Chip amps used in "push pull"

[ChrisA]

Originally posted by leadbelly


Your definition of a bridged amp is incorrect. The previous respondents are correct. Search on "bridged".

Thanks. You're right. Different terms are used by different groups. I just read National's data sheet. They say "bridged" to "bridged" it is. Thanks again

About the four compartment cabinet. You are thinking hi-fi. Many guitar cabinets are open back but not many bass cabinets. The reason is the instrument amps do NOT re-produce recorded music. They are the source and the amps "color" the sound many times dramatically altering it.

One reason for using one (bridged) amp per speakers is that I'd only have to design one PCB and then I'd have an entire range of amps for 100W up.

It will be a while until I need this. Got a few projects in the pipeline. I'm building an amp now with four 6L6 tubes and then after that a small single end amp.

[AndrewT]

if you decide to adopt an open back cabinet, then your drivers are not sharing the same volume. A failure in one will not affect the others. You will simply lose a bit of SPL.

[kscharf]

Push Pull is a term that applies only to tube amps, or transformer coupled amps.
It means two devices that conduct on alternate half cycles of the input signal.
The internal circuitry of the LM3886 already has two output transistors connected in a totem pole configuration that conduct on alternate cycles, so internally the amp already IS push pull in that sense.

Connected two chips in a bridge (BTL) configuration drives each amp out of phase from the other and the output is taken between the two amps rather than from either amp to ground. The result is that the two amps drive the load out of phase or on alternate cycles of the input signal (similar to push pull) and the output voltages of the amps are in series with the load. By Kirkoff's law this results in double the voltage across the load, which results in 4 times the power into the load vs a single amp. The amps must be able to handle a load impedance of 1/2 the value normally used in a single amp configuration.

It would also be possible to drive the load through an output transformer that would match the load to the chips. In this case the center tap of the transformer might be connected to ground and each side of the primary to it's respective amp. The turns ratio of the transformer would match the load to the amps (the impedance ratio is the square of the turns ratio). You'd probably have to have the transformer custom wound, but it would allow combining two chip amps in a bridge configuration as if it were a push pull amp while not presenting too low an impedance to the chips. A pair of amps could then be run into a 4 ohm impedance from 25 volts ang deliver twice the power. But why go through the cost of a transformer when a parallel connection will do the same thing?

[bacon665]

odd



I thought this was a solid state push pull amplifier....

[AndrewT]

Originally posted by kscharf
Push Pull is a term that applies only to tube amps, or transformer coupled amps..........................
.................The internal circuitry of the LM3886 already has two output transistors connected in a totem pole configuration that conduct on alternate cycles, so internally the amp already IS push pull

Originally posted by bacon665
odd



I thought this was a solid state push pull amplifier....

K started by saying PP ONLY applied and then recanted with 3886 ... IS push pull.
PP applies to the principle and can be SS or tube or transformer coupled

[bacon665]

I see.

[kscharf]

Strickly speaking, push pull applies to a transformer coupled amp.
Complementary output stages aren't EXACTLY push-pull, but the
idea of two devices, each working during one half cycle of the signal does apply.

[bacon665]

out of curiosity... why a center tapped transformer?
cant you tie the plates of a twin triode together and use it as a mixer tube for both halves of the wave?

[kscharf]

Originally posted by bacon665
out of curiosity... why a center tapped transformer?
cant you tie the plates of a twin triode together and use it as a mixer tube for both halves of the wave?

Sure, but that's not a push pull amplifier.
A mixer circuit with a push pull input signal to the grids, a second signal input to the cathodes, and the plates tied together for the output will CANCEL OUT the input signal from the grids and ONLY output the sum or difference between the two input signals. This is a balanced mixer circuit. But this has nothing to do with an amplifier circuit.

[bacon665]

nooo not what i meant.

p1 and p2 are tied together
G1 is tied to push tube g2 to the pull tube
Cathode and heater both get DC