DIY Transistor Amp Using NVA Boards

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Paul Barker
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#136 Re: DIY Transistor Amp Using NVA Boards

Post by Paul Barker »

Cressy Snr wrote: Thu Jan 23, 2020 4:17 pm
steve s wrote: Thu Jan 23, 2020 11:21 am It would be good to hear it Steve, I too have found what I think is a much better amp than the a20, but im sure people will laugh when I say it's built around the tda2030 chip..
I've owned them for a few years, but eventually give them a try a few months ago.. pretty good in my view.

I've been trying to build an off the net circuit, quite unsuccessfully !.. to see if the sound is recreatable.... I think I'm going to end up copying my originals when I get a chance though.

I've been planning a new valve amp and been back and forth deciding which output valve to use... still not really settled on it.. going back to one power supply.. as a tester, but with an unconventional smoothing arrangement as valve amps go.
I have picked a mains transformer with multiple by taps so switching between choke and cap loading will give a wide HT range, I think I may settle on 250vt globe px4s for testers
Let's see..
Wish I’d paid more attention back in 1975; it would have been far easier to learn about transistors then, than now, when I don’t know what day of the week it is half the time :l
I forgot a cousin’s name yesterday trying all evening last night while with someone I needed to tell. The more urgent the need to recover the momentary laps of knowledge the emptier the conscious mind becomes. Just now when I read Your post Steve her name just popped up from the dark emptiness. Always comes when you aren’t in need of it later!
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Ali Tait
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#137 Re: DIY Transistor Amp Using NVA Boards

Post by Ali Tait »

I believe it's called a senior moment. :D
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#138 Re: DIY Transistor Amp Using NVA Boards

Post by steve s »

Nice to be of use Paul
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#139 Re: DIY Transistor Amp Using NVA Boards

Post by kptseng »

Hi Everyone,

I’ve been reading this for recently and have joined this forum 2 days ago.
Seeing this project getting on well gives me a lot of confidence in modifying my old A60 into something better.

My plan is to use 100W transformer for each input stage and 250W transformer for each output stage.

I’m seriously considering using a LDR attenuator (aka Lightspeed) with a remote controllable digital potentiometer; but seems like the LDR type is only suitable for amp with input impedance no less than 100k. I’ve tried searching for NVA’s input impedance but with no clear answers. Can experienced NVA fans advise me on this?

Once I get this done I’m actually thinking of ripping out my Chord Mojo and incorporate this into the amp as well (of course with a dedicated power supply).

Many thanks in advance

KP
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#140 Re: DIY Transistor Amp Using NVA Boards

Post by kptseng »

Just a thought.......

I can see that the A80 runs on 2 separate power supplies for the input and output stages. I’ve read the write up about PSU on TNT-audio and seems like a regulated power supply for the input stage would benefit greatly while the output stage should be running on unregulated one.

Anyone have explore using regulated power supply for the input stage?
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#141 Re: DIY Transistor Amp Using NVA Boards

Post by karatestu »

Hello. I think NVA input impedance is about 47 K ohms. Richard Dunn was not a fan of regulators or optocouplers. I have run regulators on the input stages of my nva amps but only because I needed to drop some voltage. I never got to try the input stage without as that would need different transformers but I will get around to it eventually.

Good luck with your plans.
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#142 Re: DIY Transistor Amp Using NVA Boards

Post by kptseng »

Thank you Karatestu for the very useful information and sharing you experience.

Yes, 47kohm input impedance would be ok for the Lightspeed but while the circuit for LDR volume control is simple, finding 2 matching pairs of LDR will be challenging for me, there is no kits available and I can only find Lightspeed in the form of separate pre-amp which is not what I want (I prefer integrated into the power amp to minimise external cables).

I thought about using AVC from Intact Audio but the cost is quite steep for me, so I am leaving this as a consideration for further development.

Current plan is to get the Tentlabs relay volume control which seems neat and priced quite reasonably.

For the regulated power supply for input stage, my plan is to get those "Super-reg" to regulate at 35vdc. All my transformers are with 35vac secondary so I would get approx 44vdc after the LCC smoothing, which will allow some voltage drop for the regulation.

