Dissecting treble boosters

[THEZED]

Hello guys, I'm pretty much a newbie in stompbox building.
Being a huge Brian May fan I'm focused on Treble boosters and I have already built a couple of them.
Now I got interested into tweaking component values to fine tune the sound, so I would appreciate if any of you could help me to
understand how the single components affect the performance of the circuit, why are them there and so on...
Maybe this is a dumb question but I believe that this discussion would be of great help not only for me, but for a lot of other
novices as well.
Thank you

I know that C1 and R4 forms a HIGH PASS FILTER...
I know that if increasing the value of C1 more bass frequencies will pass through.
What's the effect of altering R4 value?

Why the optional R3 ?

C4 and R5 from emitter to ground?

optional C3 between R4 and R5?

C2 is related someway to frequency retention...

optional R7?

C6?

R8?

R9 and C5 (filtering current?) i found different values for C5: 47u(Fryer) or 100u(Cornish)

R6 to the collector?

R1?

R2?

[GibsonGM]

Hi, THEZED....the easiest way to cover your questions (which are good ones!) might be to go to Youtube and look up "transistor common emitter amplifer".  You'll find tons of vids on how one is designed, and what each component does.   The ones that are left (the filtering stuff) will be the things that make this a treble boost vs. just a full bandwidth booster!   You will gain in that after you understand what's going on, YOU will be able to take a transistor and whip up a gain stage with little effort!   Several ppl on there have made such nice vids, with oscilloscope measurements and more, that I can't compete - they've already aced it!

Many of the parts you're tagging are just to set the transistor's range of operation and set its gain.  Hope that helps, and welcome to the forum!

[R.G.]

Mike is right - you're digging into basic circuit theory. This is not to minimize the difficulty. The stuff you're asking was the topic of most of a three-lectures-per-week sophomore EE course back in 1970.

Re-organizing your questions a bit, you need to understand:
- how Rs and Cs affect voltage and current flowing through them
- how Rs and Cs together make a simple high or low pass filter
- how a basic common emitter bipolar transistor amplifier works
- how feedback works in simple non-ideal amplifiers
I'll give you answer to your questions in quick form, but I encourage you to go learn the basics all together, not piecemeal by asking questions in forums. It will be much quicker and more complete learning overall.

I know that C1 and R4 forms a HIGH PASS FILTER...
I know that if increasing the value of C1 more bass frequencies will pass through.
What's the effect of altering R4 value?

R4 is part of the biasing setup. R2, R3, R4, and R5 set up the DC biasing of the transistor. To a first approximation, R2, 3, and 4 set the voltage on the base; this sets the voltage on the emitter to one diode drop lower than the base, and that sets the current through the emitter resistor R5. As long as the power supply can supply the current and the gain of the transistor will provide enough current flow, that sets the current in the collector resistor R6. So for no-signal conditions, the transistor sits at these DC voltages and currents.

So altering R4 will alter the DC conditions in the transistor. Get it too high and the transistor conducts lots of current and the collector sits near the emitter. In extreme cases, you get "gating", which means that you need a big enough signal to force the transistor into amplifying. Get it too small and the collector sits near the positive power supply, again giving massive distortion and possibly gating.

R4 does some other things beyond the simple biasing.

Why the optional R3 ?

As a guess without running some circuit simulation, R3 and C3 roll off some of the very high frequencies by shunting very high frequency signals around the base-emitter.

C4 and R5 from emitter to ground?

R5 is critical to stable DC biasing, because it provides DC feedback to the base voltage signal, using the transistor's current gain to do it. R5 also reduces the gain of the transistor from it's maximum possible voltage gain down to some much smaller number. Arriving at what the lower gain actually is takes some math, but you eventually arrive at the voltage gain from base to collector being the value of the collector resistor R6 divided by the emitter resistor R5.

This gain is stable, but quite low. You can restore the gain to the higher value by "short circuiting" R5 with C4. For why that happens, you need to know about the impedance of capacitors. Capacitors have an impedance that decreases with frequency, and can be computed by Xc = 1/(2*pi*f*c) where f is the frequency and c is the capacitance. At high frequencies, R5 is "short circuited", bypassed by a very low impedance in C4.

