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Old 10th July 2011, 06:49 PM   #1431
mikelm is offline mikelm  England
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Quote:
Originally Posted by srh View Post
John Curls recommendation is an inductor of 1uH. JLH said the only way to do away with the inductor was to use mosfets in a CFP with small sig bipolars
Well I'm sure they said different things at different times but I read from John Curl that you should design so as to avoid the need for o/p chokes if you possibly can but if you really can't manage that then at least keep the inductor size down as much as possible. He was quite clear in his opinion that an o/p inductor spoils the sound. I realise many here don't like to hear this point of view expressed but that does not mean he didn't hold it

The JLH Wireless World published 80-100 Watt amplifier had MF output stage with no CFP and 0.22 o/p resistors plus snubbers - so it looks like he proved himself wrong with his most accomplished design !

mike
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Old 10th July 2011, 07:09 PM   #1432
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Andrew, you may want to include Source resistors for the Outputs. I have also faced a similar rise in offset voltage, when raising bias in an amp with Vertical Mosfets; I found that 100mA worked well in that design and set both channels. Incidentally I also eliminated Source Resistors. I'd also suggest that you try changing the Output Gate resistors to 680E and 470E respectively.
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Old 10th July 2011, 07:11 PM   #1433
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Originally Posted by Ian Finch View Post
I don't suggest that you can't design for these components to be eliminated, as Sonny advocates but I haven't seen it and I wonder how this new strategy might also affect sonics.!
JLH Simple class A, 1969 & JLH 80-100 watt amplifier published in the 1980's - not such a new strategy really.

I wonder do you refer to DNM ?

Unlike Mr Moorecroft I prefer tightly twisted speaker leads for several reasons including reduction of RF pickup but mostly because to me, they just sound better.

mike

p.s by the way add the Nelson Pass Firstwatt F5 to that list

Last edited by mikelm; 10th July 2011 at 07:22 PM. Reason: addition
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Old 11th July 2011, 02:12 AM   #1434
AKSA is offline AKSA  Australia
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Andrew,

Just saw your post.

I believe latfets do require more stabilisation of bias current...... it is only beyond about 500mA that the tempco becomes negative. Whilst cold in particular the bias creeps up rapidly and can spike before temp brings it down again.

For this reason on my pcb I have used three series 4148 diodes which can be mounted on the reverse side of the pcb, facing the heatsink, giving a thermal feedback mechanism through direct radiation and some convection. I have also added source resistors, and would actually prefer to use 0.22R. This is, I believe, a belts and braces approach.

Cheers,

Hugh
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Last edited by AKSA; 11th July 2011 at 02:15 AM.
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Old 11th July 2011, 03:10 AM   #1435
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Hugh, the indicative figure of 500mA and above to achieve negative tempco is doubtful in practice. Elektor and other designs have routinely used 100mA per output pair for the Hitachi Lateral Mosfets of which I have built too many and have never seen this phenomenon of rising bias or offset.
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Old 11th July 2011, 03:29 AM   #1436
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I think a good guide for the L/R is to use for R the minimum resistive load the amp is designed to drive, and increase the L until it is stable into any cap.

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Old 11th July 2011, 03:31 AM   #1437
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Thanks Sam,

Until I build and test, I cannot comment further.

Hugh
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Old 11th July 2011, 07:15 AM   #1438
srh is offline srh  New Zealand
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Hi mikelm, which JLH amp are you referring to? I'm looking at the WW August 1982 article and it has the whole works at its output including a fuse in the output line. Re John Curl. You are correct, that'll teach me to type before making sure my brain is engaged!
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Old 11th July 2011, 09:05 AM   #1439
mikelm is offline mikelm  England
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Sorry Steve, my mistake - it was ETI 1984

http://www.tcaas.btinternet.co.uk/ETI7-84.gif

I have the articles but they are in transit just now but I remember he mentioned that he was pleased that he managed to dispense with the o/p choke and he also, after initial skepticism, agreed that an electrolytic cap in the feedback path spoilt the sound so he designed the FB circuit it so he could use a film cap ( actually two parallel film caps ) instead. The distortion was so low he had to measure it at 10Khz to see anything above the noise floor of his measurement equipment.

This was the amp that was considered one of the best ever designed by those who heard it at the time.

mike

Last edited by mikelm; 11th July 2011 at 09:08 AM.
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Old 11th July 2011, 09:29 AM   #1440
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I'd like to hear the reasoning on all those special RC's in that amp. I'd especially like the hear the methodology in choosing R10 and C8.

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