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Old 31st May 2005, 01:15 AM   #11
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Thanks for that circuit macboy, it makes sense now. for some reason, before i did not understand you meant a seperate opamp for the global feedback.
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Old 31st May 2005, 02:48 AM   #12
jcx is online now jcx  United States
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the outer loop op amp has to be slower than the paralleled output op amps - not hard when you're using 100 MHz cf amps inside

a gain of +2 in the output amps makes for easy R matching and doesn't amplify offset too much

load C isolation may still need attention, but a high frequency lossy ferrite or the usual load isolating resistor+inductor (outside the feedback loop!) should work
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Old 9th June 2005, 09:14 PM   #13
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What about something like this: similar to macboy's schematic. Using paralleled amps at unity gain inside the feedback loop of an opamp. Would the paralleled amps still need output resistors?
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Old 10th June 2005, 01:32 PM   #14
macboy is offline macboy  Canada
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Quote:
What about something like this: similar to macboy's schematic. Using paralleled amps at unity gain inside the feedback loop of an opamp. Would the paralleled amps still need output resistors?
Yes. But you can get away with small ones, like 0.1 or even 0.05 ohms.
Be very aware that this can only be done if the amps that are used are unity-gain stable. (I only mention this because the National LMxxxx chip amps are not, and I just know that someone will wonder if it can be done with them. Answer: No.).

If you are going to use unity-gain stable amps, then you can hook it all up like the following, and eliminate the extra opamp on the input. Note all four opamps in the schematic are identical power opamps. (also note that this schematic generally assumes voltage-feedback opamps. With current-feedback opamps, you can't use the cap across the feedback resistor, and you can't configure it for unity gain by tying the output to '-' input).
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Old 10th June 2005, 02:12 PM   #15
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fwiw -- for the first set of paralleled LM4780 amps I built I matched the resistors to 0.1% -- and it worked quite well. on the second set with production PCB's I matched to 0.01% (inside of 1/10th of an ohm) -- the distortion was about 0.001% lower -- which probably doesn't mean a whole lot -- since the world of probablility, noise and statistics...is the world of noise, probability and statistics. The direction was better, that's all I can conclude, and a bag of 100 Yaego metal film resistors isn't that expensive.

I used an HP3478A DVM for the matching -- these are now cheaper than a new Fluke 177 handheld (although you are tied to the A.C. line).
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Old 11th June 2005, 05:43 PM   #16
ljordan is offline ljordan  United States
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Default Great ideas, I too would like to build a 4 to 8 piece tpa amp..

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jcx:...load C isolation may still need attention, but a high frequency lossy ferrite or the usual load isolating resistor+inductor (outside the feedback loop!) should work .
I want to avoid using any resistors in series with the load.
Would I just use a ferrite cylinder as a core for several loops of output wire?
Are these the snap-together type seen on power power lines?
Can some one post some examples?
What would be some starting point values for components in these two options for isolating the load C from the amp outputs?

My prototype TPA will oscillate if I do'nt keep the headphone cable short and still.(I use 24 ohm phones and no resistors, btw)


Lewis
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Old 11th June 2005, 07:37 PM   #17
ljordan is offline ljordan  United States
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I just tried one of those snap on ferrite tubes around the three (L,R Grd) as they come out of the chip.(just one around all 3)
I'm whipping the long cord right now. NO buzzing.

Lewis
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Old 12th June 2005, 12:29 AM   #18
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Default Re: Great ideas, I too would like to build a 4 to 8 piece tpa amp..

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Originally posted by ljordan


I want to avoid using any resistors in series with the load.
good luck. the purpose of the resistors isn't parasitic elimination -- that's for the RC network consisting of the 2.7R/100n combo -- let us know how long your chipamp lasts.
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Old 12th June 2005, 06:06 AM   #19
ljordan is offline ljordan  United States
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I meant the TI output 10r (spec'ed in the eval.brd pdf) ,not the sharing Rs in these parallel diag's.

Lewis
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Old 12th June 2005, 06:13 AM   #20
ljordan is offline ljordan  United States
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I am only using a single chip for now and it oscillates for any reason with out the output R. With short headphone wire I got it to sound better wo the R. Then I tried the ferrite and its seems I have found a way to eliminate the 10r

Lewis
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