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?

 

[Sonny]

Any further suggestions?

At the moment and after this bunch of different tests I'd be thankful for any notice, WHERE else to focus at.
For I can't not exclude the option, I might have a dummy (stupid mistake) anywhere,
I will check schematic -> vero-graphics -> stripboard the next days again.

But problem cause has to be ANYWHERE, isn't it?

According to your knowledge about IC's, circuits and based on the recording - where could be vulnerable parts/connections
in the schematic to provoke a phenomen like this?

 

[temol]

Take an audio probe and check stage by stage where it starts to appear. Input -> buffer -> 1st op amp -> 2nd -> EQ -> and so on..
I'd also build another unit but on a breadboard, just to compare them.

 

[Sonny]

Hi temol,

thank your for reply - that's good for sure.

Take an audio probe and check stage by stage where it starts to appear. Input -> buffer -> 1st op amp -> 2nd -> EQ -> and so on..

As I already mentioned, I did the audio probe the first time the problem appeared...

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).

After that audio probe (months ago now) I focused at the first signal stage (from input to IC1a/IC2a). You can read the tests and swappings
I did in this direction in the post history.

• Test Phase 2 - Biasing aspects at and before first IC (IC1a)
  For I still assumed to have heard the noise after the first IC in audio probe I checked the parts, which could produce a biasing alike noise in the first way - in this first part of the signal line - for wrong biasing
• tested every resistor in feedback network of the IC1a/IC2a in value (per pot) - no results
• tested 1M before and 100 k resistors at the JFET in value (per pot) - no results
• tested to move the 1,5 M impedance resistor at front of signal line - in my wiring I had positioned this on directly at the first switch, wasn't sure if this could destabilize the JFET - no difference
  
  Test Phase 3 - Wrong part in first IC section
  After testing the bias option without results I had to follow the path of the first signal section till the end
• swapped the TI072CP I had used for some TI072IP from an other trader - got a bit more hiss, but no results in problem zone
• swapped the coupling cap in the first place (1uF) for a WIMA MSK2 foil for I had read about possible interference of MLCC caps with irregular signal levels, buzz even got worser a bit
• swapped the coupling cap after the JFET (1uF) for a WIMA MSK2 foil for the same reason - nothing changed
• swapped the 2SK117 for another 2SK117GR from another trader - no results

But I may repeat this approach anyway - primarly for lack of alternatives. So thanks.

I'd also build another unit but on a breadboard, just to compare them.

Thats an interesting approach, though probably a bit pricey. First it's worth to mention, that I was busy with the problem on breadboard for bout 4-6 weeks, including a bunch of tests these days including IC and resistor swapping, run on battery, etc. and the "problem noise" was exactly the same as in (now) soldered version. So I'm very suspicious on a problem course as a field phenomen too, cause settings and positions changed already. As I wrote in post history...

So I think it's important to point out (one more time) that the problem we're talking about happened to appear the first time (and from that on) in a circuit WITHOUT ANY MODIFICATION: Second channel was completely decoupled and the simple low pass filter in front came long after my first analysis of the problem. So I first tried to analyse it on breadboard with a really pure circuit (REALLY EXACTLY LIKE THIS VEROBOARD https://dirtboxlayouts.blogspot.com/2020/02/baja-hiwatt-dr103-amplifier-pedal.html ). And the phenomen still sounds exactly the same way like these days.

But that maybe in the end I have to take this way if I don't want to dump the project.
But at the moment I'm still suspicious about it: thought consequently, I had to buy every part new (to exclude part issue -> pricey) and if the problem would vanish in this case - due to what?
One broken part and maybe I'd never realise what it was?
Just sincere thanks - I'll keep it in mind and will repeat the audio probe anyway.

At the moment I really like to UNDERSTAND where things go wrong. Therefore my question...

According to your knowledge about IC's, circuits and based on the recording - where could be vulnerable parts/connections
in the schematic to provoke a phenomen like this?

 

[Sonny]

Hi folks,

I got news. I promised to repeat my tests and started with a detailed voltage test (multimeter) today.

• I switched to the "normal channel" and read out every single IC pin including voltage divider (VBias)
• read out drain-gate-source at the two 2SK117 JFETS too
• read out all the pins at different input levels: 0db/unity gain/+4db/+8db/+12db

You can read the table yourself, these are peak observations myself

• voltage supply (V++) and Vbias stayed clearly stable at all levels of input and on all of the measured pins
• voltage on pins of IC3a, IC3b, IC4a and IC4b (TL062) stayed continuous through all levels of input
• non-inverting inputs of IC1a, IC3a, IC3b and inverting Input of IC1a reacted with a slight hum to contact of the multimeter where IC4a and IC4b didn't
• the multimeter measured strange voltage behavior at the Gates of the JFETS: different to other pins, where final value where reached after 1 or max. 2 skips on display, the value seem to countdown every time from about 4.21 V to about 4.04V. This behavior was independent from input level.
• the non-inverting input of IC1a (IC after the JFET, +) showed the same behavior at all input levels, but still some more:
• whith the multimeter pin in touch to non-inverting input of IC1a (+) there was a sudden voltage drop (something about half the level, I guess)
• at the same time the signal was even more distorted (more like the problem like fuzz sound) than without multimeter contact to IC1a (+),
  at higher levels I would describe the hearable effect as the signal "collapses in on itself". For may ears much alike a strongly starved fuzz with just bits of fuzzy fragments coming through. I really like this kind of sounds sometimes - but I like them to come from my fuzz tanks and not from my preamp, anyway.
  
