'Digital' HPA Project

For anything about cans, and the amplifiers that drive them
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Ray P
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#1 'Digital' HPA Project

Post by Ray P »

Here's an interesting little project that I had all but forgotten until I found in a drawer last weekend and decided to see what I could do with it.

I've labelled it as a 'Digital' HPA because it is based on a cheap Chinese Class D PWM amplifier board, (designed to drive loudspeakers); around 6yrs ago a chap on DIY Audio was experimenting with using the amps with headphones with reportedly very good results - on the basis of it being cheap and looking a bit of fun I bought a couple of the amplifier boards (about a fiver each, delivered, IIRC). I didn't complete the initial project (I seem to recall documenting some of my initial project somewhere here) because I was having problems getting it to play music properly but the last week has shown that this was down to a faulty board. Anyway, I've now installed the spare amplifier board and this afternoon I've been listening to some very nice music via it.

Basically the digital amp board is modified to remove all of its inductor based output filters and the outputs are routed, via bipolar coupling caps, to output transformers - these achieve three things, they step-down the output for HPA use, they act as low-pass filters to remove the noise on the amp outputs and they perform balanced to single-ended conversion - transformers are wound for 32ohm headphones.

The USB input can only handle 44.1 and 48KHz sample rates due to the limitations of the C-Media USB interface chip. The STA333 PWM digital amp chip can handle 88/96KHz rates and it would, in theory, be possible to feed it i2s from a different input interface but I won't go down that route, mainly because it actally sounds very good as is. With a 14V SMPS the sound is very good other than being a bit bright/tizzy but with a 19V linear power supply the sound is much improved and is very easy to listen to - how much of the improvement is down to the extra volts vs the type I don't know.

A good bang for buck result - I think the total outlay, including the chassis and transformers, was under £50.

I just need to sort out drilling some front/back panels and some final hook-up wiring to put the lid on it.

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Ray P
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#2 Re: 'Digital' HPA Project

Post by Ray P »

A few more additions and improvements. I've added a new low noise linear power supply (outputting 17.5VDC) into the chassis and wired in a couple of 3300uF caps to the STA333 board for some additional local decoupling (the STA333 amplifier arrangement means there is no PSRR). Also added the correct loading caps to the output transformer secondaries. Each iterative change improved the sound quality and I have to say that, for the expenditure, the sound quality of this project is quite extraordinary, a spacious sound, very smooth but really detailed with incredible insight to voices and instruments.

For practical aspects I've made up a rear panel with a USB socket and I'm working on drawing a front panel in CAD with a view to eventually getting it 3D printed.

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Last edited by Ray P on Sat Jan 14, 2023 4:00 pm, edited 1 time in total.
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Cressy Snr
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#3 Re: 'Digital' HPA Project

Post by Cressy Snr »

Looks good Ray.
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Ray P
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#4 Re: 'Digital' HPA Project

Post by Ray P »

It's been fun Steve, no great initial expectations, just an experiment with something different but it's turned out very well. If you were down the road I would get your Ant to give it a try.
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#5 Re: 'Digital' HPA Project

Post by Dede »

It was an experiment that went well then.
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Ray P
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#6 Re: 'Digital' HPA Project

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InDede it deed...
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Ray P
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#7 Re: 'Digital' HPA Project

Post by Ray P »

When I referenced this build in the 'New Toy thread I realised I didn't 'top out' this project, so here's a picture of it boxed up with a 3D printed front panel.

The buttons are Vol +/- and mute.

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