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Old 19th Feb 2012, 9:29 pm   #21
thermionic
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Default Re: JLH Class 'A' Amplifier

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Originally Posted by kalee20 View Post
As the amplifier has DC coupling throughout, if the signal source happened to put out some DC, you could fry your loudspeakers. Having an input cap protects against this, at least!
It wasn't the fact that there IS an input cap, its the (relatively) high value that I was pondering over.

SimonT.
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Old 19th Feb 2012, 9:36 pm   #22
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Default Re: JLH Class 'A' Amplifier

Mike, that seems pretty good value for money! I would certainly mount the power transistors off board though. Even with an L shaped heatsink bolted beneath, the heat could eventually cook the PCB.

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Old 23rd Feb 2012, 8:32 am   #23
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Default Re: JLH Class 'A' Amplifier

I was also considering the dual rail supply to eliminate the output cap but, I would always worry that if a fault in the output stage occured the resulting DC might blow my precious speakers
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Old 23rd Feb 2012, 5:07 pm   #24
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Default Re: JLH Class 'A' Amplifier

Quote:
the resulting DC might blow my precious speakers
Fast-blow fuse!

John
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Old 24th Feb 2012, 2:09 pm   #25
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Default Re: JLH Class 'A' Amplifier

What, faster-blow than the wire they wind voice coils with?!
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Old 25th Feb 2012, 9:15 am   #26
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Default Re: JLH Class 'A' Amplifier

Or this: http://www.esr.co.uk/velleman/k4700.htm

Ironic that it has a lot more complexity than the amplifier itself, but will protect your loudspeakers better than any fuse could.

Mark
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Old 25th Feb 2012, 8:49 pm   #27
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Default Re: JLH Class 'A' Amplifier

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What, faster-blow than the wire they wind voice coils with?!
Yes, I have blown a couple of voice coils over the years, they don't like it up 'em!

Can anyone honestly say they could tell whether the speakers are AC or DC coupled, on a blind test? It would be interesting to try.

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Old 25th Feb 2012, 11:59 pm   #28
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Default Re: JLH Class 'A' Amplifier

An elephant might be able to tell the difference .....

The speaker coupling cap will cause LF roll-off, but 10mF is a stingy value by modern standards and gives a -6dB point of 2 Hz, well below what humans can hear, with an 8 ohm speaker.
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Old 6th Apr 2012, 5:08 pm   #29
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Default Re: JLH Class 'A' Amplifier

Hi,

I've just assembled two of these JLH 10 watt class A classics (1969 - not quite antique!) but have not tried them yet as I need to source suitable heatsinks.

My main worry is the ST 2N3055 devices; they are not printed as I would have expected; the printing is fuzzy and looks as tho' it is on a layer of silver paint rather than directly on to the metal can. Scraping the paint off doesn't reveal any markings underneath but the surface of the can looks as though it's been scrubbed.

QUESTION Are these devices fakes? If so are they as good for this application as the pukka transistors?
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Old 6th Apr 2012, 8:50 pm   #30
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Default Re: JLH Class 'A' Amplifier

Hi Dave, let me know what you are looking for in the way of heatsinks, I may have something suitable.

It sounds as if the transistors have been re-marked; however, a lot of 2N3055's are re-marked higher power fall-outs and may work fine. This was the way of getting rid of power transisors that were not up to a much higher spec.
Try them and see, but keep close control on power and monitor device temperatures.

Ed
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Old 6th Apr 2012, 10:21 pm   #31
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Default Re: JLH Class 'A' Amplifier

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Originally Posted by G6FGPDAVE View Post
QUESTION Are these devices fakes? If so are they as good for this application as the pukka transistors?
If you've got doubts about them, why not get a new set from somewhere like CPC? They're only about a quid each.

Pete.
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Old 7th Apr 2012, 1:55 am   #32
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Default Re: JLH Class 'A' Amplifier

As Silicon said, interesting choice of a switcher for the power supply.

Linn went to switchers in the late 1980s mostly because of needing power factor correction, I believe.

For most class A/B hifi amps, the pulsing current demand requires unusually good bandwidth in the switcher's control loop and you can get into all sorts of trouble. The class A design doesn't stir up this problem.

A past boss of mine got into a protracted argument with JLH over the advantages in noise floor of series and shunt modes of negative feedback, in the letters pages of Wireless World.

I like the ziggurat heatsinks!

David
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Old 7th Apr 2012, 7:52 am   #33
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Default Re: JLH Class 'A' Amplifier

Hi Dave, I'm afraid I don't know those heatsinks, but I do have a variety of commercial heatsink extrusion in different sizes, that may do if you let me know what sizes & thermal conductivity you need.

