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Crate CR-212.....HUMMMMM

Started by matthias, July 09, 2006, 12:01:31 AM

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That D.C offset is pretty high, but not so high as to stop the amp from working or anything.

I'm wondering now if this is just the way this amp sounds. I can only suggest larger PS caps like 10,000µF or even higher but that still may not do it or even be worth the cost on such an old amp.

Sometimes designers shoot themselves in the foot by crowding things together and causing proximity effect hums and buzzes. Of course I'm guessing here cause we don't have a schematic or layout to go by here.

Hum usually means DC where there isn't supposed to be any. A Zobel network on the output might stop some oscillations. It will require a 2.7 ohm 2 watt resistor and a .1µF cap.

Like this.




Actually, DC makes very little noise compared to AC where it shouldn't be.
Life is what you make it.
Still rockin' the Dean Markley K-20X


How does a Zobel network reduce DC offset?


Heres the problem, your trying to re-invent the wheel to get this amp going.

I say scratch that, start fresh, and use the housing to fit in a new project. You may aswell, for as much as you have trouble shooted this one, its going to just cost you a lot for a little payoff.


Well, since it seems like you already have tried pretty much everything except total rebuild then maybe it's time for something far-fetched: Have you tried the amp in another room or apartment? Sometimes too long mains wiring, very long extension coords etc. causes a very irritating hum. Also, any motors or fluorescent lights nearby?

Anyway, that DC offset indeed seems too high: I'd suspect there's something breaking down and I'm sure Crate did not design an amp that would heat the voice coils with such a high DC figure. One possible source for unbalanced output is in the power amp input stage, which is very often a differential amp. I'm pretty sure this amp has a discrete one, (although a schematic would verify it). Any inbalance there will increase the DC offset - at least a bit. Maybe some components, solder joints or something have gone/are going bad in there. Besides a bad grounding, increased hum is as well a symptom of a stressed output stage. (The stage draws more current than it normally should). Are some of the transistors or resistors getting hotter than they normally should? Perhaps a busted CCS circuit somewhere in the PA is messing up the operation, causing inbalance and probably an excessive current draw - which then leads to supply sag, which causes an increase in ripple.

Can you test the supply voltages without the load of the power amp circuit to see whether you have sag even when idling? If this is the case then adding more capacitance would help in reducing the hum but would not cure the real problem. If the voltages still fluctuate notably (without the load of PA circuit) you most likely have either problems in the mains distribution or a faulty transformer.


It seems the preamp is good, so don't totally start from scratch.  Just do a new PS and power amp section, those are the easiest to build anyway!
Life is what you make it.
Still rockin' the Dean Markley K-20X



I'll try to measure the transformer with out a load and see whats up there.
Alright, I think I'm comming to my senses.
I think It's the pre-amp that I like anyway.



Did you ever resolve the hum? I'm having the same issue.
Reflowed the whole board, replaced the the 4700/35 caps.
Attached is schematic from cr-160 power section which is the same board.
I love these amps and am dying to get it running right.