Help understanding drive pedal behaviour at different amp volume levels

Started by josephfra, September 08, 2022, 11:39:38 AM

Previous topic - Next topic

josephfra

Hey everyone,

I've breadboarded a really simple drive pedal shown by this circuit (I posted about it in a previous thread too):


I've found that with the amp volume high, gain set to taste and the level set just low enough to not hurt my ears, it sounds nice. (A bit bassy, but that's not hard to solve.) But if I turn down the amp volume and raise the level accordingly, it quickly becomes very tinny and thin sounding.

My first question is what makes this happen? Is it more because of the amp, or more to do with the pedal? My thinking was that it's to do with the amp since the advice with drive pedals generally is to set the gain on the lower side, the level high and run it into a cranked amp - but I'm not really sure what's going on at a deeper level than that.
And my second question is would a tone control be sufficient to compensate for the lack of "body" at lower amp volumes? (I'm not sure exactly what's going on frequency wise so I apologise for the kind of vague terms).

Thanks everyone :)

ElectricDruid

Do you have any other drive pedals you can experiment with? Does the amp lose body at low volumes generally? Does it do it with other drives?

I can't see anything that immediately jumps out. You should probably stick another 1u cap on the output to block the 4.5V bias from that last op-amp, but that's just good practice, and I don't think that's causing your issue.

Fancy Lime

The big question is: what amp? Many amps have so-called Bright Caps across the volume pot (especially on the clean channel or, on older amps the Bright channel) to make it so that there is more treble when the volume is low. This is done because the frequency response of human ears is different for low and high volume. At low volume we mostly hear mids but at high volume we hear more evenly. The bright cap compensates for that effect (similar to the Loudness control found on many hifi systems) but is usually tuned for "just the amp" without extra pedals in front. With distortion before the volume control, it is often too harsh at low volume. In some amps, this cap is switchable with a switch usually called (no prize for guessing) "Bright". Mind sharing what your amp is? Do you happen to have a schematic for it?

A treble control would certainly help and likely fix the issue entirely and is probably easier than modding the amp (assuming it even does indeed have a bright cap). A variable cutoff control like the Rat Filter is likely better suited here than a shelving control like in the Fulltone OCD.

HTH,
Andy
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

A cider a day keeps the lobster away, bucko!

Mark Hammer

Andy is correct, but it is also worth considering where the Volume control is in the amp circuit.  For instance, if it is closer to the input and pads down what is hitting the initial gain stages then upping the output of the drive pedal shouldn't really change the tone much, if at all.  In contrast, if there are several gain stages before the amp's volume control, then those gain stages are having additional harmonic content squeezed out of them, BEFORE having that result attenuated by the amp's volume control.

antonis

Quote from: josephfra on September 08, 2022, 11:39:38 AM
the advice with drive pedals generally is to set the gain on the lower side, the level high and run it into a cranked amp..

The above should be correct in the absence of D1/D2 pair.. :icon_wink:

Here, max output amplitude shouldn't be higher than 600mV..
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

josephfra

Quote from: ElectricDruid on September 08, 2022, 12:36:18 PM
Do you have any other drive pedals you can experiment with? Does the amp lose body at low volumes generally? Does it do it with other drives?

Unfortunately I don't have any other drives I can experiment with, but I gave it a go clean and it does seem to sound slightly fuller when the volume is higher, so maybe this is just an amp thing.

Quote from: Fancy Lime on September 08, 2022, 01:34:57 PM
The big question is: what amp?

A treble control would certainly help and likely fix the issue entirely and is probably easier than modding the amp (assuming it even does indeed have a bright cap). A variable cutoff control like the Rat Filter is likely better suited here than a shelving control like in the Fulltone OCD.

