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Old 15th October 2010, 11:57 AM   #41
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by godfrey View Post
No need for anything exotic here; just scratch around the house for something that has about 50 ohms DC resistance - maybe an electric toaster or some lightbulbs wired up in parallel or .... ?
the second filament of all the broken H4 auto headlamp bulbs you did not throw out.
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Old 15th October 2010, 12:05 PM   #42
qusp is offline qusp  Australia
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well why bother with a toaster or lightbulb if its not a dynamic load why not know exactly the load you test and use a high wattage 1 or 2K resistor?
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Old 1st November 2010, 06:25 AM   #43
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I bodgied up the circuit on a protoboard over the weekend, and gave it a test with the bench power supply. All good so far. I'll give it a try with some varying (static) loads tonight. If anyone has suggestions for a dynamic load to test with, I'd be glad to hear it!
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Old 4th November 2010, 02:18 AM   #44
Bigun is offline Bigun  Canada
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If you look at the F5 and your PSU, do you find that the speaker currents flow through the last set of capacitors in the power supply ? It's often the case that they do. In which case, a virtual earth for the F5 is perhaps the same thing as a single rail supply plus an output capacitor on the amplifier ?
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Old 23rd September 2011, 01:31 PM   #45
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Just resurrecting this thread because I'm fascinated by the power supply arrangement Quad have used here! Apologies if this is just conventional stuff to many people, but judging by this thread, this PSU configuration has passed some people by (like me).

Virtual ground in power amp applications

Why have Quad used this method? Looking at it, I think it might be because it confers the following advantages:

1. The transformer secondary doesn't need a centre tap.
2. Only one fuse in the transformer secondary side means that if it blows, the ground stays symmetrically between the supply rails as the power collapses. From the earlier 405 schematics and its conventional split supply, I can see that Quad had thought long and hard about the positions of the two fuses, in relation to the input and output stages' power.
3. Although the amp output is not capacitor coupled (it behaves exactly like a DC coupled amp output), if one of the output devices goes short, or partially short, there is no DC path via the speaker to +ve or -ve (well, only a high impedance), so only the energy held in (one of) the PSU caps can be discharged through the speaker even if the fuse doesn't blow, thus limiting damage.

It only works for AC applications (not a problem for audio, obviously), but apart from that, are there any disadvantages to this PSU arrangement?
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Old 22nd April 2013, 11:23 PM   #46
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Here's a VERY INTERESTING virtual ground circuit I worked out recently, (with some help from people at head-fi.org), which should also lend well to chip amp applications, although the TO-220 regulators are going to need heat sinks, for sure. It offers positive and negative outputs with 1.5 amps of current from each, simultaneously at up to about +/- 37V. see the whole article either here:

Virtual Ground (A Regulated Rail Splitter)
or here:
A Regulated Virtual Ground Circuit
or here:
Vitual Ground (regulated!) - and Rail Splitter Circuits!

Click the image to open in full size.

Last edited by Sonic Wonder; 22nd April 2013 at 11:50 PM.
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Old 13th September 2013, 03:01 PM   #47
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Quote:
Originally Posted by godfrey View Post
Hi Andrew

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
WOW! This looks like just what I need. I do have a couple questions - not really a newbee (I've been building/modifying kits for ~50 years, but some of this is still new to me):
1- What controls the V+/V- voltage? Or is it always 1/2 the input?
I will be using a tranny with ~72VAC secondary so ~90 VDC post rectification. Currently I have a MOSFET regulator resulting in a stable 70VDC. My plan would be to add this virtual ground circuit to create +/- 35VDC to power two LM3886 amps. Do you see any problem with this?
2- This shows my lack of knowledge of the LM3886. I can follow all the connections in your schematic, but is it fine to just leave the mute pin (#8) unconnected? Also what about pin #7 which typically goes to ground?
Thanks for the help.
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Old 13th September 2013, 11:59 PM   #48
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One cool thing about the Virtual Ground circuit is that the two adjustable regulators always output 1/2 of the rail-to-rail voltage.
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Old 14th September 2013, 07:38 AM   #49
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by dmtparker View Post
WOW! This looks like just what I need. I do have a couple questions - not really a newbee (I've been building/modifying kits for ~50 years, but some of this is still new to me):
1- What controls the V+/V- voltage? Or is it always 1/2 the input?
I will be using a tranny with ~72VAC secondary so ~90 VDC post rectification. Currently I have a MOSFET regulator resulting in a stable 70VDC. My plan would be to add this virtual ground circuit to create +/- 35VDC to power two LM3886 amps. Do you see any problem with this?
2- This shows my lack of knowledge of the LM3886. I can follow all the connections in your schematic, but is it fine to just leave the mute pin (#8) unconnected? Also what about pin #7 which typically goes to ground?
Thanks for the help.
I think using another 3886 for this job is massive overkill. A somewhat beefed up version of the Quad could easily do the job.

Another option would be to put the amplifying 3886's themselves to task, and generate their own VG, without any supplementary circuit.
If you post your exact schematic, we can see how to tweak it to accomplish this
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Old 14th September 2013, 09:00 AM   #50
AndrewT is offline AndrewT  Scotland
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Hex Buffer, MOSFETs Build A High-Power, Lossless, Virtual Ground | Power content from Electronic Design

Is this any good for audio?
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