DIY Transistor Amp Using NVA Boards

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

Post by Cressy Snr »

Well It didn’t blow up:
38V on the front end, 51V on the power stage. Heatsinks are barely warm.
DC offset; zero. Noise; zero.
5C7AB57E-E85C-4382-95E5-9AACEFA15F67.jpeg
I’ll leave it soaking in the system for the rest of the day, then start on the cosmetics tomorrow. It’s pretty damn’ fugly at the moment :D

After the power supply incident, there is now, 6800uF per rail on the front end and 10000uF per rail of Supertech “Guided Current” caps on the power stage. I figured that 20,000uF of caps (power stage) would take fractionally longer to charge than 13,600uF (front end) thereby ensuring that the front end would come up a few milliseconds ahead of the power stage. This, I thought, would avoid any nasty noises at power-up. It seems to have worked a treat; this amp makes far less of a switch-on thump, than the A20.
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#107 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

I’m well impressed with the sound quality improvement over the A20.
Multiple power supplies are in. :)
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#108 Re: DIY Transistor Amp Using NVA Boards

Post by ed »

Cressy Snr wrote: Mon Jan 20, 2020 2:07 pm Heatsinks are barely warm.
never a good sign, Pussy!

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#109 Re: DIY Transistor Amp Using NVA Boards

Post by karatestu »

Cressy Snr wrote: Mon Jan 20, 2020 5:18 pm I’m well impressed with the sound quality improvement over the A20.
Multiple power supplies are in. :)
I have heard a couple of reports of preferring the sound of a single psu. I am firmly with you on this one :D

As there is no pot to change the bias then there is no inclination to bias it more in to class A and get some heat going.
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#110 Re: DIY Transistor Amp Using NVA Boards

Post by Dave the bass »

Smaller heatsink! They're obviously far too big at the moment if the skin stays on your fingers.
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#111 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Dave the bass wrote: Mon Jan 20, 2020 8:10 pm Smaller heatsink! They're obviously far too big at the moment if the skin stays on your fingers.
I think, the heatsinks are cool because of the high efficiency and ease of drive (no wild impedance swings or tricky reactive monkey business) of the speakers. As Ed put it in another thread, the amps are barely being tickled by the Fane single drivers.
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#112 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

So how does the NVA clone sound now it has been reworked with separate power supplies on the front end and power stages?
In a word, marvellous! It’s surprisingly easy describe the improvement over my bog standard A20; the clone, further gets out of the way of the music.

The biggest difference between the clone and the A20 is in terms of pitch, dynamics, rhythm, and tune playing ability. The clone is in a different league to the A20 in that regard.

It’s sweet as a nut, unfatiguing, with an expansive soundstage and is musically, instrumentally, texturally and spatially detailed without throwing everything your face. It all hangs together nicely; unforced, solid and natural.

The nearest description I can give in terms of the equipment itself, is that “The Clone” appears to imitate the presentation of my multi-power-supplied, single-tube OTL, but with the addition of a bloody great rocket up its arse.

I’d like to put three power supplies on it, but I’m not going to do that. I reckon I’ve got away with what I’ve done so far, by the skin of my teeth, and these boards are precious to me. As far as I am aware, only myself and Stu have had them as bare circuit boards, to be DIY powered; so in that sense, they are personally valuable items.

So where to go? Well if anyone on here can give me some suggestions as to where to find literature on solid state high fidelity amp circuits, how they work, and how to apply them without creating arc-welders, I’d be grateful.
Last edited by Cressy Snr on Tue Jan 21, 2020 1:39 pm, edited 1 time in total.
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#113 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

I think that and the amp is basically a class b amp so idle current is very low. Try running it into a load with a sine wave at 2/3 full power for half a hour to check the hs size.
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#114 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

For a reference work Self or Cordel would be my suggestion. Probably and instead of or in the last sentence.
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#115 Re: DIY Transistor Amp Using NVA Boards

Post by karatestu »

Cressy Snr wrote: Tue Jan 21, 2020 1:36 pm
I’d like to put three power supplies on it, but I’m not going to do that. I reckon I’ve got away with what I’ve done so far, by the skin of my teeth, and these boards are precious to me. As far as I am aware, only myself and Stu have had them as bare circuit boards, to be DIY powered; so in that sense, they are personally valuable items.

