Maganvox SS receiver evolution.

[Firedome]

Magnavox AstroSonic receivers had eliminated output transfomers (as shown in pic 1, a 1972 model receiver), by the early 70s, and perhaps earlier than that, whereas the early AstorSonic models did employ transformers (pic 2). I don't know electronics technology well enough to understand this evolution, and the reason for it, they were used for matching the speaker impedance to the output? My circa 1977 Sansui AU-717 still had 2 big output transformers, but it seems that output xfrms disappeared completely from SS audio stuff at some point, can anyone explain this to a non-electronics background person (like me) ?

[TC Chris]

I'll try an uneducated reply. Early transistor designs were impedance-sensitive. My 1966 Heathkit AA-22 amp has a wirewound resistor in series with the 4-ohm speaker outputs, for example, apparently as a current limiter. Also, without a transformers, there was an issue with DC offset--a constant DC current under the audio AC information. Putting DC into your speakers isn't good for them. I can't give a summary of circuit design because it's way beyond my knowledge, but those are the practical issues. Of course, transformers were expensive, especially ones big and good enough to give good low-frequency response, so getting rid of them solved several problems.

Chris Campbell

[Firedome]

I did figure there'd be a money saving angle, as always, but not how they got around the impedance issue technically. Did it go beyond cost, is there improved performance by eliminating output xfmrs as well? From what I understand it can be done with tube amps (ie OTL), but with much more difficulty and complexity than can be done with SS amps for some reason.

[electra225]

The output transformer's job was to match the impedence of the output tube to the impedence of the speaker. Wasn't one of the alleged advantages of transistors that the impedence match is not such a stretch?

[TC Chris]

The output xfrmr is there to match impedance--high tube plate circuit to low speaker voice coil. The plate is high voltage, low current. Early radios used high impedance headphones without a transformer and early speakers were basically a headphone with a horn attached. Apparently transistor amplifiers are low-impedance devices--low voltage,high current. We have reached the limits of my knowledge and we need somebody better-informed to give s a tutorial.

By the way, it seems that the horn is an impedance matching device as well--matching the driver diaphragm to the room air.

Chris Campbell

[Motorola minion]

The output transformer's job was to match the impedence of the output tube to the impedence of the speaker. Wasn't one of the alleged advantages of transistors that the impedence match is not such a stretch?

That is very true, making this a productive discussion. Maximum power transfer is the ultimate goal of matching impedances, but so is cost. An easy answer might be that as transformers became less practical, capacitor sizes decreased and so did cost.

Early car radios that used the Delco DS-501/502/503 transistor also used a small transformer to match impedances which were higher for those early germanium transistors than for silicon. IIRC, car speakers were rated at lower impedance than some early solid state audio equipment. In most cases, cheaper transistorized equipment used low power germanium transistors in push-pull to drive a little speaker via transformer - OR - used the speaker impedance as part of its output circuit load. This last "shortcut" required the speaker impedance of 16 ohms or higher. It also made for some difficulty servicing without "blowing up" transistors and I remember THAT well.

Other than this, rarely seen are push-pull output transistor amps that couple to speakers without large-value electrolytic capacitors. The impedance of any speaker is not constant over the desired range of frequencies and neither is the impedance of the capacitor. DC blocking is #1 job of this capacitor but secondarily, matching its capacitance to a given speaker.

While it be a moving target, working toward a flatter response across the audible range reduces the need for engineering good old-fashioned advanced compensation in all stages including volume and tone controls (emphasis and de-emphasis tailored to overall system response) and THAT Engineering is what actually makes our tube stuff sound awesome on speakers it was designed for.

In theory, the capacitance and the inductance in series with it will yield a resonant frequency at which their series impedance is minimum, an important design parameter needed first.

[Firedome]

Thanks for that explanation! As always I figured cost was one of the driving factors being that trannies are probably the most expensive part of any amp/receiver.

Being that I doubt finding the rest of a console to put it in, this R231 receiver will hopefully become the main part of my "office" (which sounds a lot more important than it is, being just a small desk + bookcase + computer in an otherwise unused bedroom and no real "business" going on) system, if it works well it'll replace the Scott receiver it's sitting on. Luckily my uncle labelled all the buttons on the side (not visible).

Looking at it's chassis it's very evident that any service required will be waaay above my pay-grade (which ain't much being retired!) given 24 blue and black caps of various sizes and a big multi-section mounted on 4 different PCBs. I'd imagine that kind of complexity being the chief reason so many are just junked if problems arise.

I think I may have some NOS Delco 501/2/3 transistors out of my late uncle's pile.

[Moto_TJ]

I didn't think those were output transformers, but interstage transformers between the driver and the output transistors.

[TC Chris]

My Heathkit AA-22 uses driver transformers (big ones, too, given the small signals involved).

After I built it I was disappointed in the hiss levels. Sent it back to Heath. They tested it and pronounced it performing to specs and suggested maybe it could be my speakers. Those were GE "Extended Range" 8-inchers in my homemade bass reflex cabinets, from a Stereo Review magazine article. I thought that was a poor excuse. But shortly after, I bought some used KLH Model 17s and the hiss went away. Oh....

When Heath checked out my amp, they also incorporated a production change, a lead dress on the driver transformers. They were serious about their products.

Chris Campbell