Noisy Dynacomp...

[dr. jimmy]

Hi, after too many years to count I've decided to put my all original '76 Dynacomp back on my board.  Sounds great except for one thing: when not playing there is a quite audible sound coming through the amp that is best described as rustling paper or whooshing air.  If I turn it off and CRANK the amp the noise is still there but not as loud. I've tried cleaning pots/jacks/switch to no avail and am leaning towards it being the result of a leaky cap.  Question is, do i replace the tant. caps in the circuit, the ceramics or all of them?  Also, assuming it does end up being one of the tants, if I replace them, what kind of effect will it have on the overall tone?  I love the way this pedal sounds and would hate to change it, but it is kind of unusable as is due to the noise.

Thanks, I really appreciate the help!
Bill

[idy]

That is usually what a compressor does, if no signal it boosts your noise as far as it can. Max gain.  Some folks use a noise gate. Can you borrow another compressor from a friend to compare? What about when the compression is turned down halfway?

Tantalum do have limited lifespan, not so much ceramic (they are sometimes microphonic, different problem).

I have made a few of these that sound good and people love and use them, but never used tantalums. Film up to 1uf, electro above that. "Jangle box mods" for a switchable more highs, leave a little attack (slow clamp.)

maybe we'll get more opinions.

[dr. jimmy]

I have tried different comps in the same spot, including a Dynacomp from a little later (I think it's a '79) and it was pretty noise free.  I get the rushing wind noise regardless of the where the sensitivity is set...

[dr. jimmy]

Ordered the tant caps, should be here Saturday.  I guess it's worth a shot since I can't use the pedal as it stands.

[Mark Hammer]

As idy correctly noted, that is what compressors do and are generally designed to do.  It is typically referred to as "breathing", when the hiss gradually comes up in level and sounds like someone inhaling.

While limiters clamp the peaks of the signal and don't do anything to softer passages (their role is simply to protect against excessive transients), compressors provide gain to low-level signals in addition to reducing the gain of peaks.  This is exactly how they provide the illusion of "sustain".

The trouble is that the "decision" to apply gain is generally made solely on the basis of average level, without regard to actual frequency content.  So incoming hiss is treated identically to the soft decay of a string.

Cures?

1) Always feed your compressor the cleanest and quietest signal you can.  While placing it after a high gain pedal can, in some circumstances, yield a sound that is musically useful and valid, high-gain devices also generate a lot of hiss, so should be avoided before a compressor.

2) Transistors can vary in their noise specs, though I couldn't say how much or which specific ones are best bets.  There is only one before the OTA/3080, that might affect incoming noise level.  None of the others would have any noticeable impact on hiss.  Perhaps some good folks here might have viable suggestions.

3) My own view is that MXR used tantalum capacitors when a normal electrolytic would end up being "too tall" to fit in the box behind the (non-16mm) pots.  Because of their general shape and size, tantalums could be easily bent over to provide a lower-profile board.  There aren't many polarized caps in the Dynacomp, and certainly those associated with the power supply would have no bearing on hiss.  That said, electros can be VERY small these days, compared to their predecessors from the 1970s, so ordering a couple of 1uf/16V units wouldn't be a terrible idea.

4) Low-noise metal film resistors in the input wouldn't be a terrible idea either.

5) If you do go about replacing ANY components on the board, BE CAREFUL.  Not just to avoid accidental solder bridges, but to avoid the lifting and fracturing of PCB traces when heat has to be applied to both remove components and install new ones.  My point #1 above is always the safest strategy.

6) The Ross compressor is basically a Dynacomp with a few added components.  I don't know what impact they have on noise levels, but I think it is a relatively safe guess that they were added specifically to try and minimize that aspect of the basic Dynacomp design.  Perhaps someone who has compared the two can comment.

I realize the post/thread concerns a vintage Dynacomp, but will mention that the SSM2166 and 2167 chips provide excellent compression, and include downward expansion to significantly deaccentuate hiss when you stop playing.  A VERY quiet compressor.

[dr. jimmy]

Thanks for the reply Mark, I appreciate it!  This isn't the usual compressor breathing noise, it's more of a low level crackling/rustling paper sound similar to either a noisy pot/jack sound which is constantly occurring (kinda like there's DC where there shouldn't be, hence why I assumed caps). I'm almost 100% sure it's neither pot or the jacks as I've cleaned them out and it's not like I can make the noise happen by manipulating them, the noise is just always there.

Also, I'm only using this comp on my cleanest of clean sounds, no distortion or gain, just using it for very clean funk stuff so the signal going in is essentially straight guitar with nothing else.

[Mark Hammer]

Well, it IS around 40 years old, if not older.

If it's not the pots themselves, perhaps the trimmer inside is showing its age.

[dr. jimmy]

Well it wasn't the caps.......