DIY Transistor Amp Using NVA Boards

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

Post by Cressy Snr »

So here we have the input stage as it is now:
LTPFbCAPS.png
LTPFbCAPS.png (24.31 KiB) Viewed 7223 times
Feedback resistor matches the input resistor value and the distortion cancelling bipolar cap arrangement is shown. Only the component values of my mods and the 470R value Stu mentioned in the other thread are given. The rest, I've left out and given the CCS a generic symbol.
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Nick
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#377 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

In theory, the two resistors in the base path, the low pass filter and the resistor after that should be added to the 10k that need to be matched on the other side, but they are probably small enough not to matter. Or you could move the low pass filter to the other side of the input blocking cap.

With the 10k/470r ratio, I would expect the amp to have a AC gain of 27dB. Which sounds about right.

20log(470/(470+10000))
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Cressy Snr
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#378 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Cheers Nick,
The gain is just right for my system with “bought” speakers. It was way too high before (32dB) with the original value of RFb. Thanks for the formula that allowed me to work that out.

That was how all this the mod business started. I was trying to get the thing into a decent range on the volume control rather than a 1deg turn of the volume pot blowing the windows out. Then it was the power supply upgrades and finally tweaking up the circuit itself.

I’ve just this afternoon swapped a pair of Faital Pro 10” bass/mid drivers with our Ant for my old A20, which of course has two more boards in it. Stand by for monoblocks with all the current mods to the circuit incorporated and the Fb resistor upped to match the input plus base resistor and low pass resistor. This will increase the gain by 1dB - barely noticeable.

The empty A20 case will be converted into a passive preamp incorporating my Khozmo shunt stepped attenuator. I’m thoroughly enjoying this solid-state lark.
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Cressy Snr
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#379 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Two boards are ready for modification. When I start doing something with them, I’ll start a new thread in ‘Everyones Projects.’
Need to get the heatsinks bought first, so I can size the projects then I might, no will probably, ask our Ant to make a pair of cases in the style of either the QUAD 303 or the old Crimson Elektrik monoblocks.
IMG_0214.jpeg
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radio_free
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#380 Re: DIY Transistor Amp Using NVA Boards

Post by radio_free »

Hello Cressy,

Quite a bit of change this time.
I'm a bit surprise that you have reduced the value of the capacitors on the feedback and added the input coupling caps.
I read somewhere (can't find it now :oops: ) recommending increasing the feedback cap value to ~100uF, and also I thought this amp works best without input cap? And since the input cap is "hanging" anyway, have you considered using a film cap instead?

I took out my old and broken A60 after reading your experiences. A friend of mine managed to fixed the blown output Darlingtons, and it now have the BDW93/94C.
DC offset were a bit on the high side ~63mV. We noticed that some of the TO-92 transistors at the VAS ran very hot, but due to space limitation we only managed to fit in those clip on heatsink on the one in the middle (MPSA92 IIRC). Not sure if it is the heatsink but we recently tested the DC offset again and now ~54mV. You may want to consider this.
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Nick
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#381 Re: DIY Transistor Amp Using NVA Boards

Post by Nick »

100uf would be better. But they were what I had on hand for Steve to try. But 22uf has -3dB of less than 1Hz
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Cressy Snr
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#382 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

radio_free wrote: Wed Jun 14, 2023 1:56 am Hello Cressy,

Quite a bit of change this time.
I'm a bit surprise that you have reduced the value of the capacitors on the feedback and added the input coupling caps.
I read somewhere (can't find it now :oops: ) recommending increasing the feedback cap value to ~100uF, and also I thought this amp works best without input cap? And since the input cap is "hanging" anyway, have you considered using a film cap instead?

