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JLH 10 Watt class A amplifier
JLH 10 Watt class A amplifier
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Old 20th February 2020, 10:00 PM   #5961
Alan Frobisher is offline Alan Frobisher
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Join Date: Jun 2010
Great to hear that the JL-H is still arousing interest.Split rail amp. is much better,eliminates the op. cap. Tried building the amp. with Toshiba 2SC5200s (watch out for fakes!). The sound was extremely "bright",not surprising as the op. was still flat at 250KHz.!It is well worth reading the JL-H section on the Class A Amp. Site. now hosted by Elliott Sound Products.
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Old 21st February 2020, 01:38 AM   #5962
grunf is offline grunf  Croatia
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I played with JLH69 for over two years, the starting position was a dual-power version. I then added power sources for Q3 and Q4. I spent most of my time trying out various transistors, primarily for the Q3, which I realized had the most impact on amplifier sound. I dropped the C3 and adjusted the current through the Q4. For Q3, low Cob fast transistors are good.Q3 with low Hfe will not sound good. Q1 and Q2 must have higher Hfe.
Of the equipment, I only have an oscilloscope and a voltmeter, but on scop I have an FFT analysis and you can see the difference in Q3 selection.
The other important thing is the power supply, it has to be as good as possible with less ripple and noise, initially I only used capacitors, so I added the resistors so I had a CRCRC combination with a 2V voltage drop, now I added the regulators with an additional 3V voltage drop. I have a total of 180,000uF in power supply.
The JLH69 amplifier really sounds great and I have no intention of changing anything for now.
In my opinion it is not necessary to change a lot according to the basic scheme, it is enough to add CCSs, but special attention should be paid to the choice of parts (especially Q3) and power supply quality.
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File Type: gif 1969 design with dual supply rails (Option 1).gif (6.9 KB, 169 views)
File Type: png JLH69 new.png (55.7 KB, 175 views)
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Old 21st February 2020, 07:07 AM   #5963
Rallyfinnen is offline Rallyfinnen  Sweden
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Quote:
Originally Posted by Rallyfinnen View Post
....
0.00x% should be possible with some tweaking, but maybe not with the 3055. I have seen figures like that in Arta around 1W into 8ohm load, but I prefer to look at the FFT and not plain numbers. EDIT: not sure now, maybe it was only 0.01 something.. anyway, distortion around -90dB is definitely possible.
Just did a quick test of my PNP JLH to refresh my memory. Load is actually 7.2ohm, not 8.
There are some spikes from noise pickup that should be disregarded at 8kHz and 16kHz, and some 50Hz and multiples of that.
Attached Images
File Type: jpg PNP JLH 1W 8ohm 1kHz.jpg (202.6 KB, 154 views)
File Type: jpg PNP JLH 8ohm multitone.jpg (258.8 KB, 144 views)

Last edited by Rallyfinnen; 21st February 2020 at 07:10 AM.
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Old 24th February 2020, 10:42 PM   #5964
horu5 is offline horu5  Germany
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Join Date: Apr 2012
Location: Munich, Bavaria
Hello you Guys,

I am bulding the high power esl version from Geoff Moss. http://www.kaschei.com/classa/jlheslfig1.gif

I've measures output transistors Type MJ15003 with my Lab-PSU and a thermometer to get the hfe at 85°C. -> Post 5901 Link: JLH 10 Watt class A amplifier ..Thanks for your advice fredbloggstwo!

The transistors have gains between 102 and 129.

Does it make sense to always combine a strong and a weak one on each "side" for Example Q1, Q1A gains = 109 + 125 = 234 Q2, Q2A gains = 104 + 129 = 233.

As far as described in the TCAAS-Design Notes for the 1996 Version the Q1 and Q2 should be matched, Q1 should have higher gain.

Do gains sum up the way I've been thinking of? Is it a "clean" solution? Hope you could give advice! Help would be very much appreciated!
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Old Yesterday, 10:56 AM   #5965
nigel pearson is offline nigel pearson  United Kingdom
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I've been doing some thinking. The JLH design is frustrating because as far as I can tell it was arrived at by experiment and finds a sweet spot that removes the need for an extra stage of amplification. Designs using op amps and current buffers don't give dramatically better paper distortion as proof of concept. The main hang up most seem to have is wanting to use the rules of a class AB designs to update the JLH.This does no harm but when it works is for different reasons.

After some thinking I remembered two designs from the past. Teleton GA202 and PE Texan. The hybrid I suspect could work is the Texan output stage and a single transistor VAS. The output can have any gain you like. This gives options that should favour a gain of one. It might not and most likely will be three. We might have an open loop gain of 700 or greater from two stages.

As the output buffer has massive current gain the VAS current might be lower than usual. 2mA? The distortion of the buffer will be an order of magnitude lower than the JLH. Bias would be two diodes and a resistor. Current between 0.4 to 0.8 amps would be fine as loud stuff would be class AB like old cinema amplifiers. If 0.5 amps 30 watts might be possible. 2 watts class A then AB might sound better than the JLH due to PSU effects.

The big advantage of an amplifier like this is the highly desirable input shunt feedback is used. The beauty of this is if your preamp has the current and the output certainly does the feedback is warp factor 9 fast. I would use the speaker capacitor as bootstrap as 2 mA won't hurt the speaker. It possibly is better.

I always remember tests I did with the OPA604 and this output stage. It was almost impossible to measure any distortion from the buffer if running 0.7 amps bias. If the heatsink large enough the bias was stable with two diodes and a resistor. It's often said this bias method is hopelessly bad. Not really, if so the heatsink is too small. An smps makes that more certain as it is a regulator into the bargain. I suspect even stability will be fine. The output pair can have 100% local negative feedback. Very little loop feedback required to correct it. The distortion is a single transistor with active current source with a gain of twenty perhaps. The current source makes the transistor think it's working from hundreds of volts. Therefore the amplification curve looks like a straight line. An mpsa 44 at 300 VDC no better if as good.

The JLH still assumes a valve preamp would be used. This Idea assumes a preamp that might drive 32 ohms used. One might even make the amplifier gain of minus three and make the preamp work harder.
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