DIY Transistor Amp Using NVA Boards

[Daniel Quinn]

They offer a 30 day money back trial, stop pissing me off and try it.

Me I won't

[pre65]

They offer a 30 day money back trial, stop pissing me off and try it.

Me I won't

So, you don't know, and don't want to know.

I thought you were ignoring me.

[Cressy Snr]

On second thoughts: I can’t be arsed.

In the end I talked myself into actually finishing the project properly, rather than having the phono stage hanging off the VAS/buffer supply. So now the amp is on two power supplies and the phono stage is on the one with the toroid fitted to the rear panel and standing vertically:

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[Cressy Snr]

Final schematic.That's yer lot:

TriplePSUFinal.png (85.47 KiB) Viewed 8218 times

Thoroughly enjoyed getting a bit deeper into the solid-state realm. I've gained a bit of knowledge from various sources and come out with a great sounding project. No qualms about bringing it to Owston that’s for sure

[radio_free]

Never thought you could manage to squeeze in 3 sets of PSU into that chassis, but they look really good.
Would you consider putting in RC filter at the end of the power rails for the 2 smaller PSUs?

I'm still wondering if the input coupling caps are necessary especially when you have the source in the same box, you may want to do an AB test with and without them at some point.

I just shorted my PSU over the weekend and amp is not working now, I suspect the output transistors are also fried
Looks like there is now a reason for me to do an overhaul.
This amp was a "B-stock" bought from RD almost 20 years back. The top plate of the perspex chassis was a little deformed when it first came (seems like the original one was toasted which was then sent back to RD who repaired it and sold as B-stock), I think its time to get one of those nice looking aluminum chassis from China.

[Nick]

I'm still wondering if the input coupling caps are necessary especially when you have the source in the same box

If you go back over the thread(s) Steve wanted a constant and low output dc offset. Not sure how to do that with a pot on the input without a coupling cap or adding a servo.

[Cressy Snr]

…Would you consider putting in RC filter at the end of the power rails for the 2 smaller PSUs?

I'm still wondering if the input coupling caps are necessary especially when you have the source in the same box, you may want to do an AB test with and without them at some point.

I just shorted my PSU over the weekend and amp is not working now, I suspect the output transistors are also fried
Looks like there is now a reason for me to do an overhaul...

Yes, I think it might be worth filtering the power rails of the other 2 power supplies, but It’s not a priority at the moment. The main source of any nasties (power stage) has been dealt with using both the PSU filters and the Zobel/inductor output combination, and the shielded input cap goes a long way towards preventing RFI getting into the preceding stages. I may however put X-class caps across the AC terminals of the bridge rectifier packages at some point in the future.

As Nick says, under DC conditions, the input cap buffers the LTP against the influence of the passive pot and ensures that all the work done matching the feedback resistor with the input resistor in order to ensure a low DC offset at the output, is not undone by the variable resistance of the pot in parallel.

If you are going to overhaul your amp, You could do the mods I did. The inductor and Zobel network on the output was the first thing, followed by the feedback resistor change, then the input cap and input resistor matching with the the feedback resistor and cap values. As Nick also said, 100uF bipolar caps would be best if you are going to do the mods. Once I get the project with the other boards off the ground, 100uF is what I will use.

[Cressy Snr]

Complete power supply below. I've applied symmetrical HF filtering to the input/VAS/buffer stage supply, using what I had to hand and working out on the back of a fag packet, so that both amplifier supplies now have HF filters on their outputs. I also put an X-2 suppressor cap across the primary side:

SSPSUFin.png (88.06 KiB) Viewed 7947 times

[Nick]

What job is the 1R and 3R9 resistor doing?

[Cressy Snr]

Hi Nick, quote below and URL to the explanation.

Also, please note the two symmetrical RC networks. They serve to get rid of the residual capacitor inductance, which should improve high frequency performance. In my experience, it always does, though to what extent remains open to debate, trial and error. No matter how good the capacitors may be, they always have some inductance left over; the better they are, the lower the value, and vice versa. Therefore, this is always good to have, even if its greatest effects will show up with the worst of capacitors. A side benefit is that the amp will tend to be even more stable with complex loads, although this is primarily something the amp design should deal with.

Here:
https://www.tnt-audio.com/clinica/ssps2_e.html

Third paragraph down. The filters do (to my ears) as the guy says improve the quality of the treble. The top end is worse without them. With the filter on the power stage, the treble got better, adding one to the other stage improved it further. The top end is as good now as any valve amp I’ve heard.

With all this lot done to it, including all the rest of the mods (feedback, cap input, Zobel + output inductor) this no longer sounds like an NVA amp. I personally think it’s better but that’s just me. Don’t know why I should feel worried about saying so but it’s difficult not to feel the history.

[Nick]

OK, but normally to match impedance with a Boucherot cell, you have the R equal to the ESR of the caps, so I would think far less that 1R, and the C = L/(R.R)

But if it works, all good.

[Cressy Snr]

Cheers Nick.
I might be able to work out something a bit better using the cap specs and the formula you provided. It works well as it stands, but might as well try and do it right when I start to work with the other boards.

[Cressy Snr]

Ok, so we are now a couple of months into life with the messed about with NVA amp amp boards with twin power supplies and built-in separately powered phono stage, so I thought I might try the single-ended pentode amp for a while.

I let it warm up over a one hour time period before letting it loose on a few choice recordings. Oh dear

I know I've done a bloody good job of breathing on the NVA boards, but even so, I was not prepared for the sheer crapness of the sound of valve amp in comparison. In every department, the class B solid-state amp wiped the floor with it. Great on the one hand, sad on the other, but that's the situation.

If I want to build something with valves that will be able to live in the company of the SS amp, I’m really going to have to go some. Whether I can be arsed is another question entirely. I mean, what would be the point?

[izzy wizzy]

If I may take you back to an earlier post of yours ....

"With 85dB efficient speakers, a solid-state amp of decent power is the obvious choice."

Is the valve amp crappy or is it just not a match for those speakers?

I would also wager that the valve amp you have there is hardly as max'd out as the NVA.

SET and SEP amps are a very different sonic matrix to class AB SS amps as are 85dB and >95dB speakers Just an observation from reading your threads.

Cheers,
Stephen

[Cressy Snr]

I don’t think there is a mismatch. The Missions are 8R - an easy load and the valve amp certainly goes loud with them. Comparing apples and oranges is, I suppose, a big danger with any sort of A/B comparison like this. But my own single-ended tube efforts have proved to be no match for what I have with this solid state amp. I reckon that to get anywhere near it now, I’d need to go down the push-pull KT88 and above route.