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Old 6th October 2010, 08:44 AM   #11
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by godfrey View Post
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
Quote:
.........the resistors only have to deal with DC offset current, which should be very low.
is not true for an audio Power Amplifier.
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Old 6th October 2010, 11:33 AM   #12
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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...
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Old 6th October 2010, 12:01 PM   #13
AndrewT is offline AndrewT  Scotland
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Originally Posted by aspringv View Post
I suppose the question of where to connect the virtual ground to chassis ground would go is a good one too.
do you need to?

The exposed conductive parts must be connected to Safety Earth.
That includes the RCA input/output socket barrels and any metal control knobs.
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Old 6th October 2010, 01:56 PM   #14
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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.
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Old 6th October 2010, 02:03 PM   #15
qusp is offline qusp  Australia
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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

Last edited by qusp; 6th October 2010 at 02:06 PM.
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Old 7th October 2010, 07:10 AM   #16
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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...
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Old 7th October 2010, 10:04 AM   #17
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by aspringv View Post
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
Yes, extra capacitance is good.
A CRC could be added before or after the virtual ground.
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Old 7th October 2010, 10:17 AM   #18
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by qusp View Post
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.
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Old 7th October 2010, 12:12 PM   #19
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... I'd sat that makes the lm3886 a proper possibility. I believe the max voltage supply that it can handle is 84V, so it'll be happy in the circuit above.

http://www.national.com/ds/LM/LM3886.pdf

hmmm... any other options
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Old 8th October 2010, 06:12 AM   #20
godfrey is offline godfrey  South Africa
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FWIW: If I was doing this, I'd use the simple passive circuit, or at least try it first.

An active virtual ground adds a lot of complexity and expense in return for little or no improvement.
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