Zenith "Extended Stereo"

[davet753]

I posted an earlier thread about my newly acquired 1961 Zenith console stereo. Now that it's working well, I am wanting to satisfy my curiosity about something and thought maybe someone here could provide some answers.

The design on my Zenith isn't what you would normally see in a purely "stereo" receiver/amplifier set-up. To begin with, there is a 3 position switch on the console marked "Mono", "Stereo" and "Extended Stereo". The mono position works like you would think (sound coming out of both sides equally), but the others is where things begin to change. The "sound stage" does seem to extend outwards when going from stereo to extended stereo, but the equalization changes, too. The extended stereo sounds nice with instrumental music, but when listening to vocal music, there is a noticeable drop in the frequency range of the human voice.

A quick look at the schematic reveals a very odd design. Being a stereo amp, there are two separate audio output transformers, but they are connected on the secondary sides. The transformers themselves are very different (one has two center-tapped secondary's and the other only has one). The transformer with 2 has both center taps connected to the other channel's output.

Something may be learned from a look at the factory service literature. While the Sams Photofact folder labels the channels "right" and "left", the Zenith literature labels the channels "Sum" and "Difference".

I will try to attach a schematic of the amp with another post.

[davet753]

The only way I could figure out how to post this is a PDF file. Scroll down to the page with the amp for model SFF2585H.

[old_tv_nut]

The schematic will tell the tale, but I believe the extended stereo is mainly a boost in the difference components (left minus right and right minus left) of the left and right speaker drives. So, for example, something mostly in L tends to be made entirely in L, and something entirely in L actually produces some out-of-phase output in R. Not sure about the drop in the mid-range - could it be a drop in common-mode (centered) content rather than a mid frequency drop?

[old_tv_nut]

Not seeing the file (?)

[davet753]

Well, I can't seem to upload the file on this website. I'll try and figure out how to do this. I'm getting an error message with the PDF and don't know of any other way.

[davet753]

Here is a JPEG

[davet753]

And here is a copy of the factory Zenith service manual amp diagram

[davet753]

Originally Posted by old_tv_nut

The schematic will tell the tale, but I believe the extended stereo is mainly a boost in the difference components (left minus right and right minus left) of the left and right speaker drives. So, for example, something mostly in L tends to be made entirely in L, and something entirely in L actually produces some out-of-phase output in R. Not sure about the drop in the mid-range - could it be a drop in common-mode (centered) content rather than a mid frequency drop?

You know, it might indeed be just a lowering of the center stereo portion. Most vocals are split between the 2 channels. I've got a Shure phonograph cartridge test record that I can use to see if that's the case.

[old_tv_nut]

OK -

The top amplifier string produces a common mode signal (L+R) for all speakers. In the case of 2-way speakers, it produces separate common mode drives for the woofers and tweeters. In the case of three-ways, it produces a single common mode drive for all speakers.

The bottom amplifier then has center tapped output windings with the center taking the common mode L+R signal, so the end taps have ((L+R) + (L-R)) for the left and (L+R) -(L-R)) for the right.

I don't see the "extended stereo" switch on this page, so I assume it's on an earlier page where the (L+R) and (L-R) signals are developed, and controls the gain of (L-R). Is that right?

[davet753]

Here is the page showing the mode switch (bottom right hand corner).

[old_tv_nut]

Hard to read.

Are the two amp strings in the middle of page 6 labeled L and R, or sum and difference?
It seems they might be L and R and then the circuitry associated with the plug (lower right of page 6 and upper left of p. 8) could be cross-coupling to the first two triodes on p.8 to form the sum and difference. Or are the sum and difference already present on p.6?

[davet753]

The photofact labels the channels "left" and "right".....Zenith labels them "sum" and "difference". There are two dual section pre-amp tubes in the main chassis, and there is some cross-coupling going on in that section. Unless I missed something, the amp chassis keeps things separate until you get past the final output stage.

I also noticed it odd that there are minor differences in certain component values between the two channels.