TillEulenspiegel said:
Peavey had more designs then Hienz has pickles. Most of their power heads use 3055s or similar. They have some of the weirdest ◊◊◊◊ you will ever see. The main designer (Hartley Peavey? ) puts some of the oddest circuit designs out , I don't know why he does half of the stuff he does.
One clusterfuge is that he likes to separate chassis ground from circuit ground...now this is not medical electronics folks. That causes soooo many problems. The best one tho is in the CS-400 and CS-800 he put in a crowbar circuit AT THE OUTPUT stage! In other words if there is a dc current at the output the SCR shorts the outputs (all of them ) to ground instead of popping the fuse all the transistors must be replaced. Ugh
The triac crowbar circuit is a rip-off of the BGW 750 series. It's actually a good idea; if the amp develops a fault that jams the output against one of the supply rails (a shorted output or driver can do this; I've also seen a couple of CS-800s where a resistor in the +&-15V supplies, which are derived from the main rails, opened up, killed one of the LV rails and sent the output up against one of the HV rails) it will try to put 80VDC through the load.
It's a lot less expensive to replace output transistors than it is to recone whatever speakers it was connected to at the time. BTW, in my experience if the original fault was a shorted output device then the fuse will blow immediately, if the original fault was in the front end it's usually only one output device that dies, along with the triac.
Peavey doesn't seem to be terribly strong on originality. If you examine the sches for the pre-facelift CS-800 you'll see that the circuit topology is pretty much a dead lift from the Crown DC-300A, scaled up a bit in output voltage and current capabiity. Even the output stage current-limiting circuitry is lifted from Crown's SPACE circuit.
The one spot in that product line where Peavey came up with something both original and cool is the DDT distortion detector. It works on the same principle as most power amp clipping detectors- monitoring the magnitude of the amp's internal error signal- but Peavey came up with a neat way of arranging the gains of the front end, back end and overall amp gain to permit the clip detector to cancel out the "normal" error, allowing for a more sensitive detection of abnormal conditions.
Incidentally, the internal grounding scheme on the CS-800 actually uses the chassis as the ground reference; the power supply common is connected to chassis at the speaker terminals, and the front end takes its ground from the chassis via the input connectors; front end and output stage commons are joined internally by a low-value (2.7ohm, IIRC) resistor. This arrangement prevents output current returning to the power supply common from producing a potential difference between the output stage ground and the front end ground and is also found in the DC300A and other power amps.
While I'm on a subject I actually know something about (I've only been fixing pro audio gear for twenty years now), I'll add- if you're in the market for an audio power amp and value things like reliability, serviceability and good warranty policies-
CROWN, CROWN, CROWN, CROWN, CROWN.
(Opinion based on several years of working in a shop that did warranty repairs on pretty much all of the major brands of pro audio equipment)
And, to get to the original subject of the thread, a browse through the
Mix Bookshelf catalog shows several books that are likely to be helpful to someone wanting to learn about troubleshooting audio amps. Check the "amps" and "electronics" categories in particular.
