#1 'Digital' HPA Project
Posted: Sat Jan 07, 2023 7:18 pm
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.
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.
