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Bernhard 2nd March 2009 08:32 PM

Virtual Ground ?
I want to built something with 30V rails and would like to have nice encapsulated transformers.
Unfortunately most of them have much higher voltages.
Now I found some nice pieces but they have only one sec. winding without center tap.

Bad idea ?

ceharden 2nd March 2009 11:03 PM

Go onto the Crown Audio site and have a look at how they do the output stage in the Macrotech/Microtech range. It's essentially a bridged amplifier with one side driving the output and one side creating a virtual ground which moves in the opposite direction.

traderbam 2nd March 2009 11:15 PM

You could use 2 transformers with primaries in series to mimic a dual secondary transformer.

unclejed613 3rd March 2009 02:09 AM

the Crown and QSC amps do use a center tapped transformer. the center tap is left floating, and the rails are driven, which the return of the power supply follows and provides the output to the speaker. the output devices are connected between the supply rails and ground, and "walk" the supply rails with the audio. it's a half bridge amp, just as a standard class B is, except the transistors drive the "other half" of the bridge (the power supply) rather than driving the speaker directly.

it's very similar to the principal of the "circlotron" amp, but in solid state form.

the other option would be to operate the transformers using a capacitive or resistive (or active) divider to create a ground return. generally, you could build a crown-like amp without a center tap on the transformer, and the power supply filter caps, not only would serve to couple the signal to the speaker, but also provide a capacitive divider and so a "center line" for the power supply. it would definitely be worth investigating, since it would simplify the power supply a little bit.

the "active" divider option would be the equivalent to a high power version of the opamp "battery splitter" used in guitar effects boxes to create split 4.5V rails from a 9V battery, but your "opamp" would actually be a power amp and a voltage divider at the input to provide a low impedance virtual ground.

Elvee 3rd March 2009 08:14 AM

You can lift the circuit from the Quad amplifiers:
To be on the safe side, use somewhat larger components.

pacificblue 3rd March 2009 08:22 AM

Nothing suitable on this site?

unclejed613 3rd March 2009 11:45 AM

2 Attachment(s)
i've posted this jpg before (probably about 2 yrs ago....). it shows how the qsc and crown output stage works...

if you notice, the only thing that's been moved is the power supply and ground. the emitters still drive the load (this time through the power supply). one advantage to this is that the drivers and outputs can be driven with an op amp operating from +/-12V without the use of a VAS operating from high rail voltages. the only thing really required for full output is a lot of current gain. the voltage gain actually is developed by the output stage and the floating output rails.

KSTR 3rd March 2009 12:11 PM

in case you wanted to build something active to get a "virtual ground", then consider taking this just a little bit further and build a true bridge amp. "GND" then carries no spkr-load currents and is only a reference potential. Many of eg Nelson Pass designs qualify.

@Uncle: I wouldn't label the QSC schemes (both with and without ctr tap) a cirlotron, but that's of course a semantics question. IHMO, it's a plain single-ended design, exept that the OS is cleverly split up, or "wrapped inside out", so to say. However, it still is, the classic "passive bridge" topology, either with a DC path or without (the latter case with DC blocking in the input to prevent static imbalance/runaway).

- Klaus

ceharden 3rd March 2009 04:15 PM

The Crown and QSC output stages are not the same.

Crown use what they call a 'grounded bridge' configuration which is described here: There are two sets of output devices and a single rail supply.

QSC just use a very common rearrangement of a standard push-pull output stage to allow the output devices to be electrically connected to a grounded heatsink. This has many benefits, especially heat conduction from the transistors to the heatsink.

wg_ski 3rd March 2009 04:48 PM

The Crown and QSC are related to each other. The "high side" of a Crown grounded bridge is a normal run-of-the-mill amplifier. The "low side" of the Crown which generates the virtual ground is essentially a QSC amp (with darlington outputs instead of CFPs, but the operating principle is the same). They couple together through the power supply.

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