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.

 

[temol]

With "the second op amp" you refer to IC2a in the original schematic

I refer to the original schematic, but it doesn't really matter. First, second - counting from input.

So what now? Does that finally mean, original circuit is never meant to be pushed by higher levels?

I'm afraid so.

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...

It' does not look like typical crossover distortion. But take a look here.
The thing is that I tested three different opamps (not fakes) with the DR103 and the result was the same with each of them.
btw - in the video the problem is related specifically to the LM358

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

I have no idea...

 

[Sonny]

I refer to the original schematic, but it doesn't really matter. First, second - counting from input.

I thought it might matter, cause second opamp counting from input is an TL062.
In my consideration, if two different opamps (types) in different positions show the same phenomen (the nick) - doesn't that mean they are affected by someting external third, which concerns them both?
Does the reading in oscilloscope telling you wether the second opamp clips itself or take over the clip from its predecessor?
Sorry if that's a dumb question, I'm really not into that.

The thing is that I tested three different opamps (not fakes) with the DR103 and the result was the same with each of them.

So it's not an issue of the opamp quality. I swapped them some times myself, so I believe in that.

I'm afraid so.

Oh. That would be bitter. I surely will have to face the truth in the end...but before I'd be happy to find a way to fix it anyway.
If there is anybody with an idea, what could cause that kind of clipping, I'd be thankful.

 

[Sonny]

Hi folks,

so in this forum seems to be dead silence after the conclusion the circuit was never meant to be blown by an increased level via a boost.
I can understand that but due to lack of alternatives for my musical needs I'm still resoluted to fix it.
Thanks to everyone who joined me to this point, especially temol and brett .

After all, I finally go a new perspective on it in the last days. Let me take you into my considerations:

• According to the actual state the mistake may be found within the circuit (inkl. choice of parts) itself. Maybe it was never meant to be blown by an increased level via a boost.
• So we come to TL072. According to this article
  
  https://www.eetimes.com/Op-amps-in-smal ... reviewed-/
  
  ...there might be the possibility of generally lack of TL072 in handling increased input voltages due to its design.
• AND -according to measuring results in the circuit by temol- signals stays stable at a small input voltage (500mVp) at least. The strange clipping starts with increasing this input.
• So, assuming the opamp in the first Place (IC1a/IC2b) hasn't got the funcion of producing overdrive, there might be a way to bypass it's special problem range due to the discussion in this post
  
  https://www.diystompboxes.com/smfforum/ ... ic=96006.0
  
  
• One approach might be to decrease/limit input level in front of the opamp radically (2:1 or even 4:1) and increase it again in it's gainstage. This way, the TL072 would be settled within its comfort zone, without loosing overall level to push following gainstages and clipping diodes. (There's an other approach of inverting the signal too, but this sounds much more complex to me - especially for not knowing about its impact until having tested it.)

First, does this sound conclusive to you?

If, I got two questions about details:

• Decreasing Gain: Where would be the correct place for a limiting resistor in front of the opamp? Directly at the input of schematic OR in front of the input pin of the opamp (IC1a)?
• Increasing Gain: My assumption is, this would be done by increasing the 3,3 K feedback resistor. Is that right at all?

Thanks and greetings.

 

[temol]

How do you use the DR103? As an overdrive for a clean amp, as a preamp for a power amp, as a preamp for an audio interface?
Maybe you need to make sure that the input signal does not exceed a safe level and implement the additional drive in a different way?
For example, check if it is safe to increase the gain of the 3rd or 4th opamp. Or do not change anything but push the device after the DR103?
Or use overdrive before DR103 and find a safe level with a satisfying drive level.
Unless you want to keep digging into DR103 for sport

 

[Sonny]

Hi Temol,

thanks for your reply - these are really important questions. Indeed I got I quite clear vision of the DR103 usage.
I need it for shaping preamp stage in front of a power amp pedal (EHX Magnum 44 aczually) and so as a pedal platform, too.
For my requirements it should act as natural as possible like a preamp in the given options and borders a pedal emulation, of course.
We don't have to go into the discussion, wether it is possible to emulate an amp realistic true to nature or not - that's not the point, at least for me.
I use a JHS Charlie Brown actually for this purpose. Great pedal, but I like HiWatt tones much more than the marshall/bassman-thing.

So the DR103 don't have to produce tons of gain, but it must saturate and NOT clip with being blown by a boost or a fuzz. So I got a real motivation for some more digging. It's all about getting the music in your head into your fingers in the end.

But to combine it with a transparent OD is really an option, if everything fails - thanks!

But what to you think about my last approach? Does it make sense in any way?

 

[temol]

Couple things you can try.
1. Lower the gain of the first op amp. Replace 3k3 with 2k or even 1k5. With 1k5 you need around 2Vpp at the input to see the cliping of the 1st opamp. Seems to be a safe margin.
2. Increase gain of the 5th op amp a bit. Replace 1k5 with higher value. Judge by ear.
3. Increase value of the feedback resistor that sits between inverting input of the 4th opamp and 1N4148 diodes after 5th opamp. It's 2k7 on the schematic. Try higher values, judge by ear.

