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Old 9th August 2007, 02:42 PM   #1
AMV8 is offline AMV8  United Kingdom
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Default Musical fidelity P270 and A370 fets

Hi

I am rebuilding 2 A370's and 2 P270's and would appreciate help with replacing the output devices.

In rebuilding I have decided to change the output devices as these run very hot. (In the rebuilt ones I will run them at about 50 degree centigrade as the mark 2 versions of each ampifier did.)

At present the amps all contain different output devices as follows;
P270 - 2sk1058 and 2sj 162 ( sound very good )
P270 - 2sj83 and 2sk227
A370 - buz - 905p and buz 901p
A370 - 2sj118 and 2sk413 ( sound good )

I would like to standardise on the output devices so that I can match sets for each amplifier economically and without having to buy different devices for each amplifier. I have a LOT of irfp240 and irfp9240 ( which I like the sound of ) and would prefer to use these if possible - although they have a greater capacitance. Has anyone tried these in the MF amps?

Otherwise any advise on the best FETS to use would be appreciated.

Don
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Old 9th August 2007, 03:08 PM   #2
AndrewT is offline AndrewT  Scotland
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Forget the vertical FETs unless you are tooled up to properly debug the resulting abomination.

1058/162 are easy to get and can run fairly hot, but they are plastic devices so be sensible.

I got 16pairs a while back and manged to get 7 well matched sets of four, the Nchannel are consistently closer than the Pchannel . The 8th set had one device that was a few more mV different (well actually less mVgs) from it's partners and it shows up as extra quiescent current in that odd device.
I can only run 75mA per device otherwise the sinks get too warm.
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Old 13th August 2007, 08:29 AM   #3
AMV8 is offline AMV8  United Kingdom
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Andrew

Thank you for your reply. It is very helpful.

Have you tried the buz 905 and buz901? If you have any comments on those devices that would aso be helpful.

Don
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Old 13th August 2007, 08:39 AM   #4
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Hafler used Exicon after Hitachi quit making their parts.

http://www.profusionplc.com/subsites/exicon.htm

The plastic only run GBP 4.15~5 in small quantities.
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Old 13th August 2007, 03:57 PM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by AMV8
Have you tried the buz 905 and buz901? If you have any comments on those devices that would aso be helpful.
Hi,
no experience of any of the other Lateral FETs.
I have heard that they are all very similar and probably made under licence from Hitachi.
The specs from the old To3 devices to the whole range of current devices are VERY close.

I wonder if the same silicon shop is selling wafers off to be cut up and soldered to the backplate of these different "manufacturers"?
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Old 13th August 2007, 04:51 PM   #6
AMV8 is offline AMV8  United Kingdom
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Andrew

Thanks again.

I will try the ones you suggested earlier. I might also have a go with the vertical fets since I have a lot already.

In general it is the front end of the P280 and A370 that has an unusual IC arrangement and ( I find ) sensative to changes. The output architecture is fairly standard.

I will be making changes in any case to run the amplifiers at lower temperatures - about 55 degree C - as the MK2 versions did.

Don
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Old 14th August 2007, 07:25 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
Tc=60deg C is very different from running the heatsinks at 60degC.
If the sinks were at 55degC the worst Tc at quiescent could be >=65degC

What I am saying is that 55degC is very hot if you want good SOA from these plastic devices.
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Old 14th August 2007, 09:00 AM   #8
AMV8 is offline AMV8  United Kingdom
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Andrew

I hope you do not mind my asking for more informastion but what heatsink temperature would you recommend?

The original versions of the p270 and A370 ran at high heatsink temperatures - around 70 degrees. The mk2 versions ran at around 55 degree heatsink temperatures. I had planned on 55 degrees after rebuilding but I can go lower if that is advisable.

Do you in practice recommend 75ma per device as you mentioned earlier. This would drop the temperature considerably but limits the class A operation.

Out of interest this is about the current that the Dr T Mk2 used and it sounded very good. The circuit on that amplifier was the same as the P270 but with resisters to drop the voltage for the input circuit instead of the pass transistor that the P270 used.
The Dr T Mk2 used 2 output devices instead of 3 in the P270. However the DrT Mk2 sounds a little thin compared to the P270.

Don
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Old 14th August 2007, 12:11 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
how hot your run the FETs is determioned by the loadline (the number of parallel devices) and the temperataure de-rated SOAR. For four pair on +-69Vdc and Tc=50degC the amp can safely drive a 60degee phase angle 8ohm load to about 195W until the heatsink starts to rise even higher.
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Old 14th August 2007, 02:09 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
had to dash to a meeting.
The 75mA I ran was limited by the dissipation of the sinks.
Most designers say that FETs get better as the bias is raised all the way to full ClassA.
Borbely states 100mA minimum per output pair and 500mA total minimum Ib.
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