[R-390] OT: Pass Transistor Question

[Ian Gallimore]

Has anyone had experience using power mosfets as pass elements? From what 
I've read, equalizing resistors are not needed, but I wonder, being REALLY 
CONSERVATIVE, if a small resistor, say .1 ohm, in each source lead might be 
a good idea. I have a plan germinating to re-do a big old GRC PSU putting 
out 28V at 50 A as a variable voltage PSU using a multiturn pot, a 
three-terminal Voltage regulator and mANY parallelled power fets as an 
improvement on an SCR pass element.

Ian VA3ODA
----- Original Message ----- 
From: "ews265" <ews265 at rochester.rr.com>
To: <r-390 at mailman.qth.net>
Sent: Friday, October 17, 2008 7:00 PM
Subject: Re: [R-390] OT: Pass Transistor Question


> Don,
>
> Thanks for the info on the MJ15003s.  I've already purchased a handful or 
> 2N3773s since they were cheap and available so I'll give them a shot 
> first.  They are Brand X by the way.  Could be one of the thousands of 
> processes Motorala must of spun off to the 2nd line fab houses.  If I end 
> up with flight at/near current limit I won't be too concerned since I 
> don't plan to run the amp anywhere near there anyway.  And yes I am the 
> curious type.  Sounds like this could turn into an interesting exercise.
>
> So how did Crown Audio get the kind of recognition they received?  Given 
> the parts available at the time, dealing with basic design problems is one 
> thing.  The packaging  however is such a disappointment.  I think I could 
> have done better in my garage.
>
> Regards,
> Jon  WA3MVM
>
>
>
> 2002tii wrote:
>> Jon wrote:
>>
>>> This is an old beast that I just happened upon that has one bad channel. 
>>> Right now I'm resisting working on it just for the mechanical reasons 
>>> you describe.  Sounds like I don't need to worry about matched/selected 
>>> parts and your comments do explain the parasitic suppression components. 
>>> Also sounds like the older transistors may have had a lot of phase shift 
>>> at their upper limits with the potential for creating havoc with the 
>>> amp's feedback loop.
>>>
>>> The mechanical packaging really is pretty bad.  I was unpleasantly 
>>> surprised.  Documentation's nothing to write home about either.  Digging 
>>> in to the docs still leaves ???  Anyway I'll be sure to sweep the output 
>>> for any signs of life after I get it back together.  Any special 
>>> conditions that's apt to make it go into flight; level, freq etc?
>>
>> In all honesty, unless you are just obstinately curious, if you haven't 
>> put the effort into it yet -- don't bother.  After hassling with the 
>> worst mechanical design you've ever experienced in an electronic product, 
>> you will have a thoroughly mediocre amplifier (at best).  Sell it on eBay 
>> and buy an Ashly FET-200/2000 or a Bryston 4B.
>>
>> If you insist on repairing it, use MJ15003 output transistors (real 
>> Motorola ones) instead of the original 2N5631s.  If the TO-66 drivers are 
>> broken, good luck -- very few manufacturers make TO-66 packages these 
>> days.  You can make a TO-220 transistor fit, but being plastic-cased 
>> transistors, they don't last.  Whatever you do (including using stock 
>> parts), be prepared to adjust the compensation to maintain stability. 
>> (Note that Crown provides two schematics, one for each brand of output 
>> transistors they used.)
>>
>> Run it up to clipping without a load, with an 8 ohm load, and with a 4 
>> ohm load, using any convenient sine wave in the 500 Hz to 2 kHz range. 
>> Look for bursts of oscillation as you do this (it will appear at the same 
>> spot each cycle -- check particularly as the amp enters and leaves 
>> clipping).  Don't push it very far into clipping without a load.
>>
>> Do not try to reach full power above 10 kHz unless you are willing to fix 
>> it again.  I generally ask power amplifiers to drive 2 kHz square waves 
>> into an 8 ohm load resistor paralleled by a variety of capacitors from 
>> 0.001 uF to 1 uF, but Crowns will not tolerate this, either.
>>
>> Good luck!
>>
>> Don
>>
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