Class B feedback resistors originally missing in a vintage integrated amplifier - diyAudio
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Old 27th October 2014, 08:30 AM   #1
ygg-it is offline ygg-it  Italy
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Cool Class B feedback resistors originally missing in a vintage integrated amplifier

I got from my family and now own the stereo integrated amplifier National Panasonic SA-73

Technics /National/Panasonic SA-73

that is in excellent and mint condition. Sound is good, a sort of a vintage warm sound, but fatiguing at high volume, so something didn't convinced me, then I thought it was just due to the old components (the amplifier is from 1971!!).

I did some test and THD was 0,5% on 8 Watts on 15 ohm load. So for me the circuit was good, no heating, capacitors still working after forty years, no noise, clean printed circuit, without leakages etc..

So I got the service manual in order to understand what modifications I could do to make the sound less fatiguing.

By chance I discover from the original schematic that 4 resistors (3300 ohm) were missing in the power amplifier section !!. The four resistors (two for each channel) had to be mounted manually on the bottom of the printed circuit.

I'm pretty sure that they have always been missing from the beginning, because there was no sign of de-soldering on the back of the printed circuit!

See pic.

How was possible such mistake, which nobody discovered after forty years...?

Being National (now Panasonic and Technics) an excellent trusted brand, my question now is: the four resistors have been intentionally missing ?? Why mounted on the back?? Which was their functions, with or without? Or it was an initial error, afterwards manually adjusted, but just I own a preserie amplifier?

I think resistors were part of a voltage divider of the negative feedback network of the class B stage and their presence modifies in someway the feedback signal.

I soldered the four resistors on the back and fatiguing sound disappeared immediately!
Any idea??

PS: by the way also the two 120pF C312, C362 capacitors have been manually mounted in a different place, but effect was negligible after I put them in the correct place.
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Old 27th October 2014, 11:26 AM   #2
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TR304,5 are VI limiter transistors. They reduce drive to the output transistors in the event of a shorted output leads, excess power level etc. The resistors R323, 324 provide a DC bias to the transistors to adjust their operating point. This is a protection circuit, not an essential part of the amplifier itself.

The components that you think are missing look to be circuit revisions, as they are squeezed into the schematic. If your amplifier precedes the revisions, it will be perfectly understandable to be the way it is, before the revisions were made. Since the amplifier has survived over 40 years, I don't think it needs tinkering with now to make those minor amendments to the protection circuit. Leave it as you found it.

Unless you really know what you are doing in shifting parts from their original locations, don't do it either. It can only invite instability which may result in disaster. What you should focus on, is replacing all the electrolytic capacitors, like in any amplifier this age. That will improve your sound much more effectively.
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Old 27th October 2014, 02:35 PM   #3
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Quote:
Originally Posted by ygg-it View Post

Being National (now Panasonic and Technics) an excellent trusted brand, my question now is: the four resistors have been intentionally missing ?? Why mounted on the back?? Which was their functions, with or without? Or it was an initial error, afterwards manually adjusted, but just I own a preserie amplifier?

I think resistors were part of a voltage divider of the negative feedback network of the class B stage and their presence modifies in someway the feedback signal.
You think wrongly - the resistors are part of the S/C protection for the output stage, not feedback at all - actually the bottom half of an attenuator.

Fitting them increases the current output of the amp before protection cuts in - so if it's improving the sound, then you must be running it hard enough (and into a fairly low impedance load) to bring the protection in to play.

As Ian said, they look to be later modifications, so probably wouldn't have been done on an early example. Japanese manufacturers were well known for 'bodging' lots of extra components on the bottoms of their PCB's

Removing TR304/5 would possibly improve things even more?, by removing the protection entirely, as is often done in higher quality amps.
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Old 27th October 2014, 04:10 PM   #4
JMFahey is offline JMFahey  Argentina
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Agree they are part of the short protection circuit, and that adding them will allow higher current output into lower impedance loads.

Which may be good or bad

If they wanted to improve reliability making the protection more sensitive then no 3k3 resistors; if somebody complained the amp was harsh they would add them, design is always a compromise.

If you are careful with speaker wiring, a softer protection is fine.

