DIY Transistor Amp Using NVA Boards

[Cressy Snr]

Anyway, what has been bugging me for a while with this amp is that the output stage has a higher voltage transformer than the input/driver stage, which I’ve realised is a bit pointless as this isn’t a valve amp. The output stage (current amp) can only produce as much current as the swing of the driver stage (voltage amp) will allow it to, so having the output stage at higher voltage is a waste of electricity if the driver is going to bash into its own rails first.

I was going to remove both transformers and install a 30-0-30 for the driver and a 25-0-25 for the output stage, but both the damn’ things together were too big to fit in the case. The compromise was to keep the 120VA, 25-0-25V on the VAS and driver stage and simply fit a 300VA, 25-0-25V to the output stage PSU.

It turns out that both power supplies produce exactly +/- 37V when under load, which is nice, as if the output rails sag under duress, below 37V, the driver rails will stay up courtesy of their own PSU.

[Cressy Snr]

New 300VA, 25-0-25V toroid at bottom right:

IMG_0201.jpeg (117.72 KiB) Viewed 487 times

Whilst I was at it, I removed the crap quality shielded input cables going to the selector and replaced them with simple straight wires.

I find loads of these blue/yellow twisted pairs lying around next to BT street cabinets when I’m out walking. I found a metre of it the other day due to the engineers not cleaning up after themselves. Shame to let it go to waste. The 0.5mm jumper wire makes good signal wire and my old BT Lindstrom long nose pliers strip it perfectly.

[Cressy Snr]

Revised power supply voltages:

SSPSU4.png (84.27 KiB) Viewed 472 times

[Cressy Snr]

Really pleased with this now. Lovely relaxed, detailed and dynamic presentation. It's certainly worth making the trip to Owston with it.

[radio_free]

Hello,

Looking at Post#364 and #365, I see you now have a RC after the main cap bank.
If I read the TNTAudio article correctly (https://www.tnt-audio.com/clinica/ssps3_e.html) these are best implemented near to the board since it is supposed to take away any remaining inductance on the power rail.

Few questions:
- What are those big film caps hanging over the amp boards?
- I did some measurement on my old A60, the VAC across the emitter resistors are only 25mVAC, and if my math is correct the quiescent current works out to be approx 18mA which seem very small. Have you measured yours?
- so what value of FB resistor you ended up with?

Very nice work, I've been reading your posts for a while

[Cressy Snr]

Hello,

Looking at Post#364 and #365, I see you now have a RC after the main cap bank.
If I read the TNTAudio article correctly (https://www.tnt-audio.com/clinica/ssps3_e.html) these are best implemented near to the board since it is supposed to take away any remaining inductance on the power rail.

Few questions:
- What are those big film caps hanging over the amp boards?
- I did some measurement on my old A60, the VAC across the emitter resistors are only 25mVAC, and if my math is correct the quiescent current works out to be approx 18mA which seem very small. Have you measured yours?
- so what value of FB resistor you ended up with?

Yes, the RC network is supposed to go near the boards, but space was at a premium as can be seen from the photo, so they went on the end of the output stage PSU. There was still a very useful improvement in the treble clarity, sweetness and definition. I would not be without those filters.

Other questions in order:
1. See post #300
2. Yep 18mA quiescent or thereabouts. These are class B amplifiers. With the monolithic TIP142/147 Darlington output stage as applied in this particular circuit, class B operation is (from what I’ve read) the best option RD had for stability and thermal management of the output stage (I think.) I’m still early in my learning though, so I could be wrong.
3. I ended up with around 9K for the RFb.

[radio_free]

Thanks, good to know that my old amp's quiescent current is not abnormal

The RFb on my boards are the original, 18kR. I will try to parallel another 18k to see how it turn out.

[Mike H]

Mike, do you remember the Maplin 50W Mosfet module amp circuit ?

I think you can still get similar on Ebay. A pair of them was very nearly my first amp build.

Yes and I once foolishly tried making them on plain matrix board – can't remember why now, most likely just being a skinflint – didn't work, the seemingly 'overly ornate' copper layout of the kit board is what keeps them RF stable. Despite all attempts to fix, mine stubbornly remained as RF signal generators, so that was the end of that.

There was also a 1kW MOSFET amp kit – it had a Zobel network across the output. At some point the capacitor could no longer be sourced, but buying office found an equivalent. Guess what, with the different cap it was RF unstable, customer kits were being returned as 'faulty'. The development lab had a high old time finding a capacitor that would work, and I think the board layout had to change because the footprint was different.

[Cressy Snr]

…Yes and I once foolishly tried making them on plain matrix board – can't remember why now, most likely just being a skinflint – didn't work, the seemingly 'overly ornate' copper layout of the kit board is what keeps them RF stable. Despite all attempts to fix, mine stubbornly remained as RF signal generators, so that was the end of that.

I would love to build a few circuits out of books myself but the fear of oscillators, arc-welders and speakers reduced to smoking rubble needs to be overcome first….ulp

[Cressy Snr]

I’ve had an issue with this build for a while now, where the sound disappears from the left channel. One culprit was a duff cable where the centre conductor had snapped inside the phono plug. I thought I had cured the problem (well you would wouldn’t you.) Then the bloody thing reared its head again, but this time it was intermittent It would go for days perfectly fine, then the left channel would disappear again. To cut a long story short, I finally traced the problem to the selector switch and replaced it. So far there has been no left channel going AWOL.

Apart from the phono card wiring, which I left well alone, I replaced the other two bits of input wiring with solid copper:

IMG_0207.jpeg (128.73 KiB) Viewed 18 times

What a fecking criss-cross quiz! RD would have been proud…well apart from the Zobel network and the inductor after the output stage, for which he would have chased me down the street with an axe.
Getting at the boards is almost impossible. But there’s nothing I would want to do to them.