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-   -   Impedance matching (http://www.ultimate-guitar.com/forum/showthread.php?t=1581273)

voltafan485 01-05-2013 07:36 AM

Impedance matching
 
i am building a passive guitar volume pedal without all the extra mechanical moving parts and adopting the Rotovibe idea of a large knob on the side to control the potentiometer. i am using a 1M pot straight to the pedals output. i chose 1M because it should retain the most trebble, and its what they use in amplifier inputs. my reasoning for this is that the high impedance of the pedal input should be roughly 10 times higher than the guitars output creating a bridged impedance that provides maximum voltage with minimum damage to the signal. my problem is here:

since the 1M pot goes straight to the output doesnt that mean the output impedance is also 1M?
when connecting to the next effect in my chain (dunlop 105Q bass wah) does this high output impedance to the lower impedance of the pedal basically negate what i designed my volume pedal to do in the first place?
or is there a way to reduce the output impedance of my volume pedal without damaging the signal?

sorry for so many questions i havent really payed attention to impedance until recently and i want to make sure i get it right!

AcousticMirror 01-05-2013 03:22 PM

umm that pedal already exists...

it's called your guitars volume knob.

Mephaphil 01-05-2013 03:42 PM

Quote:
Originally Posted by AcousticMirror
umm that pedal already exists...

it's called your guitars volume knob.


Ignore this.

I think volume pedals are great. Loads of cool stuff can be done with them.

Unfortunately I don't have much knowledge on what you're asking, but it's cool.

earthwormjim 01-06-2013 01:54 AM

Quote:
Originally Posted by voltafan485
i am building a passive guitar volume pedal without all the extra mechanical moving parts and adopting the Rotovibe idea of a large knob on the side to control the potentiometer. i am using a 1M pot straight to the pedals output. i chose 1M because it should retain the most trebble, and its what they use in amplifier inputs. my reasoning for this is that the high impedance of the pedal input should be roughly 10 times higher than the guitars output creating a bridged impedance that provides maximum voltage with minimum damage to the signal. my problem is here:

since the 1M pot goes straight to the output doesnt that mean the output impedance is also 1M?
when connecting to the next effect in my chain (dunlop 105Q bass wah) does this high output impedance to the lower impedance of the pedal basically negate what i designed my volume pedal to do in the first place?
or is there a way to reduce the output impedance of my volume pedal without damaging the signal?

sorry for so many questions i havent really payed attention to impedance until recently and i want to make sure i get it right!


Amplifiers use a 1M resistor in parallel with the input, it's not in series. Their input impedance is not necessarily 1M. But it is quite high to ensure good voltage transfer, not to ensure good treble levels like you might pick on a guitar's volume knob. It's also there to act as a DC reference for the preamp grid.

The guitar signal chain can be equated as a series resistance, and the 1M resistor is in parallel with the input. A basic voltage divider circuit. By having a high resistor, 1M, you ensure that the input the amp sees is a majority of the voltage coming from the guitar. A lower than 1M value would result in a proportionally larger voltage drop across the guitar's signal chain, thus less voltage for the amplifier to see.

The impedance of your pedal is invisible to your amplifier if you have another effect (especially buffered) between the volume pedal and the amplifier. And your pedal isn't going to have 1M output impedance when you're really using it. When it's full volume, it'll be 1M in parallel with your Guitars impedance (i.e. less than 1M). When it's fully off, it'll be 1M but who cares since your signal is grounded. In between, it's a resistance in parallel with your guitar, and a series resistance in the path (also less than 1M).

Your wah is an active circuit, and it's not acting as a passive filter, it doesn't really care what impedance it sees as long as it's "High Impedance."

You also don't need to be matching impedance to your amps input. Impedance matching is done when you need efficient power transfer, like on the output stage. On the input, you really don't care at all about power, you just care about voltage levels. In fact you don't want your guitar's signal chain to be matched to the input impedance of your amp, that would cut the voltage level of the input in half.

More specifically to your question, your wah is an active circuit, it has some gain in it more than likely, you won't be losing your signal. You don't need to concern yourself with it's output impedance, it's not a passive circuit.

Quote:
Originally Posted by AcousticMirror
umm that pedal already exists...

it's called your guitars volume knob.


Quite honestly this is a stupid response. Why even bother posting?

Viban 01-06-2013 01:58 AM

Quote:
Originally Posted by AcousticMirror
umm that pedal already exists...

it's called your guitars volume knob.

+1

LeviMan_2001 01-06-2013 05:37 AM

Quote:
Originally Posted by Viban
+1


-1 :D

Viban 01-06-2013 05:55 AM

Quote:
Originally Posted by LeviMan_2001
-1 :D

/gloveslap, I Demand satisfaction, we are to duel upon the morrow.

von Layzonfon 01-06-2013 06:47 AM

Quote:
Originally Posted by Viban
+1
Oh dear. I may have to reconsider our "friend" status, which would be a shame because you're the only one I have.* :haha:

*Officially, around here.

Viban 01-06-2013 06:48 AM

Quote:
Originally Posted by von Layzonfon
Oh dear. I may have to reconsider our "friend" status, which would be a shame because you're the only one I have.* :haha:

*Officially, around here.

:haha: you know you love me.

von Layzonfon 01-06-2013 06:56 AM

Quote:
Originally Posted by Viban
:haha: you know you love me.
Hmm...

Well, remember: volume pedals are swell.

*runs*

Viban 01-06-2013 07:02 AM

Quote:
Originally Posted by von Layzonfon
Hmm...

Well, remember: volume pedals are swell.

*runs*

what do you use it for?

von Layzonfon 01-06-2013 07:25 AM

This sort of thing.

Viban 01-06-2013 07:34 AM

The only thing I saw on there that I couldn't do with my control bypass was the cool violin sounding swell thing.

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_______________
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voltafan485 01-07-2013 11:04 AM

earthwormjim, thank you for clearing everything up
everyone else, lol yall are too much


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