Maxon AD-900 Analog Delay... Modding for MORE high end??

[aarondavis]

I use PRS guitars, so this thing is pretty dark.  Is there a cap that I can exchange that will make the delays clearer?  There are also trim pots on all of the delay chips.  In addition there are 3 other trim potts throughout the circuit.  Do you know what these do?  

Thanks

[george]

I think the Maxon AD-999 is analog ...? if so there are limitations that are brought about with the BBD delays used in analog delays.

do you have another analog delay in mind as an object of comparison eg AD-9, DM-2?

It may well be that there is a cap that you can change that gives brighter delays but causes undesirable side effects such as letting through high-pitched clock whine (which typical analog delay circuitry is designed to filter out to let you have a reasonable delay time - a consequence is that high frequencies are cut).

trimpots in analog delays are typically used to set the bias on the BBD input or adjust the maximum delay time - none of which adjust the tone.

You might be better served by buying a second-hand Boss DD-2 or DD-3 and modding it to sound more "analog" ...

[cd]

George hit it on the head - none of the trimmers adjust anything tone wise.  The trimmers are typically for clock leakage (noise), BBD bias (distortion) and clock pulse width (delay time).  There can be more than 1 of each - if each chip has its own trimmer, it's likely for biasing each chip.

I would not muck around with the trimmers.  You really need a scope to set things right, it's not something you can do factory accurately by ear.

If you need more high end, stick an EQ pedal before/after it unless you can live with clock noise.  That's the problem with changing caps/etc.  When you input a signal into a BBD chip, what comes out is not a perfect replica - if you can imagine a sine wave, if you put that into a BBD, what would come out would be a general sine wave shape, but in a jagged stair step pattern.  That's the clock signal being mixed with the sine wave.  Extensive filtering (cutting high end) removes the clock signal, so you end up with your original sine wave.  Now guitar signals are not simple sine waves - there's lots of harmonics in addition to the fundamental, so you hear some high frequency loss.  This is the characteristic "warmth" of an analog delay.  It's not perfect like digital.

You could also muck around with the pre-emphasis and de-emphasis filters (if they're there - it's typical of analog delay designs, but not always present), shifting it into a different frequency range or increasing the amount of boost or decreasing the amount of cut... but again, I wouldn't mess around in there unless you really know what you're doing and have a scope.

[george]

... and a schematic :-)