CBS Simplex stereo amplifier circuit

[BYacey]

I stopped in a my dad's tonight; he was tinkering in his shop. I noticed an Electronics World issue, September 1959 on top of a stack of magazines. The cover picture / schematic caught my eye, a two tube, compact stereo amplifier. It's quite an unusual output circuit; at first glance it looks like an ordinary push pull output stage, however it has an additional transformer to decode the stereo information for the second channel.

Here's a link to the cover picture and schematic:  http://commons.wikimedia.org/wiki/File:Electronics_World_Sep_1959.jpg

I thought this might be of interest to some.

 

[ruffrecords]

Fascinating. At first sight it looks like a balanced amplifier but fed with a stereo signal. The output would therefore be the difference signal in MS terminology. There then looks to be some transformer wizardry that adds in the sum to produce left and right. Odd but neat.

Cheers

Ian

 

[abbey road d enfer]

I stopped in a my dad's tonight; he was tinkering in his shop. I noticed an Electronics World issue, September 1959 on top of a stack of magazines. The cover picture / schematic caught my eye, a two tube, compact stereo amplifier. It's quite an unusual output circuit; at first glance it looks like an ordinary push pull output stage, however it has an additional transformer to decode the stereo information for the second channel.

Interesting exercise. It's definitely not a push-pull amplifier, although it uses a push-pull OT. I don't see any significant advantage here compared to a dual SE arrangement. It has to operate in class A, so there's no increase in power. Proper matching of OT's is a concern. The push-pull OT could easily be downspec'd because the S power is generally a fraction of the total power, but the SE transformer has to be spec'd to the full power of both channels combined, with the DC rating of both tubes combined. All in all, that's much more difficult to source than ordinary SE transformers.
I don't know what are the claimed advantages of this arrangement but I can see its flaws.
I'm often impressed at the achievements of our ancients, but sometimes I wonder if they may have been carried out by their own wit.

 

[PRR]

I do not see a schematic?  {EDIT-- duh....}

The idea is old and dubious. Center/Mono is passed push-pull. Stereo/Difference is passed single-ended. Assuming "small" stereo separation, the maximum clean loudness is greater than the same bottles and iron used 2-channel. The "small separation" tidbit was dubious in 1959 and moreso with 1960s pop recording practices. IAC, the cost savings were small. It seemed like a good idea for mass-produced "Stereo!!" consoles, but wasn't good enough (in performance OR in savings) to catch-on.

 

[BYacey]

Center/Mono is passed push-pull. Stereo/Difference is passed single-ended.

One thing that I seem to be missing is how the mono signal could be push pull, without a phase inverter. Were the inputs driven out of phase?

 

[PRR]

> how the mono signal could be push pull, without a phase inverter. Were the inputs driven out of phase?

(SEE TEXT)

EW September 1959 is available here:
http://www.americanradiohistory.com/Electronic_World_Master_Page.htm

"The cartridge used by the authors is the Columbia SC-2. This commercially available cartridge is manufactured specifically for use with the CBS-Simplex circuit. It is identical to the Columbia SC-1 except for phasing of terminal voltages. Although other 3-terminal cartridges cannot be used with this circuit, 4-terminal cartridges having a sufficiently high output can be used by properly phasing their output voltages externally (reversing leads to one side).

Stereo pickups are usually two mono pickups, one on each slant. If you bring out all four wires, you can connect L+R or M+S. The SC-2 is apparently M+S, unlike the L+R connection used for twin-channel stereo rigs.

Also of note in that issue: a sub-Watt transistor audio amplifier by a guy named Richard S. Burwin. Specs are decent for the era, but it puts about 4 Volts DC right across the speaker. It claims greater reliability than a tube-amp, after noting that a momentary short will burn it up.

 

[BYacey]

It's interesting to read about these pioneers in electronics. I believe there is much to be learned by close examination of their efforts.

When I as in technical college, we barely touched on tube technology. In fact, I mentioned the synchro-guide  horizontal sync circuit to one of the instructors; he had no clue what I was talking about. Audo tube output circuits were foreign territory too.

 

[PRR]

Heath offered, circa 1959, a CBS-patent M+S stereo console rated 10 Watts, larger than the usual plans.

> Audo tube output circuits were foreign territory too.

I don't know that automobile tube outputs were much different from table radios. Some difference because the expected supply voltage variation is larger than expected wall-voltage variation; but still the same '42 and 6V6-series tubes, SE in the base Plymouth, P-P in the Buick.

The buzzer to raise 6V to 250V was pretty specialized (and trouble prone).