Virtual ground in power amp applications

OK, so I was running around getting a quote on some transformers for a F5 amp build, and I came across a transfo (625VA, 38-0, 38-0 secondaries) which was for sale for a song... so I bought it figuring it'd be good for a class AB build. The I got to thinking (always dangerous!) about my f5 build and started wondering about whether I could use the transfo I have somehow...

So the F5 wants rails of +/- 24V and with 38V secondaries I'd end up with an unloaded rail voltages of around +/- 53V.... which is almost double what I actually need... So I started wondering about voltage doubling rectification, and so arrived at the following questions.

Is there a way to (exactly) halve the voltage output after rectification?

Crazy idea I had was to use a truckload of diodes (I have, oh, about 1000 N4004's on hand). Each one would knock ~1V off the rail voltage and I can then drag the rail voltage down to +/- 24V. I can wire them in series/parallel so that they can deal with the current they'll be seeing.

Another one was to apply some kinda voltage regulation, but my understanding is that with a Class-A build, voltage regulation is a bit of a pointless exercise, and I'd be asking the regulator to dump a -lot- of power as heat whenever it's running. I haven't looked up the current draw of an F5 as yet but i already feel sorry for whatever sort of regulation scheme I come up with...

Anywho, I'd normally experiment a bit, but playing with that kinda power alarms me somewhat, so a question seemed in order!

Any suggestions?
 
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OK, so I was running around getting a quote on some transformers for a F5 amp build, and I came across a transfo (625VA, 38-0, 38-0 secondaries) which was for sale for a song...
a superb transformer for a two channel 140W+140W into 8ohm amplifier.
It would even do for ~250W+250W into 4ohm amplifier.
It's too low VA for a single channel KSA100.
 
Hi

Wasting half the voltage seems like a bad idea - and there would be twice as much heat to get rid of.

Maybe you could do something like shown below, with the left and right channels each having their own supply (one transformer secondary per channel).

It looks a bit funky, but Quad uses this arrangement in some of their amps so I guess it works OK.

I'm not sure what resistor values would be best - with 100R as shown, they'll each waste about 6W as heat. For a class A amp that won't make much difference though.

Cheers - Godfrey
 

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... <browses internet> Virtual Ground Circuits is a pretty good primer around what you're talking about as I understand it, except that we're talking about 20x the current.

Ok, so I need to find some sort of high current active virtual ground schematic... Or would finding some sort of regulated PSU circuit that doesn't feature a CT on the transfo be an easier task?

Hmmm...
 
hi,
those 100r will allow a low current unbalance for an opamp or similar low current circuit.

Each F5 could require 5Apk of unbalance when still in ClassA and more if the amp is taken into ClassAB.

I think you need an active virtual ground.
Can some recommend a current capability for the active virtual ground?
Hi Andrew

The high AC currents will go through the capacitors, so the resistors only have to deal with DC offset current, which should be very low.

Performance should be acceptable although an active solution would be better.

The circuit below could be good. The opamp is used as a unity-gain buffer. R3 and R4 simply limit it's loop gain for stability.

R5 is included to ensure that the bulk of the current flows through the caps, so the opamp will only have to deal with the relatively small DC offset current (and some low frequency ripple).

R5 should also help with opamp stability, and is low enough to ensure minimal DC offset voltage.

Regards - Godfrey
 

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your solution shows a 7A chipamp. Why, if it only has to handle the very low DC offset?
In the active circuit (post 8), all of the loudspeaker return current flows through
C3//C4//R5. Most flows through the capacitors but at low bass frequencies a significant amount flows through R5 (and the opamp).

For example, with the values shown:
C3 = C4 = 10000 uF, R5 = 1 ohm.
At 32Hz the reactance of the capacitors is about 0.5 ohms each, i.e. 0.25 ohms for both in parallel, so the current through R5 will be about a quarter of the current through C3//C4, i.e. roughly a quarter of the total loudspeaker current.

e.g. for a loudspeaker current of +-2A (1.4A RMS) at 32 Hz, the current through R5 will be about +-0.5 A (350 mA RMS).

The suggested LM3886 may be overkill, but some sort of power device is needed, ideally capable of at least a couple of amps peak current.

As always, component values can be changed to taste. e.g. although I'm not sure what the recommended PSU for an F5 looks like, I expect it involves more than 10000 uF per rail.
 
In the active circuit (post 8), all of the loudspeaker return current flows through
C3//C4//R5. Most flows through the capacitors but at low bass frequencies a significant amount flows through R5 (and the opamp).

For example, with the values shown:
C3 = C4 = 10000 uF, R5 = 1 ohm.
At 32Hz the reactance of the capacitors is about 0.5 ohms each, i.e. 0.25 ohms for both in parallel, so the current through R5 will be about a quarter of the current through C3//C4, i.e. roughly a quarter of the total loudspeaker current.

e.g. for a loudspeaker current of +-2A (1.4A RMS) at 32 Hz, the current through R5 will be about +-0.5 A (350 mA RMS).
thanks for this explanation.
It shows that
.........the resistors only have to deal with DC offset current, which should be very low.
is not true for an audio Power Amplifier.
 
Cheers for all the input so far folks; I've always put the amplifier after the PSU circuit, so for me this is all new...

ok, the F5 PSU is a bit unusual (to my eyes) http://www.firstwatt.com/pdf/prod_f5_man.pdf (last page), but the key point is getting stable rails at +/- 24v so I'll give this layout a shot.

I guess my next question then is around the capacitance after the opamp virtual ground, and whether putting some kinda CRC filter in will be an issue. Having a look I can't see why I couldn't put in a filter, and use scads of capacitance - I suppose the question of where to connect the virtual ground to chassis ground would go is a good one too.

Any thoughts all? I'll draw up a schematic (and if eagle plays nice) a provisional circuit board as well...
 
Having been the recipient of 240VAC on two separate occasions I'm quite the enthusiast of safe grounding of any projects I've done. The question was more around where should I make my ground connection on the rail that will be ground.

There's a good chance this is irrelevant, and something I don't need to worry about, but I remember the first power amp build I ever did specified which side of the cap bank the chassis ground would be attached to, relative to the load, and I've followed that topography since. With the virtual ground circuit in place I would guess that the appropriate place would now be just after R5... I've a random memory that it's due to the current loops, but, hey, I could just be making things up.

Anyhoo, I'll build this up sooner rather than later, and we'll see how we go.
 
LT1970 active +/-500ma current sink/source controller with mosfet driver would make an excellent contemporary high current Vground (it becomes the ground channel, rather than buffering it) VERY little capacitance is needed. also linear has other opamps such as the 1.1A LT1210 which would also do nicely. IMO linear are really all out on their own in this area. the devices arent cheap or all that available though. farnell will have them, but we all know how farnell like to charge. you can buy small quantities direct from linear though
 
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Hi Qusp,
I'll need asome help in utilizing that IC for this purpose... When you say "becomes the ground" what sort of circuit are we talking about; the same as has been featured above?

Having had a quick skim of that data-sheet it looks like the V+/V- input range is 36V (total), so does that mean I'll have to derive a regulated voltage to drive that chip, given I'm starting at ~53V between the rails?

I'd appreciate any pointers that could give me some learning around the use of this chip...
 
LT1970 active +/-500ma current sink/source controller with mosfet driver would make an excellent contemporary high current Vground [snip] also linear has other opamps such as the 1.1A LT1210 which would also do nicely [snip]
Unfortunately the absolute maximum supply voltage for both LT1970 and LT1210 is 36V (+-18V), so neither can be used here.