DIY Transistor Amp Using NVA Boards

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Cressy Snr
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#331 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

IslandPink wrote: Mon Dec 12, 2022 5:48 pmNice work !
Thanks Mark.
There are so many conflicting opinions about negative feedback that it is hard to know who to believe. The only answer seems to be to try it yourself at different levels/settings and see/hear what happens by measuring and listening. I've done this on two completely different amplifier technologies and fail to understand the bad mouthing some designers give it. Provided you compensate the bloody thing properly it works beautifully - end of story! It's not bleed'n rocket science.
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#332 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

OK,
So I added a bit of capacitance to the power supply feeding the output stage, so that it now has 16800uF per rail as opposed to the 10000uF per rail it had before and the result is good.
SSPSU.png
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Authority, midband performance, depth, width, transient response, treble delicacy and overall musicality have been boosted.

I was only trying to improve the bass definition by adding the 6800uF of capacitance to the power stage, but it seems to be a waste of time looking up opinions on how much capacitance is really required and how much is too much. I don't know a great deal about solid-state amplification and was only able to build something decent because of the late RD's generosity in sending me the boards in the first place. When I asked him about power supplies for SS amps he told me the boards were all I was getting, that I should buy some 200V, 25A rectifier packages, use 25-40V toroidal transformers, and to 'feck off and find out the rest,' and that was it for the info. :lol:

That I must have had slightly too puny a power supply to run class B effectively is all I can conclude from this.
Last edited by Cressy Snr on Thu May 04, 2023 7:06 pm, edited 1 time in total.
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Cressy Snr
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#333 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Decided to put another wood fascia on the amp. On/off switch has been moved to the back in order to clean up the looks:
IMG_0193.jpeg
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Tomorrow, it’s back to a bit of plasterboarding a short stud wall with door frame.
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Cressy Snr
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#334 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Read a bit about power supplies for SS amps on Rod Elliot's site and a 3-part article on the same (TNT - Audio) during a break from the plasterboard job.
Found from both sites and a few threads on DIYAudio, that I was sort of right about the 10,000uF of capacitance per rail in the original version of the NVA based amp, except that the 10,000uF figure was actually the value for a mono amplifier. OK, so I was completely wrong in that case.I only had half the story! There that's better. Confession is good for the soul.
As it happened, I had a bag of 10 CDE (Cornell Dubilier) 6800uF-63V doing nothing, and three of them in parallel, give 20,400uF which is near enough. So last evening, the caps got installed, complete with new bus bars:
Inside.jpeg
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SSPSU2.png
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The result is not the major leap up in sound quality that happened in the previous post, but there was a slight improvement in the solidity of the soundstage. Everything is locked in place, solidly grounded and everything stays separate and easily followed, no matter how loud or complex the going gets.

That there is more that could be done in terms of HF filtering and inductance killing networks is a given, having read up a bit, but I'm enjoying the solid qualities so far. As RD would have put it, "A bit more muck cleaned off the window."
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Cressy Snr
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#335 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

More mods:
I've redrawn the power supplies in a better format, as done by Rod Elliot, put the 10000uF slit foil Supertech caps on the input and driver stage supply and put a symmetrical RC filter across the output of the output stage supply because a different solid-state bloke on the internet sez. I've made sure it all works but have not yet listened to the results.
SSPSU3.png
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#336 Re: DIY Transistor Amp Using NVA Boards

Post by jack »

Might be an interesting exercise to power the unit off of a known "quiet" source with low ESR, e.g. batteries, and then see if a) you like that sound and b) if you can approach that with a mains-driven supply.
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#337 Re: DIY Transistor Amp Using NVA Boards

Post by Mike H »

Only issue I can see with increasing the cap values is, if/when discharged due to large signal peaks, the transformer is going to have to recharge them again, which will take a finite time due to its windings' impedances. There is probably an ideal ratio of max. transfomer secondary current vs reservoir capacitor value, but I don't offhand know what it is. Personally I'd be uncomfortable about adding more caps ... not saying I never have ...

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#338 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Rod Elliot works it all out as around 3500uF per Amp of current delivered to the speakers.

3.5A into 8 Ohms is about 100W (98W actual) so 3.5 x 3500 = 12250uF per rail for a mono setup, which gives 24500uF per rail for stereo. If you want to be able to drive 4 Ohm speakers then the requirements double.
I configured for 50W into 4 Ohms so ended up with the 24500uF per rail as above for stereo. In fact I’m slightly under at 20400uF as I have 2 power supplies, so the output stage is relieved of the requirements of the input and driver transistors, which have their own power supply with 10,000uF per rail.

Regulation of the power to the pre/driver stages might ultimately be a better engineering solution than the pair of 10000uF caps, but it works nicely as is. Regulation of the smaller power supply remains a possibility for the future.

Of course I don’t know a great deal about SS power supplies so by necessity am relying on the info given by Mr Elliot being good, but at the moment, I’m listening to the results of everything done using his advice and information and in terms of absolute sound quality, micro and macro dynamics and transient behaviour, the amp certainly seems to work very well indeed.

Edit - Mains transformers are 220VA for the driver stages and 300VA for the power stages. The power stage transformer is out of an old active subwoofer.
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Nick
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#339 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

Mike H wrote: Sat May 06, 2023 6:31 pm Only issue I can see with increasing the cap values is, if/when discharged due to large signal peaks, the transformer is going to have to recharge them again, which will take a finite time due to its windings' impedances. There is probably an ideal ratio of max. transfomer secondary current vs reservoir capacitor value, but I don't offhand know what it is. Personally I'd be uncomfortable about adding more caps ... not saying I never have ...

Image
The will be fully charged up again in 10ms by the end of the next 100hz rectified sine wave. Adding more caps won't require more charging. All that needs replacing in the cap is the power used by the amp in the last 10ms. More or less capacitance won't alter that. All that will be different is how low the voltage is discharged down to.
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#340 Re: DIY Transistor Amp Using NVA Boards

Post by brig001 »

Most current is required for low frequencies, and these are usually mono. Inverting one channel can improve things, but it’s not always easy to implement. Only works if outputs aren’t bridged obviously

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#341 Re: DIY Transistor Amp Using NVA Boards

Post by Mike H »

Nick wrote: Sun May 07, 2023 8:34 pm All that will be different is how low the voltage is discharged down to.
Which reminds me of a Hi-Fi magazines advert from the '70's – "Does yours sag?"

:lol:
 
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#342 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Finished off the amp today with a major tidy up of the power supply mains wiring.
IMG_0197.jpeg
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I'm not particularly happy about the length of the leads that run from the PSU cap banks to the board. To me, after reading all about solid state circuits, those leads are waaaay too long, but nothing short of a complete rebuild and change of layout, will solve that particular problem.
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#343 Re: DIY Transistor Amp Using NVA Boards

Post by Daniel Quinn »

So long wires are detrimental to sound ?
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#344 Re: DIY Transistor Amp Using NVA Boards

Post by jack »

Shod.... Hang on!

No... Very nice actually !
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Cressy Snr
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#345 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Daniel Quinn wrote: Thu May 11, 2023 8:25 pm So long wires are detrimental to sound ?
Dunno to be honest Dan. The opinions seems to be that in a transistor amp, power and signal paths need to be as short as possible, but the amp sounds fantastic as is, so maybe I’m obsessing over nothing.
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