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Old 20th January 2018, 03:24 PM   #21
xrk971 is offline xrk971  United States
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It's a temperature effect on probably the output 2SA1837 or maybe even the LTP? But I took several measurements over about 30 min after the lid wad replaced. The temp of the output BJT was 97C with cover on and 67C with cover off. It looks like only the second harmonic distortion is reduced at higher temperatures. Or it could be that temperatures inside the cover are more homogenized and the LTPs are closer together in temp. As they are not thermally bonded, they may have differences when the lid is off. It's definitely a thermal effect though. THD went from 0.026% with lid off to 0.0063% with it on. The relative balance of H2 and H3 will make it sound sweeter with the lid off and more analytical with lid on. Adjustment of the LTP degeneration resistor values and also the R9 (make 68k or even 50k) can raise relative level of H2 vs H3.
Aksa Lender HPA-aksa-lender-hpa-130ma-bias-2-0vpp-43ohms-hp2760-temp-effect-jpg
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Last edited by xrk971; 20th January 2018 at 03:30 PM.
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Old 20th January 2018, 09:14 PM   #22
xrk971 is offline xrk971  United States
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Way to go Vunce!

Click the image to open in full size.

Add a headphone jack and change a few resistors and you have a headphone amp.
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Old 21st January 2018, 10:34 PM   #23
AKSA is offline AKSA  Australia
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Brilliant work, Vunce!
You could easily put the preamp into a box three times smaller!
Nice use of a SMPS and elegant wiring on the selector and sources.
Some very serious caps in there.......
I never expected this preamp would be so good, but the drive on the voltage amp is very fast and I think it helps with the very low THD. XRK has championed this circuit and added his own versatility to it.
Thanks to XRK, J-P, and Vunce for making it reality.

Hugh
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Old 21st January 2018, 11:22 PM   #24
Max Headroom is offline Max Headroom  Australia
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Quote:
Originally Posted by xrk971 View Post
It's a temperature effect.....
Ok, you have uncovered one instability/dependancy, good work.

Dan.
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Old 26th March 2018, 11:20 PM   #25
badboygolf16v is offline badboygolf16v
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I'm just building mine and have a few questions for X please...

- Is it necessary to increase C124/C134 to 1000uF as you have done in your HPA build?

- Is there any benefit from matching Q5s on the daughterboards?

- Should both C132 (or C1032) and C133 (or C1033) be installed? What are the options here?

- Should I build the CLC filter? I see it's not on your HPA build. If I use it would it affect the input voltage required?

Thanks

Antony

Last edited by badboygolf16v; 26th March 2018 at 11:42 PM.
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Old 28th March 2018, 01:44 PM   #26
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by badboygolf16v View Post
I'm just building mine and have a few questions for X please...

- Is it necessary to increase C124/C134 to 1000uF as you have done in your HPA build?

Yes, use as big of a value as you can fit here while still using a high quality brand (Elna Silmic II, Nichicon KA, Panasonic FG, etc). The values affects the bass extension depending on your phone impedance. For 50ohm phones, set the -3dB corner frequency to 2 octaves below where you want the amp to play bass flat to. Say you want flat to 20Hz, then 2 octaves below is 5Hz. Use equation Fc=1/(2 pi R C) and solve for C given impedance of phones in ohms and Fc of 5hz. So that gives 640uF. So 1000uF should be plenty.

- Is there any benefit from matching Q5s on the daughterboards?

Not really, as long as bias currents on both channels is within 5% you are fine.

- Should both C132 (or C1032) and C133 (or C1033) be installed? What are the options here?

It's your choice, but many people don't think a bypass cap on the inputs is a good idea as they say it smears the sound compared to even a plain electrolytic cap like an Elna Silmic II 10uF 35v cap. Use a high quality film cap here. Follow the same equation for setting the value. In this case, the input impedance is set by the 22k resistor to GND. So a 1uF cap here gives flat response to 29Hz. I have since changed this value to a 5.6uF film cap so my input bass extension is flat to 5Hz.


- Should I build the CLC filter? I see it's not on your HPA build. If I use it would it affect the input voltage required?

If you do not have a noise issue with the DC-DC converter adding noise to your amp, then not needed - however, it is a good idea to have it so that RF/EMI from the DC-DC converter in the amp gets radiated back out to your mains or environment outside of the box. I have used 47uF and 1mH here with a 51R parallel to the 1mH for stability.

Thanks

Antony
As the TH and SMT MELF and SMT 0805/1206 boards have different designators, let's assume you have the TH one, as that is assumed to be needed for the 2SA1837's to be used.

Click the image to open in full size.

Main board (MB) schematic:
Click the image to open in full size.
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Last edited by xrk971; 28th March 2018 at 01:57 PM.
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Old 28th March 2018, 05:14 PM   #27
badboygolf16v is offline badboygolf16v
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Thanks for the detailed reply X. The examples are very useful, as I'm lacking basic electronics knowledge. I've just bought The Art Of Electronics, so I'll read up on the filter section with this schematic in mind.
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Old 18th May 2018, 09:49 AM   #28
yourownbuzz is offline yourownbuzz
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where does one get the 2SA1837 nowadays?

I had a look on ebay but I guess anything from china and surrounds are suspect. what about ones from the UK? (5X 2SA1837 High Frequency/HF/RF PNP Amplifier Transistor PACK OF 5 | eBay)
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Old 18th May 2018, 10:10 AM   #29
xrk971 is offline xrk971  United States
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where does one get the 2SA1837 nowadays?
I bought the last remaining batch of genuine Toshiba from Mouser a year ago. They are becoming rare... I can sell you one when you order the Aksa Lender pre MB's. 2SA1837 are $10/pair.

The photo in that link is all wrong. The 2SA1837 are all black epoxy covered, no metal tab.
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Old 18th May 2018, 04:33 PM   #30
tonza75 is offline tonza75  Europe
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Quote:
Originally Posted by xrk971 View Post
I bought the last remaining batch of genuine Toshiba from Mouser a year ago. They are becoming rare... I can sell you one when you order the Aksa Lender pre MB's. 2SA1837 are $10/pair.

The photo in that link is all wrong. The 2SA1837 are all black epoxy covered, no metal tab.

You could try 2SA1837LB-TF3T at 2SA1837LB-TF3T - Power Transistor -230V -1A
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