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Improve a Rotel amp THD by 20dB!
Improve a Rotel amp THD by 20dB!
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Old 1st June 2015, 02:41 PM   #1
AngelP is offline AngelP  United Kingdom
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Default Improve a Rotel amp THD by 20dB!

Question: How good can an old Rotel get? I have had dozens of Rotel amps through the repair workshop over the years, and this RA-802AX came in as a “speaker killer”. It turned out to be fully working – except it had a solid +16Vdc on both Left and Right speaker outputs. In a Rotel, the 12A rated power transistors and the 4A output “protection” fuses will easily deliver the resulting constant 2Adc into 8ohm speakers (even more into 4ohm), or 32W heat dissipation which indeed can destroy most static standing woofer coils. Finding the unusual fault involved a thorough circuit analysis – and in turn led us to discuss Rotel's power amp philosophy which has changed surprisingly little over the years. Then came the wager. I put on a bet that I could easily modify the Rotel amp design to improve the Total Harmonic Distortion (THD) from the 0.03% in the specs by a decade – (i.e. a factor of 10 or -20dB in audio terms) – transforming this “entry level” amp into an audiophile class of less than 0.003% THD. We even added three tough rules: 1. Fully reversible changes, i.e. no cutting of PCB tracks. 2. Budget: max GBP 40, i.e. faulty components could be replaced, but no upgrades to any fancy audiophile high-end components. 3. Preserve the “Rotel sound” i.e. no major changes of power amp audio signal path topology. In short, it was down to basic good audio engineering and circuit analysis. Ho Hum. We agreed that the changes should be staged and at first only applied to the Left channel, so we could follow the improvements through comparison RTA distortion measurements. If anyone is interested in whether such a dramatic reduction in Total Harmonic Distortion can be achieved, I will describe each stage and the THD improvements in subsequent posts.
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Old 1st June 2015, 04:04 PM   #2
JohanH is offline JohanH  Netherlands
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Please do
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Old 1st June 2015, 05:46 PM   #3
AngelP is offline AngelP  United Kingdom
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Default RA-820AX mod stage 1

Ok, please do comment or ask questions if this becomes too boring or if there are things that needs extra explaining. After a succesful repair of the (very unusual) original +16Vdc output fault, the DC offset level on the speaker terminals were down to 60-90mV, as commonly found on these amps. While now clearly not destructive (
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Old 1st June 2015, 05:57 PM   #4
sonidos is offline sonidos  United States
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Improve a Rotel amp THD by 20dB!
Do it! My cousin has an old Rotel integrated that I just overhauled with new caps. He's a happy camper but I'm wondering if there is more I can do it.
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Old 1st June 2015, 07:21 PM   #5
mlloyd1 is offline mlloyd1  United States
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Improve a Rotel amp THD by 20dB!
sounds interesting ... do tell.

but before you get started, just so i understand, please comment on " fully working – except ... +16Vdc on both ... outputs"
are you saying it had 16Vdc superimposed on top of the usual audio output signal?
what did you fix to cure it and what do you think caused it?

mlloyd1
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Old 1st June 2015, 08:48 PM   #6
AngelP is offline AngelP  United Kingdom
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Ok mlloyd1, since my original post attempts seem to have gone haywire anyway, I'll start by replying to your question.
Yes, this was something I'd never seen before. All the amp transistors seemed to measure out ok, you could even set the bias - and send a small audio signal through the amp - but there was a solid +16Vdc on BOTH outputs, frying innocent woofers - and almost also my test resistor setup. It did take quite some circuit analysis time, but eventually it turned out that both the base resistors (R611/612) in the input current source were o/c! Say what? Completely surrounded as they are by much more fragile semiconductors - they would absolutely be the very last component you'd expect to fail in the case of e.g. some surge/overload - or whatever. I have no explanation, but as these are the only two 6K8 metal film resistors in the amp, my only guess is that perhaps Rotel may have got a faulty 6K8 resistor lot?
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Old 1st June 2015, 08:58 PM   #7
sbrads is offline sbrads  United Kingdom
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If those resistors are through-hole, were the leads bent too close to the body? I've seen many go o/c in tightly packed (non-audio) designs where space was being 'saved'.
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Old 1st June 2015, 09:11 PM   #8
AngelP is offline AngelP  United Kingdom
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No, same pitch mounting as all the other metal film resistors in the circuit, nothing visually untoward whatsoever. Perhaps I should have kept them for a professional investigation - but then again, for what purpose?
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Old 1st June 2015, 10:27 PM   #9
minkuni is offline minkuni  Norway
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Interesting. I currently have an old RB960BX on the kitchen table and have been considering some tweaks. My first thought was to add a current mirror to the input stage. It will be interesting to follow your progress.
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Old 1st June 2015, 11:34 PM   #10
mlloyd1 is offline mlloyd1  United States
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Improve a Rotel amp THD by 20dB!
wow, that's interesting.
about 20 years ago, i fixed a friend's old pioneer receiver that had that exact same fault, but only on one channel. the resistor had a hairline fracture that caused an open circuit. however, for this amp, the affected power amp channel appeared to have no power supply at all, so the output had no voltage of any kind, even the output devices were cold. it was like someone had pulled all power fuses.

i think adcom even used that as an approach to protectively shut down a power amp in case of a fault.



Quote:
Originally Posted by AngelP View Post
Ok mlloyd1, since my original post attempts seem to have gone haywire anyway, I'll start by replying to your question....
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