Volume control for Cornish Treble Booster

[Sparky]

Anybody slap a volume control at the end of the TB-83 Treble Booster?   It wouldn't hurt to have a bit more control over this circuit.

[GibsonGM]

I'd throw a 100kA pot in place of R10, and take the output from the wiper.   That's a pretty high-octane boost, no wonder it needs a volume pot!

[antonis]

I second sir Mike's suggestion but for 10k pot..
(no "character" altered..)

[Sparky]

My first thought was to just wire a 100K pot right at the end of the circuit before the output jack.
What is the function of C7 and R10?  Are they needed?

[duck_arse]

I second sir Mike's suggestion but for 10k pot..
(no "character" altered..)

a coffee for antonis. R10 is shown 47k - we'd just replace that with an A50k and take the OUT from the wipe, would we not?

[antonis]

a coffee for antonis. R10 is shown 47k - we'd just replace that with an A50k and take the OUT from the wipe, would we not?

A hard day for Antonis  today.. 

Yeaaarr.. What Stephen said..!!

[antonis]

What is the function of C7 and R10?

They form a HPF together with C6..
(C7/R10 resonance at 720Hz..)

Without C7, C6/R10 HPF would have a corner frequency of 72 Hz (all guitar frequencies passed through) where now it has a variable corner frequency according to (X_C7//R10) / (X_C7//R10 + X_C6) = 1/2, where X_C = 0.159 / (C*f)

[Sparky]

I had a 250K pot handy so I put it at the end of the circuit before the output jack and it seems to be working well.   I don't know what the difference in sound would be if I swapped a 50k pot for R10.

[antonis]

I had a 250K pot handy so I put it at the end of the circuit before the output jack and it seems to be working well.   I don't know what the difference in sound would be if I swapped a 50k pot for R10.

IMHO, minor difference..

250k Volume pot is set in parallel with 50k resistor resulting into 41k6 equivalent resistance which offsets upwards HPF's corner frequency by 20%..

[m4268588]

R10, R11, C6, and C7 networks are Wien bridge BPF.
Transfer function is
(I've written about this BPF in past, but you can't read Japanese, right?)

"720Hz" is nowhere to be found. It's a meaningless calculation.

Simplified calculations.

• Fc(HPF) = 1/(2*pi*(R11+R10)*C6)
• Fc(LP)F = 1/(2*pi*(R11||R10)*C7)

More sloppy.

• Fc(HPF) = 1/(2*pi*R10*C6)
• Fc(LPF) = 1/(2*pi*R11*C7)

However, only when rising

[Lino22]

Can you just place 50k pot where the 50k resistor is?

[Sparky]

Can you just place 50k pot where the 50k resistor is?

    Yup, Gibson and Duck Arse mentioned that.  I don't have any 50 K pots so I tried puting a 250 K pot at the end of the circuit.  It works [as far as a volume control] but I was just not sure if it's affecting the tone.

[Lino22]

It won't. Combining a pot with a resistor in parallel is an old trick. It together creates a log taper. That's why you should use a linear pot, if you use a log, you will get a super log. Also, the bigger is the pot than the resistor, the more log taper is the result.
Also, the bigger the volume pot, the bigger gets the output resistance when it is turned down to some extend. I would go for the 50k replacement if you eventually have a chance to get one.

[GibsonGM]

I assumed the R would be removed and replaced with a pot, but yes...that would make the response dependent on pot setting.  Half the filter would be variable lol.  I said 100k just cuz we all seem to have them around, but yeah - a log pot might act strange with the 50k resistor there! 

Maybe they didn't include a vol pot because of this?

[antonis]

"720Hz" is nowhere to be found. It's a meaningless calculation.

If it's meaningless for you, it's more than OK with me..

P.S.
Maybe I'm wrong but knowledge demonstration isn't quite on the point for a plain query about a simple CE amp..
There are many guys here who could analyze it according to hybrid-π, T-model or h-parameter (including parameters like r_b and r_μ) but you can see that they don't do it..

As for the BPF, your analysis is sketchy 'cause r_o (BJT Collector output resistance) is omitted..