[net.audio] son of ampzilla bridge

mikek@tekid.UUCP (Mike Kyle) (09/20/83)

In real life: Mike Kyle
              14065 S.W. Domino St.
              Beaverton,Oregon 97005
              503 646-5429

I have recently aquired a second Son of Ampzilla amplifier and wish
to bridge the two together.  These amps were built in California
by GAS (Great American Stereo).  GAS also built a passive bridge
unit called The Bridge.  I'm looking for one of these bridges or
just the schematic to the amplifier so that it would be easier
to construct an active bridge.

mikek@tekid.UUCP (Mike Kyle) (09/20/83)

I have recently aquired a second Son of Ampzilla amplifier and wish
to bridge the two together.  These amps were built in California
by GAS (Great American Stereo).  GAS also built a passive bridge
unit called The Bridge.  I'm looking for one of these bridges or
just the schematic to the amplifier so that it would be easier
to construct an active bridge.

In real life: Mike Kyle
              14065 S.W. Domino St.
              Beaverton,Oregon 97005
              503 646-5429

jsf007@trsvax.UUCP (09/25/83)

#R:tekid:-154600:trsvax:55100012:000:763
trsvax!jsf007    Sep 23 08:02:00 1983

The way bridging works is seperatly amplifying the positive and negative
parts of the input through different amplifiers.  Since most preamp out-
puts use common ground (i.e. unbalanced outputs), it is hard to do the
conversion to balanced outputs with a simple passive circuit effectivly.  
An active circuit is a fairly trivial matter, however.  Two opamps per
channel are required.  One is configured as a voltage follower, the other
as a voltage inverter.  The outputs of each of these opamps are fed into
the unbalanced inputs of your amps.  The speakers are connected across
the positive outputs of the amps with the two negative outputs wired together
(this may be unnecessary if your output stages are common ground).
			Steve Fintel
			...trsvax!jsf007

rzdz@fluke.UUCP (Richard Chinn) (09/27/83)

I'm posting this reply to the net since it may be of general interest.

Bridging an amplifier is no big deal. I don't like to do it for a couple of
reasons. 

    1. If the load impedance stays the same (say 8 ohms) then each amp
       channel must drive 4 ohms. If the output stage has enough SOA (safe
       operating area), then fine. Otherwise, expect smoke.

    2. There's twice the opportunity for problems if one of your amp's goes
       south.

    3. Neither side of the speaker line can touch ground, since both are hot.

I must admit that most of my prejudice comes from having been in the touring
PA bizzness, where reliability is paramount. 

On the positive side, you get twice the power into twice the impedance (nice
for driving 16 ohm speakers) and 4 times the power into the same impedance
(nice for subwoofers).

If you can live with the above mentioned shortcomings, heres what you need
to do.

I don't know if the ampzillas phase invert. If they do and if GAS sold a
*passive* bridging box then they *must* phase invert (unless the box had a
transformer in it), then build the following:

    A voltage divider whose division ratio is equal to the amplifier's
    voltage gain.  That is, if the amplifier has a gain of 28 then the
    divider must be a 28:1 divider.  

    The output impedance of this divider must be *at least* 10 times less
    than the input impedance of the amplifier. (If the amplifier has a 22 k
    input impedance, then the shunt arm of the divider should be 2.2k ohms
    maxixmum.) 

    The division ratio shoud be accurate (use 1% resistors). 

    Connect the input of the divider to the output of the first amp channel,
    connect the output of the divider to the input of the second amp channel. 

    Connect the load across the *hot* terminals of each output. Don't use
    the ground terminals for anything. Neither terminal of the load can come
    in contact with anything else. 

Example:

    The amplifier has a voltage gain of 33. The input impedance is 22k.
    I assume you know what a voltage divider looks like.

    The divider must have a division ratio of 33:1, thus the resistor values
    must be in the ratio 32:1 (value ratio = 32:1, then division ratio =
    33:1)

    If the shunt leg must be 2.2k, then the series leg is:

	  2.2k * 32 = 70.4k. The nearest 1% value is: 69.8k.

    this results in a division ratio of: 31.73, probably close enough.


If your amplifier doesn't phase invert, then there is no way to do what you 
want passively (except to use a center tapped transformer). I don't 
consider transformers truly *passive*, especially for a high performance 
stereo system. 

You can do what you want actively. It will take one op-amp. I suggest 1/2 
of a ne5532 (can't use the 5534 as it is not unity gain stable) as an 
inverting amplifier. this time it can have unity gain. Make the gain 
setting resistors the same value, say 22k ohms, 1%. The input of this goes 
to the output of the preamp, and one amp channel. The output of the 
inverter goes to the remaining amp input. 

One reply to the net recommended using two op-amps, with one running as a
voltage follower. While this will work, the voltage follower has been
condemmed as a source of distortion, both slew induced and common mode,
since it is running at 100% feedback. If your preamp will drive the combined
loads of the inverter and power amp input in parallel, this is far superior.
If your preamp won't, then I suggest running the non-inverting stage with
some gain, preceeding it with a voltage divider to exactly offset that gain.

In either case, the speaker (load) goes between the two hot terminals of 
the amplifier. The ground terminals are *not* used.

I hope this helps. If you want more details, reply by mail or usmail or 
phone:

	 Rick Chinn
	 John Fluke Manufacturing
    	 MS-232E
	 POB C9090
	 Everett WA  98206
	 (206) 356 5232

or fluke!tpvax!rzdz

	 

jj@rabbit.UUCP (09/28/83)

I must beg to differ with the opinion recently offered about
bridging amplifiers on several grounds:
	1)   An amplifier that is conditionally stable, and
ampzilla's certainly qualify, (I know of some that are
not even conditionally stable, grrr)  will have much
more trouble in a bridged configuration.
	2)  Using a single op-amp invertor is reported to
be "far superior".  This opinion is quite wrong, as the
distortion introduced by a reasonably designed voltage follower
is on the order of the distortion introduced by a
voltage invertor, at MAXIMUM, if a good op-amp (and the
Signetics NE5532AFE certainly qualifies) is used.  Using the
inversion in only one channel risks having phase problems
at the output related to the phase shift (mostly transit delay,
if the circuit is designed right) of the op-amp circuit.
While this delay is small, the results, especially when combined with 
1) above, can lead to surprising results, up to and including 
oscillation that would not occur with a different delay/phase shift.

The problem in 1) comes about due to the fact that an amplifier's
output impedence has several poles and zeros.  These poles and zeros
are also (not too suprisingly :->) the system poles of the amplifier.
Combining the two sets of poles additively through another
complex impedence (the speaker) can lead to various kinds of problems.
In some amplifiers that are quite stable, the problem will be that
the phase margin is reduced, leading to undamped ringing at
a VERY high frequency.  In a few, very well designed amps, the
output impedence is nearly enough resistive (at least at
frequencies less than a MHz) that combinations do not matter.


good luck
rabbit!jj (through allegra, harpo, or research)