The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all products and all products

    ST-70 and bridged center channel?


    Posts : 1
    Join date : 2018-03-04

    ST-70 and bridged center channel?

    Post by jehovaswettness on Sun Mar 04, 2018 11:54 pm

    Hello!  New user, so can't post pix of my st-70 for another week, but I do have a question about these amps and applying a two-channel bridged matrix for center channel.  Is it worth playing around with?  The method I'm looking at is from a Dope From Hope article by Paul Klipsch where the L and R speakers are wired normal at 4 ohm terminals, and the 8 ohm taps are combined with 8 ohm 10W resistors, with a common ground.  My ST-70 currently drives a set of '65 Klipschorns and I had a Cornwall as a center from my previous setup, but it's too big for the current space; now employing an Acoustic Research AR1 for experimentation.

    I've seen other more complex matrices applied with some really oddball wiring (other than Klipsch) but has anyone here played around with pulling a center channel (technically three-channel stereo) with this amp?

    Also:  I will need help identifying my driver board.  It's been upgraded but it seems to be a very early version of something that resembles nothing else currently available.  Tube complements are 2-Mullard EF86 and 1 JAN 6189W (12AU7).  No trim pads, and the board traces are really fragile and looks home-etched, no mfg markings.  I upgraded the caps and tube sockets when I first got the amp some 10 years ago.    I'll post pix in a week.


    Edit: pretty sure it's an early Ned Carlson/Triode board.

    Last edited by jehovaswettness on Mon Mar 05, 2018 12:06 am; edited 1 time in total (Reason for editing : further research provided more information)

    Posts : 155
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: ST-70 and bridged center channel?

    Post by daveshel on Mon Mar 05, 2018 1:36 pm

    The derived center channel was pretty much a Dynaco innovation. Their method was to connect the center channel to the positive terminals of the right and left channels, and the idea was it would get only the stereo program information by eliminating the common program information through phase cancellation. This principle grew into the Quadapter, which added some resistors and essentially connected two speakers in place of the center channel. The two were connected in series between the positives for the right and left. The idea now was that the left of these would get the left stereo program information minus the common signal, and the right would get right stereo minus common signal. These speakers were placed behind the listener, and created an four-channel ambience without having to double spending on amplifiers, which was the case with the discrete 4-channel systems of the day. Later technologies, from the 'wide stereo' switch on boom boxes to the Carver Sonic Holigram generator and eventually today's surround sound, have accomplished the same phase cancellation effects in the circuitry instead of in the room.

    I haven't heard it since the early 80s, but I recall it was more realistic than modern surround sound, which to me sounds ticky-tacky.

      Current date/time is Sat Dec 15, 2018 6:23 am