  So that's the actual state for now. I'm very warily on making assumptions to fast. So what do you think?
  What are this voltage numbers and behavior is telling us (please keep my already done tests in mind, thanks!)
  With best regards!​

 

[Brett]

At a quick glance, your voltages all seem to be correct based on the schematic. I'm going to second the recommendation that @temol made and suggest building the stock circuit without any modifications on a breadboard. I understand that this isn't the desired suggestion, but it's the only one I have at this point. Yes, I understand that you have done this already and had the problem and went on to build your vero board, but it's going to be easier to solve the problem(s) when you remove all the added complications.

Once you can dial out the problem on the breadboard, it may provide some insight into fixing the issue(s) with your vero build.

 

[Brett]

One more thing I did just notice, and it may be nothing:

SW3 and SW2.C in your schematic have DC across the lugs (which will cause an audible pop when changing states). Have you tried adding a jumper here to bypass the switch entirely to rule out any possible influence that switch bounce could be having on your problem(s)?

I'd also try to isolate either your normal or bright channel for the purposes of testing. You said you're running a buffer in front of the pedal, but I'm not sure of the impact you'd have when you're loading down the input going into both IC1.A and IC2.A from the same source without a unity buffer immediately before splitting the signal.

 

[temol]

I've just build the circuit on a breadboard using original schematic (normal channel). No issues, no farting sounds. Not much gain (less than I expected). It's a bit late now to properly test it with a guitar.

 

[Sonny]

Dear Brett, dear temol,

thanks for your postings, both of you. May I ask some points to understand...

At a quick glance, your voltages all seem to be correct based on the schematic.

That means...behavior at the gate of the JFET and the noninverting input of IC1a (+) is correct to the schematic? Even with the
voltage drop and the further more distort reaction? If you say so, it'll be right for me... I just want to understand.

SW3 and SW2.C in your schematic have DC across the lugs (which will cause an audible pop when changing states). Have you tried adding a jumper here to bypass the switch entirely to rule out any possible influence that switch bounce could be having on your problem(s)?

I see what you mean...discharging of capaciators? I'll keep that in mind. If I go for breadboard, I'll exclude that second channel anyway - again .

I'd also try to isolate either your normal or bright channel for the purposes of testing. You said you're running a buffer in front of the pedal, but I'm not sure of the impact you'd have when you're loading down the input going into both IC1.A and IC2.A from the same source without a unity buffer immediately before splitting the signal.

That's a very good hint, but again... second channel was already missing completely in my first testing phase on breadboard (4-6 weeks).

I've just build the circuit on a breadboard using original schematic (normal channel). No issues, no farting sounds. Not much gain (less than I expected). It's a bit late now to properly test it with a guitar.

That's very good, it's quite a clean sound. I really would love to hear about your experience with the following setting:
Set Gain to 2-3 a clock and run it with a boost in front of it. 10 dB+ should be sufficient to reach my "problem range" maybe even
a humbucker equiped guitar. I'd love to hear it works.

I'm going to second the recommendation that @temol made and suggest building the stock circuit without any modifications on a breadboard. I understand that this isn't the desired suggestion, but it's the only one I have at this point. Yes, I understand that you have done this already and had the problem and went on to build your vero board, but it's going to be easier to solve the problem(s) when you remove all the added complications.

Thanks. I see that option, especially after all the testing. I may even have had the original on breadboard. But after all, maybe I have to go there again.

 

[temol]

So.. there's definitely something going on.

With single coils - no issues. With humbucker and stronger strumming - some kind of fizz. Quite familiar sounding, like crossover distortion. So I took the oscilloscope and signal generator. Up to 500mVpp nothing interesting. Then signal starts to clip gradually (1st and 2nd op amp) and barely visible nick in the sinewave appears (also 1st and 2nd opamp).

Yellow trace - output of the 1st op amp, cyan trace - output of the 2nd op amp.
500mVpp


800mVpp


1.1Vpp


Yellow trace - output, cyan trace - output of the 2nd op amp.
500mVpp


800mVpp


1.1Vpp

 

[Sonny]

Hi Temol, dear Brett,

that's really interesting and seem to connect with my connections. With "the second op amp" you refer to IC2a in the original schematic
(https://www.freestompboxes.org/viewtopic.php?p=269389#p269389) and the IC3a in my modified scheme - the first TL062 op amp. Is that right?

So what now? Does that finally mean, original circuit is never meant to be pushed by higher levels?
Are there approaches left to handle with that behavior of the mentioned op amps - or does this regarding the circuit as a whole?

Thanks a lot for all your effords and time.
Greetings.

 

[Sonny]

that's really interesting and seem to connect with my connections.

...seem to connect with my experience in tests - this is what I intended to write .

 

[Sonny]

One thing from my side: I'm not an expert in electronics at all and didn't got an oscilloscope, so temols test is a real enrichment to the process, thanks again.

But I may contribute my observations from my audio probe (on breadboard those days AND in soldered version):
First point I recognised misaligned signal was right after the IC1a. In further gainstages my impression was that the clipping essentially stayed the same but got shaped (tonestack) and gained by following stages. Did a quick recherche on crossover distortion and found out this could be a quite familiar process in multi-stage circuits. On the other side the nick in the sinewave doesn't look like a typical crossover distortion graph like described, showed and eliminated here for example...

So where could the disturbance of the sinewave could come from in this case? Some mess from the Vbias?