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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 18th March 2019, 12:02 PM   #411
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
How is your source grounded? Is the hum bad when source is not connected? Try adding a connection from amp board ground to chassis via a 1W 10ohm and pair of diodes (typical GLB) and connect chassis to mains earth ground.

To debug, remove connections from all parts except main and PSU. Connect HP jack directly to board not touching the case.

Do you have insulating shoulder washers on your MOSFETs? Those screws look really big. The washers have to insulate the shaft of the screw as well as the cap.
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Last edited by xrk971; 18th March 2019 at 12:25 PM.
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Old 18th March 2019, 01:21 PM   #412
avtech23 is offline avtech23  Australia
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Problem is I am using isolated supply so mains earth ground not readily available.

I'm using a mobile phone 3.5mm jack to RCA to test. Does this amp require a grounded input device?

I'll try the resistors tomorrow, thanks.

MOSFETs are isolated with the insulating pad and the shouldered plastic bushing. Nil continuity between fet and chassis. The screws are M3 10mm through the aluminium spacer into the chassis.
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Old 18th March 2019, 05:47 PM   #413
xrk971 is offline xrk971  United States
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
With the amp powered off and not connected to a source or headphone, measure the resistance between the case and signal ground.

You may have a cold solder connection from one of your RCA gnd to the volume pot, etc. When touching the case causes a large change in noise or hum, it indicates an improper or open circuit on one of your ground returns. You may have a wire strain issue and one of the ground wires is internally broken.

I like to use small RG170 coax cable from the RCA jack to the pot and from the pot to the board. I find this reduces noise pickup as the shield is grounded and signal wire is inside.
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Old 19th March 2019, 11:49 AM   #414
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Open circuit between case and signal ground. Wires are all without tension and buzz out fine.

I've finally managed to kill the noise. I had to:

1. Connect PSU input 0V directly to chassis. - This made the amp silent except when touching anything from the (floating) Source to RCA.

2. Loop breakers on both audio grounds to the connection made at 1. (1n400x anti-parallel, 1W 10R, 100nF). - I get a very faint buzz if I touch the mobile phone (cell phone) but otherwise the amp is now dead silent and sounds great again.

I'll take a look at the PSU when I get a chance - perhaps there is something wrong with the resistors that tie Audio Gnd and Power Gnd. I probably cooked them..

But for now, thanks X for all your help getting this sorted. And thank Brytt for taking the trouble to test out his amp for my benefit.



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Old 19th March 2019, 12:11 PM   #415
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
That’s good news to hear! Thanks for persevering and solving it!
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Old 20th March 2019, 03:09 AM   #416
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Just one other observation, one channel doesn't fully go to zero volume with the pot turned all the way down.

I measured 9.27k and 9.35k on the pot tracks which is within the 20% stated for that Alps pot.
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Old 20th March 2019, 03:27 AM   #417
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
A pot wiper should be able to be fully shorted to gnd (0 ohms) on the fully CCW turn - thus forcing the input volume to zero. If you gnd the input, I assume the sound goes away? Perhaps, your pot is hitting a mechanical stop before it fully goes to gnd? Or, since you added 10R gnd lift to the RCA, it can never go to zero?
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Old 20th March 2019, 03:46 AM   #418
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Hmm true.

I'll check it out when I get a chance. No big deal but just another oddity I'd like to address.
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Old 24th March 2019, 05:28 AM   #419
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I swapped in the MELF 2R7 resistors - 0207 resistors *just* fit on the pads, but it looks pretty ugly. On the other hand... hey, it fit. also swapped R108 & 103 to carbon resistors. will have to listen and see if I can hear any differences

I've really been liking the sound of this amp, more than the DCA or the AKSA HPA!
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Old 24th March 2019, 05:50 AM   #420
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
What power supply are you using with yours?
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