Help with muffled sounding Timmy clone

[Ben N]

I used the pedalpcb RTS board. Here is Bugg's schematic:
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I expected, based on Paul's descriptions of the concept, that at minimum gain, bass and treble I'd have an essentially flat, unchanged signal--just a clean boost.  Then, as you turn up the gain you can cut the bass and treble accordingly. But at that setting, mine is a bit muffled sounding, with a definite loss of sparkle. I tried subbing a TL072 for the 1458 I had in it originally, and that helped a bit, but it's still not quite right. It sounds great with the gain up, but I'd like to have the full spectrum to play with. The only components I can see that would affect this are R2 (510k) at the + input to the first opamp, C2, the feedback loop cap, although that looks to be on the small side already at 100pf, and R5/C5 (1k5/10n) in the tone circuit.

Any suggestions, my learned masters, for what I might have gotten wrong, and what I might profitably change, and to what value, to get my sparkle back?

[antonis]

<as you turn up the gain you can cut the bass and treble accordingly>
Gain pot only cuts treble 'cause it forms a LPF togheter with C2.
C2 doesn't affect anything lower than 400kHz (!!) for 0% gain setting,  15kHz for 10% gain setting, 7,8kHz for 20% gain setting, 5,2kHz for 30% gain setting, etc.
(max gain setting roll-off frequency is at about 1,5kHz)

Max-Min Treble setting (via Treble pot) is 10,6kHz - 308Hz with a 12dB/oct (40dB/dec) slope for frequencies equal or higher than the lower of ones set by Gain+3k3/C2 & Treble+R5/C5 (both LPFs..)

Maybe you have to halve C2 value (47pF) to see if that makes things better but only if other filters settings are higher than frequencies of "clearing"..

[Ben N]

Thanks, Antonis. Perhaps you could explain a bit more.

Gain pot only cuts treble 'cause it forms a LPF togheter with C2.

Makes sense. Because an HPF in the feedback loop = a LPF, right?

C2 doesn't affect anything lower than 400kHz (!!) for 0% gain setting...
Maybe you have to halve C2 value (47pF) to see if that makes things better but only if other filters settings are higher than frequencies of "clearing"..

Not for those frequencies!

Max-Min Treble setting (via Treble pot) is 10,6kHz - 308Hz with a 12dB/oct (40dB/dec) slope for frequencies equal or higher than the lower of ones set by Gain+3k3/C2 & Treble+R5/C5 (both LPFs..)

So the treble control doesn't roll off at a fixed frequency, but changes the point of the roll-off? How is it affecting the signal at minimum?

[antonis]

<Because an HPF in the feedback loop = a LPF, right?>
Yes..!!
(highs are "leaking", through C2, the Gain pot so they aren't amplified..)

<Not for those frequencies!>
Can't figure what frequencies you refer on by "muffle"..!! 

<So the treble control doesn't roll off at a fixed frequency, but changes the point of the roll-off?>
That's exactly the reason for calling it "Control".. 

<How is it affecting the signal at minimum?>
It's a LPF with cut-off frequency set by 0.159/[(Treble pot setting + R5) X C5] ..
Do the maths or see previous post for Max-Min settings cut-off frequencies.. 

P.S. 0.159 is equal to 1/2π, in case of been confused..

[Ben N]

link=topic=122547.msg1156338#msg1156338 date=1560340113]
Can't figure what frequencies you refer on by "muffle"..!!  [/quote]

I'm not sure either, but I am quite sure it's not in the range of 400kHz!

<So the treble control doesn't roll off at a fixed frequency, but changes the point of the roll-off?>
That's exactly the reason for calling it "Control".. 

Good point. 

It's a LPF with cut-off frequency set by 0.159/[(Treble pot setting + R5) X C5] ..
Do the maths or see previous post for Max-Min settings cut-off frequencies.. 

P.S. 0.159 is equal to 1/2π, in case of been confused..

Thanks again for the lesson! I did the math, and found a couple of ways to boost freq of the min roll-off without too much effect at the other end (actually, I don't mind increasing that low end cutoff a bit, to leave open string fundamentals alone). I'm not at all sure anything over 10.6khz matters feeding a speaker that rolls off signal starting at 5-6khz, but no harm in trying.

[antonis]

I'm not sure if I got you right so, you may ignore the present post.. 

Treble is a passive Low-pass filter so it can only cut highs..
(it can't boost anything..)

When set at lower setting, 300Hz say, it lets anything equal to or lower than 300Hz (all the way down to DC..) to pass without attenuation..
e.g. a signal of 80Hz freq equally passes through it both at extreme pot settings (300Hz - 10kHz..)

P.S.
Lows amplification is set by Bass pot setting together with C4 & R3 (ingnoring C3 2nd pole part for the moment..) so there is the place you may deal for open string fundamentals..

[Ben N]

Yes, except there is a slope.

[antonis]

Yes, except there is a slope.

There always is a slope..!! 
(the rate of which is determined by filter order..)

As far as frequency of your interest lies between +/-3dB you don't have to mind..
There shouldn't be any audible issue - signal voltage drop of 0.707 (or less) times intinial signal voltage can't be perceptible by human ear..
[-3dB = 20log(Vout/Vin) = 20log(0.707)]