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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 13th March 2019, 12:58 PM   #401
avtech23 is offline avtech23  Australia
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Quote:
Originally Posted by xrk971 View Post
If you look carefully at the photo on post 1 you can see which pins go to which pins. Follow the yellow wire - thatís +ve 18v.

The schematic will also give you the correct pin and reference pin 1.
Thanks X.

The problem was that I prefabbed 1:1 leads expecting the modules to be 'murphy-proof' (I need all the help I can get sometimes!) .

Easily fixed by installing X112/113 on the amp board rotated 180 degrees.

Hoping to get some sound through it tomorrow!
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Old 13th March 2019, 01:40 PM   #402
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Glad you got that sorted out and hope no permanent damage was done. What is the quiescent bias current? If that is flowing correctly, you will most likely have a functional amp. Since DC offset appears close to zero, it’s probably all good.

I think in the future, for DC power inputs, we will add a label of + and GND and - to the silkscreen to ensure this sort of stuff doesn’t happen. This seems to happen all too often. Even on my DCA which is just single polarity people mix those up. All one has to do is check for continuity with the ground plane. But that’s an extra step.
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Last edited by xrk971; 13th March 2019 at 01:42 PM.
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Old 14th March 2019, 06:39 AM   #403
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Just the damaged BJTs it seems.

Measured 0.31v over R117 and R137, so 0.31/2.2 = 141mA bias.

I temporarily connected up the audio side and used a pair of knock off 'Beats' headphones and hit the power button...

And she's alive

Both channels sound great (even with a crappy audio setup - croc clips and all!). There is definitely a decent amount of detail in the lower ranges and the circuit really does sound good as others have said. Can't wait to get it properly loomed.

One thing I don't like is the DC dump to headphones when the amp is switched off. Is that a normal behaviour?
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Old 14th March 2019, 10:12 AM   #404
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
I don’t recall a thump with power switched off or on. The slow ramp on the PSU takes care of that. The slow discharge through the caps should eliminate any thump on turn off and the cap multiplier takes care of it for the turn on. Describe it more...
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Old 14th March 2019, 11:11 AM   #405
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
No DC on the output while switched on.
When power switched off, DC charges up to 1.25v, holds for a couple of seconds then dissipates.
The headphones make crackling sound as the voltage dissipates.
This is on both channels.
I'm using the CLC board if that makes any difference (could the inductor do that?)
Edit: no, just bypassed the CLC and switch boards and same.

Otherwise the amp is silent in operation (noise wise) and no appreciable DC offset.
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Last edited by avtech23; 14th March 2019 at 11:15 AM.
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Old 14th March 2019, 12:56 PM   #406
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
As I gave away both of my amps, I can’t test this anymore but sounds like it may be an inherent “feature” of this topology. So perhaps adding a DC protect circuit or a turn on/off relay that instantly disconnects phones moment power switch is turned off. I will have to ask Byrtt if his amp exhibits this. In the meantime, just leave the amp on - it’s not bad for it, and in fact will last longer than switching on and off (if you don’t mind the continuous 10w draw) .
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Old 15th March 2019, 12:27 PM   #407
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Hi Avtech,
I checked with Byrtt and he tried turning off the unit with headphones plugged in. There is no turn off thump, just a gradual decrease in sound to nothingness.

Quote:
...never tried a power off with head phones connected so had to set it up and do the test, didn’t notice what I call a real DC thumb at power off just little bit of sound stream change for one second or so and then output level drops to a very low level for about 15-30 seconds before sound stream dies totally...
However, if you unplug the headphone and plug it back in while it is in process of turning off and discharging, you may hear a mild thump. He measured it and it is about 0.22v. This persists for a several minutes as the caps discharge.
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Old 15th March 2019, 01:56 PM   #408
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Thanks for checking that out. In the meantime, I installed a DC protection module.

I seem to have a problem somewhere. Although I have music, I also have a wicked 50Hz hum and AC on the chassis.

It is extreme when using an isolated plug pack - I am getting 75v+ on the chassis despite there being no continuity between chassis and Neg. The lab supply has 5v ac WRT GND and this is present on the chassis when I use that to power it up.

I cant explain how it gets there.

Oh and when I touch the chassis, the frequency doubles
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Old 15th March 2019, 02:16 PM   #409
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Hi Avtech,
This amp was designed to be absolutely hum-free if setup properly. You have an inadvertent connection to ground somewhere and the 75v is extreme - seems like you have a live chassis - be careful!

Please sketch a block diagram showing connections between the amp and the chassis and power supply, etc all knobs, jacks, swithes, pots should be shown. Are you using isolated RCA jacks and isolated headphone out jacks? There should be a plastic insulator ring around the GND on the headphone jack and the RCA input. The volume pot is another place - cheap pots are metal shell and that is connected to the gnd of the pot. Also take several closeup photos from multiple angles to show how everything is laid out. Ground loops are troubling - but with careful work, can be fixed.

One question is how are you supplying the 12v input DC to the amp? I used an isolated 5.5mm barrel jack. The GND on the DC input should not touch the case. It will be isolated internally via the DC-DC converter chip inside. That is how any mains or high DC ground loops care isolated from the amp.
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Old 18th March 2019, 09:15 AM   #410
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Simple High Performance DC Coupled Class A HPA with sub PPM THD
Sorry for the delay in replying (and the forthcoming wall of text), I've been trying to fault find over the weekend to figure out what is going on.

The amp is set up as follows:

Power:
12-18VDC* -> CLC Board -> Switch -> CLC Board ->AMP & to DC Protect board.

Input:
RCA -> Twisted Pairs / unconnected shield -> 10k Vol Board -> AMP

Output:
AMP -> Twisted Pairs / unconnected shield -> DC Protect Board -> H/Phone Out Board


Volume Pot is touching Chassis - but leaving it floating has no effect, and no continuity on board to chassis.

H/Phone out jack is isolated from chassis.

No continuity between power switch body and pins.

Power regs and their heatsinks are isolated.

There are no connections between chassis and any of the boards.

There is no connection to earth and the DC jack is isolated.

* 12VDC 2A SMPS 'Wall Wart' (Multiple tried) / 12-18VDC 10A Bench Supply

*****************

When connected up to the lab supply:
I get a low hum.
The 2 input capacitors are very sensitive to touch.
If I touch the top case of the capacitors on the Traco board, the noise level drops.
If I touch the H/Phone output GND, the noise level drops.

When using a switch mode supply:
I get a loud hum.
Same as above.. but if I touch the chassis, I get 'Bwaaaaaaaaaaaaaa'.

*****************

There is no connection between chassis and earth.
When measuring the chassis WRT to House Earth, the lab supply measures about 6VAC, the SMPS shows 75-90VAC. I don't understand how there is a difference when I can find no continuity between chassis and Earth.

Any SMPS I use is horrifically worse than using the bench supply.

I'm pretty sure I have a nice big antenna between the RCA and the Input Vol board but I have not been able to find a combination of grounding that will kill the noise. I have tried various combinations of wires and shield connection points but I'm not winning this battle.

I've attached a photo of the latest layout.

(I have taken the DC protect out of circuit but this makes no difference. I have fed it with a different power source also but no change).
Attached Images
File Type: jpg Sherpa1.jpg (644.3 KB, 49 views)
File Type: jpg Sherpa2.jpg (917.7 KB, 51 views)
File Type: jpg Sherpa3.jpg (799.3 KB, 45 views)
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