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a single output transformerstereo tube amp ??

Discussion in 'Electronic Repair' started by Boborann, Oct 28, 2006.

  1. Boborann

    Boborann Guest

    I bought a Columbia680 stereo today on Craigs list which I assumed was mono
    as it had one output transformer and a pair of 6L6's and a mono tuner input
    When I tore it down and found the schematic I found that it was in fact
    stereo and used a single 6L6 for each channel and they shared an output
    transformer .
    What looked like a traditional Push Pull design is actually fed with a
    channel per 6L6 and in phase .
    The output winding is Center tapped and has a speaker attached to each side
    of the winding.
    Curious whather anyone has ever seen this and could comment on how common it
    was
    Thanks
    Bob
     
  2. I vaguely remember a single ended push pull designation but this seems to be
    used for OTL amps, like the Philips that used 800 ohm speakers. I do recall
    seeing a design like this many years ago.
     
  3. Boborann

    Boborann Guest

    Best I can tell these are 4 ohm speakers
    I'm actually embarassed because I assumed mono and told teh seller that her
    stereo was mono based on teh single output transformer
     
  4. Jim Land

    Jim Land Guest

    I've never heard of a circuit like this, and my experience goes back to
    the pre-stereo days of "hi-fi". To me, the circuit doesn't pass the
    smell test. Two stereo channels out of a single output transformer? I
    have grave doubts.... Sam, what do you say?
     
  5. Boborann spake thus:
    [JPG was here]

    So it looks like a stereo amp and smells like a stereo amp ... but I say
    no way would it actually function as one. Seems like someone's smartass
    idea to save a few nickels on iron by sharing an output transformer, and
    sell "stereos" to folks who probably didn't know better.

    My guess is that there would be tons of "crosstalk" between channels; no
    way is the output winding going to "ignore" what the "other channel" is
    driving on its half of the input winding.

    Any experts here have an opinion on this?


    --
    Just as McDonald's is where you go when you're hungry but don't really
    care about the quality of your food, Wikipedia is where you go when
    you're curious but don't really care about the quality of your knowledge.

    - Matthew White's WikiWatch (http://users.erols.com/mwhite28/wikiwoo.htm)
     
  6. That's a hoot but you are right. The difference signal is small in
    comparison to the main signal so that's how it works. Not a popular design
    methinks.
     
  7. Guest

    Since the output depends on changing flux in the transformer, unless
    the transformer has some sort of a split core, the output from each of
    the secondary windings on the transformer will be identical. Let's see
    the schematic if you have one.

    H. R. (Bob) Hofmann
     
  8. Guest

    Since the output depends on changing flux in the transformer, unless
    the transformer has some sort of a split core, the output from each of
    the secondary windings on the transformer will be identical. Let's see
    the schematic if you have one.

    H. R. (Bob) Hofmann
     
  9. Guest

    Quite. I vaguely wondered if 2 mag fields at right angles could be
    persuaded not to interact too heavily, but that would make a very funny
    looking and harder to make transformer.

    I suspect the amp is in fact stereo with crosstalk... about 100% of
    crosstalk.


    NT
     
  10. Meat Plow

    Meat Plow Guest

    Plenty of single ended amps out there but not one have i seen that had one
    common OT.
     
  11. Boborann

    Boborann Guest

    I have a JPG of the schematic that for some reason I can't post here ( 90k)
    .. I can send it along to anyone interested or better yet can someone tell me
    how to post it here or elsewhere
    BOb
     
  12. Meat Plow

    Meat Plow Guest

    Post it to alt.binaires.pictures
     
  13. Boborann

    Boborann Guest

    Just posted the schematic (90k) and a photo of the amp with transformer (
    1M ) as different post on alt.binaires.pictures
    Bob
     
  14. Meat Plow

    Meat Plow Guest

    It's a push-pull mono amp according to that schematic.
     
  15. Boborann wrote:


    This is like some Grundig amplifiers, where they separately amplify the
    sum and the difference signals. That other transformer you see in the
    B+ lead extracts the difference signal and then adds it to one side of
    the secondary ( L+R ) + (L-R) => 2L, while subtracting it from the
    other half of the secondary: ( L + R ) - ( L - R ) => 2L. Clever.

    I wonder if they licensed thepatent or what? You might try looking
    up the patents at ww.uspto.gov and see if anything interesting pops up.
     
  16. This one has two really - a main one for mono and a small one for the
    difference.
     
  17. Meat Plow

    Meat Plow Guest

    Yeh I read that in another post. I looked at the schematic albeit I didn't
    study it for long.
     
  18. Better to post it on alt.binaries.schematics.electronic

    The other one is a porn group.
     
  19. Nope. Stereo. It looks like one set of speakers is in the cabinet with the
    amp.
     
  20. Guest

    I have never seen such a design, I didn't see a link to the pic, so I
    am flying by the seat of my pants here.

    First of all such a thing might be stereo, or hi fidelity, but not
    both. To explain, my definition of stereo is that there are two
    seperate, discreet and identical channels. Perhaps is is very high
    fidelity when used as a mono amp, but actually was capable of stereo.

    Using a large main output transformer for the mono signal, typical push
    pull, but the if the primaries are split up and phased just right I can
    see how this can be accomplished. Of course then you have the
    secondaries of the small transformer in series with the secondaries of
    the big transformer. The way I see it, the arraingment of the primaries
    is important, otherwise you get alot of second harmonic distortion, IMO
    an unacceptable level.

    The only way this can work is to feed L+R to the 6L6s in differential
    mode, and the L-R in common mode. By splitting the primaries so that
    the differential mode current is bucked effectively in the primaries of
    the secondary output transformer, and the common mode current is
    doubled, acceptable performance could be achieved IMO.

    The whole thing sounds quite inefficent to me. If the seconday output
    transformer is not equal in output to the primary, the amp cannot
    deliver it's rated power into one channel only, even though that is
    rarely needed.

    I can see it is simply a matrix. This is pretty much the same way FM
    stereo, or NTSC/PAL colorplexing/decoding works. The difference is that
    the levels are higher.

    On the drive side of such a circuit I bet the negative feedback network
    is a beauty to behold. (lol). Actually if I were to design such a
    circuit, each 6L6 would have it's own completely independent feedback
    system. Even then I would expect quite a bit of second harmonic
    distortion in the difference channel, i.e., anything that comes out of
    only one side, and to a lesser degree when panned partway to one side.

    Music mixed in modern times often has some phasing effect, sometimes
    they "pan" using a time delay. When used judiciously it doesn't sound
    like an effect, but it allows them to spatialize the sound left or
    right without a great change in the levels coming out of each side. So,
    I wonder how one of these things would handle modern music.

    It probably sounded good on orchestra music, basically anything that is
    miked, but I have heard some music that would probably sound terrible
    on it. Some of the old Beatles comes to mind.

    At any rate, I think this an interesting topic, thanks for bringing it
    up. Today, this is thinking outside the box. :)

    JURB
     
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