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zenith console amplifier (a stereo mono amplifier :) )
zenith console amplifier (a stereo mono amplifier :) )
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Old 14th February 2015, 02:24 PM   #1
dr_skoobie is offline dr_skoobie  United States
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Join Date: Jan 2005
Location: Fat City, CA
Default zenith console amplifier (a stereo mono amplifier :) )

I have been rehabilitating an old (1959) Zenith console, SFD288. To date I have replaced the capacitors in the amplifier and preamplifier/tuner, as well as resistors that had drifted well out of specification, using the following Sam's photofacts as my guide:

https://app.box.com/s/jnjt1ahxrap1nqb9nx00

I am an electronics newbie, but I can follow a schematic (given enough time ), am handy with a soldering iron, and have built a few components (Foreplay preamp, several headphone amplifiers) and speakers over the years. I fell in love with the Zenith and I'm going to bring this fantastic old MCM console back to life (with a few updates here and there, most likely).

The amplifier seems to be functioning properly, and is surprisingly quiet (the tuner is another matter, currently hunting down a hum in one channel).

However, its design is a bit of a puzzle. If you look at the schematic attached, you'll notice that while it has a 2 channel PP topography, with ECC83 drivers and EL84s, the output transformers are "tied" together (note the "yellow" and "white" wires connecting the two). As a result, input from either channel is sent to both speakers. I'm a little flummoxed about why anyone would design a circuit this way... There are probably folks who recognize it instantly (and the intentions behind the design). The taps on the transformer with three windings feed both the L and R main channels as well as additional speakers that could be purchased (surround sound ).

Most simple PP El84 circuits seem to have symmetrical, independent output transformers (one per channel). I am considering replacing one or more of the Zenith transformers and making the channels independent-- doesn't seem to make sense to have stereo input otherwise?

I am also wondering if there are simple modifications I could make to bring its performance up to more modern standards? There is room in the chassis to do so, though I'd like to keep the changes manageable considering my skill level and my limited experience/knowledge.

I'm looking for advice...!

-vince
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Old 14th February 2015, 04:42 PM   #2
Keit is offline Keit  Australia
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Join Date: Feb 2014
Location: Australia
The amp is designed for use with sum & difference signal sources.

Normal stereo operation from a record player cartridge is two discrete channels carrying the right and left channels, or L & R signals. A cartridge provides such signals directly if its' sensistivity axes are -45 degrees and +45 degrees - that is each channel responds to the modulation on either the inner groove wall or the outer groove wall.

However, some cartridges for whatever reason were made to respond to 0 and +90 axis - that is side to side and up and down motion. One output (side to side) is thus L+R from both walls adding sideways and the other (up and down) is L-R from both walls acting together to lift or drop the stylus.

With a 45-45 cartridge, manaufacturers could make a cheap mono system by connecting both cartridge outputs in parallel - but this cross feeds the signals and can cause distortion. With 0, 90 cartridge, a mono system can be made simply by ignoring the L-R output and there is no penalty.

Thus to recover the individual L & R signals, a circuit is needed to add and subtract. That's what the transformer connections in your amplifier does. One speaker gets L+R minus L-R = 2R and the other gets L+R plus L-R = 2L.

The output of the simplest possible FM tuner circuit is also L+R and L-R. However the vast bulk of tuners made do the addition and subtraction inside the tuner.

In the early days of stereo records (1950's) there was a modicum of debate on how to handle it. Because human hearing is insensitive to direction with bass frequencies, and mono cartidges had poor compliance in the vertical direction, the record companies adopted a convention to record bass equally in both channels, and use the higher frequencies to give a stereo effect. This makes the record compatible with old mono cartridges, which would otherwise tend to damage the groove. It also means that in an amplifier such as yours one side handles mostly bass and one side handle mostly trebble, lowering intermodulation distortion. However in a normal stereo system, the bass, which needs more power, is shared beween the two channel, lowering harmonic distortion. As they say 6 of one and half dozen of the other.

Last edited by Keit; 14th February 2015 at 05:01 PM.
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