Adjusting output level - Volume controls

[Sonny ReVerb]

Hi all,

I've got Doug H's Hwy 89 circuit on my breadboard at the moment. (btw - thanks, Doug!). I get some very nice tones out of it, but it seems to have way too much output. I barely have to turn the volume control up at all and I get more level than I need. The signal output measured over 3VAC (rms) with the gain and volume maxed.

Paul Marossy touched on a similar problem in an earlier post about the Blackfire,

...Mine has way, way too much boost, though. I am going to have to play with the voltage divider at the end to get the output level that I want out of it.

My question, in general, is about controlling signal output; What is the best way to adjust the output of a particular circuit? Volume control (voltage divider) is the obvious answer, I guess. So, how do different size volume pots (say 100k vs 500k) effect output level? They're both used as a voltage divider, so the only difference would seem to be across the pot - from full signal to ground. What else does volume pot size effect? Probably has an effect on output impedence? So, what should one consider when choosing a particular value volume pot?

In my case, with the Hwy 89 - how would I go about changing the voltage divider? Seems that I need to send more signal to ground rather than to the output. Would changing the 500k pot to a 100k help? And, is altering the voltage divider the best method here? I also tried lower gain transistors (2N5088, 2N4401, 2N2222A) but they had little effect on overall signal output.

I'll play around with all this on my breadboard, but I was hoping someone could help me understand a little of the theory as well. I guess it's back to Electronics 101 for me... :roll:

Thanks,
Sonny

[Sonny ReVerb]

Found a good article on this subject at the ESP website:

http://sound.westhost.com/vda.htm

Forgot to mention, I am using a log/audio pot rather than linear taper for the Hwy 89 volume control.

Sonny

[Ge_Whiz]

Changing the value of the pot won't help. All you need to do is to put a fixed resistor in series with the output of the last stage to the volume pot. At the moment you have up to 3 VAC arriving at the output put via a 500k pot, right? If you put a 500k (well, 470k) resistor in series to the pot, you'll only have access to the first half of that signal, i.e. 0 to 1.5 VAC. That will halve your overall signal output. A 1M resistor will only give you 500k/(1000k + 500k) or one third, etc. It's the ratio of the fixed resistor to the pot value that is critical, so if the fixed resistor value gets too high, drop the pot value. A 200k (approx) fixed resistor in series with a 100k pot will similarly drop the output level to a third of normal.

It doesn't matter if you're using a linear or audio pot.

[Sonny ReVerb]

Thanks Ge_Whiz.

After thinking about this last night, that was what I was going to try next on the breadboard. There's already a smaller series resistor (10k I think) I can adjust in line with the output. I may have to tweak the adjacent filter cap values to maintain the frequency response, though.

I mentioned the log taper because in my first post I said,  

I barely have to turn the volume control up at all and I get more level than I need.

I figured someone might think I was using a linear taper vs log.

Sonny

[Ge_Whiz]

You shouldn't need to tweak the caps if you keep the total resistance (10k + 500k?) in the same ball park, e.g. 290k fixed + 220k pot, 410k fixed + 100k pot etc.