 

[Sonny]

Hi Temol,

thank you! That approach seems to follow the idea of divide-down-and-amplify-again but with some turns - in my lackely understanding of electronics. So I go for all of that, but may you answer me some questions - just for my understanding:

• Decreasing: so increasing the feedback resistor would increase the susceptibility of the opamp for clipping? Am I right that this is due to the assumption, that clipping is emerging in the output rather than in the input of the TL072 (as discussed here https://www.diystompboxes.com/smfforum/index.php?topic=96006.0)?
• Increasing: will the increasing of the parallel 2.7k resistor (from inverting input of the 4th opamp) increase the level the two serial to vBias diodes after the 4th opamp see? For I would try to hold to the harmonic clipping behavior of the circuit, this would be advantageous. You suggested try increasing gain of 3th and 4th Opamp as well some posts before...is there a reason you didn't note that again? Are there notable risks in trying that?
• TL072: Due to the thesis of part caused issue - does it make sense to try developed versions of the opamp like TL2027 or TLC2272?
  
  Thanks a lot for hold on this thread.
  Best regards

 

[temol]

• Decreasing: so increasing the feedback resistor would increase the susceptibility of the opamp for clipping? Am I right that this is due to the assumption, that clipping is emerging in the output rather than in the input of the TL072 (as discussed here https://www.diystompboxes.com/smfforum/index.php?topic=96006.0)?

By increasing the value of the feedback resistor , the gain of the opamp increases. The equation for the on-inverting op apmp is quite simple

Av=1+(R2/R1) where R2 is our feedback resistor. So, with default values we have 1+(3k3/1k) = 4.3. 1V in = 4.3V out.
By clipping I mean cutting top and bottom of the signal.
Here's the ltspice simulation screen from the DR103. Input signal 1.4V pp. Blue trace - op amp input, green trace - output of the op amp.
R2 values - 1k, 1k5, 2k, 2k7, 3k3, 4k7.

As you can see - with R2=1k signal is nice and clean. Then, starting from 1k5 you can already see the rounding of the tops. Now - with 1k5 it is a very short time, but with higher values it lasts longer and the top is flattened more and more. Notice that the amplitude does not increase - this is the ceiling for the signal. You have two options here - decrease the gain (by lowering R2 value) or increase power supply voltage. Higher supply voltage = more headroom.
Here's the same setup. but for 12V supply.


Also note that the main problem here is the high input signal. When does it occur? - mainly at the very beginning, right after the strings are struck. The attack itself is quite strong. You could use some kind of limiter/compressor pedal to even out the signal.

• Increasing: will the increasing of the parallel 2.7k resistor (from inverting input of the 4th opamp) increase the level the two serial to vBias diodes after the 4th opamp see? For I would try to hold to the harmonic clipping behavior of the circuit, this would be advantageous. You suggested try increasing gain of 3th and 4th Opamp as well some posts before...is there a reason you didn't note that again? Are there notable risks in trying that?

I may be completly wrong here, but that's how I understand purpose of the 2k7 resistor - by connecting output of the IC3b to the inverting input of the IC3a (through 2k7) we have negative feedback, like in the "real" tube amps. This lowers the gain. Value of the feedback resistor controls amount of the signal that's going back. It's a bit too complex circuit for me and I'm there's more going on for sure.

As for the 3rd and 4th opamp - you can try this. But - in my opinion - every modification alters the original design and you may end up with "amost DR103". On the other hand - I don't know how close frequency response of this circuit is to the real amp.

• TL072: Due to the thesis of part caused issue - does it make sense to try developed versions of the opamp like TL2027 or TLC2272?

You can try, but I would not expect miracles. I'd rather stay with tl072, decrease the gain of the 1st opamp and keep input signal at safe level.
Unless you have those fancy opamps and you don't have to buy them.

 

[Sonny]

Hi Temol,

thanks for your reply and your explanation. I think I got the effect of the feedbackresistor on the gain of the opamp now.
And I think I can follow your considerations much better now.

Lower the gain of the first op amp. Replace 3k3 with 2k or even 1k5. With 1k5 you need around 2Vpp at the input to see the cliping of the 1st opamp. Seems to be a safe margin.

By clipping I mean cutting top and bottom of the signal.

So decreasing the value of the feedbackresistor follows the assumption, the irregular clipping

is related to the amount of regular clipping going on (in addition to input level)?
I think that's really a possibility.

For controlling signal level...i think I want to try it internally first. Finally I'm gonna test it all: Decreasing the level at the input AND/OR decreasing the gain of the opamp. I'm far from understanding it theoretically all at once, so I'm gonna go by ear at this point.

 

[Sonny]

Hi folks,

did the all-in-all-test today and literally failed across the board .

• decreasing the feedback resistor lowered gain AND level as expected The lowered level is quite a lot beyond unity .
• but neither increasing the 2.7k (OpAmp3-4) or the 1.5k (OpAmp 5) did anything to increase the level or (best wish) to bring back the desired diode clipping again
• did try to limit signal level a) at the beginning of the circuit (no effect, even on level) (502k/1000k) and in front of the OpAmp via a resistor. In the the latter case, sounds became thinner and overall hum louder - but that doesn't decrease the irregular clipping.
  I have to admit I don't get it. Again.

There was really some hope again with that new hint to bypass the opamps problem. But now there comes a feeling of having fought bravely enough to be able to finish with it in a good way. If anybody has an exploding new approach, which wants to burst out - you're welcome.
But at this time I'm still very thankful to the people, who joined me, especially to brett and temol. Thank you.