Capacitor and resistor in parallel - what's going on?

[Greenballs]

Hi all.

I've been tinkering with an electra overdrive circuit involving a darlington pair (two BC108s). In between the emitter of Q1 and the base of Q2 I've placed a capacitor in parallel with a resistor (470n and 150k) and it's effect is to add a bit of mid 'honk' to the sound, which is not altogether displeasing to my ear. Smaller value caps produce a higher mid frequency, larger value caps the opposite. Changing the value of the resistor has minimal impact anywhere up to 470k where there is, unsurprisingly a slight drop in gain.

I've researched online why this pairing of capacitor and resistor behaves the way it does but can't find anything specifically linked to an overdrive circuit or that doesn't present me with equations that, frankly, baffle me. Maths was never one of my strengths! If someone could explain to me in layman's terms what is going on here I'd very much appreciate it!

Cheers.

Phil.

[antonis]

Did you also placed a resistor from Q1 Emitter to GND, for Q1 acceptable h_FE..??
(for Q1 current set by Q2 V_BE/R..)

But, better post a schematic of your circuit..

[Greenballs]

Hi Antonis

No, I haven't put a resistor to ground from the emitter of Q1, there is a 330R resistor boot strapped with a 22uf cap to ground from the emitter of Q2.

I've attached a version of the schematic so you can see the placement of the 470n capacitor and 150k resistor in parallel between Q1 emitter and Q2 base. The values of all the other components don't seem to affect the effect that these two components have on the sound, believe me I've switched them ALL out at some point!

The thing that surprises me is that 470n is quite a large value yet it seems to filter out quite a lot of low frequency from the circuit. Why would this be?

[idy]

So you are controlling gain...sometimes people put a variable R there, with cap attached between wiper and ground. Cap doesn't let DC through, so the DC gain is less than the AC....down to a frequency that is too high for you and what you want. Traditional fuzz puts a bigger cap there, 20uf? Fuzz face. Might deepen things out.

[antonis]

The thing that surprises me is that 470n is quite a large value yet it seems to filter out quite a lot of low frequency from the circuit. Why would this be?

Q2 can be considered grounded Emitter amp whose input impedance is considered very low..
(0.026/I_Collector times h_FE
So for Q2 either low h_FE or large quiescent current (or both..), corner frequency of 470nF/input impedance HPF may be high..

Additionally, cap shunt resistor "resticts" Q2 Base current (at low frequencies, where 407nF cap impedance is greater than resistors value) so there is more than 2 V_BEs drop between Q1 Base and Q2 Emitter..

P.S.
I'd try to either connect resistor's right leg on Q2 Emitter (acceptable values from 1k up to 10k say) or connect another resistor between Q2 Base & GND (maybe replace that resistor with a pot with its lu 3 to Q1 Emitter, wiper to Q2 Base and lug 1 to GND)..

[Greenballs]

The thing that surprises me is that 470n is quite a large value yet it seems to filter out quite a lot of low frequency from the circuit. Why would this be?

Q2 can be considered grounded Emitter amp whose input impedance is considered very low..
(0.026/I_Collector times h_FE
So for Q2 either low h_FE or large quiescent current (or both..), corner frequency of 470nF/input impedance HPF may be high..

Additionally, cap shunt resistor "resticts" Q2 Base current (at low frequencies, where 407nF cap impedance is greater than resistors value) so there is more than 2 V_BEs drop between Q1 Base and Q2 Emitter..

Thanks Antonis, that makes sense, I think?!!

[antonis]

Just take also in mind what idy said about DC & AC gain..

[Dormammu]

smthwrng

[Dormammu]

Hi Antonis

No, I haven't put a resistor to ground from the emitter of Q1, there is a 330R resistor boot strapped with a 22uf cap to ground from the emitter of Q2.

I've attached a version of the schematic so you can see the placement of the 470n capacitor and 150k resistor in parallel between Q1 emitter and Q2 base. The values of all the other components don't seem to affect the effect that these two components have on the sound, believe me I've switched them ALL out at some point!

The thing that surprises me is that 470n is quite a large value yet it seems to filter out quite a lot of low frequency from the circuit. Why would this be?

Basically, this "mod" breaks down the whole idea of the Darlington pair.

[antonis]

Basically, this "mod" breaks down the whole idea of the Darlington pair.

Not entirely..

http://diy.smallbearelec.com/HowTos/BreadboardGeDarlingtonFFs/BreadboardGeDarlingtonFFs.htm