To be certain, you should scope the output, that kind of VI protection chops the sinewave peaks in a "spikey" way, buzzier sounding than classic squarewave or flat topping.
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Old 27th October 2014, 04:13 PM   #5
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Originally Posted by Nigel Goodwin View Post
.......Removing TR304/5 would possibly improve things even more?, by removing the protection entirely, as is often done in higher quality amps.
I also thought about that "rip out the limiters" trick for purist modders of old amps but it then requires much more care in use and no little fingers playing with the gear. I don't encourage people to do this unless they are the careful type or have a plan for fitting an alternative overload/DC protection scheme. I've had to replace quite a few output stages and more in some old and irreplaceable parts models because of that dangerous mod.
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Old 28th October 2014, 02:47 AM   #6
CBS240 is offline CBS240  United States
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Has the filter caps (and any other electrolytic cap for that matter) been replaced since 1971? They have a life expectancy and do dry out over time resulting in inept functionality.
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Old 28th October 2014, 11:51 AM   #7
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I sure wouldn't bet my life on 40-year-old electrolytics. High ESR on the bootstrap cap, for example, wouldn't be exactly beneficial. 0.5% at 8 W into 15 ohms sounds like too much in any case. How are the supply rails doing under load?

The circuit also has other issues though.
Input stage current is quite unbalanced, even with the correct value of R306. (Despite the looks of it, it's an LTP.) Impedance balance is similarly off. And of course, no current source.
It's a quasicomp output stage without a Baxandall diode. Adding one in parallel to R316 would be easy though.
Output stage emitter resistors are 1 ohm, which is on the high side of things. Presumably that was needed in order to keep the circuit from becoming a thermally unstable mess with the simplistic diode bias. You can't really get a zero or negative temperature coefficient when using 2 diodes and some series resistance to bias 3 B-E voltage drops. You would need something to sense TR306, TR308 and TR307 temperatures.
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Old 28th October 2014, 02:29 PM   #8
ygg-it is offline ygg-it  Italy
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Default What about the C312 Cap?

This discussion is become interesting thanks!!

R306 is wrong in schematic: correct value is 3k9

The amplifier has been revisoned by sure!

Just found that R303 in my ampli is 100k, here 68k (so more biased circuit I suppose). May be this the reason of the 3k3 add.
Yesterday I have changed it to 68K. Added the four 3k3 resistors. I put very old Allen Bradley resistors. Moved the 120 pF capacitor.

Wonderful sound!

Electrolitics still works very well, I don't want to ruin inside vintage look..and nowadays cap are half size.......(and currently no 50Hz noise, no rumble...). I have to take courage to do this...Everithing is still original....sorry

Still question: why they put the 3k3 to the rail + - and not to ground? Why they just reduced R312 and R313 values instead of adding 4 extra resistors? More easy?


And, overall, why I have found the C312 120 pF capacitor in a different place (between ground and the bases of TR307 (or was TR305?, now I don't remember anymore after I moved in the right side, as per the current schematic)?

Last edited by ygg-it; 28th October 2014 at 02:33 PM.
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Old 28th October 2014, 04:11 PM   #9
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Electrolitics still works very well, I don't want to ruin inside vintage look..and nowadays cap are half size.......(and currently no 50Hz noise, no rumble...). I have to take courage to do this...Everithing is still original....sorry....
Courage or bravado? I think you seriously underestimate the danger in not replacing capacitors well past their service life. As well as poor performance, you risk some severe damage if the amplifier has not been used much for some years and being wet cells, capacitor life is definitely limited. Please replace them regardless of appearance aesthetics.

You have already changed and added parts. Only you will be interested in what the amplifier looks like inside and the components fitted will be quite hidden with the cover replaced, as it must be.
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Old 28th October 2014, 08:17 PM   #10
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Still question: why they put the 3k3 to the rail + - and not to ground? Why they just reduced R312 and R313 values instead of adding 4 extra resistors? More easy?
Because it gives more 'attenuation' of the protection signal, and it's easier to bodge resistors on the back of the board than to remove and replace already fitted ones.

As for this, and the other changes you've mentioned - as I've already explained, it's commonplace in Japanese designs to have loads of bodges to correct their design defects. It's pretty rare for them ever to redesign the PCB and build it correctly - more a case of 'chuck it together any how', and if it works that'll do.
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