Other thoughts are:
1. Use Vishay metal foil resistors for those on signal path - this will get very expensive.
2. Use all film caps on amp board - I have some 41uF JFX caps which will work but need to do some adjustment to create space for them.
3. Add a pair of VU meters for that classic feel :-) - need to find the right circuit first.

Well, have to get moving and stop planning for far ahead.............
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#143 Re: DIY Transistor Amp Using NVA Boards

Post by karatestu »

Hello.

I would be very careful when swapping parts on the nva amplifier boards unless you really know how it operates. Resistor swaps may not be too risky but capacitors are another story. I think Richard had this circuit optimised so becareful you don't make the sound worse. One modification at a time and then listen. Swapping back to the original can often be illuminating and show where you have "imagined" an improvement.

What exactly are the super regulators you refer to ? They may not be able to cope with the voltage you are intending to use .35V ac secondary transformer winding will give you a shade over 50 V dc and any drop over the smoothing caps will be very small. How did you get the 44 V figure ? A super reg will probably cope with the load of the front end no problem.

Stu
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#144 Re: DIY Transistor Amp Using NVA Boards

Post by kptseng »

Hi Stu,

For the regulators, I am looking at one of those from Teddy Pardo or this one (https://diyaudiostore.com/products/super-regulator).

I'm planning to use LCC after rectifiers, the L before C would not give the peak voltage of 50vdc but around 90% of it so I'm having around 44vdc on my paper for now.

Thanks.
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Nick
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#145 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

If you are using choke input make sure you are pulling enough current for your choke inductance.
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#146 Re: DIY Transistor Amp Using NVA Boards

Post by rowuk »

kptseng wrote: Sun Aug 23, 2020 5:00 pm Hi Stu,

For the regulators, I am looking at one of those from Teddy Pardo or this one (https://diyaudiostore.com/products/super-regulator).

I'm planning to use LCC after rectifiers, the L before C would not give the peak voltage of 50vdc but around 90% of it so I'm having around 44vdc on my paper for now.

Thanks.
Maybe listening as originally intended would be a good start - and then IF any flaws are noticed, thinking about what could address those flaws. I think that the NVA boards got a LOT of listening during the design stage. Sure, someone elses preference but certainly without serious issues. Basically we "listen to the power supply". Changes there will dramatically affect the sound.
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#147 Re: DIY Transistor Amp Using NVA Boards

Post by kptseng »

Thank you Stu, Nick and Rowuk for your input.

Power supply will be my first priority and will try out both regulated and unregulated power supply for the input stage, output stage will be fed with unregulated ones.

We agree that Richard had designed the circuit so well, which is why we (NVA fans) love the sound of NVA so much.
In one of the old article I read, I remember Richard briefly mentioned that the amplifier will benefit from better components - not that his choice of components were poor, on the contrary I believe he had chosen each component very carefully taking into consideration of space and affordability for us, and that he had done so well.
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#148 Re: DIY Transistor Amp Using NVA Boards

Post by pre65 »

Nick wrote: Sun Aug 23, 2020 6:10 pm If you are using choke input make sure you are pulling enough current for your choke inductance.
Is the current calculated as V/H ?

And does a "tuning capacitor" of (say) 0.5uf have any benefit and still keep it in choke input ?
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#149 Re: DIY Transistor Amp Using NVA Boards

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Nick wrote: Sun Aug 23, 2020 6:10 pm If you are using choke input make sure you are pulling enough current for your choke inductance.
Hi Nick,
I read about the need to have enough current for the LC filter to work, but I’m not sure what I need to do to ensure that.

My plan is to have 30H/10Amp choke, followed by 2x10,000uF for each rail. The transformer size will be 250VA with 2x35vac secondary for the +ve and -ve rail. Would that be ok?

Also, would adding a soft start module before the transformer cause any issue (to the current pull)?

Thanks
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#150 Re: DIY Transistor Amp Using NVA Boards

Post by ed »

kptseng wrote: Thu Aug 27, 2020 11:07 am

My plan is to have 30H/10Amp choke,
??? wow, how big is it?
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