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Valve DAC from Linear Audio volume 13
Valve DAC from Linear Audio volume 13
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Old 28th February 2021, 02:10 PM   #1491
MarcelvdG is offline MarcelvdG  Netherlands
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Join Date: Mar 2003
Location: Haarlem, the Netherlands
Today I first got sound using an LX45 module, with my solid-state DAC that is supposed to be compatible with the valve DAC. There is still a set-up time issue related to the CDIN line of the DIX4192 to solve and I need to tweak some coefficients because it now regularly runs into integrator clipping when using the loud mode. I guess the first integrator needs a bit more headroom now that the images around 600 kHz are not suppressed anymore at the sigma-delta modulator input.

This morning the JTAG programming cable refused to work, later it worked fine, no idea what changed. Then the DAC didn't do anything at all, but that was because I had forgotten to short the R102 pads on the FPGA module.
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Old 28th February 2021, 03:24 PM   #1492
SonnyMarrow is offline SonnyMarrow  United States
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I appreciate this extra effort. If it's too much of a hassle, the LX75 stock date just got updated to May from September, so hopefully I can get that by summer if the LX45 route doesn't happen.
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Old 28th February 2021, 10:12 PM   #1493
MarcelvdG is offline MarcelvdG  Netherlands
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Join Date: Mar 2003
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It's not the aliases, apparently I introduced some bug in the sigma-delta modulator that causes asymmetrical behaviour. It can handle much larger positive than negative input signal excursions without integrator clipping. Fortunately it can be simulated with reasonably short run times, so I should be able to figure it out.
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Old 2nd March 2021, 11:16 PM   #1494
MarcelvdG is offline MarcelvdG  Netherlands
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Join Date: Mar 2003
Location: Haarlem, the Netherlands
I made a mistake with the quantizer clipping levels. In the PWM9 mode, the quantizer output range is -5...+4, but I had put the clipping levels at -4 and +5. In the simulator, it works a lot better now that's corrected.
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Old 4th March 2021, 10:04 PM   #1495
MarcelvdG is offline MarcelvdG  Netherlands
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Join Date: Mar 2003
Location: Haarlem, the Netherlands
Attached is a zipped .mcs file meant for TE0630 LX45 modules. I solved the bugs, but I can't test it in real life at the moment because my JTAG programming cable now really stopped working. Resoldering its connectors didn't help, so I guess I'll just have to order a new one.
Attached Files
File Type: zip versie3_ochtend4maart2021.zip (652.5 KB, 7 views)
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Old 4th March 2021, 10:10 PM   #1496
SonnyMarrow is offline SonnyMarrow  United States
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Join Date: Dec 2016
You're an angel! The expected date of the LX75 got pushed back again, so this is amazing. I have an audio meet this summer and now I can have the ValveDAC there. Before I pull the trigger on the LX45, there aren't any downsides or other changes I need to make?

Last edited by SonnyMarrow; 4th March 2021 at 10:24 PM.
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Old 4th March 2021, 10:29 PM   #1497
MarcelvdG is offline MarcelvdG  Netherlands
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Join Date: Mar 2003
Location: Haarlem, the Netherlands
There are a couple of technical advantages and disadvantages, but they are all very minor:

The LX45 version has less suppression of far ultrasonic images (> 100 kHz, particularly 500 kHz...700 kHz).

The LX45 version has slightly more out-of-band quantization noise in the PWM modes.

It probably has a slightly better signal-to-noise ratio over the audio band in the PWM modes.

Aliasing effects in the quantizer of the sigma-delta modulator in the PWM modes are completely absent in the LX45 variant, rather than negligibly small.

The chaotic sigma-delta mode of the LX75 versions 1, 2 and 2.1 has a time-varying noise level when playing absolute silence, I implemented a workaround for that in the LX45 version.
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Old 4th March 2021, 10:31 PM   #1498
SonnyMarrow is offline SonnyMarrow  United States
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The disadvantages appear to be inaudible? 100khz and above isn't really a concern, I wouldn't think
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Old 4th March 2021, 10:32 PM   #1499
MarcelvdG is offline MarcelvdG  Netherlands
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Join Date: Mar 2003
Location: Haarlem, the Netherlands
Indeed, even your cat couldn't hear it.
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Old 5th March 2021, 01:11 PM   #1500
SonnyMarrow is offline SonnyMarrow  United States
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Join Date: Dec 2016
Great, I ordered the LX45 and the JTAG cable! Getting fairly close to having a functioning DAC.

I had a question regarding TH1. If I'm using a separate soft start circuit for the entire unit, is TH1 redundant? I was wondering if I should do a wire jumper at TH1 in that scenario.
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