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Old 16th August 2012, 10:05 AM   #231
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It is very confusing, i know. C23 and C24 are the low frequency feedback capacitor designators on the EBAY chinese version or "JHL" headphone amp. Pretty cynical alteration of JLH to avoid possible patent infringement etc. In the original JLH design, the capacitor designator is C4 (100uF) - note the voltage polarity (+ve to GND), this is correct. To avoid any further confusion, you could post your final FidoCAD schematic here and the good people on this forum can check it over before you build?
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Old 16th August 2012, 10:27 AM   #232
oz7aff is offline oz7aff  Denmark
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Quote:
Originally Posted by robertor View Post
@ oz7aff:in the original scheme (circuit 1, jlh 1979) there are not C 23 and c24.
the capacitors ranging from C1 to C8.
C23 and C24 is the ID on the Schematic that comes with the PSB's from eBay, on the original JLH schematic it have the ID C4.

Sorry for the confusion.
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Old 16th August 2012, 08:53 PM   #233
lung is offline lung  Germany
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MilesCampbell explains on post #133

correct polarity on the picture
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Old 16th August 2012, 09:00 PM   #234
lung is offline lung  Germany
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Quote:
Originally Posted by mkusan View Post
Hello lung,
congratulations for your build. I was just wondering without some of the caps and variable resistors what is your DC-offset now?

Thanks
Marko
DC-offset 150mV each channel after 30 min. open case
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Old 17th August 2012, 06:52 AM   #235
mkusan is offline mkusan  Croatia
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Quote:
Originally Posted by lung View Post
DC-offset 150mV each channel after 30 min. open case
Hi lung,
thank you for your answer. I asked because at the time I was contemplating should the trimpots stay or should they go. I decided to keep them for the same reason as roclite, I did not want to damage small IEMs I sometimes use.

I was wondering whether two common big heat sinks for both sides of the board would lower the offset to acceptable level in which case I would remove the trimpots. If that does not turn out as expected I will desolder them, measure their resistance and replace them with a resistor of measured value.

One more thing puzzles me, although I am using over sized EI transformer (60VA) it does get very hot, what is your experience?

Price aside, this Chinese edition is not the kindest tribute to Mr. Hood and if Chiara boards were available I would have taken that route. Perhaps someone who used siliconray boards could chime in?

Kind regards
Marko
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Old 17th August 2012, 06:14 PM   #236
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For what its worth, here are some voltage readings from my Headphone amp for output dc offset, no input signal or load R. Initially, i set up the trimpots on the first day of use but have not adjusted since. They were set to less than +/- 5mV initially:

CH1 CH2
Switch-on 120mV 110mV
After 1 min 60 54
After 30 min -11 -8
After 60 min -14 -11
After 120 min -12 -10

First thing to note, it might be advisable to switch amp on with headphones disconnected to reduce risk of damage, the dc offset voltage then falls rapidly.
I could probably trim again but these readings are pretty acceptable, compared to the non-trimmable circuit which has typically 100 - 150mV offset. I dont think i would spent any time trying to improve on this. The 4 series 1N4148 silicon diodes used to provide the 10K trimpot Vref of -2.25V (nominal) are mounted inside case on the pcb not on the output transistor heatsinks (two seperate heatsinks) so you wont get ideal temperature tracking anyway. Someone with more time could probably optimise the circuit and improve. If its a worry, you could always check the output dc voltage offset after being driven for several hours into headphones. My guess is that it will be still be much better than the non-trim version.
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Old 17th August 2012, 10:25 PM   #237
lung is offline lung  Germany
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Quote:
Originally Posted by mkusan View Post
Hi lung,
thank you for your answer. I asked because at the time I was contemplating should the trimpots stay or should they go. I decided to keep them for the same reason as roclite, I did not want to damage small IEMs I sometimes use.

I was wondering whether two common big heat sinks for both sides of the board would lower the offset to acceptable level in which case I would remove the trimpots. If that does not turn out as expected I will desolder them, measure their resistance and replace them with a resistor of measured value.

One more thing puzzles me, although I am using over sized EI transformer (60VA) it does get very hot, what is your experience?

Price aside, this Chinese edition is not the kindest tribute to Mr. Hood and if Chiara boards were available I would have taken that route. Perhaps someone who used siliconray boards could chime in?

Kind regards
Marko
Hi muksan,

i bought a few "boards only" from this seller.

JHL Class A headphone amplifier pre-amp Preamplifier PCB | eBay

Using 50 VA EL transformer got warm, but not hot after 60 minutes
testing.

If i have time i will make a second one with bigger heatsinks and use trim pot.

I am using small cheap IEMs only for testing. To listen music i take my beyer DT 300 Ohm headphone.

Kind regards

lung
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Old 19th August 2012, 11:09 AM   #238
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first step, first test: connected to ........... is completely silent.
no noise, buzz, bursting capacitors, no nothing.

surely something wrong.
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Old 19th August 2012, 12:20 PM   #239
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if we forget to connect the output , all amplifiers will be muted.

step one: one channel works properly.
step two: copy the first step on the second protoboard.
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Old 20th August 2012, 10:32 AM   #240
mkusan is offline mkusan  Croatia
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Hi guys, sorry for not replying sooner, been away for the weekend.

@rocklite

My measurements are consistent with yours, only had to account for 20mV offset change with the lid of the case closed (trimming was obviously done with lid off). Would using LEDs as a voltage reference bring any benefits in this case?

@lung

Thank you for checking transformer for me, I joined two secondaries to create center tap, perhaps small voltage difference between the two is causing additional heat.

I was referring to a diyaudio member siliconray and his dual mono design of this amp, considering the very attractive price of his boards it is surprising there is lack of builders.
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