Hi. I have a load of blown PV1500's and 900's. The power amp sections in these amps are built of the topology mentioned in the title, and as a result, the circuit simulates some surprisingly poor signal performance. I have the option to bring these back to original spec or completely hack them. Making serious improvements seem to require trashing the original circuit boards. My dumb question is, does anyone know of a particular circuit of this type that gets better than 80dB down harmonic distortion (at -3db or so), because in my opinion, there's no good reason for a modern amplifier that can't do that, and these don't. :/
Thanks for any input at all...
Thanks for any input at all...
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Peavey's specification was:
"20 Hz to 20 kHz, 10 dB below rated power Less than 0.03%
"20 Hz to 2 kHz, at full rated power - Less than 0.03%"
That's -70dB on my abacus?
The "wacko" topology is not a problem. I was fooling with it long before Peavey made these amps. My main objection is that it is hard to think about. And a few monitoring details get tricky. Performance can be very fine.
That series was Peavey's "value" series; i.e. cheap. The 900 was known as "the bomb" because it would go into thermal runaway. I know the product manager and he says he actually *added* parts (tsk, tsk) for the 2000, and reduced service troubles. I believe this was a minimal change (it would have to be, for a value amp) so you might look to rebuild these as 2000s.
Having my best speakers that have "2nd harmonic distortion down 20 db at all frequencies @ 1w", I don't see the .03% HD of an amp as a problem. 1% HD, now that is clearly audible from my "legendary" dynaco ST70.
Having crawled through a PV-1.3k one blown part at a time I see problems as 1: Thermal protection of a snap action switch on the heat sink, plus a double diode as temp sensor in the idle bias stack, is rather primitive. It cries for a transistor bias limiter.
2. Speaker protection of the triac crowbar is non-existant, as it won't blow the main breaker.
With the invention of the high current nfet after 1990, speaker & even output transistor protection can be an add on board. I put some disconnects in my supply rails. It also detects excessive speaker current and shuts down, a pretty common problem with 1/4 phone plug speaker connections. Ditching the 1/4 phone sockets for banana jacks or speakons strikes me as the most urgent need for upgrade.
All the parts dedicated to the VI limiter I don't quite see the point.
The flying ground took a lot of learning, but is a slick way to provide bridgability without a lot of money. The two separate windings on the transformer for the two channels don't necessarily have to be separated from each other if you decide to redesign. The silence of the copper banded tranformer and the cool faraday cage it is isolated in strikes me as elegant, cheap production engineering.
And as for the op amp, 4558 is just okay. At 50X gain in my disco mixer they were quite hissy, so I imagine the lower level details of a hifi signal are being lost in the hiss. For $1 more than the $.28 they cost there are a lot of better ICs. My op amps were socketed, a cool feature. The feedback wire to control the output signal back to op amp - input are there in the design.
Having crawled through a PV-1.3k one blown part at a time I see problems as 1: Thermal protection of a snap action switch on the heat sink, plus a double diode as temp sensor in the idle bias stack, is rather primitive. It cries for a transistor bias limiter.
2. Speaker protection of the triac crowbar is non-existant, as it won't blow the main breaker.
With the invention of the high current nfet after 1990, speaker & even output transistor protection can be an add on board. I put some disconnects in my supply rails. It also detects excessive speaker current and shuts down, a pretty common problem with 1/4 phone plug speaker connections. Ditching the 1/4 phone sockets for banana jacks or speakons strikes me as the most urgent need for upgrade.
All the parts dedicated to the VI limiter I don't quite see the point.
The flying ground took a lot of learning, but is a slick way to provide bridgability without a lot of money. The two separate windings on the transformer for the two channels don't necessarily have to be separated from each other if you decide to redesign. The silence of the copper banded tranformer and the cool faraday cage it is isolated in strikes me as elegant, cheap production engineering.
And as for the op amp, 4558 is just okay. At 50X gain in my disco mixer they were quite hissy, so I imagine the lower level details of a hifi signal are being lost in the hiss. For $1 more than the $.28 they cost there are a lot of better ICs. My op amps were socketed, a cool feature. The feedback wire to control the output signal back to op amp - input are there in the design.
