Squealing EH Deluxe Memory Man

[rf7]

This pedal squeals/oscillates when the delay is set to it's longest setting. It's clock noise coming from the oscillator circuitry on the pcb. When you shorten the delay, the oscillation goes past 20Khz and you can't hear it. Do they all do this? Is there a fix for that?

[Lee_ranaldo]

yea even my polychorus does the same no max settings
that's the clock that is getting in the audio path

u can adjust the clock trim to make it dissapear but u may get a shorter delay time

[Mark Hammer]

ALL BBD-based pedals are obloiged to sacrifice some bandwidth at the top end in order to keep clock noise out of the output signal. When delay pedals were initially produced, it appeared that occasionally some manufacturers would aim for greatest possible bandwidth at most delay settings, permitting clock bleedthrough at longest delay.

I say "appeared" because I have no idea if that was their deliberate strategy or if the long-delay-whine was simply a matter of cutting it close and component tolerances would mean that the whine might be successfully suppressed in some units but slip through in others. My own feeling, back in 1978, was that they were aiming for maximum bandwidth, and simply assuming that the user would turn the treble on the amp down if the whine became too objectionable. It may have been much less premeditated than that, however.

As time went on, more and more units began using compander chips (NE570 or 571), which helped a lot with noise suppression, and filtering became fairly standardized. Your basic DM-2 is not dramatically different from earlier DMMs, but it contains companding and good filtering.

It is also the case that clock whine can be cancelled out (to some extent) via a balance trimpot on the outputs of the delay chip. For instance, in the upper right hand corner here - http://sites.google.com/site/electro...MM-Factory.jpg - you will see a 5k balance trimpot on the output of the MN3005 delay chip. The delay chip has two complementary audio paths internally. I like to describe them as one for the "tick" and another for the "tock". If the two outputs are perfectly balanced in level, then the tick and tock clock signals cancel each other out, leaving just the audio signal, somewhat like a humbucker pickup cancels out 60hz hum but still leaves the string sound (though via different means that involves no winding or magnetism!).

It can happen that the manufacturer declines to use such a trimpot and simply uses two equal-value resistors to do the balancing. I fact, a lot of them do. Of course, if those are the regular 5% tolerance resistors used throughout the circuit, that may only come close to achieving balance and not really nail it. The ideal IS to use a balance trimpot, but the price you payu is that over time the trimpot can be jostled out of place. This is true for both the bias trimpot that sets the bias voltage going into the delay chip (a common source of people saying "Why does my analog delay sound distorted?") as well as the balance trimpot on the output of the delay chip.

So, without knowing much more about your particular pedal and that issue of DMM, it is possible that the whne is a product of design and needs maybe a little bit of "help" from some added capacitance to improve the filtering. It may be the case that the clock cancellation is not optimized anymore. Either way, the good news is that the situation IS soluble.

[Lee_ranaldo]

Damn mark that's a lot of helpful info

I've been into this forums like for about 10 years including the old ampage forum and i do appreciate this kind of info.

Thanks, seriously

[Mark Hammer]

You're welcome. If all those years of buying and reading magazines, and taking stuff out of the library, don't allow me to be helpful, then it would have been wasted effort.

[rf7]

The problem was caused by a badly implemented true bypass mod. You can't run the footswitch wires over the back of the PCB, they've got to be away from the rest of the circuit as much as possible to keep the clock noise from bleeding into the audio. The guy acted like his pedal just started squealing on its own. Yeah, right.

[Mark Hammer]

Annnnnnnnnnnnddd THAT would be the other reason for the squeal.

Layout isn't always critical, but when you have a circuit that either incorporates a hi-frequency clock or high gain, you really do need to watch out for where the wires run, or else make sure you've shielded them well.

I guess in this instance, the previous owner was unaware of this and simply did the best job they could installing the switch.