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Re: Building a DIY amp with NVA amplifier boards
Posted: Sat Nov 06, 2021 9:54 pm
by karatestu
Sorry I can't say anymore. It is just what I read on here and other forums from figlet who worked with RD for a while to produce a streamer.
I have never had a board blow in anything and I aim to keep it that way.
Re: Building a DIY amp with NVA amplifier boards
Posted: Tue Nov 09, 2021 11:00 pm
by r3xj0hn570n
And here now have a mostly conventional power supply.
- mod_amp.jpg (193.87 KiB) Viewed 8408 times
Re: Building a DIY amp with NVA amplifier boards
Posted: Wed Nov 10, 2021 10:06 am
by karatestu
Hi Rex,
You have wooden side panels and base in that enclosure.?
I often have the thought of rather than giving front and back end a separate psu that I could just drop a LM317 / 337 regulator in the power rails between the two parts like where the diode and resistor are in the pic
Would have to ensure that the regulator has enough dropout so that it still regulates if the power rails sag. It would enable the use of a higher voltage secondary as you can drop voltage via the reg.
I may give it a go at some point
Re: Building a DIY amp with NVA amplifier boards
Posted: Wed Nov 10, 2021 8:58 pm
by r3xj0hn570n
Yep, ply from an old packing case for the base and sides. I've made a top out of hardwood ply, but that is going to have some holes cut in it above the transformers, they get warm. Front and back are black acrylic from an old pizza house sign. Fixings are mainly brass and stainless. Transformers are now from a JVC VCR (seriously!), well, 2 actually. The whole thing is made from recycled stuff, bar a transistor and cap or three and a bunch of screws.
Re: Building a DIY amp with NVA amplifier boards
Posted: Wed Nov 10, 2021 9:34 pm
by r3xj0hn570n
And onto the circuit modifications.
Here is what i measured before messing about with values. There is more 2nd and 4th harmonics than you would normally see as i've used asymmetric emitter resistors. Measured at about 2vrms output.
- unmodified_amp_1kHz@-10dB_distortion_spectra.jpg (151.34 KiB) Viewed 8305 times
and here is what we've ended up with.
- modified2_front_amp_1kHz@-10dB_distortion_spectra_L.jpg (142.64 KiB) Viewed 8305 times
Next step is to install it and compare it to my other standard amp.
Re: Building a DIY amp with NVA amplifier boards
Posted: Fri Sep 23, 2022 3:35 pm
by Radio_Free
I've read about isolating the power rail for input stage and output stage with a diode+220R bypass with 100uF+100nF.
Would having separate wires from the shared PSU to the respective power input points of each stage be a sensible alternative? Of course need to cut away the existing wires that currently link the power inputs for each stage.
Re: Building a DIY amp with NVA amplifier boards
Posted: Fri Sep 23, 2022 8:44 pm
by karatestu
Swings and roundabouts. The diode and resistor thing is a clever trick. Separate rectifier and cap bank off the same transformer winding is probably better than one . Not as good as two transformer Windings though.
However, sometimes simplicity is better. Depends on your speakers though.
But i would advise the route of Just enjoyng the music and forgetting about all the ocd bollox.
Re: Building a DIY amp with NVA amplifier boards
Posted: Sat Sep 24, 2022 6:34 am
by karatestu
By the way, the 100uf and 0.1uf caps are to ground (decoupling) and not between the stages. Nva don't have the 100uf, just the 0.1uf.
Re: Building a DIY amp with NVA amplifier boards
Posted: Sat Sep 24, 2022 6:45 am
by Radio_Free
Thanks Stu,
Of course, doing like the A80s would be the ultimate, and I would get a used pair at some point when I have a chance to.
I’m still running my old A60 with a volume pot. Had some issues with a few transistors and diodes but Tom gave some useful advice and I got a friend to fixed it for me, and took the chance to recap at the same time.
Yes, I see that each board has only a pair of 0.1uF for the power rails, so splitting the power rails would also mean the need to add one more pair of 0.1uF to the power for the output stage. I think adding the 100uF will going be a challenge in terms of space (probably on the underside of the board). Which is why I thought if pulling another wire from the same cap bank would be an alternative but seems not. Anyhow I’ll probably leave them as they are for now and enjoy the music (like you said)
Re: Building a DIY amp with NVA amplifier boards
Posted: Sat Sep 24, 2022 8:50 am
by karatestu
Radio_Free wrote: ↑Sat Sep 24, 2022 6:45 am
Thanks Stu,
Of course, doing like the A80s would be the ultimate, and I would get a used pair at some point when I have a chance to.
I’m still running my old A60 with a volume pot. Had some issues with a few transistors and diodes but Tom gave some useful advice and I got a friend to fixed it for me, and took the chance to recap at the same time.
Yes, I see that each board has only a pair of 0.1uF for the power rails, so splitting the power rails would also mean the need to add one more pair of 0.1uF to the power for the output stage. I think adding the 100uF will going be a challenge in terms of space (probably on the underside of the board). Which is why I thought if pulling another wire from the same cap bank would be an alternative but seems not. Anyhow I’ll probably leave them as they are for now and enjoy the music (like you said)
Yes you could decouple the output stage separately but there aren't that many designs that do so. The power supply rejection ratio of the output stage is much higher than the input stage which after all is kind of like an active preamp. Look at what lengths some people go to for supplying clean power to pre amps.
However , my advice would be don't lose sight of the design choices RD made. As little filtering and regulation as possible. But, you may personally prefer the sound of whatever you decide to do. It is down to your ears and nobody else's. Hifi forums are full of opinion but that is all it is, opnion. Not gospel.