JLH 10 Watt class A amplifier

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.
 
Quiescent current adjust

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?
FAX : (011) 493-9319
sales@mantech.co.za
Mantech Electronics | Leader in Electronic and Industrial Components, Tools, Test and Measurement

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
 
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.:D
 

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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
 
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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Thx Nico,

The Guy who developed this amp is called Tim Andrews and I've rarely heard of a design that was developed so painstakingly component by component to end up with sound that he thought most accurately reproduced the natural sound of acoustic instruments ( he is a professional pianist so gets to hear the real thing quite often )

I have not heard this version so cannot offer a personal comment but I have to confess I am somewhat amused that such opposite views are excited by a given design and I suspect the F1 / fiesta comparison may be a bit OTT :)

I thought some people ( like nelson pass ) like the sound of 3rd HD

cheers

mike
 
I am building one again :) The chassis i am making weights 15Kg already without anything inside of it :) I expect total weight of about 25Kg. Found a very nice 2SC2922 output transistors. It needs about 10pcs to match them for a stereo amplifier. Not that many at all. The expected Power dissipation should be around 130W per chassis.

Just have no idea how much capacitance ill be needing with 30V power supply voltage. So far I ordered 90.000uF total. But i think it will be too little.
 
Hi Nico & Mike,

I am currently driving my stacked ESLs with a Bedini 25.25, itself driven by a vintage Audion Megavox Pre II valve pre (circa 1985 as Audion was forming out of Audio Inovations). They have been an excellent combo. The Bedini needs a major overhaul as the circuit boards are starting to deteriorate. I need to strip one channel down to the bare board and have new ones made albeit slightly modified. The Bedini currently has MJ802s instead of 2n3055Hs and BC550/560 in place of MPS8098 etc. These transistors were replaced some three years ago when lightning damaged the one channel. These new components gave the Bedini a complete facelift - the mids improved by a fair factor, but the bass extension and "resolution" was profoundly better. The top end also improved but not as much. The gent who repaired the Bedini drew the circuit diagram for future reference, and I will post it here soon, as a matter of interest. Anyway, long story short, I want to build the JLH Class-A for ESL so that I can still get the sweet sound of class-a while repairing/modding the Bedini. BTW I asked Bedini USA for assistance with PCB layout and all I got was the middle finger (send it to us a we will repair it - $2000 later). No thanks, I'll do that myself, and have fun doing it.

Regards,

Kevin
 
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Thx Nico,

The Guy who developed this amp is called Tim Andrews .... :)

I thought some people ( like nelson pass ) like the sound of 3rd HD

cheers

mike

Hi Mike,

I did not mean to say that Tim amp was worse or better that JLH but it is a different design in my opinion.

I built a JLH only late last year purely to experience the charismatic sound that many enthusiasts claimed, but I did not become a converted. It does sound very nice but not for too long. (In my opinion) Most likely it does not partners well with my very inefficient speakers.

Have a look at the harmonic distribution that Tim creates with his design. Certainly how he perceives is tonally more accurate and many may prefer his design over that of JLH.
 
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