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Old 19th December 2008, 01:49 PM   #11
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Default cannot answer that OS

Quote:
Originally posted by ostripper
Sakis, what purpose is r46 ,C24..??
OS
this is not my design .....

what do you have against 2N5401 andrew ???? and also do you have any suggestion for replacement ????

your comment about 56 volt rails and 2 pairs driving 4 ohms is very correct i think its almost risky to do so ....
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Old 19th December 2008, 02:10 PM   #12
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ahh , more H2/3 distortion (not much).. but faster amp
loop gain unity and square wave better with CFP..
OS
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Old 19th December 2008, 02:20 PM   #13
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Quote:
Originally posted by AndrewT
2pair of 1943/5200 on 56Vdc supplies cannot reliably drive a 4ohm speaker.
Even an 8ohm speaker would need to be of moderate reactance to keep this output stage reliable.
It will drive a 12ohm severe reactance speaker.

reduce the supply voltage <=50Vdc

and get rid of those 5401 in the input LTP and the driver being used as a VAS.
Have you tried it and have had reliability problems with it or just looking at the SOA plots?

I'd agree it maybe won't take full power sine into a serverly reactive |Z| = 4 ohm load at low frequency, but with music and a speaker (Re Z > 3 ohm or so most of the time) you would be ahead of many amps in terms of SOA with two pairs.

A NAD2200 has 2 pairs of similar devices and +-60V low voltage supply and +-90V high rail (rail switching). It's even rated to drive 2 ohm loads, but I wouldn't push it that far though...

The schematic presented does have a major flaw - no emitter resistors. There is not a chance of current sharing at +-56V without emitter resistors...
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Old 19th December 2008, 02:25 PM   #14
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by megajocke
Have you tried it and have had reliability problems with it or just looking at the SOA plots
I'm referring to SOAR plots. when the devices are near cold, i.e. with a very large heatsink.
Using 100mS rating rather than my preferred DC or 1second rating at 55degC.
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Old 19th December 2008, 03:12 PM   #15
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Default never tested this amp

it is here for evaluation ....

thank you
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 19th December 2008, 03:42 PM   #16
alex mm is offline alex mm  Europe
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Default never tested..? look here...

Hi,
maybe, this picture will say something else http://i44.tinypic.com/jzdu35.jpg and PCB http://i40.tinypic.com/107q2hj.gif .
Regards alex mm
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Old 19th December 2008, 04:56 PM   #17
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Quote:
Originally posted by AndrewT
I'm referring to SOAR plots. when the devices are near cold, i.e. with a very large heatsink.
Using 100mS rating rather than my preferred DC or 1second rating at 55degC.
Nothing wrong with being conservative of course but I believe your face will turn pale in horror if you compare the SOA plots to what is done in most commercial amps out there (especially PA amps like QSC and Crest which use these devices)

Still, they work and are reliable... Modern transistors are much more immune to second breakdown than the old types. Temperature is mostly the limiting factor for them if voltage is relatively low. And really nothing special happens at 150 (though the plastic in the transistor case won't like more than 150 degrees average junction temperature) or 200 or 250 degrees C junction temperature, just that life is reduced (pretty fast though) the hotter it gets. So an occasional unlikely transient bringing the chip to 250 degrees won't hurt it. IIRC the thermal protection for the output array in the LM3886 (or was it the LM12?) activates at 225 or 250 degrees. It also has protection at 150 degrees for the low level stuff because that part won't work correctly at high temperature.

But that is also unlikely to happen, those huge power dissipation peaks that come with reactive loads are only about 5ms long even for the lowest audio frequencies.

One needs to be careful with emitter resistors though... Too small for the power supply voltage and thermal circuit and you will have runaway positive thermal feedback if the transistor gets too hot... Not good.
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Old 19th December 2008, 05:03 PM   #18
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.22R no good for this setup??, it is the most common value
in commercial/consumer devices.
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Old 19th December 2008, 05:05 PM   #19
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Yes, at this voltage it usually is... But this amp has no emitter resistors at all. Current sharing between output devices is going to be non-existent.

Bias current will probably be stable though as it is a CFP!
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Old 19th December 2008, 05:30 PM   #20
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by megajocke
I believe your face will turn pale in horror if you compare the SOA plots to what is done in most commercial amps out there
I did buy a cheap PA style amplifier to find out what was inside it.
I did turn pale in horror. 4pair of 1943/5200 running on +-90Vdc supply rails. I would not use it on 8ohm speaker and yet the importer specifies it for use with 4ohm speakers. I have never connected it to any of my speakers for fear of damaging them if the output stage failed to rail.

4pair of MJ21193/4 would be very acceptable but cannot be fitted to the heatsink.
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