Playing digital audio through a 1990s amplifier

[D.M. Procida]

They're very pretty, aren't they?

Daniele

[D.M. Procida]

Only if you can install Plex on an existing system, and even then I don't know whether they'd interfere with each other. Volumio (as you probably know) installs as a whole system, rather than an application, so it would need to be Plex as an application on the Volumio system.

Daniele

[Paul]

I think I'm seeing shielding on the transformer.

It doesn't have to be pretty, to work :-)

Not all the cables can be oxygen-free you know.

Paul

[misterroy]

I have a chromecast audio plugged into my 1990s stereo. I use vlc to transfer audio from the pc, and also put streams to it from the phone. works great

[Vir Campestris]

We still use the amp on our stereo at least weekly, and it's running fine. We bought the stereo with my wife's payoff when she stopped work to have the children.

In 1983.

The cassette deck died, we've added a CD player, upgraded the tuner so we can have DAB, and bought better speakers. But the amp is still fine.

Well, as much as I can tell with my aging ears...

Andy

[RJH]

Thanks. Have a feeling it won't be easy as Volumio has no GUI, and I can't see any guides. Just installing Plex on its own isn't trivial:

and that method won't compile on my setup.

[jkn]

Yes, they were nice looking. A dog to take apart though, they were very full inside, with IIRC multiple vertical PCBs. Very hard to get the dismantling/remantling(?) order correct. I 'guess' the tuner might be less packed.

J^n

[Caecilius]

Thanks for the encouragement - I think I will do this. I've just opened the case and it looks possible - just two electrolytics marked "STARGET ELNA 42V 10000uF(M)". Provided I can source replacements and get to the underside of the PCB I think I can replace them.

Glad to see there's no signs of bulging or leaking on the capacitors.

[Caecilius]

I don't really know if it's a high quality amp or not really. I understand what you're saying - a toroidal transformer and balanced signals over twisted pairs rather than flat ribbon cables would be better but it sounds good playing a CD without any mains hum present.

I have wondered if technology has moved on to the point where I'd be better just getting a modern amp - maybe they have digital processing for the low level signals now and only go analog for the power amp which would simplify a lot of things. But for now I'd like to get my old system running again with my digitised music collection and see what it can do.

[Caecilius]

Thanks. That's very similar to my situation, just ten years earlier. Surprisingly my cassette deck still works. I'd have thought one of the rubber drive belts would have perished by now.

[The Natural Philosopher]

Golly, it's that time of year. been refurbishing my old Marshall valve amp - seems to have a scratchy reservoir cap in there. Heck its only 40 years old!

[The Natural Philosopher]

TBH amps pretty much hit the 'good as it gets' mark around 2005. Anything more modern that that wont be much better.

[Caecilius]

I've only got about 250 albums, but my nas is an old QNAP TS-412 with

256MB RAM and a Marvell 6281 1.2GHz CPU so I just use it for SMB & NFS file sharing. I have another project to upgrade my NAS one day...

[charles]

I've had my Revox tape recorder since 1969! Caps replaced a few years ago.

[Paul]

Those would be storage for the high voltage DC rail running the output stage. I got a picture here, of that section of circuit.

(

) # Word salad

You can see the bipolar supply there, one cap for the positive rail, one cap for the negative rail. A bipolar supply being used so the output voltage is pretty close to the center tap "zero" of voltage when the amplifier is quiescent.

This is an unregulated design. If mains rises too high, it endangers the life of the big blue capacitors (and of course, everything downstream of that point). Amplifiers and receivers, that's probably one of the things affecting their lifespan, is their ability to swallow electrical insult from mains.

There will be a ratio, between the WVDC ("42 volts") of the capacitor, and the actual filtered DC on it. Some of the rating there is a "derating value". The very worst capacitors, you might take the normal voltage and multiply by five to come up with the WVDC needed. If you were to stick a multimeter on the terminals of the capacitor, you might find a voltage which isn't 42 volts, and does not appear to use all of the voltage rating. But some room must be made for the peaks of the rectified voltage from the bridge rectifier and the capacitor voltage. And some room is also needed for mains overshoot.

You can see in this example, the manufacturer used an 80V capacitor, and the measured rail value is 32.5V. Part of the headroom, is for the top of the rectified sine wave. While the person here decided to use a 75V capacitor, be aware that headroom was put there on purpose and it wasn't a "lark". Maybe this amplifier will be a little less resistant to mains overshoot, after the repair.

If you are lucky, the capacitor uses screw terminals and no solder is used. There may be a lock washer as well, to keep the screw in place for fifty years. If so, this makes it easier to remove them. The screw termination helps with the mass of the item and the potential for strain to be placed on the joint to the foils.

As brands go, Elna is pretty good.

If the caps happen to be small enough to be solder terminated, and they're a flush fit to the PCB, you'll then be hoping the holes in the PCB are large enough so the legs come out easily. Even using a vacuum desoldering station at work, I've had trouble with that. If you pull too hard, you will pull the "solder fillet" right out of the PCB. Yes, I've done that. That's how you learn not to do that, pull too hard.

Not all of the hum reduction in an amplifier comes from those capacitors. The capacitors still have mains-related ripple on the DC output voltage. The amplifier itself has a PSRR (Power Supply Rejection Ratio) and this comes from the closed loop feedback in the amplifier and the balancing of the components in it. The amplifier in a sense, "resists" the ripple and tries to make the output of the amp, only follow the input, while ignoring the hum on the V+ and V- rails.

It is fortunate, that higher values of capacitance, will not "fit" into the footprint of the capacitor. Electrolytic capacitors have "microfarads per cubic inch" relationship. If the substitute capacitor is physically a lot smaller than the original, that usually means it is a substandard build. It means the ratings stamped on it might not be correct. Because of the space limitations in the amp, you can't replace a 10,000uF cap with a 50,000uF one. And this is a good thing, because if you do move up too much, you blow a diode in the bridge rectifier.

There is inrush current, and sometimes good designs, use additional components to prevent the capacitor from causing the rectifiers to blow. Like if the amp had a toroidal mains transformer, there already needs to be an inrush limiter, just because of the transformer type chosen (info from Hammond transformers site).

You can replace a 10,000uF cap with a 12,000uF cap if you want. But don't go nuts. The capacitors themselves have some "tolerance" to the true value, and it's not tight like 5% either. They're looser than that. Only certain kinds of caps can manage to be +/- 5%. Some are as loose as +20/-80 on percent. And chances are, if you tried to put 15,000uF, the footprint of the cap might be the next size up and it won't fit. Always check dimensions (case size) before you buy, or you'll be sorry. Most of the complaints I hear, are "dammit Paul, these don't fit" :-) Yes, been there and done that.

Selecting capacitors is hard work, and one of the reasons you don't want to be volunteering to replace them all that often. The catalog has letter sizes, like the 10,000uF might be a "C" casing, the 15,000uF might be a "D" casing. Each casing has a drawing with dimensions (or a table of dimensions) to help in your selection.

Paul

[Dave Plowman (News]

Likewise.

Oldest bit of kit still in use (when needed) a Quad AM3 L,M,& SW valve receiver. Although switched so it doesn't power up with everything else.

[Caecilius]

I can't compete with you guys. I wish I kept my 1980s technics amp now, but my parents got rid of it shortly after I got married and had my mind on other things for a while.

[Caecilius]

I knew some of that but there's a lot of useful details I didn't know. Thanks for taking the time to put together such a detailed response - it's really helpful.

[Andrew]

How much did it cost ?

This one was sold on ebay by BHF Poole

[charles]

It was 2nd hand and cost me £180. It is a Mk I, original owner had to have a MkII ;-)