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Making a balanced power amp from two SE power amps.

N

new naim boy

pfm Member
Will this work?

The plan is this:

Connect hot and cold to the + input on separate single ended power amps. Connect 0V on each poweramp together.
Connect a speaker across the + outputs of each power amp.

Your help would be greatly appreciated.

Aidan
 
.....will result in almost total silence. You'll need to somehow apply signals to each amp in antiphase to each other. The simplest way to achieve this is to use an audio transformer with a centre tapped secondary winding - connect the centre tap to 0V and the ends of the winding - one to each amp.
 
Les: A few questions if you don't mind.

So hot and cold connect to the primary of the transformer?

Are these transformers expensive?

Aren't hot and cold in antiphase anyway?
 
Not at all, I'm delighted to help.

Audio transformers usually come standard with a primary winding with a start and finish and secondary with start-centre-finish. Feeding an audio signal into the primary (usually 600 Ohms impedance) results in a signal from the secondary. Connecting the centre of the secondary to 0V on both amps and start to the input of one amp, finish to the other should result in lovely sound from a loudspeaker connected between the two positive outputs of the amps.

The price of transformers varies with the quality (but not always) and such firms as RS and Rapid have them off the shelf.

I'm sorry that's a long winded explanation but I'm short on brevity this evening.

Send me a mail via my site and I'll draw a little diagram for you.

Hope all goes well.
 
I think this will work if you connect the 0v outputs of the amps together with a thick cable.

You'll also need to connect the conjoined 0v input to 'ground' on your balanced output. And if your output is transformer balanced this won't exist. Most electronically balanced outputs will work as expected. It may be adequate that the 0v found from the input impedances in series will suffice, it's a known unknown...

I think. It's a background project to try exactly this...

Paul
 
The other issue to remember is that balanced operation halves the effective impedance seen by the amps, so be very careful that they will be happy driving a much more difficult load; you can deliver (nearly) twice the voltage to the load, but no more current.

In particular, remember that many speakers fall to 2/3rds or less of the nominal impedance at some frequencies, and this may get down to distressing levels. So nominal 8Ohm speakers look like maybe 5 or 6 ohms at DC, and through the mid bass. Running this balanced means the amps see 2.5 to 3 Ohms, and this may give current limiting, or even damage.
 
LesW said:
.....will result in almost total silence. You'll need to somehow apply signals to each amp in antiphase to each other. The simplest way to achieve this is to use an audio transformer with a centre tapped secondary winding - connect the centre tap to 0V and the ends of the winding - one to each amp.
Click to expand...

But Les - hot and cold from a balance source are already in antiphase - thats the idea so the original suggestion should work provided the output grounds of the two power amps are conneted.

The naim 250 was originally designed so that it could be used professionally in exactly this way. Plug a standard balanced XLR lead into its socket and you're ready to go (the ground link is already made on a 250).

Speaker accross the two + terminals
 
I have just built a pair of the AAK Symasym 5.3 monoblocks from diyaudio. Apparently they are rated at 60Watts into 8 Ohms and 100 into 4 Ohms.

The schematic for this amp is shown here:

http://www.lf-pro.net/mbittner/Sym5_Webpage/symasym5_3.html

Firstly they will be used SE while I find out what they sound like. But I now have a Buffalo DAC which has balanced out and a Volumite vol control so I will definately be trying a balanced setup. I have another pair of pcbs so I can get a pair of balanced poweramps this way. (However, buying a Sympatico from TP would have been cheaper). I hope to achieve lower noise too.

If I follow my plan above, the 0V in the balanced input will be connected to earth in the preamp. So each poweramp input will be connected to earth in the preamp. If I connect the 0V outputs of the amps I will need to make sure I don't get some sort of ground loop. The amp power supplies come from two separate trannies.

This method will give me 2V across the speaker, where V is he voltage across a single amp. This means I should get 4 times the power output - 240 watts max (8 Ohm load ). Yes this needs twice the current, which I need to think about. The main device drivers are 2SC5200 and 2SC1943 placed on huge heatsinks.

Thanks for all your comments. It will probably be xmas when I give this a try.
 
colasblue said:
But Les - hot and cold from a balance source are already in antiphase - thats the idea so the original suggestion should work provided the output grounds of the two power amps are conneted.

The naim 250 was originally designed so that it could be used professionally in exactly this way. Plug a standard balanced XLR lead into its socket and you're ready to go (the ground link is already made on a 250).

Speaker accross the two + terminals
Click to expand...

Sorry, you're wrong. Connect a balanced line in such a fashion and all you'll experience is screaming howl and the smell of burning tweeter coils. The configuration of single end is not of itself, sympathetic to a balanced input as there is no 0V reference for either amp. The strategy I outlined earlier maintains the 0V ref by means of the transformer centre tap. Balanced amps have a dedicated signal input where the 0V is carefully internally referenced to ensure stability. Believe me, I've been through all the above trials and tribulations in developing a fully balanced system - perhaps for release at a later time.

The 250 is a conventional single input amp which cannnot be used as one half of a bridged amp nor will it accept anything other than a conventional unbalanced input unless other components are added to provide a balanced config - sorry again.
 
The balanced output from my preamp would drive a NAP250 in this way. Because the 0v in the three pin output is actually an 0v for both hot and cold. It will drive a single ended input using 0v and hot. Earthing 'cold' would be bad for it.

FWIW.

Paul
 
wether you invert the phase of the input or output what you are rtying to create is a bridged amplifier.

this configuration has the obvious disadvantage of an increased output impedance.
 
Yes bridging is what I was originally thinking about, then I thought why not use a balanced input?

I've looked at Rapid and there are some 1:1 600 Ohm audio transformers for just a few quid. I will take your advice thanks Les and try this, as I'm not sure if connecting the third pin in the balanced input to earth (in the preamp) will actually provide a 0V reference for hot and cold. I guess the disadvantage of the transformer is its nonlinearity.

All this may well be a waste of time. However, a few quid spent on a trannie will be worth it for interest's sake. Nothing ventured....

If anyone knows of any articles on balance audio I would be very keen to read and learn.
 
Of course you can only do it when the two power amp modules are powered by the same supply, otherwise there is no complete circut for the output.

hence a 250 can do it but quite a few commercial stereo designs cannot

A list of those which I know definitely can't (and there are lots of others) would include

naim 160
Avondale 260z
exposure super VIII
Exposure IV (any version)
most Crimson/Quantum designs

any pair of mono amps
any stereo amp with separate rectification or capacitors for each channel.

and of course bridged (which is what most of us are describing) isn't quite the same thing as balanced but most things accepting pro XLR balanced lines are bridged, and the same amps usually have a jack for single ended input which drives a buffer and inverting buffer to generate signals for the inverting and non inverting inputs.
 
I now have 4 power amp boards. Each has its own PSU onboard. They sound great in SE mode but I just had to put the balancing idea to test.

I have fed a pair of these boards from a single trafo. So 0V on each board is common. One board got the hot and the other the cold signal. The speaker connected across the two + outs. It seems to work. I used a crappy speaker to test but it sounds good.
 
New Naim Boy

Can you draw a schematic of exactly what you are doing please, especially showing how the single amps become one channel. Much appreciated if you can.
 
Here is my amp wired for SE use.

ampbefore.jpg


Here is how I have connected to make a balanced amp.

ampafter.jpg
 
I have another amp which is identical except for the fact that each board has its own trafo with dual 30V secondaries. I haven't tried this idea yet on this amp. I would need to connect the 0V lines together.
 
That won't actually work since there is no complete circuit from the non ground output from one board to the non ground output of the other. The two boards must run off the same power supply!
 
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