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Old 8th January 2006, 12:28 PM   #1
mohan72 is offline mohan72  India
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Default JLH with a differential input stage

Hi,
I have built the 15W JLH amplifier and it produces the best sound I have heard.

My system was a Marantz CD63 + NAD 3225PE Integrated amp + Wharfedale Delta 1 speakers. I tried the JLH15W and the sound was so good I have sold the NAD!

I am interested in experimenting with this amp further (I will build one more for experimenting).

Has anyone modified the input stage of the JLH to a 'differential input' ? What was the result?

Cheers,

Dr. Mohan
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Old 9th January 2006, 08:43 AM   #2
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Graham Maynard published a series of articles entitled Class-A imagineering in Electronics World Magazine back in 2004. The September 2004 issue had a schematic for a 25W 8ohm version with bipolar supplies and differential input. Supposed to be faster than the original JLH, better power supply rejection ratio to simplify power supply design, and he claimed less phase error from 12Hz to 25Khz.

He invited folks to put one together using a pizza box for the circuit board (very quick, very cheap) and try it for yourself.

I haven't gotten to try it yet, but since I don't want to build a power supply that's bigger or more complex than the amplifier, and I don't like 60 Hz hum very much, I would like to build a pair of these in the future.

I don't know if the schematic is available on line or not but you might search for 25W-8W which seems like a misprint, but is what is in the magazine.
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Old 9th January 2006, 12:47 PM   #3
mohan72 is offline mohan72  India
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Thanks Carl,
It's good to know people have so much interest in this basic amp design and to help each other.

I did a web search on the said words. The Class-A Imagineering seems to be a tube project. Let me do some more digging using Google and maybe Mr. Maynard himself might provide some clue as I have seen many posts by him.

As far as the 'power-supply being more complex than the amplifier' I can tell you that it is not true. Take the simple capacitance multiplier circuit by Rod Elliot using the TIP142/147 pair. Feed the power through this multiplier to the JLH. You would be amazed. The amp is completely silent. Even at full volume with no feed you cannot tell if the amp is running by listening to the speakers with your ears glued to the diaphragm. It is stone-silent. 60Hz mains hum is certainly not going to reach your amp. I would definetely want you to build it if complexity is the only thing holding you from doing it.

I have a few PCB layouts for the JLH and the power supply. I can send you the images if you want it.

Dr. Mohan
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Old 9th January 2006, 12:58 PM   #4
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Hi, Dr Mohan,

If you already love the sound of the original JLH which uses singleton input stage, putting differential input will changing the sound so much, you will may even dislikes the result.
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Old 9th January 2006, 01:11 PM   #5
mohan72 is offline mohan72  India
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Dear lumanauw,

I understand it. There are so many topologies that provide good sound and one of the accepted input topology seems to be a differential input pair. Sadly I am a physician but have experience soldering and setting up amps (basic setups like bias Iq etc) so am unable to change the design and experiment.

The differential input change should be easy on the JLH amp. If someone has built it and it had sounded worse then I wouldn't want to do these changes. But i haven't seen one post telling that the differential input is sounding better or worser. That's what intrigues me.

Dr. Mohan
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Old 9th January 2006, 03:58 PM   #6
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Hi, Dr Mohan,

There are alot of members here that have built JLH (original). But I don't know if someone has change the input into differential, and how do they sound then?

I can only guess that it will change the sound sooooo.... much.

You can put the question in the JLH thread by Tschrama.
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Old 9th January 2006, 04:52 PM   #7
marziom is offline marziom  Italy
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what about the GEM? is similar to the JLH?
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Old 9th January 2006, 06:18 PM   #8
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Mohan

Most of Graham Maynard's amp schematics are on this diyAudio site. Just use the search tool with his name. DestroyerX in Brazil has a long thread on one of Graham's A/AB designs. I favor the earlier 50 watt Class A design he also published here.
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Old 9th January 2006, 07:42 PM   #9
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Default GM25-8

You could try this topic:


best topo for 25W class A into 1 ohm resistive load?

Graham Maynard circuit is excellent. I prefer this one over JLH69,
which is bettered by Rod Elliot's DOZ, which is bettered by
Graham's GM25-8. (just opinion)

Regards
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Old 25th February 2006, 03:45 AM   #10
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here the circuit of my JLH Variant. it's running well on my EWB/Multisim.

Any comments are wellcome

regards:
ragil.hastomo
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