed some time to get used to this. I would take this - but the problem noise -this biasing battery creaking, tap tap tap- really doesn't change in a notable way.
I really wanted to hear a difference and did nearly 10 or more A-B tests on several riffs and single note lines.
But I can't hear a notable difference as much as I would love to hear it. But I keep the 12V option in my mind for a "maybe time after".

Does it make sense to stay tuned to the "unwanted IC clipping" -thesis and what would this mean for other possible causes?