I think the good Dr had this circuit more or less as good as it gets. I won't be messing with mine. There are a few things to try but they would probably be a step back from what makes the sound of these boards so good (all IMO ofc). Decouple the output transistors, increase bias current.

From what i gather Steve you have one transformer for the output stage of both channels and one for the remainder on both channels ? If so you may get some gains from having two single secondary centre tapped transformers per channel like the NVA A80s. I have transformers with two centre tapped secondaries for the first four amps i built which achieves the same thing (almost) but for my last two i am using two transformers per channel.
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#116 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Nick wrote: Tue Jan 21, 2020 1:41 pm For a reference work Self or Cordel would be my suggestion. Probably and instead of or in the last sentence.
Cheers Nick,
Plenty to get one’s teeth into, with those two volumes. :)
karatestu wrote: Tue Jan 21, 2020 3:12 pm
I think the good Dr had this circuit more or less as good as it gets. I won't be messing with mine. There are a few things to try but they would probably be a step back from what makes the sound of these boards so good (all IMO ofc). Decouple the output transistors, increase bias current.

From what i gather Steve you have one transformer for the output stage of both channels and one for the remainder on both channels ? If so you may get some gains from having two single secondary centre tapped transformers per channel like the NVA A80s. I have transformers with two centre tapped secondaries for the first four amps i built which achieves the same thing (almost) but for my last two i am using two transformers per channel.
Thanks Stu. Yep, I suppose mono-blocks would be the way to go, for further exploration. Unfortunately, stereo is the only option at the moment, due to space considerations, and the recycled CD player chassis looks really good, even with its stripped out, full of holes, front panel on view, until I get a new front made up.
Last edited by Cressy Snr on Tue Jan 21, 2020 4:50 pm, edited 1 time in total.
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#117 Re: DIY Transistor Amp Using NVA Boards

Post by ed »

Cressy Snr wrote: Tue Jan 21, 2020 1:36 pm
So where to go? Well if anyone on here can give me some suggestions as to where to find literature on solid state high fidelity amp circuits, how they work, and how to apply them without creating arc-welders, I’d be grateful.
Steve
did these suggestions not provide any, or enough ideas?.

http://www.audio-talk.co.uk/phpBB2/view ... =17&t=7239

Nick beat me to a reply for the earlier post. The sinks are not getting warm because the output darlingtons do not have enough bias to allow the current between C and E to start cooking at idle. In a modest push pull like Richards circuits they are biased just enough to keep them out of cut-off. This is of course a simplistic explanation. You have the boards to deconstruct the circuit but if you'd like me to send you a copy of the circuit you could play with it in spice.
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#118 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

ed wrote: Tue Jan 21, 2020 4:10 pm
Cressy Snr wrote: Tue Jan 21, 2020 1:36 pm
So where to go? Well if anyone on here can give me some suggestions as to where to find literature on solid state high fidelity amp circuits, how they work, and how to apply them without creating arc-welders, I’d be grateful.
Steve
did these suggestions not provide any, or enough ideas?.

http://www.audio-talk.co.uk/phpBB2/view ... =17&t=7239

Nick beat me to a reply for the earlier post. The sinks are not getting warm because the output darlingtons do not have enough bias to allow the current between C and E to start cooking at idle. In a modest push pull like Richards circuits they are biased just enough to keep them out of cut-off. This is of course a simplistic explanation. You have the boards to deconstruct the circuit but if you'd like me to send you a copy of the circuit you could play with it in spice.
Bloody hell, I’d forgotten I’d even started that thread. :oops:
Yes, I wouldn’t mind seeing the actual circuit itself. Thanks Ed.
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#119 Re: DIY Transistor Amp Using NVA Boards

Post by Ant »

Have a look in that wireless world 'high fidelity designs' book I lent you, there's the jlh circuit and a whole load of theory in there to go with it although it does seem to assume a certain amount of knowledge, but the theory bit did seem relatively simple even to me who knows less than the square root of sod all
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#120 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

It’s almost fully cased up now:
413A493E-C3C4-47AC-91F2-877816A52E04.jpeg
Holes punched in top surface of case will be backed with aluminium mesh, sprayed black.
Still to decide on a front panel. Whatever I decide upon, the panel will be attached to the Sony front panel we see in the picture. It’ll be blank with the LED poking through a 4mm hole.
I’m pleased with the overall effect and though it took a lot of drilling, cutting, filing and bleeding (copyright Mr I) repurposing the old CD player case was worth the hassle and it’s good to recycle.
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