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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Simple High Performance DC Coupled Class A HPA with sub PPM THD
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Old 22nd August 2018, 07:18 AM   #211
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Hi JP,
Those 3324J’s are $4ea! More than an RK09 volume pot. Not kidding. I know the TPS7A’s are about $5ea, and the 10uF x7R caps are not cheap either. So all in all, $20 ea is a very fair price for these. These also have a small inductor on the enable pin. I wonder what that’s for?
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Old 22nd August 2018, 07:43 AM   #212
JPS64 is offline JPS64  Germany
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Hi X,
seems that inductor is at input/output (don‘t know exact pinning) so for filtering purpose?

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Old 22nd August 2018, 07:49 AM   #213
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Looking at pins it is connected to input pins (2 on positive and 3 on negative). 1 is enable and can be removed and shorted. I assume pin 4 is out on either and 5 is optional feedback pin. So looks like an LC filter on the input to clean up the source even more.
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Old 23rd August 2018, 12:37 AM   #214
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
I just looked up the pinouts. The first one (from left) is the enable, next 3 conform to 78xx and 79xx (that is +input, GND, +output for positive 78xx; GND, -Input, -Output for 79xx), and pin 5 is GND.

So should be simple matter to snip off pins 1 and 5 and jumper pin 1 pad to input.
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Old 26th August 2018, 09:00 AM   #215
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Default Verification build completed

I finally had a good day to work on this amp. The build went together well. It's a big project with all the little sub boards. There is an error on the volume pot board - one of the pads on the smaller right channel is not connected to anything - jumper it to the right input. The crossfeed filter has some errors - I managed to fix it partially so that it works but the defeat feature does not fully defeat it. Sounds great. Bias is 116mA with +/-17.5v under load. DC offset is -2mV and 6mV.

Completed amp before buttoning up:
Simple High Performance DC Coupled Class A HPA with sub PPM THD-shpa-gb-build-01-jpg

Closeup:
Simple High Performance DC Coupled Class A HPA with sub PPM THD-shpa-gb-build-02-jpg

Closeup of ebay voltage regulator (negative one):
Simple High Performance DC Coupled Class A HPA with sub PPM THD-shpa-gb-build-03-jpg

Closeup of outputs and heatsink mounting:
Simple High Performance DC Coupled Class A HPA with sub PPM THD-shpa-gb-build-04-jpg

Closeup of Xfeed:
Simple High Performance DC Coupled Class A HPA with sub PPM THD-shpa-gb-build-05-jpg

Closeup of vol pot board and headphone jack board:
Simple High Performance DC Coupled Class A HPA with sub PPM THD-shpa-gb-build-06-jpg

Amp with headphones and Cayin N3 playing:
Simple High Performance DC Coupled Class A HPA with sub PPM THD-shpa-gb-build-07-jpg

Underside panel showing all the screw holes used to mount parts on the other side:
Simple High Performance DC Coupled Class A HPA with sub PPM THD-shpa-gb-build-08-jpg
Attached Images
File Type: jpg SHPA-GB-Build-01.jpg (354.4 KB, 3611 views)
File Type: jpg SHPA-GB-Build-02.jpg (473.7 KB, 1506 views)
File Type: jpg SHPA-GB-Build-03.jpg (403.1 KB, 1688 views)
File Type: jpg SHPA-GB-Build-04.jpg (475.7 KB, 1513 views)
File Type: jpg SHPA-GB-Build-05.jpg (345.4 KB, 479 views)
File Type: jpg SHPA-GB-Build-06.jpg (365.6 KB, 366 views)
File Type: jpg SHPA-GB-Build-07.jpg (332.9 KB, 895 views)
File Type: jpg SHPA-GB-Build-08.jpg (289.2 KB, 366 views)
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Last edited by xrk971; 26th August 2018 at 09:09 AM.
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Old 26th August 2018, 10:11 PM   #216
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Default Some Measurements

It's a pretty clean amp. I am using my Cayin N3 with 1kHz test tone. The N3 and Focusrite sort of limit the harmonic distortion measurements as the self THD of the DAC is about 0.003% at -1dB FS. I am keeping it in the -10dB to -20dB region.

Here are some FFT's driving a 51ohm 1W metal thin film dummy load resistor.

1.03Vpp into 51ohms:
Simple High Performance DC Coupled Class A HPA with sub PPM THD-shpa-gb-verif-build-fft-1-03vpp-51ohms-png

2.76Vpp into 51ohms:
Simple High Performance DC Coupled Class A HPA with sub PPM THD-shpa-gb-verif-build-fft-2-78vpp-51ohms-png

14.0Vpp into 51ohms:
Simple High Performance DC Coupled Class A HPA with sub PPM THD-shpa-gb-verif-build-fft-14-0vpp-51ohms-png

One thing to note is how quiet the power supply is for being a wall plug supply. There is essentially no mains or its harmonics - quiet like a battery.

There is absolutely no audible noise/hiss/hum with the source turned off and the volume turned all the way up with my OB-1's which are 55ohms and 96dB. Normal listening voltage levels with OB-1's are about 100mVrms, so you can get an idea that the data in the plots are for ear blistering levels, or for very insensitive phones.
Attached Images
File Type: png SHPA-GB-Verif-Build-FFT-1.03Vpp-51ohms.png (73.4 KB, 2173 views)
File Type: png SHPA-GB-Verif-Build-FFT-2,78Vpp-51ohms.png (74.4 KB, 683 views)
File Type: png SHPA-GB-Verif-Build-FFT-14.0Vpp-51ohms.png (79.5 KB, 343 views)
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Last edited by xrk971; 26th August 2018 at 10:23 PM.
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Old 27th August 2018, 12:59 AM   #217
AKSA is offline AKSA  Australia
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Exceptional work, X. A work of art...... and a very nice case too.

At -15dBA input, H2 looks to be around -105dB, H3 maybe -110dB. Very good figures. It should sound very natural?

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Last edited by AKSA; 27th August 2018 at 01:01 AM.
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Old 27th August 2018, 01:27 AM   #218
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Thanks, Hugh. It sounds very neutral and very revealing with no caps in signal path. The amp has plenty of headroom for crescendos and will make my headphones recoil on bass kick drum. The casework was a royal pain as you can imagine. Took me better part of a day to assemble. I’m happy with the measured performance and how it sounds. The DC-DC PSU with low noise regulator works very well. It can drive power hungry planar magnetic cans quite admirably. I am using Fairchild TO220 FQP MOSFETs for first time - they sound great.
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Old 27th August 2018, 06:15 AM   #219
Morde is online now Morde  Finland
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Hi X, great work!
Did you measure temps inside the case after running it for a while? Seems to be quite closed up enclosure, but i guess there's no that much heat to dissipate with the 116 mA bias...
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Old 27th August 2018, 09:12 AM   #220
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Hi Morde,
Thanks! With the case open, the actual MOSFETs were 42C and the voltage regulator heatsink was 55C. With case closed I would guess a bit warmer but external temp of case no more than 40C. I would have to quickly remove the lid and take temperatures to know true internal temp.
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