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Class D Switching Power Amplifiers and Power D/A conversion

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Old 6th December 2020, 12:13 PM   #11
eboissier is offline eboissier  France
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Join Date: Dec 2004
Location: Paris
Hi Jaytor

Cool project, I have a very close one (and also coming from a nc400 build), sticking to switching power supply but going directly to valve input buffer... The latest being obviously the major hurdle, I am looking forward your progress on the matter !
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Old 11th December 2020, 12:37 AM   #12
Jaytor is offline Jaytor  United States
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Join Date: Oct 2019
Default Buffer/Op Amp power supply

The Purifi module requires a +/- 12v Op-Amp supply (~27ma). In addition, the buffer board needs a low voltage supply. The Neurochrome board that I am starting with has the ability to plug in regulator boards to generate these voltages from an unregulated input connection, and will also use the regulated outputs for it's own buffer circuitry.

My future plans include buffer boards that will require higher current draw and will be able to operate at somewhat higher voltage +/- 15v or so. So I decided to implement a higher current unregulated linear supply which can be regulated on the buffer.

I found some bare PCBs from jimsaudio.com which were the right size and provided adequate functionality. I'm using two of these boards for each monoblock. The boards are designed to provide a +/- supply, but I decided to implement a CRC supply by daisy-chaining two boards with resistors between them. Each board has 13600uF for each rail, and I'm using two 3ohm resistors in parallel between the banks.

These boards did not have the transformer snubber connections I wanted, so I added the caps to the underside of the board connected to the transformer.
Attached Images
File Type: jpg opamp PS.jpg (225.1 KB, 307 views)
File Type: jpg op amp PS snubber.jpg (262.2 KB, 302 views)

Last edited by Jaytor; 11th December 2020 at 12:44 AM.
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Old 12th December 2020, 10:14 PM   #13
phase is offline phase  United States
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Join Date: Oct 2004
Good thinking using snubbers, is a great global improvement for low $.

Your snubber circuit will be more effective if it is directly applied to the rectifier diodes, the shorter distances help.
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Old 27th December 2020, 12:30 AM   #14
Jaytor is offline Jaytor  United States
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Join Date: Oct 2019
Default Progress on first monoblock

I haven't had a lot of time to work on the amps over the last couple of weeks, but I managed to get one of the power supplies installed in the chassis today and got everything checked out.

After doing some testing on the main power supplies, I found that my output voltage is a bit on the high side with my home mains voltage. I'm measuring 69V on the power supplies with no load. These transformers are huge, so they aren't going to sag much once I've got the amp modules hooked up, particularly since idle current is so low. So I decided to increase the first-stage resistors in my CRCRC supply up to about 0.2 ohm. It was closer to 0.05 ohms. I'm hoping that this will add a bit more drop under high current and reduce the chances that I'll trigger the over-voltage protection on the Purifi modules. Worst case, I'll have to swap out the transformers.

My speakers are pretty efficient (current speakers are 92.5db/w and the speakers I am planning to build over the next several months are 98db/w, so I don't expect to be pushing the Purifi modules too hard.

The main supply has 134KuF for each monoblock, with an additional 54K for the low voltage supply. I'm using an AMB omega-11 regulated supply to generate the 15V gate drive. And I've got a 12V supply mounted to the back panel to drive accessories - the LED on the power button and an LED power meter which will mount in the front panel. Each of these has it's own switch to disable when I want the amp to be dark.

I'm using a Hypex soft-start module to control power. This provides a low-voltage power switch option as well as the soft start.

I decided to eliminate the audio-level-sense power-on feature, partly because it wasn't compatible with the Hypex module without making some changes and I decided I probably wouldn't use it much anyway.

Photos show the LED power meter I'm using and the progress on the first of the two monoblocks. I have all the PCBs assembled and the backpanel wired for the second amp as well, but not installed in the chassis.
Attached Images
File Type: jpg LED Power Meter.jpg (885.3 KB, 141 views)
File Type: jpg ps top 1500.jpg (437.9 KB, 138 views)
File Type: jpg ps front 1500.jpg (345.9 KB, 137 views)
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Old 10th January 2021, 09:24 PM   #15
Jaytor is offline Jaytor  United States
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Join Date: Oct 2019
Default First channel is running

One channel complete (almost). I still need to get a laser cut acrylic panel for the front panel in front of the LED power display. But otherwise everything seems to be working. It's only been powered on for an hour, so it still has some a lot of burn-in left. I'm just playing it through a cheap speaker, so can't comment on how it sounds. I'm going to wait until I have the second channel finished before I try it on better speakers.

Please excuse my messy work area. I don't really have enough space to work right now. I've got a new electronics workbench on order which will have a shelf above the desk to move some of the test equipment up.
Attached Images
File Type: jpg top 1200.jpg (255.3 KB, 43 views)
File Type: jpg front 1200.jpg (220.6 KB, 41 views)
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