The frequency where you can start conceptualizing R5 as being bypassed is F = 1/(2*pi*R5*C4) which computes to 1.411Hz. So for signals in the guitar's range of 82Ha and up, you get the transistor's full voltage gain back. Exactly what that "full voltage gain" might be is highly dependent on the current gain of the transistor, which varies a lot. That variation is one of the big issues in basic transistor design.

optional C3 between R4 and R5?

As above, cuts some very high frequency gain. Most modern transistors have usable gain up into the hundreds of MHz, which is very unwanted in this circuit. This high frequency gain cut doesn't kick in until the impedance of C3 is about equal to the impedance of R3, which happens at a frequency of F = 1/(2*pi*R3*C3).

Side note: you've seen F = 1/(2*pi*R*C) again. That is the nominal rollover frequency of every simple RC filter. Circuit theory course again.

C2 is related someway to frequency retention...

C2 couples the DC+AC signal out of the collector to the external world. C2 in combination with R7 and R8 couples every frequency higher than F = 1/(2*pi*(R7+R8)*C2) out to the outer world. There's that F= equation again. In this case, F = 68Hz, enough to couple all of the normal guitar signal out.

optional R7?

C6?

R7 and C6 work together to cut high frequency output signal. This cut happens at F = 1/(2*pi*C6*(R7||R8)). Since R8 is much bigger than R7, (R7||R8} is effectively the same as R7. That gives the high frequency rolloff as F = 15,400Hz -effectively everything above audio. The designer was really worried about not amplifying RF.

R8?

R8 has some subtle effects. Overall, it keeps C2 from creating clicks and pops when the booster is switched out of circuit by some kinds of switching. There are other side effects.

R9 and C5 (filtering current?) i found different values for C5: 47u(Fryer) or 100u(Cornish)

R9 and C5 filter power supply >voltage< by C5 acting as a local "bucket" of stored power supply charge for the circuit to use, and reducing any noise that is on the power supply. The frequency where this starts to happen is ... yes, that's right F = 1/(2*pi*C5*R9) [to a first approximation] or about 15.9Hz. It keeps power supply noise out of the amplifier to some extent.

R6 to the collector?

The action of the transistor's current gain forces the emitter voltage to follow the base voltage. The emitter resistor R5 converts this voltage to a current. The current through the emitter can only come through the collector, and R6 re-converts this current back to a voltage - the primary purpose of R6 in this circuit.

R1?

R1 is a "pull down" resistor that forces any leakage of capacitor C1 to ground when it's R1 end is opened by switching. It also forms a predictable load for the guitar. There are several side issues involved in this resistor.

R2?

Part of the DC biasing, as noted above.

There are several section-by-section teardowns of effects in the "Technology of..." section of geofex.com. Since the late 1990s geofex has hosted a lot of basic howto and explanations of effects circuits. It is a good place to look at more of this kind of information.

[THEZED]

Well, what can I say...you guys are amazing, thanks a lot!
I will investigate further following your advices.
Thanks again!

[PRR]

What they said, all of it.

Side-track: when an experienced circuit-person is looking at a circuit, s/he "filters out" things that "don't matter much", or are "very ordinary". R3, R7, C3 can't have major effects in clean audio (tedious to prove, comes with experience).

Most of it is a VERY ordinary amplifier. Emitter impedance is like 50 Ohms, so Base impedance is like 7k. Combined with R2 R4 (R3 is too small a complication to face) the input impedance is like 5k. For Full Bass we would pick C1 as >2uFd.

But C1 is 10nFd-5nFd, 0.01uFd-0.005uFd!! 200X smaller than "Full Bass". Figure it out: the "Bass roll-off" is like 5KHz!

So C1 and the amp input takes "a diagonal cut" at the signal, cutting treble very slightly and bass a LOT. Then the amp boosts it up so the bass level is near normal and the treble is !BOOSTED! (and surely distorted).

R7 is actually a minor detail; the output impedance is mostly R6. Except in severe clipping when the transistor pull-down near-shorts the output. The asymmetry would charge-up C6. R7 makes both sides more equal. R6-R7-C6 make about a 3.7KHZ low-pass so the super-highs don't ice-pick the ears (or the mail amp!) so bad.