The 4 channel amp I built 50 years ago

[Sidecar Bob]

I'm not sure if I should post this here or make it a project log. If it should be a log maybe a moderator can move it for me.

In 1972 I built a 4 channel stereo amplifier as my major lab project in electronics at college. The preamps are a design from the 1971 edition of the RCA Transistor, Thyristor & Diode Manual and the power amps are the "L'il Tiger" design published in the December 1967 Popular Electronics.

It has been stored for about 40 years for various reasons (for the last 22 mostly because it was in the attic and I just didn't think about it). Now that I'm getting involved in electronics again I decided to revisit it and see if I can get it working (no idea what I'll do with it if I do but I'll think of something).

When I built it I planned to use a simple transformer-bridge-capacitor power supply as recommended in the article but I couldn't get a 34V transformer as specified (for 4 amps I figure it would have had to be 5A) but I could get 40V 1A transformers so I decided to build separate power supplies for each amp. Naive 18 year old me thought I could just use a series dropping resistor but someone pointed out that if I did that the voltage at the current drawn by the amps would vary depending on how hard they were working at any given moment, which would change how much the resistors dropped and at low current it could be enough to kill the power transistors.
Being 1972 this meant using zener diodes to regulate the supplies so I moved things around inside the cabinet to make room for another heatsink and, after seeing how much heat they produced, added a big grill in the top.

Amazingly, I still have my notes plus the test reports for the power amps and preamps (I got a B overall) and was able to find them.
And even more amazingly, I was able to find the PE article and the RCA book online. I've attached the articles here. I'll post pics of the actual amp next time.

 

[Sidecar Bob]

This monster is 23.75 x 16 x 5" (60 x 41 x 13 cm) and weighs about 18 Lb (8.2Kg) My Dad was a carpenter/cabinet maker so he helped me make the cabinet for it. Note the use of Dymo tape for labelling the controls; That was state of the art at the time but now it looks so ancient


A lot has changed in the last half century. When I built this project design was still very much influenced by construction methods developed in the days of vacuum tubes; We never thought about unplugging parts of a project so we could test other parts so there are no connectors of any type inside this.
I really wish I could unplug a few connectors and test things on their own without cutting wires. I also wish I had invested in some coloured wires instead of using #16 black heater wire for half of it and whatever that white stuff is for most of the rest


Note that the volume pots have centre taps for the loudness control (boosts bass at low volume settings). It also has a Scratch filter for playing old vinyl (no matter what anyone tells you vinyl does NOT sound better, especially after it has been played a few times)


I couldn't have made the cabinet much smaller. And no, I don't remember why 3 of the preamp boards are phenolic but the 4th is fibreglass like the power amp boards. Or why the spare fuse holder is empty.

 

[Sidecar Bob]

I spent yesterday evening separating the wires for the transformers so that I could power each channel separately and digging out a speaker to use for testing.
Preliminary results:
Channel 1 seems to work OK and sounds OK with an audio signal from the computer.
My plug in power meter says it is drawing about 25W whether it is playing pretty loud or the volume is turned right down and my infrared thermometer says the zener is running at about 60c.
The power transistors run at 18-25c depending on how loud the music is (as expected).
The transformer is about 30c
The power supply's series dropper resistors are about 55c (1 was briefly 70 when I turned the volume up for a few minutes)
The power amp's supply voltage varies between 40V when the music is quiet and about 36 when it is loud.
The preamp's supply voltage is 11V (loud) to 11.5V quiet.
For reference, other surfaces in the amp measured about 15c.

Channel 2 does not work. The fuse was blown so I borrowed one from another channel. The power meter says it is drawing 56W, the voltages are quite low, including the AC from the transformer so I suspect the transformer is faulty. Its zener did warm a bit.
Channel 3 plays very quietly. Its power amp is getting 41.5V. its preamp is getting 12.5 and its zener is warming up so I suspect the signal isn't getting through somewhere.
Channel 4 is distorted I measured 58V at the power amp and saw light smoke from somewhere on or near the power amp board (nothing felt hot) so I shut it down. Its zener was warming up.

Now I have to make some decisions.
- If I am going to use it for something I would definitely change to a modern power supply.
- I'm a lot less interested in 4 channel stereo than I was back then. I did find drawings for building a quad encoder/decoder (no source mentioned) that I did around the same time with the stuff about the amp (4 transistors and a couple of dozen resistors & capacitors), but a couple of years later I learned how to produce quad from a 2 channel stereo with a 2nd pair of speakers and some wire so I'd be a lot more likely to do it that way anyway.
- I wouldn't mind upgrading the stereo in the rec room but we don't use it all that much these days so there isn't much incentive.
- On the other hand it would be neat to get some use out of some of it after all these years.
- On the other other hand I'd have to replace (or at least test) all of the components in whatever parts of it I did use (50 year old caps and carbon comp resistors....)
- I keep thinking about the parts for that big PA speaker (18" woofer, horn mid & tweeter, cabinet and schematic for the crossover) that I have in the attic and one channel of this could be nice with that.
But I don't know where I would put it....

 

[Sidecar Bob]

Well, that was interesting. I thought 15c for other surfaces in the amp sounded a bit low but I was too interested in everything else to worry about it until I went upstairs and it felt cooler than in the shop (where I'd been giving off body heat and heating parts). The thermostat was set at 20 but it said the temperature was only 16. After checking a few things inside I went out to look at the gauge on the propane tanks. We've never had a problem with the automatic delivery before and I can't really complain this time because emergency delivery truck was here about an hour after I called the supplier.

So back to the amp. When I woke up this morning I thought the next step should be connecting channels 2,3 & 4 to power supply #1 and see what happens.
1 still works like it did yesterday.
2 works with the good PSU.
3 works with the good PSU.
4 still distorts.
I didn't test the PSU voltages with the music loud and quiet this time but the supply's output was about the same no matter which amp it was connected to so the issues with 2 & 3 were definitely the power supplies.

It looks like I have at least 3 good amps & preamps and I doubt it would be all that difficult to get #4 working if I had a use for all of them.

As much as this has a bit of sentimental value (something that has survived from my youth and all that), I'm not attached to it enough that all 4 amps would have to stay in the same cabinet to make me happy. But it would be nice to turn parts of it into something useful (maybe something more like what I probably should have built back then?)
And I have learned a lot about construction of projects, wiring and (apparently) soldering since I was 18 (I believe I made it before I got the soldering station so who knows what I was using then but I'm sure it wasn't very good). I thought it was so perfect then but when I opened it up I cringed so even if I wanted to keep it as one unit I'd want to make a lot of changes and improvements.
And then there are those 50 year old caps and carbon comp resistors (did I mention that last year I sorted through over a thousand resistors that I acquired years ago and scrapped a hundred and some of them because their values had drifted out of spec?)

At this point I need to think about it for a while before I do anything and I picked up the rest of the materials I need for another project today so I think I'm going to put the lid back on and put it aside while I decide.

 

[Sidecar Bob]

Apparently I'm not very good at reading dates either because while I was packing it up I noticed that they are all dated February, March & April '74 so I built it 48 years ago, not 50 and I was 20 at the time...

 

[Martaine2005]

What’s 2 years over half a century!!


Martin