Question about Op-amp, volume and gain in a drive pedal

Hi all, it feels good writing a new post on this shiny forum.
I recently acquired a second hand overdrive pedal from Marshall (Marshall Drive Master, made in the 90s in England).
It sounds good and my goal is to use it with my Shruthi or my LXR.
Only problem is, I experience a drastic volume drop when it is activated, like a loss of at least 50% of volume. Pretty sad for an overdrive pedal hu?!
The thing is, I do not know if it is because the signal coming out of a modern synth (like the Shruthi) is too hot for the circuit or if something is not working the way it should. right now if I use it, I have to crank the volume up and be careful not to bypass it at the risk of frying my speakers or sound card.
The Op-amp used is a TL027CP, like in the Shruthi.
So I was wondering if any of you have experience with this kind of pedal/circuit and if I could mod or improve the signal processing by adding a circuit or changing some components?

Here is a pictures of the boars (good condition for a 30 years old-ish pedal)

thanks!

Here are the schematics:

It has a high-impedance input with a lot of gain (1 to 31), then 68; so if the pedal works as expected, you should expect massive distortion with a line level signal.

Something might not be right… Can you check with a scope the signal on the two output pins of the dual op-amp?

Hi Olivier,
Thanks for the schematics, I tried to study them and could not find anything wrong. I also modded the distortion circuit (replaced one of the led with a diode as stated here and switched of the ceramic cap with its film equivalent) but the loss of volume was there before I touched anything on the circuit and these two mods were for me a way to experiment and try to solve the problem.
I also tried to replace the TL07CP with another one I had lying around from a previous Shruthi build, nothing changed.
I have a scope here at home, but I still haven’t started to use it. This might be the perfect occasion to start learning working with scopes.

my guess would be that the overdrive is designed to output a (overdriven) signal that has an amplitude in the range if a typical guitar signal so you can use other pedals after it.
The shruthi has a very hot output. So when that goes into the distortion it is clipped down to the “guitar level”.

Phrased differently: I think you need some make-up gain to match the output of the pedal to your high shruthi clean signal

That’s a good intuition… I see that the output impedance is fairly high (33k), which means that the signal will be attenuated a lot if you connect the pedal to a mixer or audio interface that does not have high-Z inputs. Some mixers have input impedances in the 5k range - meaning you’ll get a lot of attenuation.

What are you connecting the pedal to? If this device has a high-Z setting, try enabling it.

Thanks for you replay @TheSlowGrowth, thats what I thought at first, but as Olivier stated, it should work with line level sources.
@pichenettes I have my scope up and running, what should I look for?

thanks!

I would say, a simple opamp buffer on the output will fix this for you.
Just use a single non inverting opamp amplifier stage. You can even add in a pot so you can match the output with the dry signal. This has the benefit of giving you some gain for matching it to hot sources like the shruthi. As a plus, you get a low output impedance to drive your mixer inputs.

It’s not a problem with the input, but the output. The pedal is probably working perfectly fine with line inputs, but its output stage is such that it can only be patched into another pedal or a guitar amp.

If you have a buffer pedal, try putting it between the DriveMaster and whatever you connect it to.

Ha! I see now, I did not think about the chaining between guitar pedals!
I have a Mackie 1202 that does not seem to have a high-Z setting and no buffer pedal here.
So I guess the pedal behaves as it should.
Is there a way to reduce the output impedance directly on the circuit so it matches the input or is it better to add an op-amp buffer as @TheSlowGrowth suggested? In the latter case, would a simple buffer like the second one on this page be ok for this use? In this case, I would only use a pot as R1 (what value should this pot have?). I have an extra TL072Cp here, could I use it or do I need another type of op-amp?

Cheers!

You could try lowering the 33k resistor at the output to 330 ohm, and increasing the 0.22µF cap to 22µF. If you don’t change the output cap, the low frequencies will be severely attenuated.

The naive op-amp buffer schematics you linked to is a dual supply circuit.

You can try this, which will work with a single 9V supply:

@pichenettes Great! Thanks for the infos, i’ll study both option. It now all makes sense, I’ll be careful with guitar pedals in the future.
Funny thing is I’m using a cathedral from EHX as well as a Boss Chorus CE-3 without any impedance issue with both of my Shruthis.

Those probably have a lower output impedance.

It’s fairly rare to see a circuit with a high output impedance like this.

I checked the CE-3 schematics and they have a low (470 ohm) output impedance. Cathedral… I don’t know, but its output stage is probably an op-amp, and they can use any (low) output impedance they want.

@pichenettes @TheSlowGrowth
Noted! Thanks for the knowledge.

I’m just going through my components and could not find any 22µF caps. In fact, I cannot find any ceramic caps with this high capacitance. I have some electrolytics ones (47µF) but these are polarized so i’m not sure I can use them in this circuit?
Odd thing is, I think that the 0.22µF caps in place is “C6” on the board, but it really looks like a resistor, in fact a small amount of capacitor on the board are regular ceramic caps, but some have the same shape and color code as a resistor.

You can’t make ceramic capacitors with such a large capacitance. You have to either use bipolar electrolytics, or put your two 47µ electrolytics back to back (but it’s a hack).

Thanks, I manage to find a 18µF cap in an unsorted bag of parts that was lying around (uncoated metalized film cap that took some time for me to be recognized).
There is a good improvement, I still have a small volume loss, but nothing compared to what I had before. I can now use it without having to turn the volume of my mixer at its maximum and fearing to destroy my speakers if I step on the off switch.
Actually I’m now able to use the pedal with my send and return as an FX on my mixer.
Neat!

Thanks a lot for you time !
I’ll use it for a gig on Friday night in Strasbourg, added to the SMR4, the resonance reaches sometimes a distortion similar to the ladder filter.