mlloyd1 said:
jacco vermeulen said:
Very good guess mlloyd1 and jacco nailed it!
Sure enough this is the beastly Perreaux PMF5550.
Another boat anchor pulled from the "trash".
Now, before I get too deep into yet another big amp repair (the Rowland Model 7s I am repairing are still in progress), is this thing even worth fixing from a modern day performance perspective?
mlloyd1, I have only put a meter on the outputs so far and the AC voltage is very low in both channels (2mV).
I have only had a chance to clean it up (years of dust and cobwebs inside), check for any toasted parts, check the power supplies, etc.
Any ideas how to measure bias on this design with no source resistors?
Any idea what the correct bias current is for this amp?
Thanks!
ostripper said:
Michael Chua said:
I was thinking the same thing, but surely the same error could not have been made on more than one Perreaux schematic?
See this post for an identical DC offset arrangement:
http://www.diyaudio.com/forums/showthread.php?postid=895876#post895876
I should also note that I find it strange that both channels are showing a nearly identical high DC offset.
I will go through and check/replace all of the small electrolytics first to see if leakage is the issue.
jacco vermeulen said:
jacco, you don't happen to have any service documentation for this or any other Perreaux amps of this vintage, do you?
I read in another thread about setting bias on these amps that the prodcedure is a little more involved than normal for setting DC offset and bias. Something about using an ammeter in place of the rail fuses, etc.
Thanks!
If there are no emitter resistors to measure the output stage bias, then solder 10r0 across the fuse holders in the supply rails. Tiny 1/8W or 1/4W resistors are ideal here. Leave them in permanently. They will blow with the fuses on abuse/overload/failure.
Remove the fuses and measure the volts drop across the 10r0.
This gives the quiescent current of the amplifier. Output bias current < amplifier quiescent current.
If the quiescent current >100mA then use 4r7 or 5r1 resistors.
Remove the fuses and measure the volts drop across the 10r0.
This gives the quiescent current of the amplifier. Output bias current < amplifier quiescent current.
If the quiescent current >100mA then use 4r7 or 5r1 resistors.
help please cause here i miss something
OK !!!! more or less everybody knows that my theory is not really enough .... but looking at this schematic there is a lot of things that i dont really understand ....
A)what about output resistors ..... amplifier i worked with (Legend4) didnt include any and all forum members were screaming about this ..... similar to this is the discusion about the JOY 200 amplifier ( an sziklai with 4 out transitors with out any out resistors ) so what's the catch here ????
B) since legend 4 was didnt include ccs in the vas or ltp stages was called a "POOR " design and not really stable as far as i can see from this schematic at least in the ltp stage there is no ccs here also . ......
C) can somebody tell me if this design is stable ???? then again how much power ???? and what is the rail voltage ...????
D) is this amplifier takes as a very strickt fact that outputs must be very well matched ????? or wtf is going on here ?????
answers please i am very confused now .....
OK !!!! more or less everybody knows that my theory is not really enough .... but looking at this schematic there is a lot of things that i dont really understand ....
A)what about output resistors ..... amplifier i worked with (Legend4) didnt include any and all forum members were screaming about this ..... similar to this is the discusion about the JOY 200 amplifier ( an sziklai with 4 out transitors with out any out resistors ) so what's the catch here ????
B) since legend 4 was didnt include ccs in the vas or ltp stages was called a "POOR " design and not really stable as far as i can see from this schematic at least in the ltp stage there is no ccs here also . ......
C) can somebody tell me if this design is stable ???? then again how much power ???? and what is the rail voltage ...????
D) is this amplifier takes as a very strickt fact that outputs must be very well matched ????? or wtf is going on here ?????
answers please i am very confused now .....
It is balanced , I have built/ simmed this topology and if properly laid out and compensated it CAN be very stable.. seems to run off
of +- 118 V rails
and 113v for VAS/LPT. CCS wouldbe better for PSRR.
Outputs would have be matched.. A wire is very bad at
current sharing.. this is the same as the HH1200MX power amp
but a little more crude. (see attachment)
OS
Attachments
thanks OS
but your answers dont look really enough ......
what i mean is ....How come an amp like that runing at so much high rails doesnt requier CCS and out resistors ????
the circuit i was working with had only 4 mosfets and rails at max of 60 volts and i never managed to make it work propelry ( forum members also tryied to advice me but always we end up that deisgn is poor since dont have resistors vas and ltp without CCS )
what do you think that is the expected output of this amp ???? my calulation say that 500W is very possible given as a fact a very good psu ...am i correct ????
but your answers dont look really enough ......
what i mean is ....How come an amp like that runing at so much high rails doesnt requier CCS and out resistors ????
the circuit i was working with had only 4 mosfets and rails at max of 60 volts and i never managed to make it work propelry ( forum members also tryied to advice me but always we end up that deisgn is poor since dont have resistors vas and ltp without CCS )
what do you think that is the expected output of this amp ???? my calulation say that 500W is very possible given as a fact a very good psu ...am i correct ????
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