stranger than fiction audio output stage...

[unclejed613]

i didn't want to hijack a thread to post this, but there was a discussion in another thread about grounds and power supplies and power amp circuits. i stated that there were some very strange designs that looking at the schematic , don't look like they would work at all, but they do, and are very successful designs. here's one of them. the amp is made by QSC, which uses this design in almost ALL of their PA amps, and it is so successful that other companies have copied it shamelessly. in this design, the power supplies float, and the supply common is NOT ground, but is the speaker output. the output transistors are grounded, and the collectors attached to the rails. the amp works by using the output devices to "walk" the power supply rails in reference to ground in unison with the audio input, and so the power supply common also changes voltage with the supplies. the negative feedback is applied to the noninverting input of the input stage (which in this case CAN BE an op amp running from +/-15V rails driving the driver transistors directly since drive current with hardly any voltage swing is required to drive them)



notice the transposing of the ground, and that the power supplies are floating in the QSC amp.

 

[Nigel Goodwin]

As I've mentioned in the other thread, ground is completely relative, you can choose pretty well any point you like and use that as a reference - it's just that some places make more sense than others.

I don't know as people have copied QSC's circuit, I suspect it's more a question of buying amplifiers from the same Chinese manufacturer as QSC do, and it may even be a Chinese design?.

I don't see as it gives any particular advantage?, although it could be to stop people trying to bridge them?.

 

[unclejed613]

QSC is an American company. interestingly enough, American Audio is a chinese company. Samson is an American company, and Behringer is a European company (at least they were at one time) and Tapco was a Japanese company (probably chinese now). all of these companies make at least a series of amps with this design (and QSC and Behringer also make class G versions). one of the advantages of the design is that the input and driver stages can be run from low voltage rails, and only the output devices need to be able to withstand higher voltages. another advantage is that the speaker is coupled through the filter caps in the power supply, and DC offset is nearly impossible. overcurrent detection is also simplified since the common point of the emitter resistors is at ground potential. both this and the standard output stage are known as "half-bridge" output stages, as opposed to a "full-bridge" amp where both output terminals are driven, 180 degrees out of phase. this design with floating supplies has it's roots in an Electro-Voice series of tube amps that used cathode drive of the output transformer. the two output tubes had floating supplies. when drawn as a simple conceptual schematic, the tubes and power supplies appear to be all in series in a circular fashion, hence the nickname "circlotron" for the circuit. so this type of design is neither "oddball" or new. i'll admit it's a bit of a "Whiskey Tango Foxtrot" moment the first time you put a scope probe on the power supply rails and see full output audio riding on them...

 

[Jony130]

I don't see as it gives any particular advantage?

Now you get a voltage gain from emitter follower.
So you can drive the output stage from the circuit that has the lower supply voltage.
So know you can drive the output stage with opamp supply only from ± 15V.
https://www.electro-tech-online.com/custompdfs/2010/03/usa370.pdf
And Peavey a similar technique.
PCB Update & Back to Power Boosters (Diamond Buffer Topology)
High perfomance audio power ... - Google Ksi??ki

 

[Nigel Goodwin]

QSC is an American company. interestingly enough, American Audio is a chinese company. Samson is an American company, and Behringer is a European company (at least they were at one time) and Tapco was a Japanese company (probably chinese now). all of these companies make at least a series of amps with this design (and QSC and Behringer also make class G versions). one of the advantages of the design is that the input and driver stages can be run from low voltage rails, and only the output devices need to be able to withstand higher voltages. another advantage is that the speaker is coupled through the filter caps in the power supply, and DC offset is nearly impossible. overcurrent detection is also simplified since the common point of the emitter resistors is at ground potential. both this and the standard output stage are known as "half-bridge" output stages, as opposed to a "full-bridge" amp where both output terminals are driven, 180 degrees out of phase. this design with floating supplies has it's roots in an Electro-Voice series of tube amps that used cathode drive of the output transformer. the two output tubes had floating supplies. when drawn as a simple conceptual schematic, the tubes and power supplies appear to be all in series in a circular fashion, hence the nickname "circlotron" for the circuit. so this type of design is neither "oddball" or new. i'll admit it's a bit of a "Whiskey Tango Foxtrot" moment the first time you put a scope probe on the power supply rails and see full output audio riding on them...

I know QSC are an American company, but these amplifiers are made in China - and may well be designed there?, as (like you said) various other manufacturers sell pretty well identical ones - in fact you can use the QSC circuit diagrams to repair Behringer versions

 

[unclejed613]

actually it's not voltage gain from an emitter follower (common collector), but the output transistors are actually operating as common emitter amplifiers. that's where the voltage gain between the drivers and output. the speaker terminal is inverted with respect to the input and drive voltages, and that's why the negative feedback goes back to the noninverting input.

the advantage of having low voltage drive is that i can significantly simplify the amp to having an op amp driving a pair of mosfets, and the mosfets drive the output devices. there is a big disadvantage, however. each channel of the amp needs it's own floating power supply.. this can come from two separate windings on the same transformer as shown in the QSC schematic, or (to satisfy the "purists") from two separate power transformers. a transformer with dual supply windings is more expensive than a transformer with only one. a second bridge rectifier and pair of supply caps also increase cost as well as taking up more chassis real estate..


unfortunately, just about everything is made in china these days.... here in the US we have taxed our stateside manufacturing into oblivion, and given tax breaks to companies that have stuff made "offshore"......

 

[Nigel Goodwin]

here in the US we have taxed our stateside manufacturing into oblivion

Same here