Author Topic: Volume Pot Overdrive Pedal  (Read 2911 times)

aeroemi

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Volume Pot Overdrive Pedal
« on: February 12, 2015, 04:04:33 AM »
Hi guys,
It's an overrated subject I know  ::), but I really wish to know your opinion on that... Most of the OD pedals project you can find, end up with volume pots values in the range of 50k to 250k max (as well as all the MB overdrive projects... except for the Neutrino); I know someone who said that this is due to the low impedence the volume pot "sees" from the output of the amplifying stage.
From this poin of view, it could be see as a mistake using a high resistance pot (such as 500k)...

But I've also noticed that: the higher the pot value, the higher the overall output.

I'm not talking about frequencies cuts (lows or highs), I'm just interested in the overall output you can get out of an overdrive pedal without adding an additional gain stage (which could introduce more undesidered saturation)...

Thanks!!  8)

ps: sorry for my english...   :-[

luks999

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Re: Volume Pot Overdrive Pedal
« Reply #1 on: February 12, 2015, 04:13:33 AM »
you mean the GAIN pot right, not the volume?
volume is normally 100k (A or B) for most drive circuits i know. the timmy uses 10k

ive built a tubescreamer with a 500k and a SRV special ts with a 1M drive pot...
a little bit more gain, but not extremely more ;)

my experience is that the overall gain depends not so much on the pot, but more on the  circuit (transistor stages) and of course the voltage
the klon and the catch22 are louder than my tubescreamer that operates under 9V ;)

aeroemi

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Re: Volume Pot Overdrive Pedal
« Reply #2 on: February 12, 2015, 05:00:43 AM »
you mean the GAIN pot right, not the volume?
volume is normally 100k (A or B) for most drive circuits i know. the timmy uses 10k

ive built a tubescreamer with a 500k and a SRV special ts with a 1M drive pot...
a little bit more gain, but not extremely more ;)

my experience is that the overall gain depends not so much on the pot, but more on the  circuit (transistor stages) and of course the voltage
the klon and the catch22 are louder than my tubescreamer that operates under 9V ;)

Actually I'm not talking about the drive pot (which is a 1M)... I'm talking about the volume pot you can find at the end of any OD pedal... I've noticed that with a (say) 250K set to max, I do not have the same output as if I used a 500k one...
I have less output....

Why is that? Is it because with higher values, less part of signal is shunted to ground?  :-\

luks999

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Re: Volume Pot Overdrive Pedal
« Reply #3 on: February 12, 2015, 05:14:54 AM »
ok never built a drive pedal with a vol pot higher than 100k so far

hmm i would say the only thing that changes when increases the vol pot is that the range differs.
so the higher the vol pot, the less volume you have on the lower vol settings.
but on max vol level (=0R for all pots) they all behave the same

so, yes its correct, not more output when changing the volume pot

TGP39

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Re: Volume Pot Overdrive Pedal
« Reply #4 on: February 12, 2015, 07:52:35 AM »
It turns out there are many things to consider when choosing a volume pot.

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

I hope this helps.

Steve.
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RobA

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Re: Volume Pot Overdrive Pedal
« Reply #5 on: February 12, 2015, 07:59:32 AM »
This really depends on the circuit you are looking at. The typical volume pot at the very end of the circuit does a couple of things. It's a voltage divider, so the volume setting aspect of the pot is going to be the same no matter what value you use. It has an impedance matching function between the output of the last stage of the circuit it's in and the input of whatever it goes into next. The efficiency of the power transfer between the two is at a max when the impedance is matched. It's pretty much impossible to cover every input situation, so the practical thing to do is design the pedal so that last amplification stage as an output impedance that is in the typical range of input impedances and then use a pot that's in that range. But, if for some reason you need the output impedance of your last amplification stage (and that can include tone control and buffer stages) to be lower or higher, then you might need to use a different value pot.

There's also the interaction between the volume pot and the output cap in many OD designs. Changing the corner frequency of this filter could be perceived as volume difference in some situations. 
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TGP39

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Re: Volume Pot Overdrive Pedal
« Reply #6 on: February 12, 2015, 08:08:57 AM »
RobA is the guy who taught me about looking at a volume pot as a voltage divider (Thanks Rob) and Mr. Beavis is the guy who solidified that concept for me......

http://www.beavishifi.com/articles/Volume_Control/

Steve.
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RobA

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Re: Volume Pot Overdrive Pedal
« Reply #7 on: February 12, 2015, 08:13:28 AM »
RobA is the guy who taught me about looking at a volume pot as a voltage divider (Thanks Rob) and Mr. Beavis is the guy who solidified that concept for me......

http://www.beavishifi.com/articles/Volume_Control/

Steve.

You're welcome Steve. There's a good chance that I first learned about it from the ESP site.

Do read the article at ESP that Steve linked to. The information there is always good. They are a great resource in general for audio related electronics.

In that article it details how to calculate the input impedance and output impedance of the volume pot and then you can figure out from there how that's going to interact with circuit you are building and any input impedance your pedal might end up having to drive.
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aeroemi

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Re: Volume Pot Overdrive Pedal
« Reply #8 on: February 12, 2015, 10:16:15 AM »
Thank you guys!! I'll read the efs article!

You know, I've been confused when I saw th OCD schematic ending up with a 500k volume pot instead of a 100k one, as I would expected...