Change impedance

[Thelonious]

Hello,
I build some FX (with this site), all for guitar (high impedance), but I use them with analog synth (low impedance)
How can I change the impedance of this FX for use with synth ?
I think it's maybe just with the 1M input resistor, but what can I put, or there is maybe other change to make ?

I have too, an Ibanez AD-100, that I want to mod for use with synth, the input signal is very "low" with synth and he quickly saturate with it.

Thanks for your help.

[IvIark]

It depends which effects you're talking about

[Thelonious]

For example :
Xfuzz : http://tagboardeffects.blogspot.fr/2014/05/x-fuzz-by-deadastronaut.html
Neptune Delay : http://tagboardeffects.blogspot.fr/2014/04/culturejam-neptune-delay.html
Blackmore ROUS : http://tagboardeffects.blogspot.fr/2014/05/blakemore-effects-rous.html

And the Ibanez AD100 ;)

[Thelonious]

This is with the Xfuzz that I have the more difference, with the signal, between guitar and synth (I tried both) With synth  the volume pot should be maximum to have a signal. With a guitar, volume work very well, the signal quickly feedback.

[IvIark]

For the X-Fuzz you'll want to reduce the Pregain pot and the 1M pulldown resistor, with the Neptune Delay you can only really reduce the 1M pulldown resistor which will reduce the maximum input impedance and so may do the trick.  The Rous input impedance is around 500K based on the pulldown resistor and 1M resistor to vref, so just drop those two values down to whatever you want.  So if you used two 100K resistors the input impedance will be around 50K.

For the AD100, reduce the 510K to vref resistor after the input cap.

[Thelonious]

Thanks for your quick answer.

Last question : What that means "vref"? I am a beginner in electronics and I do not yet know all the words

[Thelonious]

So for Xfuzz I can maybe try with 500K lin for PreGain and 100k for pulldown resistor ?
And for AD100 I use 100k resistor ?

[IvIark]

It's the virtual ground reference voltage that is used in most opamp circuits that use a simple 9V supply.  You'll often see an area in a schematic with one 10K (for instance) resistor going to the supply and another 10K resistor going to ground.  That makes a voltage divider based on the ratio between the resistor values, and when the values are both identical you end up with 4.5V from a 9V supply.  You'll often see it marked up as vbias, vref, 1/2 supply or 4.5V on the schematics.  And it just gives a central reference point between 9V and ground.

It's basically simulating a bipolar supply which may be +/-9V which then uses true ground as the central reference (half way between -9V and +9V).

[IvIark]

Thelonious wrote

So for Xfuzz I can maybe try with 500K lin for PreGain and 100k for pulldown resistor ?
And for AD100 I use 100k resistor ?

If that impedance is suitable then yes.  You may need to experiment a bit to get the best results

[Thelonious]

Thank you for the explanation.


"If that impedance is suitable then yes.  You may need to experiment a bit to get the best results "

All this is still obscure to me  
I don't want to make a mistake. And I don't really find a simple explanation on the difference in impedance, and how to calculate the right pulldown resistor.

[IvIark]

Well if you know the impedance suitable for a synth is 100K for instance, then make the pulldown resistor 100K, then the input impedance set by any other component can't exceed that.  I don't know anything about synths or their requirement and so can't offer any advice specific to them.  My main recommendation when you aren't 100% sure is to socket the positions and then you can try a number of difference values without committing to soldering and tweak the value to get it just right to your ears.

[Thelonious]

Thanks I'll do some experimentation with a socket and different resistor.

[Thelonious]

I found this :
Synth/Keyboard - 1k Output Impedance
Guitar/Bass - 100k - 500k Output Impedance.

And there is a 10x rule : the input impedance should be 10 times the output impedance of synth or guitar.

This is a good basis for start experimentation.