Ed
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Old 7th Apr 2012, 11:43 pm   #34
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Default Re: JLH Class 'A' Amplifier

Quote:
Originally Posted by Radio Wrangler View Post
A past boss of mine got into a protracted argument with JLH over the advantages in noise floor of series and shunt modes of negative feedback, in the letters pages of Wireless World.
I have forgotten who was JLH’s antagonist in this debate. Later on though, JLH seemed to soften his position, and in his 1982 Wireless World modular design used a two-stage series-shunt circuit. In his 1997 book he focussed on the HF error inherent in the series feedback approach (which could be, but seemingly often was not corrected), but was neverthless quite complimentary of Baxandall’s 1977 treatment of the subject, even though the latter came down in favour of the series feedback approach.

Back in 1969 I guess it would have been possible to combine the Cambridge P40 Darlington emitter follower input buffer with a lower impedance version of JLH’s shunt feedback triple for gain and equalization, and so have had the best of all worlds. But that would have required 5 transistors, as compared with the 2 (Dinsdale) or 3 (Bailey) typical of series feedback designs.

Cheers,
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Old 9th Apr 2012, 10:45 am   #35
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Default Re: JLH Class 'A' Amplifier

After getting the amp board kits & single rail power supply board mentioned earlier in this thread I decided to try & lash a JLH amp together using the parts I have knocking about, namely a 25v 300 v/a toroid (greater than 18v but my speakers are 15ohm so I hope this will work), a black hammond chassis from an old valve amp project & a couple of massive heatsinks I got cheap on ebay (they were being used for Aquarium LED lighting). It's only about 70% done. The boards went together very quickly & the parts quality seems very good for the money, caps look to be decent quality etc. The metalwork has taken quite a bit longer than expected as the heatsinks are quite unwieldy. Although the parts kit with the PCB's is very good there are no insulated sleeves for the transistor mounting bolts for offboard mounting on the heatsink ...which I didn't realise I needed (knowing not much about transistors)...I've now ordered these & will get back on it when they arrive. The amp boards also included a speaker protection board kit with a relay...may also build this & incorporate it into the build. Although I've managed to successfully build a couple of valve amps from PCB's before this is my first go with anything transistor & first use of a toroid...potential for disaster is high so stay tuned!

I could do with a steer on the power supply...

-my single rail supply is CRC using 8 4700uf 50v caps...will this be enough to power both boards....?

Some pics of progress to date....I will make a mesh cover when it's done & adjusted....

Mike
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Old 10th Apr 2012, 11:36 pm   #36
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Default Re: JLH Class 'A' Amplifier

Hi Ed,
I was comparing the stacked heatsinks to a stepped pyramid!

The debate between JLH and Hugh was some time ago. Hugh was professionally involved in some of the highest dynamic range analogue systems ever designed. He knew what he was talking about and had the maths to back it up. A mild and quiet man, a very good guy to have as a boss. He retired very early and lives in a house with a turret with his family.

David
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Old 14th Apr 2012, 6:11 pm   #37
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Default Re: JLH Class 'A' Amplifier

I have also built the 69 linsley hood Very fine amplifier for such a simple circuit the drawback is massive heat-sinking is required but non the less well worth building
my version used a simple thyristor regulated supply with good results
regards Trev
ps hope you are doing well these days wham 68
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Old 15th Apr 2012, 11:40 am   #38
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Default Re: JLH Class 'A' Amplifier

Hi Trev

I remember your build from a couple of years back which is partly what got me interested in it. Still working on the wiring on mine...for a relatively simple amp it still takes some time....or I am getting older!

PM sent.

Mike
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Old 17th Apr 2012, 9:07 pm   #39
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I am building a clone quad 405 been working on it for 3 years now and i am about half way there
I think we are all getting slower Mike
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Old 18th Apr 2012, 12:53 pm   #40
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Default Re: JLH Class 'A' Amplifier

Yes I know what you mean Trev...

Well I finished the JLH apart from making the lid.....fired it up & no beginners luck I'm afraid! I had one channel only getting warm ....after a bit of probing I managed to get this down to no chnnels getting warm I think the problem probably lies with my off board mounting of the transistors..as the power supply is putting out 33v I'm away for a few days now but will get back on it next week ...re check all my soldering & drill out the mounting holes on the heatsinks a bit more...I have a feeling I may also have toasted a couple of the transistors as I heard a muted double crack on powerup like a static spit....ordered some spares...good job they are only 79p each!!

If anyone has any thoughts on simple JLH diagnostics let me know!

regards

Mike
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