Re: Parallel stereo = more output?

Joseph2k wrote:
> meow2222@xxxxxxxxx wrote:
>
> > Fred wrote:
> >> GregS wrote:
> >> > In article <o1vst19g0dnevqmq5tute0u0e7e934iblq@xxxxxxx>, Steven Goring
> >> > <stevegoring@xxxxxxxxx> wrote:
> >> > >I have a TDA 2009 audio amp chip that does 10W on each channel.
> >> > >
> >> > >Is there any reason why I cannot connect the output of both channels
> >> > >in parallel to produce a total 20W output?
> >> > >
> >> > >Obviously, the same signal would be used to drive both inputs, and amp
> >> > >output would be mono.
> >> > >
> >> > >I do not want to use a bridge configuration for design reasons.
> >> > >
> >> > >Thank you for any advice.
> >> > >
> >> > >Steven Goring
> >> >
> >> > I didn't look up the specs, but generally you can't do this.
> >> > They have to have identical poles and stuff, but by
> >> > adding a little lossy resistance on each output, they
> >> > may be stable enough to opperated with the resistor legs
> >> > tied together to form the output.
> >> >
> >> > greg
> >>
> >> The output of one channel would see, as a load, the very low output
> >> impedance of the other channel.
> >
> > Only if the 2 amps try to fight each other at some point. Take care
> > that the response across the spectrum is the same with both amps and
> > things are ok.
> >
> >
> >> And when you add series resistors you
> >> lose the advantage you are trying to gain by introducing new losses.
> >
> > power doubled then -10%, so no, you get 80% more power.
> >
> >
> >> Not a good approach.
> >
> > It does increase output impedance, and the careless may be tempted to
> > do it with unmatched circuits. And if any minor circuit fault changes
> > the closed loop characteristics you can end up with 2 chips trying to
> > cook each other, hence a few more failures. It works though.
> >
> >
> > NT


> The original device is two (stereo) amplifiers on one chip.  How much more
> well matched do you think you are going to get?

Matched characteristics depends also on the circuit applied to each
amp, it is here that people often get into murky water. For example
configuring one amp as a gain of -100, with the other as a gain +1
follower. Not best practice, as response at frequency roll off is
likely to be different.


>  OP may get 41% more usable
> voltage

10% less usable voltage, not 41% more.

> but not 41% more usable current;

100% more usable current. I wonder if we're talking about 2 different
circuits?


> and power dissipation remains as a
> limiting factor.

As with every amp ever made. But this time, its twice what it was with
a single amp, with about 10% then lost to the Rs, so apx 80% power
output gain.


NT

.

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