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JLH Class A headphone amplifier
JLH Class A headphone amplifier
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Old 15th September 2010, 07:28 PM   #11
owenhamburg is offline owenhamburg  Germany
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Default Delighted with the sound quality!

Dear all,

The headphone amp I ordered from ebay the "JHL Class A headphone amplifier earphone AMP / preamp" has finally arrived from China.

I added little PCB pins to the cables to the PCB easier. Unfortunately I could not find a Headphone socket in electronics bits and bobs box. I connected it to my preamp power supply, (regulated +15V 0V -15V)I did not adjust the output voltage and it was a few millivolt from 0V.

So I could now test it, I found a pair of reasonable headphones with a broken cable so solder them to the board with some bell wire.

I was immediately very impressed with the sound. Although their was an annoying hum. When I disconnected the 0V supply the hum became inaudible, seems it was a ground loop.

Looking at the PCB it is first rate construction. The components are of cause not the highest audio grade at this price (20 Euro including shipping) and one of the electrolytic capacitor bypass capacitors was missing from one channel. I have not fixed this.

I don't keep my best headphones at home. Speakers are for home most of the time and these old headphones get a little hot on the ears, while I spent an hour or two enjoying "Rokia Traoré" latest album "Tchamantché" and the sound was so good I heard things that I had missed with my speakers (Quad ESL 63's). I would characterize the sound, by what is not present, the harsh treble I often find unpleasant, and common with many amplifiers is not present. The second thing to say about about this amplifier is it sounds very pleasant. The headphone amplifier still needs its own transformer and a box and headphone sockets, and wont get a AB comparison with my best headphones until it has these. I may later try driving it with a 32 Ohm and 300 Ohm resistor ands look at the output via my oscilloscope but thats for some time in the future. Playing into these headphones the headphones definitely distorted before the amplifier went into clipping and it was well above volumes that I should want to wear the headphones.

I feel convinced that this will become my main headphone amplifier. So box and connectors and transformer to finish the project off.

I will look into the amp a little more some time and check it is really a JLH amplifier variant, but at a casual glance it does look like it may be a scaled down variant of the 1969/1996 ETI class A 15W power amplifier with duel rail supplies at 15 V. Its heat-sinks get hot but not excessively so and idle or playing music their seems to be no difference in the heat generated.

I am sure jamesfeline's headphone amplifier is better built but if you want some of his amps quality this is a cheap and exceptionally option.

Even if its not 100% class A it is so good sounding that I will have to consider building a Class A power amplifier for my speakers again.

Regards

Owen
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Last edited by owenhamburg; 15th September 2010 at 07:33 PM. Reason: Gramma
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Old 22nd September 2010, 04:11 PM   #12
oohms is offline oohms  Australia
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I ordered one for 1 pound (+ 16 pounds shipping )

Once i get it, ill post up some impressions too
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Old 1st October 2010, 12:42 PM   #13
joselito is offline joselito  Canada
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I also ordered one!
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Old 2nd October 2010, 03:53 PM   #14
oohms is offline oohms  Australia
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I have received it, and quickly hooked it up to test it out

Some quick notes in point form:
- You have to be VERY careful with DC offset - the pots let you adjust it, but they are FAR too sensitive - replace these with 10 or 25 turn 10k trimpots

- The DC offset takes a very long time to settle when it is first powered on - ~200mv initially, and over about 5-10 minutes, settles to what you adjusted it to, +/- 5mv

- The supplied schematics are not correct


It sounds really good, detailed, and the treble is better defined than most other amps i have played with. Very 'punchy' sound also

I think if i can fix the DC offset issue, it is definately a keeper
(I think it is one of the 100uF capacitors that needs to be reduced in size - will post up a schematic soon)
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Old 19th October 2010, 06:28 AM   #15
oohms is offline oohms  Australia
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A quick followup on that kit

The DC offset at the output varies by as much as 0.1v, and is entirely temperature dependant. The only safe way to keep using this amp is to build a DC servo, which got the DC offset down below 1mV

For more details, see here:
Rock Grotto Audio Forum - For Headphones - Headphone Amps - Amplifiers - X-Can V2 - Musical Fidelity - headphone Discussion - Amplifier Discussion - DIY - Amplifier Kits - Projects - SCHA - Sennheiser - Beyer - Grado - Audio Technica - Headphone amp

Great sounding amp, using it as my main amp now
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Old 19th October 2010, 10:21 AM   #16
AKSA is offline AKSA  Australia
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You can avoid this by using a quality film cap coupler to drive the headphones.....

This will be easier to fit, and much simpler than a servo. Sounds like a thermal drift issue, so passive offset control = best sonics.

Hugh
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Old 19th October 2010, 11:14 AM   #17
colin.hepburn2 is offline colin.hepburn2  Scotland
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You could just Email Geoff he’s you man on any of the JLH amps he is also a member here
The Class-A Amplifier Site - JLH Headphone Amplifiers
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Old 19th October 2010, 11:43 AM   #18
sandyK is offline sandyK  Australia
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I disagree with the suggestion to use an output capacitor. Very few capacitors are transparent sounding.
A properly implemented offset corrector,connected at the other side of the 68 ohm output resistor as shown in the amended circuit will also be transparent sounding.
I fail to see the need to contact anybody else on this subject.
The solution given will work well, and for no more than the cost of a high quality output capacitor. The poster is obviously more than happy with the results of his efforts, but perhaps the PSU area could now be investigated to obtain further gains ?
SandyK
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Old 19th October 2010, 12:26 PM   #19
colin.hepburn2 is offline colin.hepburn2  Scotland
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Quote:
Originally Posted by sandyK View Post
I disagree with the suggestion to use an output capacitor. Very few capacitors are transparent sounding.
A properly implemented offset corrector,connected at the other side of the 68 ohm output resistor as shown in the amended circuit will also be transparent sounding.
I fail to see the need to contact anybody else on this subject.
SandyK
Well it was just a suggestion you dont have to but take a proper look at the site and you will see why i posted the link i also have the original builders manual for this JLH headphone amp you Will also find the JLH schematic i posted is likely the better amp the Little brother to the JLH speaker Amp
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Old 19th October 2010, 12:37 PM   #20
oohms is offline oohms  Australia
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I have updated that post to show the correct point for the DC servo

I think sandyK is very familiar with the JLH
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