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EH Hum Fix?

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5/6/1997 9:24 AM

EH Hum Fix?

My EH Deluxe Memory Man has a pretty noticeable latent hum. In looking at the schematic I see that it has a 220 uf @50v filter cap after the voltage regulator. In some of the reportedly quieter 9v supply schematics the first filter cap is a 1000uf with an additional 0.1 uf cap in parallel for better high frequency filtering. Do you think this would significantly reduce the latent noise of this thing if I were to make this modification?

Also...still no answers to my question of the thump that follows the nicely reproduced wet signal when using heavy attack and partially muted strings. The thump can be loud enough at times that I'm worried about my speakers.

Rob

5/6/1997 9:51 AM

Can't speak from experience, but theory suggests that the larger PS filter cap should do the trick (in tandem with the small one). Fortunately, large value caps are getting smaller in size these days, so replacing the 220uf with a 1000uf in the same space out to be just fine. From what I know of these beasts, a 50v rating is overkill. Mike Matthews must have gotten a deal on these from a surplus house or something; a 25V rating is more than enough insurance.

The thump is sort of mystifying, but you might consider altering the low end roll-off of the delay section. You can do this by finding out which lug of the relevent control is taking in the delay signal, and tracing it to a capacitor one component earlier in the circuit. This can apply to virtually any analog delay device since the vast majority of bucket-brigade chips need a bias voltage to work well and that DC bias voltage has to be filtered out before the delay signal hits the mixer stage or whatever op-amp lies next in line. Conventional wisdom selects the capacitor value to be low enough to preserve full bandwidth (20hz-about 15khz), but you can easily afford to lose a few octaves of that at the low end. Stick in a cap about a quarter of the value, and you should still have a low end rolloff of 100hz or so, which should make the sound a little more ambient-like, less "grunty", and less challenging to subsequent stages. You can do the same thing with the regeneration/feedback control. Hell, if you can figure out which cap limits low end going into the BBD, make that one smaller too.

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