I took out my old and broken A60 after reading your experiences. A friend of mine managed to fixed the blown output Darlingtons, and it now have the BDW93/94C.
DC offset were a bit on the high side ~63mV. We noticed that some of the TO-92 transistors at the VAS ran very hot, but due to space limitation we only managed to fit in those clip on heatsink on the one in the middle (MPSA92 IIRC). Not sure if it is the heatsink but we recently tested the DC offset again and now ~54mV. You may want to consider this.
Hi, glad you have got your A60 fixed and running again. :)
Yes, as Nick says 100uF would have been better as the feedback cap, but the 22uF I have in there is adequate as the -3dB point is 0.7Hz, which is OK.

Re the DC offset. Fitting the input cap and matching the feedback and input resistors has reduced DC offset to -5.2mV on the left channel and -5.8mV on the right. My output transistors are TIP142 and TIP 147.

The MPSA92 transistor below the VAS transistors is very warm to the touch. The max junction temp for this transistor is 150 deg C, but in this circuit, something would have to be very wrong somewhere for it to be that hot. As it is, the MPSA92 transistors in my boards are quite warm but I don't think they are anything to worry about. Not in my case anyway. A little clip-on heatsink would be useful. Perhaps RD was saving a bit of money in production costs and made a judgement call regarding the robustness of the MPSA92.
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Cressy Snr
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#383 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

I did a bit more work yesterday on the power supplies and reduced the capacitance in the input stage from 10,000uF per side down to 6,800. I thought that much reservoir capacitance for an input stage, VAS and emitter follower buffer was overkill and just an excuse on my part to use audiophile slit-foil caps for the sake of it. All the caps in both power supplies are now of the same make (Cornell Dubilier) and the sound and noise performance is no worse than it was before.
IMG_0215.jpeg
IMG_0215.jpeg (131.45 KiB) Viewed 6943 times
Looks a bit less crowded at the PSU end too. Though of course in engineering terms it doesn't mean a lot.
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Cressy Snr
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#384 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Revised power supplies schematic:
RevisedPSU.png
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Last edited by Cressy Snr on Wed Jun 14, 2023 1:49 pm, edited 1 time in total.
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Cressy Snr
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#385 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

After a months worth of mods and tweaks to this amplifier using references such as Douglas Self, Rod Elliott and our Nick, I think the inductor, Zobel, output mods. the feedback mods that match the input resistor value and the introduction of the input cap matched to the feedback cap value, have frankly transformed the performance of the whole setup.

Not saying the improvements are night and day, merely incremental, but taken together they add up to quite a lot. The quality of the treble, for example, always good with NVA amps, is now quite sublime. The soundstaging is wide deep and high, bass is fabulously detailed and the mids are detailed, and super clean. I could go on but let's not.

RD might have argued that by carrying out these mods I have increased the 'hi-fi' aspect and decreased the music, which is not what he would have wanted, but with due respect to Richard, without whose generosity I wouldn’t even have had these boards to play with, I honestly don't think so. I think it is possible to obtain the best of both worlds.
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Daniel Quinn
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#386 Re: DIY Transistor Amp Using NVA Boards

Post by Daniel Quinn »

Richard would say " you have unnecessarily increased your component count and your bloody deluded if you think your Frankenstein amp sounds better than a similarly specced NVA amp I make "

Alas we will never know . I miss Richard .
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#387 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Next move with this could well be 3 power supplies. And possibly a big bang!
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Cressy Snr
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#388 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Madness.png
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The solid state version of Simon's "Madness" amp?
Or maybe just stupidity.
Glad I have two spare boards and a spare valve amp.
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#389 Re: DIY Transistor Amp Using NVA Boards

Post by Daniel Quinn »

Power supplies , something mr dunn could agree with.

But 3 instead of bigger va ?
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#390 Re: DIY Transistor Amp Using NVA Boards

Post by simon »

Cressy Snr wrote: Wed Jun 14, 2023 2:03 pm The solid state version of Simon's "Madness" amp?
I was truly mad as I threw in 3 stages of DHT filaments too. Bonkers really.
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