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Old 29th September 2008, 03:35 PM   #1
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Unhappy Passive mixing problems - guitar pedal

Ok, so here's the thing. (sorry if this is in the wrong thread...)

I've been trying recently to make a kind of 'bypass' pedal for a delay unit i've already got. The idea is that i can set the feedback high on the day, make a rhythmical loop, hit the bypass pedal, and play something else over the top (without the new stuff being added to the loop).

The problem comes when i try to passively mix* the output from the pedal with the output from my guitar. They should be both around the same volume, but as soon as i plug the pedal in, whether it's turned on or off, it cuts the volume of the guitar.

I'm guessing that it's something like the output impedance of the pedal is very low so it kind of "sucks in" some of the sound from the guitar.

Is there any small circuit i could add to the output of the pedal to stop it from eating all my through-level?

or anything else i can do?

sorry for the totally untechnical, incomprehensible language. hope it makes sense somehow.... and thanks in advance for your advice!



*by passively mixing i mean, literally joining the +ve and gnd poles together. I also had a 1M resistor accross the master output, not sure why i needed to do this but the A/B box i half copied from had one!
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Old 29th September 2008, 04:45 PM   #2
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I'm thinking a unity gain buffer might be my answer?
like this one: http://www.muzique.com/news/jfet-buffer-on-stripboard/

...looks simple enough for me to make without messsing it up too...
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Old 30th September 2008, 04:08 AM   #3
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try something like this....... (sorry for the crudeness of the drawing, it's a "quickie" MS paint drawing)....
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File Type: jpg passivemix.jpg (8.9 KB, 72 views)
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Old 30th September 2008, 02:50 PM   #4
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Cheers mate,
i'm having problems with mismatched impedences though...
will that help?

i tired using an old distortion pedal as a buffer and it helped a lot.
I'm now thinking to put a unity gain buffer on the input to see if that will work.

Do you know how i can measure the impedance at the output of a pedal? And how i can set a unity gain buffer to match it? does the output of the UGB depend on the input load? or does it just draw what it needs?

sorry for all the confusing questions.... cheers!
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Old 30th September 2008, 05:52 PM   #5
sreten is online now sreten  United Kingdom
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Hi,

Your unity gain buffer can also be a mixer.

Passive mixing involves signal loss and [b]not[b]
connecting any outputs directly together.

http://www.colomar.com/Shavano/intro_opamp.html see fig 9.

/sreten.
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Old 30th September 2008, 05:54 PM   #6
sreten is online now sreten  United Kingdom
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Hi,

Your unity gain buffer can also be a mixer.

Passive mixing involves signal loss.
Do not connect any outputs directly together, does not work.

http://www.colomar.com/Shavano/intro_opamp.html see fig 9.

/sreten.
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Old 1st October 2008, 12:53 AM   #7
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a unity gain buffer has (theoretically) infinite input impedance, so no matching needs to be done there. the buffer also has (theoretically) zero output impedance, but is usually about 2k ohms. since most amps have at least 10k or higher input impedance, there isn't a problem there either. impedance matching isn't that much of an issue, voltage matching is the issue with buffers. you could use a buffer on the guitar output, and that would lessen the contention between your sources, or you could make an active mixer using a single op amp and a resistor for each input going to the inverting input of the op amp and an equal resistor as feedback. the noninverting input would be grounded. the mixer will be an inverting mixer.
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