DIY Transistor Amp Using NVA Boards

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

Post by Cressy Snr »

Anyway, what has been bugging me for a while with this amp is that the output stage has a higher voltage transformer than the input/driver stage, which I’ve realised is a bit pointless as this isn’t a valve amp. The output stage (current amp) can only produce as much current as the swing of the driver stage (voltage amp) will allow it to, so having the output stage at higher voltage is a waste of electricity if the driver is going to bash into its own rails first.

I was going to remove both transformers and install a 30-0-30 for the driver and a 25-0-25 for the output stage, but both the damn’ things together were too big to fit in the case. The compromise was to keep the 120VA, 25-0-25V on the VAS and driver stage and simply fit a 300VA, 25-0-25V to the output stage PSU.

It turns out that both power supplies produce exactly +/- 37V when under load, which is nice, as if the output rails sag under duress, below 37V, the driver rails will stay up courtesy of their own PSU.
Last edited by Cressy Snr on Tue May 23, 2023 8:07 pm, edited 1 time in total.
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#362 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

New 300VA, 25-0-25V toroid at bottom right:
IMG_0201.jpeg
IMG_0201.jpeg (117.72 KiB) Viewed 6554 times
Whilst I was at it, I removed the crap quality shielded input cables going to the selector and replaced them with simple straight wires.

I find loads of these blue/yellow twisted pairs lying around next to BT street cabinets when I’m out walking. I found a metre of it the other day due to the engineers not cleaning up after themselves. Shame to let it go to waste. The 0.5mm jumper wire makes good signal wire and my old BT Lindstrom long nose pliers strip it perfectly.
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Cressy Snr
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#363 Re: DIY Transistor Amp Using NVA Boards

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Revised power supply voltages:
SSPSU4.png
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#364 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Really pleased with this now. Lovely relaxed, detailed and dynamic presentation. It's certainly worth making the trip to Owston with it. :)
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#365 Re: DIY Transistor Amp Using NVA Boards

Post by radio_free »

Hello,

Looking at Post#364 and #365, I see you now have a RC after the main cap bank.
If I read the TNTAudio article correctly (https://www.tnt-audio.com/clinica/ssps3_e.html) these are best implemented near to the board since it is supposed to take away any remaining inductance on the power rail.

Few questions:
- What are those big film caps hanging over the amp boards?
- I did some measurement on my old A60, the VAC across the emitter resistors are only 25mVAC, and if my math is correct the quiescent current works out to be approx 18mA which seem very small. Have you measured yours?
- so what value of FB resistor you ended up with?

Very nice work, I've been reading your posts for a while :-)
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#366 Re: DIY Transistor Amp Using NVA Boards

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radio_free wrote: Wed May 31, 2023 8:15 am Hello,

Looking at Post#364 and #365, I see you now have a RC after the main cap bank.
If I read the TNTAudio article correctly (https://www.tnt-audio.com/clinica/ssps3_e.html) these are best implemented near to the board since it is supposed to take away any remaining inductance on the power rail.

Few questions:
- What are those big film caps hanging over the amp boards?
- I did some measurement on my old A60, the VAC across the emitter resistors are only 25mVAC, and if my math is correct the quiescent current works out to be approx 18mA which seem very small. Have you measured yours?
- so what value of FB resistor you ended up with?
Yes, the RC network is supposed to go near the boards, but space was at a premium as can be seen from the photo, so they went on the end of the output stage PSU. There was still a very useful improvement in the treble clarity, sweetness and definition. I would not be without those filters.

Other questions in order:
1. See post #300
2. Yep 18mA quiescent or thereabouts. These are class B amplifiers. With the monolithic TIP142/147 Darlington output stage as applied in this particular circuit, class B operation is (from what I’ve read) the best option RD had for stability and thermal management of the output stage (I think.) I’m still early in my learning though, so I could be wrong.
3. I ended up with around 9K for the RFb.
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#367 Re: DIY Transistor Amp Using NVA Boards

Post by radio_free »

Thanks, good to know that my old amp's quiescent current is not abnormal :o

The RFb on my boards are the original, 18kR. I will try to parallel another 18k to see how it turn out.
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#368 Re: DIY Transistor Amp Using NVA Boards

Post by Mike H »

pre65 wrote: Sun May 14, 2023 8:22 pm Mike, do you remember the Maplin 50W Mosfet module amp circuit ?

