One for the tech heads - resistor values in circuits

MtB

Do you think that the effect of tolerance compounding (of resistor values) in an amps or pedal circuit has a significant effect on the tone and response of the circuit? 

I have done some reading on amp and pedal build forums where it has been claimed that some builders take the time to measure and select the value of each resistor, ensuring that it is the exact value as that on the schematic of the original, prior to soldering it into the circuit. 

Taking that each resistor in a circuit has a tolerance of 5% or 10%, would that have that much of an effect on what the amp sounds like?

Cirrus

I'm not a pro tech or anything but my feeling has always been that yes, when you add up all the different tolerances of the resistors and caps you use, the variations in supply voltage and current draw, and the effects of lead dress, it all adds up to make two amps that are ostensibly the same sound different.

BUT... Who's to say that getting the component values bang on with the schematic is going to sound better than whatever values random chance might give you?

I've always assumed that builders who do screen every component for tight tolerances do so because that kind of attention to detail gives great consistency and consumers will respond positively to that.

LastMantra

I'd say so, but I'd also say that closer to the value on the schematic doesn't necessarily mean it will sound better...or more like the original. Cap values too. Some are even 20%, which is a fair bit.

ICBM

Exactly what Cirrus said. Component tolerances certainly affect tone, in the same way that deliberately changing values does. Using better-tolerance parts simply produces a more consistent sound, not necessarily a better one.

jpfamps

Unless you are using some very exotic resistors (unlikely), then 1% metal film resistors are cheap and easy to source.

I would be hard pushed to imagine that greater tolerance than this is required.

Commonly available capacitors have lower tolerance than this.

10% is typical for film caps, whereas electrolytics tend to be +/- 20%, although historically +100%/ - 50% is not uncommon. Using electrolytic capacitors in parts of a circuit where exact value is important is bad engineering practice.

Capacitors (and resistors) are used to set turn over frequency of filters (tone controls), so you capacitor tolerance could affect the "tone" of pedal/amp.

Inductors (used in wah wahs etc) are the most variable tolerance wise, and the only way of accurately winding them is to overwind them and remove turns until the correct inductance is achieved, but this costs.

The most variable components generally used in pedals are geranium transistors, and JFETs, and these often benefit from being "specially selected".

ecc83

I have run response checks (latterly with the Neutrik A1 testset, no slouch!) on scores if not over a hundred amps and pedals.

If a response in the 100Hz to 10kHz band is out by more than 2dB it was always because the board was wrongly stuffed (V rare) or (rare but more common) the specification was out of whack due to wrong revision or typo!

IIRC the resistors were 5% jobbies and the caps 10% but I will try to check that. If anyone wants to do the math take a CR combination and check the response at all four tolerance "corners" with "F(3dB)= 1/2pi.C.R" and I am pretty sure you will find that for the above tolerances it don't matter a lot. I can't be fekkin' arsed!

Then, where you have cut and boost circuits around amplifiers that go into distortion the frequency response becomes level sensitive! And what guitar amp does NOT get distorted?

AND! As if all that bllx wasn't enough the classic tone stack is VERY interactive!

If you are building audiophile RIAA pre amps tolerances become important * but we's only into Rock and Roll!

*D.Self: Small Signal Amplifiers

Dave.

ICBM

jpfamps said:

The most variable components generally used in pedals are geranium transistors

Watch that flowery language .

MtB

Thanks for the responses. With respect to the germanium transistors, I've built a couple of tone bender clones using 3 old mullard oc81d trannies and found (particularly in the winter) that the sound of the pedal was better if it had been "warmed up" - a better sustain and stable decay of the fuzz sound.

martinw

MtB said:

Do you think that the effect of tolerance compounding (of resistor values) in an amps or pedal circuit has a significant effect on the tone and response of the circuit? 

With respect to amps, no, not significant in my opinion.

jpfamps

MtB said:

Thanks for the responses. With respect to the germanium transistors, I've built a couple of tone bender clones using 3 old mullard oc81d trannies and found (particularly in the winter) that the sound of the pedal was better if it had been "warmed up" - a better sustain and stable decay of the fuzz sound.

Geranium transistors are very temperature sensitive, and this will definitely affect their operation.

Dave_Mc

^ Mine definitely don't like frost, that's for sure. Most are only half-hardy.

(Yeah I know ICBM got there first, but still)