JLH 100W L-MOS 2nd Life

Salas · 10th July 2009, 10:04 PM

I have this J.L.H. Mosfet Amp DIYed in the mid 80s. Its been stashed away since Duran Duran were making hits.

Its in great condition and has very good parts quality. I have found the schematic on the Web and noted the values on it. See it attached. *Q13 is drawn upside down, drain should go to -V.

After I have given it a look inside, I saw that C1 is bypassed (but there is a DC sensing plus time delay, working relay circuit at the output), R1 is 47k, R2 is 1k8, R18 is 27k, there is no L1,R10. Also there is only +/-60V both for drivers and output but with split bridges and filter caps from a common 300VA toroidal secondary. So it must be giving less power than spec.

Bad thing is that is giving 0.2% THD at 1W on 8R. I have found that Q1,Q2 are not 65V Vco BC546Bs on the actual boards but probably due to some mistake, 45V Vco BC550Cs. Those are not supposed to hold the DC (just 0.7V drop across the 5k6 loads) but the amp works. The grounding and noise are excellent. No hum bands and -130dB noise floor relative to 1V RMS 0dB. The offset is negligible on both channels and there is proper 100mA bias through the venerable power Hitachis.

My questions are:

1. Can the high THD be a function of the improper(?) BC550s? I will get some BC546s soon and disassemble the amp. Can there be any other reason to look for, if the THD does not improve?
2. Are there any expert suggestions on changing something for the better? The PCBs are nice green epoxy, maybe not changing the schematic but manipulating some values or doing some small scale easy changes?

Thanks in advance.

jaycee · 11th July 2009, 12:06 AM

Definitely replace teh bc550's! I'd probably upgrade the MPSA43/MPSA93's to the MPSAW42/92's as they have a 1W package that dissipate a bit more heat.

I'd definitely replace the electrolytic capacitors as well... the PSU ones might really need it.

Salas · 11th July 2009, 12:12 AM

Will do. I wonder how it works with over max Vco NPNs though... maybe due to the very low current. I run a sim for the first and second stage including the feedback to the tail Njfet CCs gate and it actually confirmed the low voltage drops. They may very well be pushed I guess. Do you have any experience with this John Linsley Hood design and its proper performance?

Here I found the original if it rings a bell to UK guys mainly

AndrewT · 11th July 2009, 10:15 AM

that schematic says V mosfet.
But the output devices that JLH specified are Lateral devices.
JLH published a number of variations of this Lateral FET amplifier.

The later ones were called 80W amplifiers.

JLH was a brilliant designer.
I hold him in high esteem.

Search under his name John Linsley Hood and you will find many of his designs of this Forum.

Q6 needs to be changed to a PNP type and oriented to place the VR on the BE side of the multiplier.

This early circuit could be updated to the very similar 80W version which is still made with minor modifications by Williams Audio?
I doubt you will want to copy the 4u7//4u7F and the very high resistor values used in the NFB leg.
Note the lack of Cdom (Miller comp cap), a very British way of designing amplifiers.

Salas · 11th July 2009, 10:57 AM

Hello Andrew. Thanks.

Is the high THD indeed coming from the false use of BC550s in the particular build?

You mean the one with the 150k Rf so to use polyprop 10uF for the DC/AC cap in the feedback as it was suggested to JLH by Bobken?
The current one has feedback there though (22k) and I think it helps the situation.

forr · 11th July 2009, 12:20 PM

Hi AndrewT
---JLH published a number of variations of this Lateral FET amplifier.---

This one is quite different from the other JLH Mosfet Designs published later and which had a Mos VAS.
Here there is positive feedback around the NFB loop. To me, the most intriguing design of his author.
I built it around the date of publication. I can't say much about the sound, but when first trying it I was very surprised by its extremly low DC offset, less than 1 mV. I have to state that I matched the pairs and did not use the 470 Ohm DC balancing pot at the LTP.

Salas · 11th July 2009, 01:32 PM

I measure 0.9mV and 1mV DC offset, and it has to be trimmed since the 80s!

AndrewT · 11th July 2009, 01:59 PM

Quote:

Originally posted by forr
Hi AndrewT
---JLH published a number of variations of this Lateral FET amplifier.---

This one is quite different from the other JLH Mosfet Designs published later and which had a Mos VAS.

I can see many similarities between this and JLH's other versions.
So much so that if it had not been identified as a JLH many who know his work would guess it as one of his designs.

Salas · 11th July 2009, 02:03 PM

I just got 4XBC546Bs with hfe between 260-330. But I also pulled out old Philips ones with hfe between 80-110. What would be best to try? I guess that the old design was tested with lowish hfe 447s of that era.

AndrewT · 11th July 2009, 02:08 PM

I would not use low hFE devices around the front end of any power amplifier.

11th July 2009, 10:15 AM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	that schematic says V mosfet. But the output devices that JLH specified are Lateral devices. JLH published a number of variations of this Lateral FET amplifier. The later ones were called 80W amplifiers. JLH was a brilliant designer. I hold him in high esteem. Search under his name John Linsley Hood and you will find many of his designs of this Forum. Q6 needs to be changed to a PNP type and oriented to place the VR on the BE side of the multiplier. This early circuit could be updated to the very similar 80W version which is still made with minor modifications by Williams Audio? I doubt you will want to copy the 4u7//4u7F and the very high resistor values used in the NFB leg. Note the lack of Cdom (Miller comp cap), a very British way of designing amplifiers. __________________ regards Andrew T.

11th July 2009, 02:08 PM	#10
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	I would not use low hFE devices around the front end of any power amplifier. __________________ regards Andrew T.

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11th July 2009, 12:06 AM	#2
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	Definitely replace teh bc550's! I'd probably upgrade the MPSA43/MPSA93's to the MPSAW42/92's as they have a 1W package that dissipate a bit more heat. I'd definitely replace the electrolytic capacitors as well... the PSU ones might really need it.

11th July 2009, 12:12 AM	#3
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Will do. I wonder how it works with over max Vco NPNs though... maybe due to the very low current. I run a sim for the first and second stage including the feedback to the tail Njfet CCs gate and it actually confirmed the low voltage drops. They may very well be pushed I guess. Do you have any experience with this John Linsley Hood design and its proper performance? Here I found the original if it rings a bell to UK guys mainly

11th July 2009, 10:57 AM	#5
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Hello Andrew. Thanks. Is the high THD indeed coming from the false use of BC550s in the particular build? You mean the one with the 150k Rf so to use polyprop 10uF for the DC/AC cap in the feedback as it was suggested to JLH by Bobken? The current one has feedback there though (22k) and I think it helps the situation.

11th July 2009, 12:20 PM	#6
forr diyAudio Member Join Date: Dec 2004 Location: Next door	Hi AndrewT ---JLH published a number of variations of this Lateral FET amplifier.--- This one is quite different from the other JLH Mosfet Designs published later and which had a Mos VAS. Here there is positive feedback around the NFB loop. To me, the most intriguing design of his author. I built it around the date of publication. I can't say much about the sound, but when first trying it I was very surprised by its extremly low DC offset, less than 1 mV. I have to state that I matched the pairs and did not use the 470 Ohm DC balancing pot at the LTP.

11th July 2009, 01:32 PM	#7
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	I measure 0.9mV and 1mV DC offset, and it has to be trimmed since the 80s!

11th July 2009, 02:03 PM	#9
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	I just got 4XBC546Bs with hfe between 260-330. But I also pulled out old Philips ones with hfe between 80-110. What would be best to try? I guess that the old design was tested with lowish hfe 447s of that era.

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