Grounding one half of a bridged amp?

[unclejed613]

the bridged configuration will have the cumulative noise of both amps whether you use the second amp driven 180 deg out of phase or use it as a virtual ground. another drawback of using the second amp as a virtual ground is that the virtual ground may suffer from output impedance variations in the crossover region, adding crossover distortion. the output impedance goes up in the crossover region, resulting in less current sinking ability. the increase in impedance goes up with frequency because of feedback loss above the input stage corner frequency. the crossover distortion increase would be more noticeable with a virtual ground configuration than with bridging because the second amp will spend more time in the crossover region than with bridging.

[sangram]

You're my favourite uncle. I'll try to keep away from a bridged configuration if I can help it. This calls for a good discrete kit, I guess.

Thanks

[Andre Visser]

I don't understand your problem, it sounds like you want to build your own amp. The Lm4780 is a stereo amp, you can only use one channel but don't you have stereo from the sound card?

[sangram]

Sorry for the confusion Andre, I'll try and clarify.

I have a 4-chip stereo BPA of the LM4780. This is configured with each chip having outputs paralleled, then two chips are used to drive the speaker in a bridged configuration. A stereo amp thus has 4 chips. Hope it's clear so far.

To simplify circuitry and connections, I use a balanced soundcard to provide the input through a TRS connector. The ground (S) is used as a common signal ground for both amps, and the hot (T) and cold (R) of the soundcard are used as signal input for each amplifier. The load is connected between the outputs of each amplifier.

I have two soundcards with this capability. One is an E-mu 1212m. This has an active 4-pole filter on the output, which results in a true differential output. So far no problem. The second card is a Delta66, which actually uses a resistor and capacitor to 'float' the return signal off the signal ground. It is actually a single-ended output which uses a trick (I linked to it earlier) to create a 'balanced' output.

When I drive the amplifier using the Delta, I usually get a pretty high output offset, as the impedance of the cold line is high. I wanted to instead drive the amplifier using the hot (T) input, and instead of connecting the cold (R) to the other amplifier, ground it instead. The load would remain connected as is, i.e., between the outputs of the amplifier.

As unclejed pointed out (I vaguely knew about it but did not know the implications of such a thing), this results in a virtual ground amplifier. The speaker's return load current is not absorbed by the power supply ground directly, but through the second amplifier. That amplifier also works a little bit (I do not have an oscilloscope to test this but I see it clearly, as the feedback network will attempt to correct the output terminal).

I basically posted to figure out the tradeoffs of such an approach, and though it is a chipamp I am applying this to, I guess it's a broader question, so I posted it in SS.

[Andre Visser]

If I understand correctly, you have a stereo balanced amplifier with balanced inputs.

You can wire the input + to signal, and input - to gnd (short gnd and - input) and use the outputs as normal or you can connect the "zero" output directly to power supply 0 Volt (directly to PSU capacitors). Either way, you will loose at least half of the amplifier's rated power.

Hope this help.

André

[Workhorse]

By looking at the description of this topic, i was thinking that, you must have been talking about the grounded bridge amp, in which one half of the output is grounded and it runs on a single floating power supply.........but things changed unexpectedly!!!

[EWorkshop1708]

WHY so complicated?

[KSTR]

^^^ My thinking, too. Why not, for example, build a super-balanced line receiver (one NE5532 dual will do that, and please don't ever think that this or a DRV134 would degrade the sound of a LM4780 in standard config ;-) and then you have always a balanced output and can hook up on the input whatever you have, balanced or not.

- Klaus

[sangram]

Originally posted by KSTR <snip>please don't ever think that this or a DRV134 would degrade the sound of a LM4780 in standard config ;-) <snip>[/B]

I guess you're right - I just wanted to avoid building another circuit, another power supply, however simple. Which was really the whole point of the OP.

I guess I'll first try without, and if it is not too great (my monitors should be able to tell me some of the story) I'll ditch the whole thing and go discrete or single-ended, or press my DRV134s into duty.