Not necessarily daft because it is speaker safe in the event of a fault, and it could be seen as being more rugged under arduous live/stage use. It is also cost effective.
Beyond being speaker-safe, and *maybe* rugged, it needs only very small drive, so half a '5532 is all the voltage gain needed before the final. (Against this: it needs a separate PT winding and rect and cap-pile for each channel.)
A conventional emitter follower would need 126V p-p drive, so a fairly elaborate driver, what Doug Self calls "VAS".
It can't hold real DC in a load, as has been said.
The NFB will attempt to zero the output DC. Initially there is an offset because two (four) giant caps can't ever match. If they are 10% different it will come up several Volts off-center. The NFB quickly reduces this but on an exponential decay: 3V now, 0.14 later, 0.007V much later.
This is not A Problem. This is a Big Stage Amp. If your speaker can not stand a few-Volt thump and say 0.2V steady, then it sure will not like 60V peaks of program. The say-0.2V is 0.3% of what you plan to hit the speaker with. No speaker is centered to 0.3%. No likely program material has 0.3% (49dB!) peak to average ratio. This isn't even a pea under the mattress.
Yes, we "like" to see <10mV output offset because very simple inexpensive conventional techniques do this consistently, and any other result is "wrong", *for these amps*. But the QSC has bigger trade-offs (high power, low price) and the <200mV settled offset is a reasonable consequence.
I have used the brother of this amp to drive very expensive 2" horn drivers in a music hall. Fearlessly, and no trouble over the years.
Sound? I never noticed. I had the QSC, some Crowns, and a couple old-skool transformer output transistor amps. The OTLs were markedly more transparent (the transformer jobs more versatile and reputedly reliable). Between the Crowns and the QSC, over many working hours listening critically, all I noticed is that the QSC had 3dB more power than the Crown D-150s which sometimes hit the goal with less/no clipping. When not clipping I considered them "the same".
Yes, it may be less wise to use this big beast with little 3-inch 8-ounce shelf speakers. I still do not think the 0.2V offset (which gets lower) is a problem. The turn-on thump, mildly audible in big speakers in a big room, may be frightening on a small cone with the grille off.
A conventional emitter follower would need 126V p-p drive, so a fairly elaborate driver, what Doug Self calls "VAS".
It can't hold real DC in a load, as has been said.
The NFB will attempt to zero the output DC. Initially there is an offset because two (four) giant caps can't ever match. If they are 10% different it will come up several Volts off-center. The NFB quickly reduces this but on an exponential decay: 3V now, 0.14 later, 0.007V much later.
This is not A Problem. This is a Big Stage Amp. If your speaker can not stand a few-Volt thump and say 0.2V steady, then it sure will not like 60V peaks of program. The say-0.2V is 0.3% of what you plan to hit the speaker with. No speaker is centered to 0.3%. No likely program material has 0.3% (49dB!) peak to average ratio. This isn't even a pea under the mattress.
Yes, we "like" to see <10mV output offset because very simple inexpensive conventional techniques do this consistently, and any other result is "wrong", *for these amps*. But the QSC has bigger trade-offs (high power, low price) and the <200mV settled offset is a reasonable consequence.
I have used the brother of this amp to drive very expensive 2" horn drivers in a music hall. Fearlessly, and no trouble over the years.
Sound? I never noticed. I had the QSC, some Crowns, and a couple old-skool transformer output transistor amps. The OTLs were markedly more transparent (the transformer jobs more versatile and reputedly reliable). Between the Crowns and the QSC, over many working hours listening critically, all I noticed is that the QSC had 3dB more power than the Crown D-150s which sometimes hit the goal with less/no clipping. When not clipping I considered them "the same".
Yes, it may be less wise to use this big beast with little 3-inch 8-ounce shelf speakers. I still do not think the 0.2V offset (which gets lower) is a problem. The turn-on thump, mildly audible in big speakers in a big room, may be frightening on a small cone with the grille off.
Thanks for sharing the schematic on this interesting amplifier. Do you have the rest of the manual? I disagree with the remark that "this is just the way it is". There are no tricks or shortcuts with any amplifier and especially with this one. But, you know that the main work of keeping offset low is going to be the input differentials, or in this case the input IC.
No surprise, start with careful d.c. checks, power supply first, and keep notes. Compare one channel to the other. If those checks don't lead you anywhere I would replace the input op-amp on the faulty channel. One thing at a time or else you will get lost. Post results as you go along.
RestAssured
No surprise, start with careful d.c. checks, power supply first, and keep notes. Compare one channel to the other. If those checks don't lead you anywhere I would replace the input op-amp on the faulty channel. One thing at a time or else you will get lost. Post results as you go along.
RestAssured
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