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Ac coupled power amplifier capacitor questions
Ac coupled power amplifier capacitor questions
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Old 14th October 2021, 10:42 AM   #21
The Lord Flashheart is offline The Lord Flashheart  United Kingdom
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Originally Posted by Mark Tillotson View Post
Schematic? Popcorn noise (yes its a thing) is rare in modern devices BTW. Hiss may be fixable by altering some resistances or substituting quieter transistors in the input section.
Thanks Mark, these are vintage devices and so I think It might be the popcorn noise I've read about it.

The problem is mainly in the phono section, though the aux does hiss and hum more than more modern stuff. I'm not sure what to expect from gear of this vintage, the signal to noise specs are certainly much worse than stuff made only a decade later.

Please find the schematic attached.

I have some SS9014 that should substitute for the 2sc458s that I understand to become noisy over time.

Do resistors become noisy over time too?
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File Type: pdf hfe_sansui_350_schematic (1).pdf (3.24 MB, 10 views)
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Old 14th October 2021, 10:47 AM   #22
The Lord Flashheart is offline The Lord Flashheart  United Kingdom
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Originally Posted by wg_ski View Post
Looks like an ancient metal can (TO-1?) transistor where I suppose an input stage is. Could even be Ge - that case was used on types like the 2SB56. I wouldnít put it past the small signal transistors of the era to sound like popcorn. Like the old infamous 2SA720 and 2SC458 in that horrible air-cavity TO-92. Every single one of those Iíve seen goes bad and sounds like popcorn or distant thunder. Old resistors can get noisy too, so donít discount those either.

There is probably nothing *wrong* with those old 3055ís, but a ruggedized device isnít really needed here unless you use that for short circuit protection. I might pull them just to use them where they would be uniquely suitable, but if itís working you could just leave it alone. Those old 2N587x types are more ďaudioĒ suitable - especially on a 52 volt supply. I have seen gain values all over the map on those - if you have a bunch to choose from you may want to match up a pair, and select for a gain of say 70. The PNPs will get into the 200 range at low current. Those old D180ís arenít exactly what Iíd call ďfastĒ either. Nothing like todayís sustained-beta flatpacks. Again, nothing wrong with them in the application. Todayís amps would be using TO-220ís and really have a problem with burnout.

Thanks 'ski,

I'll give the 2N5873s a go, do you think it's worth keeping the originals in the good channel or changing everything and keeping the 2SD180s as spares for some, admittedly unlikely, future repair?

I'll find out what transistors the weird Lovecraftian looking transistors are, I seem to remember they were Hitachi and early silicon but will check. I don't like replacing things like them, they give a piece character if only visually, but if needs must I'd rather a working unit.

It would be easy to convert this to another case style for the power transistors, but again I'd rather not.
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Old 14th October 2021, 11:01 AM   #23
woody is offline woody
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Ac coupled power amplifier capacitor questions
In the old Picking Audio Capacitors article by Walter Jung he pointed out that an electrolytic began to distort a lot as it got near to the F3 point. He may have even had some oscilloscope pictures with the distortion showing at frequencies 10x the cutoff frequency. With this in mind it's usually a good idea to have the output cap much larger than you need just for a low cut off frequency. Now that article is over 40 years old and caps are a lot better now but it still may apply.
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Old 14th October 2021, 03:07 PM   #24
wg_ski is online now wg_ski  United States
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Originally Posted by The Lord Flashheart View Post
Thanks 'ski,

I'll give the 2N5873s a go, do you think it's worth keeping the originals in the good channel or changing everything and keeping the 2SD180s as spares for some, admittedly unlikely, future repair?
Just keep two extra pairs of 2N5873ís on hand for future repairs. I have something like 40 pairs of 2N5879/5881 to use for things like this. Iíd rather not kluge in TO-3Pís either.
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Old 17th October 2021, 10:35 PM   #25
Mark Tillotson is offline Mark Tillotson
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Originally Posted by The Lord Flashheart View Post
I have some SS9014 that should substitute for the 2sc458s that I understand to become noisy over time.
Its worth trying - they are the front-end. The 1k resistors on the input should probably be more like 100 ohms if you want the lowest noise.

One issue I see with the SS9014 is the noise performance is not guaranteed at all, the maximum NF is much higher than the "typical".... Go for the highest gain bracket D if you can find it - higher gain reduces base current noise.
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Do resistors become noisy over time too?
Not usually, but if they are carbon film they might be a bit noisy compared to metal film anyway.

The circuit is a compromize because the same circuit is used for phono and for aux, there are quite a few switch contacts in the low-level signal path that might be adding noise too if not clean and free of oxide build up.

The variable resistor VR601 (MPX separation?) is also something to check for scratchiness / dirt build up.
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Old Yesterday, 12:58 AM   #26
wiseoldtech is offline wiseoldtech  United States
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Originally Posted by Mark Tillotson View Post
The variable resistor VR601 (MPX separation?) is also something to check for scratchiness / dirt build up.

Unless the needed equipment for aligning FM stereo is at hand, I'd avoid messing with anything in the multiplex area.
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Old Yesterday, 10:26 PM   #27
The Lord Flashheart is offline The Lord Flashheart  United Kingdom
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Originally Posted by Mark Tillotson View Post
Its worth trying - they are the front-end. The 1k resistors on the input should probably be more like 100 ohms if you want the lowest noise.

One issue I see with the SS9014 is the noise performance is not guaranteed at all, the maximum NF is much higher than the "typical".... Go for the highest gain bracket D if you can find it - higher gain reduces base current noise.
Not usually, but if they are carbon film they might be a bit noisy compared to metal film anyway.

The circuit is a compromize because the same circuit is used for phono and for aux, there are quite a few switch contacts in the low-level signal path that might be adding noise too if not clean and free of oxide build up.

The variable resistor VR601 (MPX separation?) is also something to check for scratchiness / dirt build up.
Thanks Mark, Iíve ordered a bag of 100 ksc1845s and 100 of the ksc945 to try, Iíll use the SS9014 and try those when they arrive.

I note your comments about the phono and aux sharing the same components as the line stage plays ok but the phono has a bad channel. It almost sounds as if the input from the cartridge needs to be over a certain threshold in orde for it to cause sound in that channel. I thought it might have perhaps been a semiconductor in the phono stage but hadnít looked that far, to be honest. I am now suspecting the two input caps as they tantalum and therefore the only ones I didnít replace as didnít have any film or low leakage electrolytically to fit in that position.

Iím wondering whether the line input is enough to get through whereas he phono input cannot.

Thank you for your comments on resistors and noise, all very much appreciated to a neophyte like me. I may well change the resistors on the input to some metal film 100ohm ones after Iíve replaced the input caps.

Iím not sure about fiddling with the tuner, I didnít recap it in fact as I wasnít sure whether it would need realigning afterwards. I have most of the equipment but havenít really looked into it in very much depth. Something for the future I think, first to try and get a better grip on amplifiers.
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Old Yesterday, 10:29 PM   #28
The Lord Flashheart is offline The Lord Flashheart  United Kingdom
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Originally Posted by wiseoldtech View Post
Unless the needed equipment for aligning FM stereo is at hand, I'd avoid messing with anything in the multiplex area.
I have a scope, a RF sweep generator, a distortion meter and other bits but itís more the knowledge and experience thatís holding me back.

I also donít really listen to the radio, I donít even think I have access to a decent antenna.
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