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Old 30th August 2004, 08:29 AM   #1
vossie is offline vossie  Israel
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Default Multi-Channel PSU design

I am building a multi-channel amp with the LM3875 chips. Best would be to have a PSU per channel but I'm not going that route. Next best would be to have one large tranny for all the channels to feed off. Question is how to implement this as good as possible. Is there any way to isolate each channel from the other? I was thinking of using a fast diode bridge per channel but it worries me that 6 of these in one box will have a more negative effect than using one well designed bridge. There are lots of information about HF noise generated by bridges on the net so logic tells me there must be a trade-off at some point between the number of bridges in a box vs the effects of having more than one channel feed off the same PSU rails...

Any idees?
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Old 31st August 2004, 02:50 AM   #2
GLM is offline GLM  Argentina
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I'd like more info on this too, i'm building a small Home Theater with 7 OpAmps (5.1 but use 2 amplifiers for LFE's) TDA2005 and TDA2003 I get very wird things, ie the audio quality goes very low when connecting all the amps, but everyone alone sounds pretty nice.

I have high pops when turning on/off lights (with transformers and without, the same) and the worst case are fluoresents lamps, wich makes everything go to hell, mmm its start buzzing at different frequencies y solve it just unplugging and plugging it again.
Don't have this problem with fluorescent lamp when there is just one amplifier connected to the supply.

The supply is a common || transformer 220 to 12 with a little inductor and a 4700uf and 2 430 uf

i actually connect the power to amplifier in a bus kind
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Old 31st August 2004, 07:50 AM   #3
vossie is offline vossie  Israel
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...was it something I said!?
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Old 31st August 2004, 11:13 AM   #4
vossie is offline vossie  Israel
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GLM, sounds like you are experiencing what I am hoping to avoid. Could you post a diagram of your grounding? Maybe someone else have had this same problem and could help...

My guess is that there is very low currents flowing through your ground connection back and forth between the various channels. I was thinking of grounding each channel through a R/C combination to the common point. For the resistor one could use a thermistor but I don't know what the noise implications would be??? This would lower the current during normal operation but kick in when electrical malfunction happens.

Anyone else?
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Old 31st August 2004, 02:49 PM   #5
Mr Evil is offline Mr Evil  United Kingdom
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Personally I would go for a single PSU with one bridge and one set of reservoir capacitors. Having one PSU per channel may be the ideal solution, but when you need more than one or two it can force compromizes on the size and quality of the components to fit within budget and physically fit within the case. It can make for nightmarish wiring too. Use the largest reservoir capacitors you can, and large decoupling capacitors bypassed by smaller ceramic ones close to each amp to keep crosstalk to a minimum.


Quote:
Originally posted by GLM
I'd like more info on this too, i'm building a small Home Theater with 7 OpAmps (5.1 but use 2 amplifiers for LFE's) TDA2005 and TDA2003 I get very wird things, ie the audio quality goes very low when connecting all the amps, but everyone alone sounds pretty nice...
Sounds like a problem I had when building a 5-channel amp with TDA20xx chips. If you have access to an oscilloscope you will probably see RF oscillations all over the place.

Make sure you use star-grounding - don't use a common return path for more than one amp. Keep signal and power grounds seperate. Use lots of rail decoupling capacitors of various sizes close to the chips. Try different values of capacitor in the Zobel networks, and make sure they are grounded at the same point as the speaker return. Put a filter on the mains input. Don't use longer speaker cables than necessary, try not to route them where they may pick up EMI and try to use the lowest inductance/capacitance ones you can find. A well shielded case doesn't hurt either.
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Old 2nd September 2004, 07:28 AM   #6
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Quote:
Originally posted by GLM
I have high pops when turning on/off lights (with transformers and without, the same) and the worst case are fluoresents lamps, wich makes everything go to hell, mmm its start buzzing at different frequencies y solve it just unplugging and plugging it again.

Does your transformer have enough power for all of the amps?

Maybe try hooking just one up and seeing what the max current it takes is.


Also do you have the center tap of the transformer grounded to the case? Maybe try disconnecting/connecting (is this safe?)

Im working on mine but only have one channel hooked up at the moment, but I can tell it will be a rats nest. There will be so many wires going to the star ground point...
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