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Old 20th September 2012, 01:58 AM   #2541
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Thank you Ian. Helpful.

Others that it took a while to be clear about: biannual as opposed to biennial......
Isn't the net wonderful?

Cheers, Jonathan
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Old 20th September 2012, 02:08 AM   #2542
Bigun is offline Bigun  Canada
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Originally Posted by AC439 View Post
I think this has probably been asked before but I seem to be unable to find the answer - Can I use a regular small 50R blue plastic trim pot for VR2 ? This pot seems to be carrying some current (Q current / hFE) and
The original JLH puts too much current through the pot to use a 'small trimmer' but what you can do is put a trimmer in parallel with a fixed resistor so that it provides a small range of adjustment and takes only a fraction of the total current.

If you move to a CCS based design you may be able to use a CCS topology that allows you to adjust a trimmer that is not directly in the main current path.
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Old 20th September 2012, 06:51 AM   #2543
ferret is offline ferret  South Africa
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Hi Ferret,

You can also try Mantech Electronics they are in Johannesburg. You will not get a 2W trim pot easily but you could use a 2 watt multi-turn potentiometer. Why do you want to use a 2 watt pot?
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Hi Nico. I'm no electronics fundi - I'm just following Geoff's suggestion of using a 2W trimpot as per the following The Class-A Amplifier Site - A JLH Amp for the Quad ESL57 Addendum - 4 February 2002. I will first test the hFe of the MJ802s I have to match them and from there I'll try Geoff's suggestions.

I'll be in Amanzimtoti next week with my family. Could I possibly look you up?

Go well, KevinH
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Old 20th September 2012, 07:53 AM   #2544
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Kevin, you are most welcome PM me with a phone number and we can make a date.
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Old 20th September 2012, 09:10 AM   #2545
ferret is offline ferret  South Africa
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Hi Nico. Thanks a lot, see you next week sometime. Kevin
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Old 20th September 2012, 10:59 AM   #2546
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Kevin I took the liberty of simulating the Geoff Moss as well as the JLH class A amplifier for comparison.

It would seem that the trimmer potentiometer in question only need be rated at about 50 mW for the bias setting of 2 amp and supply voltage of 22V per rail.

From the simulations, both performed at 1 watt into 8 ohms and the results supports why the two designs sound significantly different, even though both having almost identical total harmonic distortion of 0.06% and 0.063% for Geoff's and John's designs respectively. In both designs I utilised identical transistors and adjusted all currents and voltages appropriately.

Interestingly enough by viewing the attached files the harmonic distribution is significantly different. John's amp reveals predominantly 2nd order- while Geoff's amp is predominately 3rd order harmonic.

John's Class A amplifier design became a reference design due to its charismatic and warm sound, while Geoff's design veers completely into the opposite direction thus one cannot say that it is an improved version of the JLH design. It is in fact a Geoff Moss design and should not be confused as being sanctioned by JLH.

It would be like placing an F1 Ferrari body on a Ford Fiesta. The question is what are you driving a Ferrari or a Ford.
Attached Files
File Type: pdf Geoff Distortion Analysis.pdf (17.0 KB, 70 views)
File Type: pdf JLH Distortion Analysis.pdf (16.2 KB, 74 views)
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Last edited by Nico Ras; 20th September 2012 at 11:02 AM.
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Old 20th September 2012, 11:45 AM   #2547
mikelm is offline mikelm  England
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Quote:
Originally Posted by Nico Ras View Post
Kevin I took the liberty of simulating the Geoff Moss as well as the JLH class A amplifier for comparison.
Hi Nico,

Could you post the Geoff Moss schematic - I couldn't find it on google

When you say JLH simple class A do you mean the original 1969 circuit ?
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Old 20th September 2012, 12:24 PM   #2548
ferret is offline ferret  South Africa
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Hi Nico.

I will be honest when I say this is a bit above my head. I'm know very little about electronics; I've just read through the "JLH modified for ESL" piece and there Geoff and Nick are talking about a quiescent current of 3.5 - 4 amps and this causes a higher than expected current through the trimpot. Geoff does say that one can connect a high wattage resistor with the trimmer to bring the dissipation down, so I'll probably do that - get another Caddock back to back with R5.

Thanks for the input ('scuse the pun), Kevin
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Old 20th September 2012, 12:27 PM   #2549
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Hi Mike,

I used the original JLH as it was presented in Wireless World but with input transistor 2SA970, VAS/Phase Splitter BD139 and MJL3281A output devices which is , same as what I assume was Geoff Moss design quoted here The Class-A Amplifier Site - JLH Class-A Update. If he did not contribute to this design then apologies to Geoff Moss.
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Old 20th September 2012, 12:31 PM   #2550
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Mike, I have relooked the site and see it was not a Geoff Moss design as I assumed. There seems to be several references to JLH amplifiers in this thread but not necessarily by JLH. Therefor I would like to correct my comment to Ferret, it would appear that it is a 5watt pot that is needed which appears here http://www.tcaas.btinternet.co.uk/jlhesl.htm as it is not the constant current loaded VAS design that I thought it was.
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Last edited by Nico Ras; 20th September 2012 at 12:35 PM.
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