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Old 2nd June 2015, 02:58 AM   #11
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Typical 3 stage gain "Sansui type" High Ft classic amp.

What is weird , they use a phase inverted driver stage (diamond like)
and then use positive feedback.
(below)

The NFB version of this amp (sansui) . is typically 20ppm-20k.
That is , with faster 35mhz sanken outputs.

OS
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Old 2nd June 2015, 03:00 AM   #12
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Originally Posted by minkuni View Post
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.
It would oscillate. 3stage IPS like this has massive CLG already.
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Old 2nd June 2015, 06:57 AM   #13
AngelP is offline AngelP  United Kingdom
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mlloyd1, may I first say that I find it fascinating to meet someone who also finds such issues interesting. Normally I only get polite smiles and rolling eyes from those around me when I rant about these things.

I should add that after replacing the two 6K81's, the DC levels came back to normal and the amp seemed to play ok. However, I always do a final check for HF stability and on one channel there were small repeated bursts of oscillation - of very low amplitude and easily missed. Reason? My guess is that it was probably a damaged p-n junction in one of the input transistors (Q603/4) that was switching between going capacitive and conducting? After all, it had seen about 10Vdc of reverse Vbe and thus about 0.5mA reverse current from the feedback network - for who knows how long.

That spelled bad news for the repair, as both input pairs were now suspect and needed replacement with hFE matched pnp's. Further, the feedback DC blocks (C605/606) would have been stressed, as they are only rated 6.3V.
So a relatively small repair (albeit a tricky one) expanded - and led to the discussions and THD reduction wager which I hope to get on with as soon as I get my posting skills up to scratch.
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Old 2nd June 2015, 07:27 AM   #14
AngelP is offline AngelP  United Kingdom
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Default RA-820AX mod stage 1 - once again

After a succesful repair of the original +16Vdc output fault, the DC offset level on the speaker terminals were down to 60-90mV, as not uncommonly found on these amps. While now clearly not destructive (<1mW/8ohm), such offset levels are not acceptable in a high end amp.

1st mod stage

So, first stage was to find a way to bring this down to single digit mV levels as there are no offset trim options in this amp. The offset is mainly caused by imbalance in the DC resistive values to the bases of the input transistors, both in the pre-amp IC and the power stage.

The preamp IC's in this 820AX were trusted old NE5532's, which I swapped to better OPA2134 audio opamps with low Ib JFET input stages, reducing this stage offset to less than 1mV.

I could then remove all electrolytic capacitors between the two stages, directly DC coupling these and, with a bit of resistor value calculations, establish a near perfect balanced DC load on the power amp input while retaining the same attenuation levels through both the source direct and the tone circuits.

I also established a better AC decoupling in the current source for the input transistor pair.

Stage 1 results:

Speaker output terminal offset: Lch (mod): 1mV vs. Rch (non-mod): still 82mV

THD level: down by an impressive -10dB to 0.0103% ….....Half way there! And already better than e.g. the NAD's!

THD+N (distortion+noise) was also reduced – but from a terrible 0.7649% to a still too high 0.3387%, mostly coming from 50Hz line noise and its harmonics. (Please note that this measurement should be taken with a pinch of salt, as our RTA setup seemed to pick up 50Hz noise during a direct tape-out loopback test – even with the amp switched off.) But the relative improvement is valid.

I enclose a spectrum analysis showing the distortion peak improvements (blue to red) and the direct loopback in grey. Note the 50Hz and odd harmonic peaks on this "reference" trace - but no even harmonics (e.g. 100Hz).
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Old 2nd June 2015, 07:46 AM   #15
AngelP is offline AngelP  United Kingdom
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Quote:
Originally Posted by minkuni View Post
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.

Yes, a current mirror in place of the single R605/606 collector resistors would greatly improve things - but was deemed "not allowed" as "being in the power amp audio path topology".
Sic, talk about moving goalposts!
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Old 2nd June 2015, 12:43 PM   #16
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at east electronics the book says that we have repaired 73 Rotel of that line the past 5 years .

All of them had cleaning issues...
From 73 there was 32 of them that had failure enough to make them not work ( beyond cleaning issues )
From these 32 the 25 had this type of failure no more than a TO92 transistor with a minor leak , resistors open , and a notorious Miller cap shorted ...

So i would say that for Rotel that doesn't look uncommon.

Kind regards
Sakis
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Old 2nd June 2015, 03:10 PM   #17
minkuni is offline minkuni  Norway
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Quote:
Originally Posted by ostripper View Post
It would oscillate. 3stage IPS like this has massive CLG already.
I forgot to add that as part of the mod I consider adding emitter degeneration resistors to the input pair. Simulations seem to indicate that it would work, but I am a novice.
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Old 2nd June 2015, 04:14 PM   #18
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Quote:
Originally Posted by east electronics View Post
at east electronics the book says that we have repaired 73 Rotel of that line the past 5 years .

All of them had cleaning issues...
From 73 there was 32 of them that had failure enough to make them not work ( beyond cleaning issues )
From these 32 the 25 had this type of failure no more than a TO92 transistor with a minor leak , resistors open , and a notorious Miller cap shorted ...

So i would say that for Rotel that doesn't look uncommon.

Kind regards
Sakis
Hi Sakis,
Thank you for your interesting stats.

Yes, I agree that these amps can be proper dust collectors, although this one was actually not too bad.

In my experience, the most common fault in the Rotels is due to people shorting the outputs, at best blowing the fuses or often the faster "Three-legged fuses", i.e. the output transistors.

BTW, which Miller cap have you found as "notorious"?
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Old 2nd June 2015, 07:09 PM   #19
mjona is offline mjona  New Zealand
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Originally Posted by AngelP View Post
Hi Sakis,
Thank you for your interesting stats.

Yes, I agree that these amps can be proper dust collectors, although this one was actually not too bad.

In my experience, the most common fault in the Rotels is due to people shorting the outputs, at best blowing the fuses or often the faster "Three-legged fuses", i.e. the output transistors.

BTW, which Miller cap have you found as "notorious"?

Another possible cause of failure although I don't know how common, is through someone increasing the Iq. In that regard r641 and r643 the 910R resistors from the driver emitters are not crossed coupled to each other but to the opposite voltage rail. There is a recent instance of failure due to excess heating, of driver stage components.
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Old 2nd June 2015, 07:56 PM   #20
AngelP is offline AngelP  United Kingdom
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Quote:
Originally Posted by minkuni View Post
I forgot to add that as part of the mod I consider adding emitter degeneration resistors to the input pair. Simulations seem to indicate that it would work, but I am a novice.
The best advice I received many years ago from my uni professor was to "always do things in stages and learn from each step - or you will find yourself having to start over and over".

Yes, emitter gm degenerating resistors can give improvements (if closely matched and the long tail pair current properly increased, etc.), but there are much simpler mods to be done first to an old amp that can dramatically improve performance - as I hope you may eventually see from my planned mods.
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