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JLH 10 Watt class A amplifier
JLH 10 Watt class A amplifier
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Old Today, 08:23 PM   #7981
chumingo is offline chumingo
diyAudio Member
Join Date: Oct 2020
Originally Posted by jbau View Post
Your headphones have a very nonlinear (uneven) impedance vs frequency. If you put a pot between it and the amp, the resistance of your pot is dividing the voltage against that uneven impedance, making the sound spectrum sound increasingly like the impedance curve. By contrast, the input impedance of the amp is very high, causing no such problem at audio frequencies.

What value (and wattage) of pot are you using?

As explained before, lowering the gain via the feedback resistors increases the amount of negative feedback which lowers the distortion. They're not separate.

I don't understand the question...

Dear Jbau and DIYers,

I'm quite happy to announce the birth of my new DAC/miniJLH, call it Gloria. That came to me in a moment were I found the sound to be just glorious

I guess I should had written before, but I was busy taking some definitions, testing and finalising the box. After burning out some resistor, I drilled more holes and now temperature seems mild and stable. Let's hope it lasts. Five heatsinks per board, bias current at 0.13A (+/-12 VDC).

Finally I solved the distortion problem soldering a 10K trimmer (0,25W) to the input of each board, bringing back the NFB resistors to their original values. These trimmers are also quite useful to correct the imbalance of the preceding input stereo pot (50K 0,1W currently, but I reckon that's too high, considering the extra 10K pots in each board). Of course, following @jbau observation and explanation, I removed the volume control after the amp's output. I wish attenuating at that stage would be possible, but frequency fluctuations is the least I want.

I replaced most Elcos with Nichicon gold, sound is much brighter than with the factory Chong-X. For the output cap I used the best I had at hand, an ELNA 4700uF 16V. It sounds correct to me, although the high capacitance compared to the schematic 2200uF might be cutting off low frequencies ?

Also increased the 220R before the VR to 1K, to bring down bias current to the actual point.

In the to-do list is to replace the film plastic input cap to something better. I tried some 2,2uF Nichicon Muse bipolar I had, quite sharp and detailed results but bass was gone. I read good things about Metallic Polypropylene. I might try some Wima MKP that are sold here but not cheap. Would be 1uF appropriate or will I need something higher? I've read people using up to 4,7uF for MKP input caps.

All in all, there are only details to be solved and a lot already learned. I'm quite surprised of the sound quality that ended up in a small box, although it was not my intention to have everything so tight at the beginning. Things just came in and the box remained the same.

All my gratitude to everyone else who helped me with their invaluable thoughts: OldDIY

Best regards and happy listening!
Attached Images
File Type: jpg TDA1387-JLH1.jpg (818.7 KB, 23 views)
File Type: jpg TDA1387-JLH2.jpg (774.9 KB, 22 views)
File Type: jpg TDA1387-JLH3.jpg (901.7 KB, 22 views)
File Type: jpg TDA1387-JLH4.jpg (916.3 KB, 22 views)
File Type: jpg TDA1387-JLH5.jpg (721.9 KB, 21 views)
File Type: jpg TDA1387-JLH6.jpg (939.2 KB, 16 views)

Last edited by chumingo; Today at 08:27 PM.
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Old Today, 10:38 PM   #7982
jbau is offline jbau  United States
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Join Date: Apr 2009
Location: Sunny New Mexico
Originally Posted by chumingo View Post
I'm quite happy to announce the birth of my new DAC/miniJLH
Congrats on getting good results, Domingo. How hot does it get there in the summer? You may need to consider drilling even more holes.

When you have a chance, I would try a different input transistor. The KSA992 is very-high hfe and more appropriate for very low-level signals, like from mics, phono carts, tape heads. I'm pretty sure you were driving the 992 into distortion, and probably still are, only less. I would suggest a KSA1015 and put a 2k7 to 3k0 resistor between the input cap and the transistor base to limit the current. And a 68-100pF cap from base to ground to filter RF. Of course, wih a wooden enclusure you might pick up radio stations anyway...
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Old Today, 11:26 PM   #7983
hanair is online now hanair  United States
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Join Date: Jan 2005
JLH 10 Watt class A amplifier
Originally Posted by sierratds View Post
Hello Santiago44, I'm riding my 1969 JLH (10W) - Class A.
See my power supply and my amplifier circuit.
Transistors in the Q1 and Q2 amplifier circuit require large heat sinks.
After my initial trials, I have decided to build a full fledged JLH1969.

I noticed that the quiescent current is 1.2A as per the schematic

How big is your heat-sink for one channel?
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