Champ Stamp "Highs" filter circuit

[The Gator]

I really like this circuit. I have noticed the Highs adjustment doesn't really get bright. I started looking at it and noticed it is definitely not your typical high pass filter design. As a matter of fact, it is like a reverse low pass filter. A parallel resistor capacitor filter. It doesn't really let more Highs through as much as roll them off.
I can see this being useful as putting anything in front of this circuit makes it bright and crunchy similar to fuzz circuit behavior.
I socketed a few things. Mainly the hard clippers D7 and D8. The lower you go with Vf on hard clippers the more low frequency tend to roll off.
Also C4. It is 680pf. Which wasn't the thing to do, unless you want it to roll off even more Highs when the value is increased. Decreasing it does not make anything brighter, just rolls off less when you turn it down.
So I am trying to understand this portion of the circuit. What really dictates how bright it gets.

 

[juice886]

This control rolls off high frequencies. In a nutshell when it's all the way up 1MOhm is added to the 680p making it roll off less as you said and only C7 is involved mainly but I assume 6p8 capacitance in conjunction with 2M2 still can roll off some high frequencies. When the control is all the way down 680p is added to the 6p8 making it roll off much more highs.
So basically adding highs here means just rolling off less. That's a rough explanation.

 

[The Gator]

Hmm. Yeah that is pretty accurate from what I have tested..
Perhaps upstream on the signal path is where I need to look to let more Highs through before getting to this "Highs roll off" portion of the circuit.

Perhaps it is designed like this to work with buffered signal pedals going into it from the guitar side. I noticed a buffer in front seems to brighten it up considerably.

 

[juice886]

If you want more highs try to remove 6p8 capacitor.

 

[The Gator]

Yeah I thought about that. But that is a tiny out of capacitance in parallel with the 680pf cap above it. Less capacitance just means it rolls off less Highs, but does not change the db level of the higher frequencies at all.

 

[juice886]

Yeah, that's true. I calculated roll off frequency with 6p8 and it shouldn't affect that much to be honest. So it doesn't make sense to remove 6p8. What you can try is to lower c5 (220p), for example, try 100p instead.

 

[The Gator]

Yeah we are on the same page. I was just Eyeballing that cap on the schematic...lol.
I might have to breadboard the circuit to find an answer. I lazily was trying to avoid doing that. I still need more education on audio circuits.

 

[The Gator]

I swapped the C3 220pf for a 47pf and it seems to make a slight difference
Not a lot though. It sounds pretty damn good that's for sure. I tend to take my darkest tone guitar and make the pedals I make sound good with it and the brightest guitar I have.
These are a Les Paul custom and an older Telecaster. The thought being if it sounds good and is adjustable to these two guitars sounds, it will work with the others as well.
If I want more, I will have to breadboard it and dig in.

 

[The Gator]

I forgot to say, thank you juice886, for your help and conversation.

 

[juice886]

I have the Champ Stamp pcb so I'll get to it at some point. I wouldn't have thought it is a dark sounding overdrive looking at the schematic but yeah, there is a couple of places where extra highs are rolled off, that's for sure.

 

[hamerfan]

I always found the description of being dark a bit cloudy. Lacking highs is shaper, but being dark can also mean being too bassy. My limited experience tells me that dialing off lows makes the sound clearer and less muddy, in some sense brighter. Try to reduce C12 to 10nF.

 

[jesuscrisp]

It doesn't really let more Highs through as much as roll them off.

That's... Basically the same thing?

 

[The Gator]

Yeah it's not a dark pedal. It is not too bassy. Descriptive words are subject to interpretation I realize. I would not describe this circuit as dark.
Yeah jesuscrisp kinda. You're quoting one sentence at the beginning of the thread which does not explain the whole situation. My point was modifying those particular caps only dictates how much high end gets rolled off, not how bright it is. I am looking to change the top end frequency ability of the circuit.

 

[jesuscrisp]

You could further mess with C3 and C5 to get more high end through but from my experience these electra style distortions are kinda one dimensional without a whole lot of depth including top end harmonics.

 

[The Gator]

Yeah I believe you are correct. Only so much can be done.
I didn't think of this before but, if I replace the R5 2.2M with a 2.7M, it brightens it up just slightly. I tried a 3.3M and it was definitely brighter, but lost too much "punch".
So I'm going with a 2.7M at R5 and a 47pf at C3. As well as a DB9 diode at D7 and a 1n34 diode at D8. This makes a nice fenderish sound with the Hi, Low, and volume all the eay up, the gain about 50 to 60%and a fender tele in the bridge position. Slightly brighter than stock.
It's a lil' hotrod of a circuit.
Gonna have to do some hot rod style graphics for this one.

 

[juice886]

@The Gator I've just assembled this pedal and now I understand what you meant. I can definitely feel a lack of brightness and I think the culprit is a low input impedance. My pedal has not only a dark muddy tone but also not enough gain. It reacts quite well to a picking attack though. A buffer before it immediately brings Champ Stamp back to life. I'm not sure changing Q1 to a higher gain transistor will give more gain, most likely no. And it looks like the Q2 stage doesn't bring any distortion. So I think this pedal will benefit from the input buffer, probably the one that can be switched on and off would be a good solution.

 

[The Gator]

Thank you.
Sometimes when I mention something seems a bit dark and I want to modify it, I get a lot of "why?" Or "doesn't seem necessary " but I do not like pedals that do not get bright enough. It is nice to hear someone recognize the same thing.
Yeah exactly, a buffer in front of it brings it up.
The mods I did do help a little. The lower vf diodes for the hard clipping actually help brighten it a little as well as squeeze out a little more clipping.