That's very interesting! The amp is an Ibanez TSA15H, and I found a schematic online here. I'll absolutely give the Rat filter a whirl and see how it sounds, thanks for the suggestion. :)

Quote from: Mark Hammer on September 08, 2022, 04:17:59 PM
Andy is correct, but it is also worth considering where the Volume control is in the amp circuit.  For instance, if it is closer to the input and pads down what is hitting the initial gain stages then upping the output of the drive pedal shouldn't really change the tone much, if at all.  In contrast, if there are several gain stages before the amp's volume control, then those gain stages are having additional harmonic content squeezed out of them, BEFORE having that result attenuated by the amp's volume control.

Ah I see, that makes sense. I'll gave the schematic a quick look and I think that the volume control is closer to the input with gain stages afterwards. I could be mistaken though. And I think whilst looking at it that I may have actually spotted a bright cap like Andy was saying, but again not entirely sure if I'm right! (I was looking at C10 on the schematic for anyone else following).

Quote from: antonis on September 08, 2022, 04:18:39 PM
The above should be correct in the absence of D1/D2 pair.. :icon_wink:

Here, max output amplitude shouldn't be higher than 600mV..

Is it normal then that this pedal is almost too loud without the level control? :S

GibsonGM

Pickups vary widely so this 'scratching on the back of an envelope'.   You will get a transient when you strike a note or chord that can be oh, 750mv (which the pedal cuts off and distorts)...but the average voltage after that initial transient is something far lower, say 150mV.   

With the diodes clamping the output at 600mv, you're looking at roughly 4x the input signal the amp was designed around, and this is 'pushing the front end' of the amp...with tube amps, this is most desirable.  It's not just distortion, it's also compression.   

Some but not all SS amps exhibit nice characteristics if you push them a bit, too.
Push too far, and it's all a fuzzy, crap sound! 
  • SUPPORTER
MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...

Fancy Lime

Aaallllright! That's one hot mess of an amp design! I assume the marketing department had a bunch of "great ideas" after sticking their noses a bit too deep into the white powder baggies and poor engineer Kobayashi had to turn that into a functioning circuit. But hey, if it works, it works.

The Volume control does indeed have a bright cap across it, called C10 on the first sheet if the schematic (near lower left corner). While this would not be too difficult to mod by making the cap switchable or adding a pot as a variable resistor in series with the cap to make the brightness effect variable, I think the most flexible option is indeed to add a tone control to the pedal. There are several simple options worth trying and I also just found a few problems with your schematic. I'll try to whip up a schematic to address those later today.

Andy
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

A cider a day keeps the lobster away, bucko!

Fancy Lime

Hi there,

on the matter of tone controls for overdrive or distortion pedals: There are an enormous number of different types of tone control, some simple, some complicated, some very useful, some less so. For a drive circuit it find that a simple passive variable treble attenuation after the clipping stage is usually very serviceable. Bass attenuation or treble boosting is often better done before the clipping stage. There are two variables you can adjust for treble attenuation: (1) the frequency above which signal gets attenuated and (2) the amount by which it gets attenuated. Theoretically, you could adjust both using two pots, but that's not nearly twice as useful as adjusting just one of those, so it is rarely done. There are ways of adjusting both at the same time with one pot, but those tend to suffer from less than ideal range and feel of the pot while providing little benefit over the simple variants. I have cobbled together the two main types: (1) a Rat type Filter control (upper schematic), which attenuates everything above an adjustable frequency (ca. 500-7000 Hz in this design) by 6 db per octave. I have modified the taper of the pot with a parallel resistor and restricted the range of the Filter compared to the original Rat circuit, which makes the whole thing much more predictable to use. And (2) an OCD type Tone control (aka shelving filter), which attenuates everything above the corner frequency by an adjustable amount. The corner frequency can be switched between ca. 650 and 1200 Hz in this example.



