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Hey Kg?

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12/12/2003 10:00 AM

Hey Ken, I'm wondering if you can answer a couple questions about using a seperate chassis for preamp and poweramp?
I'm modding an old Conn organ power amp chassis which had 3 power amps each with a seperate output transformer, and adding a seperate preamp chassis with 3 preamps in it. So when I get done, I'll have 3 amps in one head about the size of an SVT head. They used some 8 and 11 contact sockets previously between chassis, so thats what I'm using here also. I've been able to keep the power inputs to the preamp chassis and the preamp outputs away from each other and to be able to optimize the layouts for the preamps for the most part. The PI's are in the power amp chassis in all cases.
The question here is if I should put the coupling caps after the last gain stages in the preamp in the preamp chassis, or the power amp chassis? And should I use shielded wire between the chassis? It will be about 4 inches max between the chassis for the preamps out. Probably more like 2 1'2 inches. Also, I didn't quite understand why the hum is less if you put that last gain stage on the preamp chassis? Is it because the gain is at a higher level after that stage so its not as susceptible to hum if you have a long wire run before the next stage?
Thanks!
greg

12/12/2003 9:12 PM

i would put the coupling caps in the last stage of the preamp. i just feel better without high voltage on an umbilical, even if it is technically "inside" the larger chassis.

make sure you always have a DC leakage path (ie some resistance to ground) for all of your signal wires... ie don't cap couple both sides of the interconnection and have infinite DCR to ground in the middle.

i would tend to use shielded interconnects since you're sending single ended signals to the PIs in the "power amp" sections, even if they're really short. of course you could try it with unshielded wire and see how it works. i hate working with shields and i try to avoid them just out of laziness.

as far as your last question, you're on the money. since the signal level is higher after the gain stage, the same "injected" noise voltage has less of an effect on SNR.

hth
ken

12/13/2003 8:11 AM
Greg Simon	Thanks Ken. I've followed your advice so far in the layouts, so I should be ok. Now if I can just finish the layout for the 2204 stage, I should be able to make some actual build progress! Once I get the preamps done, then I have to do the layouts and rework the power amp sections. So I'll still be working on this for awhile. Greg

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