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Thanks for the comments. The amps are failures / pulls from various old theater installations. They were chosen on the basis of price at the time. I can't argue about Peavey's specifications, but I tried absolutely everything in LTSpice to get this circuit to have less than -40dB distortion at half power.
(Just to mention, that -40 db was with +20 to +30dB signal level, so Peavey's numbers are probably realistic. Apologies for my error. I'm just used to seeing the distortion peaks appear Way further down)
It's not terribly hard to understand the circuit. Signal quality-wise I'm pretty sure the relatively heavy distortion is a direct effect of using the output transistors for voltage gain, an invitation for more curves in the transfer function.
To be perfectly honest, these amps never did sound that great, even though they were direct driving a huge active 4-way system with no horns and sealed woofers. The customer wanted reliable SPL, and for the duty and age of the system I think they came through, although zero failures would have of course been dandy.
Believe it or not, I think the only reason the 1500 blew out was one of the rail clamp diodes spontaneously shorted. In 10 years of repair service I'd never seen that. The opposing polarity transistor bank was shorted and all their emitter resistors burnt open. It chewed the first two stages of the triple as well. Put in a bunch of fresh parts and everything is okay. I think I'll be leaving well enough alone now..
(Just to mention, that -40 db was with +20 to +30dB signal level, so Peavey's numbers are probably realistic. Apologies for my error. I'm just used to seeing the distortion peaks appear Way further down)
It's not terribly hard to understand the circuit. Signal quality-wise I'm pretty sure the relatively heavy distortion is a direct effect of using the output transistors for voltage gain, an invitation for more curves in the transfer function.
To be perfectly honest, these amps never did sound that great, even though they were direct driving a huge active 4-way system with no horns and sealed woofers. The customer wanted reliable SPL, and for the duty and age of the system I think they came through, although zero failures would have of course been dandy.
Believe it or not, I think the only reason the 1500 blew out was one of the rail clamp diodes spontaneously shorted. In 10 years of repair service I'd never seen that. The opposing polarity transistor bank was shorted and all their emitter resistors burnt open. It chewed the first two stages of the triple as well. Put in a bunch of fresh parts and everything is okay. I think I'll be leaving well enough alone now..
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It's the same core amplifier either way. With speaker load and tenths-Ohm emitter resistors, gain is say 20 and THD may be 10%. Driven/loaded as Emitter Follower, all the Gv=20 goes to reduce THD so it may be 0.5%. Taking a gain of 5, there's only 4:1 NFB so THD may be 3%. However you do have an added gain of 5 which may be used inside the overall NFB loop so the THD nominally comes to about the same thing. There may be real trouble with NFB stability above audio midrange, and the cure may lead to rising THD above 1KHz. Which is implied by Peavey's specs. And is not generally a problem since speech/music does not run to full power over a few KHz.
It ought to be "fine" in theater work. There's a lot of audio processing in theater, and most theater owners do not care. That's before whatever "features" Peavey threw into this model. (I don't know the feature-set, but clipping protection was popular, and if it works even a little a sharp ear will know the sound has been tampered, while the popcorn-eater is just glad the system does not go SPLATT.)
Hifi is swell for a home system, but when was the last time you heard anyone leaving a disco or even a movie theater complaining the midrange was a little grainy and ther was at least a 10 degree phase error in the tweeter?
These amps are inexpensive, and mostly they work. They may lack certain refinements, but mostly people are happy with them.
Peavey uses their DDT system, which is an anti-clipping limiter. It is transparent normally, but once clipping threshold is reached, it dials down the signal to prevent further increase in output. Sounds a heck of a lot better than clipping.
These amps are inexpensive, and mostly they work. They may lack certain refinements, but mostly people are happy with them.
Peavey uses their DDT system, which is an anti-clipping limiter. It is transparent normally, but once clipping threshold is reached, it dials down the signal to prevent further increase in output. Sounds a heck of a lot better than clipping.
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