Balanced amp methods

[PWatts]

Hi,
I'm currently designing a HUGE power amplifier for home cinema, using 10x MJ15032 & 10x MJ15033 per channel. The heatsinks, caps & toroidals are however a source of financial ruin. Anyway, the target is 2 monoblocks, each positioned next to its speaker- this means long signal leads and balanced operation becomes a must.

Now for the actual question. Where should the balanced become unbalanced? Some people use a simple op-amp based converter before the amp, keeping the amp basic single-ended. Better yet is to use a balanced input stage and then join them before the VAS. Whatever way you look at it, art some stage the balanced has to become unbalanced- unless you've balanced speakers! My idea is to use two separate amplifiers, one for pos, one for neg, and each's output to it's corresponding speaker terminal. Almost like bridging, but not. Instead of feeding the output of one amp to the inverting input of the other, use the balanced signal from the preamp. This should ensure zero amplifier interaction. A few SPICE simulations were most encouraging, with lower distortion than the individual amps'.

As it sounds almost too good to be true (all the pros of bridging with none of its cons, except the need for a balanced source), I'm sure there has to be some pitfalls. Has anyone experimented with this before?

Thanks,
Pierre Watts

[Andypairo]

I suggest you to have a look at the service manuals of the Aleph series amplifiers by Nelson Pass: some use balanced inputs, some both balanced and unbalanced.

If you use a differential input stage things become relatively easy , have a look at the schematic of the Aleph30 and tell us what you think..

Cheers

Andrea

[PWatts]

Yes I did look at the Aleph30 (Actually still busy gathering components for one). I know the different methods of implementing balanced support for an amplifier, but what I'd like to know is how it'd work to keep the two parts (+ & -) completely separate by giving each its own full amp- i.e. one stereo amp per channel. That is very much the same as bridging, but I figure with lower noise & distortion.

Pierre

[Andypairo]

In this case you only need two identical amplifiers: feeding them with a balanced signal (the + to the input of one, the - to the input of the other and the gnd to the common gnd of the amps) they will put out the same signal , but one will be inverted in phase. In this way you can achieve a sort of "balanced" or bridged operation without the need of anything else than a balanced preamp (and of course 2 amps per channel )

Cheers

Andrea

[MarcelvdG]

Make sure you use accurately equal closed-loop gains (accurate feedback networks), or you'll get only little common-mode rejection.

[mikek]

Originally posted by PWatts
Hi,
I'm currently designing a HUGE power amplifier for home cinema, using 10x MJ15032 & 10x MJ15033 per channel. The heatsinks, caps & toroidals are however a source of financial ruin. Anyway, the target is 2 monoblocks, each positioned next to its speaker- this means long signal leads and balanced operation becomes a must.

Now for the actual question. Where should the balanced become unbalanced? Some people use a simple op-amp based converter before the amp, keeping the amp basic single-ended. Better yet is to use a balanced input stage and then join them before the VAS. Whatever way you look at it, art some stage the balanced has to become unbalanced- unless you've balanced speakers! My idea is to use two separate amplifiers, one for pos, one for neg, and each's output to it's corresponding speaker terminal. Almost like bridging, but not. Instead of feeding the output of one amp to the inverting input of the other, use the balanced signal from the preamp. This should ensure zero amplifier interaction. A few SPICE simulations were most encouraging, with lower distortion than the individual amps'.

As it sounds almost too good to be true (all the pros of bridging with none of its cons, except the need for a balanced source), I'm sure there has to be some pitfalls. Has anyone experimented with this before?

Thanks,
Pierre Watts

You may want to consider the following:

1. Arrange for your preamp. to contain two of the following single ended to diff. conversion circuits.

2. Use them to drive two stereo amps, each driving it's own 'speaker in BTL mode....make sure the two channels in each stereo amp. share the same star point in the chassis.

[pooge]

You can balance your amp to you heart's content and still not get real world balance when you connect it to a preamp or other source. Go to the Jensen transformer web site at

http://www.jensen-transformers.com/

and rummage around, especially through the white papers AN002 and AN003. They will convince you that a JP-11-P-1 line input transformer is the way to go. Check out the application notes for that transformer, also.