gonna to build clone of SWR750 poweramp, have some questions

[nevetslab]

Not on a Quasi-Comp amp circuit like you have. Q12, Q13 & Q14 are all in parallel, each with their 0.10 ohm resistors. Only if you were to re-build the output stage as a complimentary circuit, having NPN's and PNP's can you do that. BUT....the transistors still MUST be insulated, as the collector is NOT at Ground, even though at idle, it is at 0V. That output bus swings from V+ to V- as the transistors get close to saturation at full clipping level..

Of course, you're asking a loaded question. Is this being built in the Crown chassis with this SWR 750 circuit, and the heat sink is electrically isolated? I'd have to really look at the Crown circuit before passing judgement on that. Usually on circuits like this, the heat sink IS grounded

[jeeptechfred]

The heat sinks will be insulated, the crown had them mounted to the pcb. I am not reusing the pcb but will insulating the heat sinks from the chassis. I attempted to add the schematic to the crown, might be crumby cause I’m useing my phone. I suspect it is good enough to get the idea.

[nevetslab]

Is that schematic for the Crown CE1000? interesting compound driver stages up and down.

[jeeptechfred]

Yes that is the ce1000 schematic, well actually it is the ce2000. The only difference I can see is that the rails are 99v instead of 85v and it uses 6 output devices in place of 4. I have the service manual with all the schematics and I uploaded the wrong one

[jeeptechfred]

I finally got the amp together and it is functioning. I can run the amp up until it clips with my signal generator. It seems very stable and sounds good. I used mje15033 and mje15032 for drivers and I used all four pairs of the crown outputs mje21194. None of the components appear to be over heating. I was able to do fair amount of testing without even noticing any significant temp increases. I used an 8 ohm load and got an easy clean 250 Watts. I want to run a 4 ohm load and I have an appropriate dummy load. Any ideas on how I should go about testing it for ruggedness? Are there any warning signs or tests I can perform to see if it is on the edge?

[J M Fahey]

1) CONGRATULATIONS on your build

2) sadly ruggedness tests are destructive.
A semiconductor is either alive or dead, and it takes 1 millisecond to go from one to the other.

Factories build , say, 10 prototypes and torture/short/cook/kill them one by one, and then perform an autopsy to find ways to correct them.
What they find then applies to thousands of amplifiers; sadly that is not practical when dealing with "just one" , so just build it according to "rules of the art", be conservative, use good parts, and pray.

First 3 factors being the significative ones.

Just a small detail: not too sure about the airflow in your heatsinks.

[jeeptechfred]

The heat sinks are basically arranged just as the original crown heat sinks are they are designed that allow air to flow through them. There are a series of louvers. I’m hoping that since they worked in the crown amp the work and this one

[nevetslab]

At BGW Systems, we did much like JM Faheysaid....a number of prototypes, ran them thru their paces, pushed at elevated temperaturs to see what failed. I had a 4X15" subwoofer (from our M2200 Powered Subs line), and used those to look at how the current limiter circuits worked under high current transients. The current limier circuit in what you built is unusual. sense current taken only from one of the positive half output xstrs, feeding the upper Current limiter circuit, while operating on both the positive and negative clamp circuits to remove drive from the upper/lower driver circuits.

Over the years, I've found 40Hz/50Hz & 63Hz 1/3 octave pink noise as an interesting test signal for driving large bass bins in checking for abnormal current limiter circuit behavior. The pink noise thru those LF 1/3 octave filters produces wide random-amplitude sinewave, with amplitude variations as much as 15-18dB. Looking to see how the amp handles back-emf from a loudspeaker load was one of the things we looked at....often an overlooked behavior in current limiters.

[J M Fahey]

Yes.
Heavy cone+voice coil woofers driven at or near resonance frequency are *deadly* :
They absorb a lot of electrical energy to start moving and when fully stretched "springs" (suspension, edge, air elasticity) push moving mass back with fury, they *generate* electrical power , voltage and current peaks which get injected backwards into power transistors, stressing or damaging them.