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JLH Update
JLH Update
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Old 6th March 2003, 07:14 PM   #1
GringoAudio is offline GringoAudio  Canada
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Unhappy JLH Update

Hi all. I am struggling to learn something about amplifier types again....I thought I had it figured out. I am looking at the JLH as my next project but am confused about its latest iteration as posted on 'The Class-A Amplifier Site' (www.gmweb.btinternet.co.uk/jlhupdate.htm)

While the original JLH amp(1969) and its contemporary modifications...even the 1996 modification are to me clearly Class A amps it is not clear to me that the modifications by Geoff Moss are indeed Class A amplifiers. They seem to me to be Quasi-complementary AB types. Please correct me if I am wrong and educate me. The +/- rails alone indicate that they are no longer Single-Ended amplifiers at the very least.
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Old 6th March 2003, 07:55 PM   #2
Asen is offline Asen  Bulgaria
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Trust Geoff. He, maybe, knows more than Mr. Hood himself about this amp. The +/- rails have nothing to do with the fact that the amp is SE.


(a JLH 1969 owner)
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Old 6th March 2003, 08:07 PM   #3
chris ma is offline chris ma  Canada
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Don't know anything about electronic myself, but if those amps that I built from Geoff's design were not class A, then I must have built a very very hot idling class AB amp wasting power to fool me that it is a class A amp.
Can a class AB amp's output transistors get to 70 degree C at idle or no input connected and with no ocillation(working stable)?
I do not know..
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Old 6th March 2003, 08:51 PM   #4
Nisbeth is offline Nisbeth  Denmark
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JLH Update
Definately no expert on this, but I'll try:

1) As asen says, the symmetric rails have nothing to do with the amp being SE or PP. That's decided by the way the output stage is designed (the 2N3055/MJ15003 transistors in the JLH-amp). Check any book of basic analog electronics/transistor theory for more information on this.

2) To the best of my knowledge, the difference between A and AB is only in the Iq. If the output current exceeds Iq (at times), then the amp is class AB. If the output current remains lower than Iq at all times, then the amp is running in class A. Now, to get any kind of useable output power from a class A circuit, you need to turn up Iq to very high values (~2amps for a 15W JLH). As I said, there are tons of books/webpages that explain this in more detail, but I'd definately trust Geoff to know what he's doing

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Old 8th March 2003, 07:09 AM   #5
NickC is offline NickC
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the top bjt is wired in emitter follower(current amp) voltage gain is near to 1. The bottom bjt is wired for constant current to fixed the top bjt to work in a particular current. Normally in this configuration the efficiency is higher than single ended i think as similar to installing a choke below the top bjt to allow the signal to swing. negative ps doesn't mean it must be a push-pull or ab. negative ps can be used for voltage reference or output voltage adjustment. With + supply the output of the emitter follower will be positive and a cap must be used to block the + voltage. But with the negative power supply, it can be used to cancel out the + voltage hence being able to avoid using the cap which should sound much better. Caps causes max 90 degree phase lag. I don't think you want that. i hope this helps clear things up
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Old 8th March 2003, 01:36 PM   #6
paulb is offline paulb  Canada
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Default Re: JLH Update

Originally posted by GringoAudio
I am looking at the JLH as my next project but am confused about its latest iteration as posted on 'The Class-A Amplifier Site' (www.gmweb.btinternet.co.uk/jlhupdate.htm)
Hi Greg, which iteration are you thinking of building? I was thinking of the Fig. 3 circuit, myself. Maybe we can do some collaboration. Having a PCB built might be affordable if we can get enough people involved...
[How's your gainclone amp doing?]
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Old 8th March 2003, 06:12 PM   #7
GringoAudio is offline GringoAudio  Canada
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Smile Thanks

Thanks all for your answers, especially NickC who gave me a nitty gritty answer (actual technical explanation). More technical dissections are welcome as I still don't believe any of you.

Yeah Paul we have to get together soon if my wife will let me out of the house I have promised her so many renovation projects....yet all I seem to get done are tasty little speakers and half-done amplifiers and power supplies. Oh well she needs those too I tell her. She actually liked my Tangband W3-871-S based bookshelf speakesr so there is hope that the WAF is attainable. Of course the slot car set I am routering is high on my list of projects too....she can't figure out why I need a loud router running to frame and insulate the basement :
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