JLH 10 Watt class A amplifier

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#2,444
Jacobthellamer
Nice case! really nice finish.

I'll post some pics of mine soon. It's been sanded down and painted black, its a bit too big, but after reworking the grounding a bit ( a star of stars, rather than a large bus bar arrangement), and snubberising, it has much lower hum.

Homemodder; I've been reluctant to try the CFP for the phase splitter because it may well oscillate (I used a CFP in a preamp recently, and found oscillation a problem), and I'm quite happy with mine as is for now. I also assume JLH would've used a CFP it he thought it would be suitable.
But a reasonable suggestion - if you can get it right.
 
#2,445
Hi all, thanks for an interesting and informative thread (long, huh!) even if a chunk of it is beyond a newbie like me. Anyway, a couple of days ago I finished my first Class A transistor amp (!) in the form of JLH's 1969 single-rail circuit. I used one of the kits available on ebay.

After much fiddling of the pots I finally got it working at 27V, 1.2A (approx.) as recommended on the seller's instructions. For PS I'm using an 350W-27V switch-mode ps followed by 3x 22uF/40V caps. The sound I'm getting is good but I'm worried that after some time (particularly when changing records /CDs) the ps fan switches on and runs for about 30 secs. This happens while the heatsinks are warm but not at all hot. Isn't this a sign that something is being stretched beyond its limit? Is this normal for an smps with this Wattage on such an amp like the JLH?

This reminds me that during setting-up the mA-meter used to beep and finally shut off and I have to start the process again. The beeping is a sign of overload, no? Usually, when this happens (but not always) the fan switch on and stop some time later.

My worry is that something could be wrong which might damage/destroy my (costly) speakers. Are there any checks which I can make (with a multimeter pls) to put my mind at rest?

Thanks for your help.
Regards.
 
#2,446
If you are using the 1969 single rail supply version there should be no problem for your speakers. The amp will have a coupling capacitor (around 2,500uF or more) and that should protect them from DC faults. I can not comment on the PS. I haven't used that type. Cheers and good luck, Jonathan
 
#2,449
your supply appears to be running well within it's stated specification - I suspect the fan is just a normal part of it's operation - so I think you do not have to turn down the current as long as your heatsinks are not too hot - and as long as the noise of the fan is not annoying you.

I just wonder how quiet the o/p of your SMPS is so I would be tempted to try a small choke between two of your caps for a final CLC filtering stage - might improve the sound.
 
#2,450
philpoole said:
Jacobthellamer
Nice case! really nice finish.

I'll post some pics of mine soon. It's been sanded down and painted black, its a bit too big, but after reworking the grounding a bit ( a star of stars, rather than a large bus bar arrangement), and snubberising, it has much lower hum.

Homemodder; I've been reluctant to try the CFP for the phase splitter because it may well oscillate (I used a CFP in a preamp recently, and found oscillation a problem), and I'm quite happy with mine as is for now. I also assume JLH would've used a CFP it he thought it would be suitable.
But a reasonable suggestion - if you can get it right.
Click to expand...

His book was written nearly 20 years later so one could think that he learned some things along the way which is natural. CFPs have this risk but its easily cured, the benefits are great and I can see why he proposes this mod in his book.
 
#2,451
Hi, thanks for all your help about my smps for my JLH.
As mikelm says, the heatsinks are not at all hot. I suppose I have ample heatsinking. (Later on I'll post a couple of photos.) The idea of inserting a small choke after the first (of 3) capacitors is interesting. Can you please suggest an L (impedance) and A (current) values to look for, as I've never done this before.

Thanks a lot and best regards.
 
#2,456
homemodder said:
His book was written nearly 20 years later so one could think that he learned some things along the way which is natural. CFPs have this risk but its easily cured, the benefits are great and I can see why he proposes this mod in his book.
Click to expand...

Sorry, I meant,perhaps he would of proposed a cfp phase splitter in his '96 variant - if he thought it would have been of benefit. And why not CFPs on the output stages?
If I recall, he also suggested Hiraga-esque phase splitters as an improvement somewhere else in his book.
I really like the fact that the JLH class A is a very stable amp - partly because there are very few stages in it, adding phase.

However, I think it might be interesting to investigate a variation of the 2000 version, perhaps with CFP phase splitter one day. I'm convinced I don't need much power (I think 10-15W is too much), so might consider a new build one day - but it will have to wait (I have a few projects stacking up at the mo).
 
#2,457
I think you are right Phil; if such had been a serious advantage perhaps we might have seen JLH' own 'Liniac' inside the input stage as well. I agree it is right to be wary of CFPs. It's another layer to tame, and while that's often not difficult - if the design does not rely on needing another decade of GBW, why bother.


(Disclosure to everyone else - I recently had the pleasure of Phil's company again, for an evening to play with his by-the-book implementation of the JLH 1996 version at my place with my speakers. And really - that is one very fine amplifier as it is 🙂

-And mindbggringly good for what is essentially 4 active devices per channel plus trimmings! ]
 
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#2,458
martin clark said:
I think you are right Phil; if such had been a serious advantage perhaps we might have seen JLH' own 'Liniac' input stages as well. I agree it is right to be wary of CFPs. It's another layer to tame, and while that's often not difficult - if the design does not rely on needing another decade of GBW, why bother.


(Disclosure - I recently had the pleasure of Phil's company again for an evening to play with his own textbook implementation of the 1996 version at my place with my speakers and really - that is one very fine amplifier as it is 🙂 ]
Click to expand...

Yep, I'm very pleased. It really ain't broke... ;-)
 
#2,459
I find it a bit odd that this CFP idea is dismissed just through a process of mental reasoning. I'm surprised that someone who has a self built "simple class A" isn't just a little curious about how it might sound.

On the other hand Homemodder, if you are that curious I guess the onus is on you to try it - it's the only way to really know how it would sound.
 
#2,460
Look, just a question "out of left field" re: temperature of heat sinks. Is there a cheapish way of taking an infra-red photograph (or other method) of a heat sink. RS and others stock packets of temperature sensitive paper strips that can be stuck to a surface and indicate a value but I was just curious if there is a more elegant version that gives a "picture" of what is going on. My current JLH is adequately "sinked" but I am contemplating fabricating half a dozen h/sinks for further versions and want a way of effectively checking them......other than the classic: "if it burns you then its not big enough" method.
Any thoughts. Cheers Jonathan
 
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