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.