I used generic values for the first stage, substitute whatever floats your boat. I gave the output buffer a bit of gain 12 db just because, why not. It's not necessary but allows more maximum volume to boost an amp or other pedal, if that is desired. If you want to use LEDs for clipping, the gain of the output buffer should be no higher than 6 db for 9V operation. You'll notice that I also moved the volume control to after the output buffer. This serves two purposes. First (and most important), in your schematic there was no output coupling cap and ground reference resistor after the buffer. This isn't a problem when used with an amp like yours but would mess with the bias of some tube or JFET input stages on some amps or effects, so best to design it more universally. Now the volume pot doubles as the ground referencing resistor post coupling cap. Second, moving the volume to the back also means that we avoid interaction between the tone and volume settings, which can be bothersome with Rat type filter controls. A different way of dealing with that has been christened the Stupidly Wonderful Tone Control by Mark, which is a worthwhile thing to read up on as well. You may also note a bunch of 22n caps. These form RC filters with the preceding 1k resistors to limit the overall bandwidth of the circuit to about 7 kHz. Nothing valuable for guitar happens above that frequency (not much at least) and limiting the higher frequencies reduces potential noise problems and makes the circuit sound less fizzy and brittle. One may even contemplate increasing these to 33n for even less fizz.

I would suggest trying to breadboard both of these filter/tone circuits. They do react quite differently and both have their pros and cons. I tend to prefer the upper example for high gain pedals and the lower for lower gain.

Cheers,
Andy
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

A cider a day keeps the lobster away, bucko!

Rob Strand

Schematic of the amp would help.

When you crank the pedal are you using the GAIN pot or the LEVEL?

Your Pedal is outputting 4.5V DC into the amp.  Who knows what that's doing.
Send:     . .- .-. - .... / - --- / --. --- .-. -
According to the water analogy of electricity, transistor leakage is caused by holes.

Fancy Lime

Quote from: Rob Strand on September 10, 2022, 08:30:52 PM
Schematic of the amp would help.

When you crank the pedal are you using the GAIN pot or the LEVEL?

Your Pedal is outputting 4.5V DC into the amp.  Who knows what that's doing.

See link in reply #5:
https://el34world.com/Forum/index.php?PHPSESSID=nlqvblfemhbhlua8i88g5a3j51&action=dlattach;topic=25027.0;attach=78767

My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

A cider a day keeps the lobster away, bucko!

Rob Strand

Quote from: Fancy Lime on September 11, 2022, 01:47:59 AM
See link in reply #5:
https://el34world.com/Forum/index.php?PHPSESSID=nlqvblfemhbhlua8i88g5a3j51&action=dlattach;topic=25027.0;attach=78767

Quote
I've found that with the amp volume high, gain set to taste and the level set just low enough to not hurt my ears, it sounds nice. (A bit bassy, but that's not hard to solve.) But if I turn down the amp volume and raise the level accordingly, it quickly becomes very tinny and thin sounding.

With that schematic the amp input is AC coupled so the DC isn't the cause.

If we assume the Pedal gain pot and Amp drive pot *aren't* changing and the two scenarious are:
- Amp LEVEL (poster calls it volume) high and the Pedal LEVEL low.   Sounds is OK.
- Amp LEVEL (poster calls it volume) low and the Pedal LEVEL high.   Sounds thin.

Then the cause for a thin sound isn't obvious since there's no change in the filtering.   We would expect the amp to sound more distorted because *amp* clipper is being driven harder.

The thin sound would make more sense if the Pedal GAIN was increased as this would cause the diodes in the Pedal to clip and then the cap C6 might start rolling off the lows due the pedal diodes clipping.

Send:     . .- .-. - .... / - --- / --. --- .-. -
According to the water analogy of electricity, transistor leakage is caused by holes.

Fancy Lime

I think "volume" in the original post refers to the knob named "volume" on the amp and the schematic rather than the one named "level". If that is indeed so, then the described behavior is expected because the filtering does change with changing "volume" setting because there is a 100p bright cap across the 1M volume pot. The "level" knob affects only the built-in Tube Screamer of the amp, not the "clean channel".

Andy
My dry, sweaty foot had become the source of one of the most disturbing cases of chemical-based crime within my home country.

A cider a day keeps the lobster away, bucko!