I think you can still get similar on Ebay. A pair of them was very nearly my first amp build.
Yes and I once foolishly tried making them on plain matrix board – can't remember why now, most likely just being a skinflint – didn't work, the seemingly 'overly ornate' copper layout of the kit board is what keeps them RF stable. Despite all attempts to fix, mine stubbornly remained as RF signal generators, so that was the end of that. :lol:

There was also a 1kW MOSFET amp kit – it had a Zobel network across the output. At some point the capacitor could no longer be sourced, but buying office found an equivalent. Guess what, with the different cap it was RF unstable, customer kits were being returned as 'faulty'. The development lab had a high old time finding a capacitor that would work, and I think the board layout had to change because the footprint was different. :shock:
 
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Cressy Snr
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#369 Re: DIY Transistor Amp Using NVA Boards

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Mike H wrote: Thu Jun 01, 2023 9:43 pm …Yes and I once foolishly tried making them on plain matrix board – can't remember why now, most likely just being a skinflint – didn't work, the seemingly 'overly ornate' copper layout of the kit board is what keeps them RF stable. Despite all attempts to fix, mine stubbornly remained as RF signal generators, so that was the end of that. :lol:
I would love to build a few circuits out of books myself but the fear of oscillators, arc-welders and speakers reduced to smoking rubble needs to be overcome first….ulp :confused1:
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#370 Re: DIY Transistor Amp Using NVA Boards

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I’ve had an issue with this build for a while now, where the sound disappears from the left channel. One culprit was a duff cable where the centre conductor had snapped inside the phono plug. I thought I had cured the problem (well you would wouldn’t you.) Then the bloody thing reared its head again, but this time it was intermittent It would go for days perfectly fine, then the left channel would disappear again. To cut a long story short, I finally traced the problem to the selector switch and replaced it. So far there has been no left channel going AWOL.

Apart from the phono card wiring, which I left well alone, I replaced the other two bits of input wiring with solid copper:
IMG_0207.jpeg
IMG_0207.jpeg (128.73 KiB) Viewed 6085 times
What a fecking criss-cross quiz! RD would have been proud…well apart from the Zobel network and the inductor after the output stage, for which he would have chased me down the street with an axe. :lol:
Getting at the boards is almost impossible. But there’s nothing I would want to do to them.
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#371 Re: DIY Transistor Amp Using NVA Boards

Post by Mike H »

:thumbleft:
Cressy Snr wrote: Fri Jun 02, 2023 9:46 am I would love to build a few circuits out of books myself but the fear of oscillators, arc-welders and speakers reduced to smoking rubble needs to be overcome first….ulp :confused1:
I have endured many years of such – often on the way home from college I would go by the Bi-Pre-Pak shop in Westcliff, and if I was working on an amplifier design at the time (when wasn't I :lol: ) and it was a Friday, I would nip in to buy 2 or 3 pairs of power transistors, 'cause at least 2 if not 4 of them could be toast by Sunday evening ... :lol:

One time, it was either a dodgy bias preset or just me misunderstanding which way to turn it, but next thing I knew, both emitter resistors are glowing red and smoking, that's another pair of transistors trashed then ... :lol:

I can laugh about it now ... but frustrating in the extreme at the time.
 
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#372 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Right! Well...carrying on from the other thread about feedback caps, the postie delivered four Nichicon ES bipolar caps, courtesy of Nick, who had prepared two of them with a copper foil shields and earth wire. This of course meant that I had to tear down the amp and get the boards out, in order to carry out the mods, which were:

1. Change the feedback cap
2. Change the piggy backed pair of feedback resistors to a single 10K one
3. Change the 47K input resistor to 10K to match the feedback resistor value
4. Fit the shielded cap in series with the first LTP transistor's base. This matches the value of the feedback cap.

Below, the mods have all been done and the boards refurbished:
IMG_0211.jpeg
IMG_0211.jpeg (157.67 KiB) Viewed 5869 times
Green replacement feedback cap on the right.
Feedback resistor is second from the top of the row of 5 resistors.
Input resistor is at the bottom below the rectangular red cap.
Foil wrapped cap can be seen also at the bottom of the board.
Last edited by Cressy Snr on Thu Jun 08, 2023 8:35 pm, edited 3 times in total.
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#373 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Boards reinstalled and all connections made:
IMG_0213.jpeg
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New sil pads on output transistors.
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#374 Re: DIY Transistor Amp Using NVA Boards

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It all works. DC offset at the speaker terminals is down from 15-22mV to 5.2 mV on one channel and 5.8mV on the other and no longer varies with the position of the volume control. I’d call that a result. :)
Gave it a quick blast with speech off the TV and it plays on both channels. I’ll see how it sounds with music later on.
Let’s just say I’m pretty pleased with the results. Is this still an NVA amp? RD would probably have said no.
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#375 Re: DIY Transistor Amp Using NVA Boards

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Testing the amp with music now and I’m well impressed. I don’t think there’s anything magical about why this is. It’s almost certainly down to reduced distortion. The better balanced LTP = less H2 and less DC offset at the output. There is reduced RFI pickup at the input due to the shielded series cap and also a drop in the noise floor.

Additionally, the amp runs cooler. The transformer feeding the output stage power supply is quite cool after 3 hours of operation, as is the one feeding the input stage, VAS and buffer. The heatsinks are barely above ambient temperature.

Thanks Nick for the advice re the LTP balance and for the caps.
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