Building a DIY amp with NVA amplifier boards

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karatestu
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Re: Building a DIY amp with NVA amplifier boards

Unread post by karatestu »

r3xj0hn570n wrote: Fri Sep 24, 2021 11:10 pm I know it's not my thread, but i thought Stu and others might be interested in a slight diversion...

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These boards have been sitting in a box for the last 20 years. I think they were old then as they are much larger than the others. The enormous transformer is from a dead onyko AV amp, an old grundig receiver has donated it's heatsink and output transistors. I'll be playing with the circuitry on this one, because, why not.
Wow Rex,. Those boards look ancient. That transformer has a fair few windings by the look of it. Don't play too much and lose the magic.
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Re: Building a DIY amp with NVA amplifier boards

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Geoff.R.G wrote: Mon Jul 12, 2021 11:20 am
Daniel Quinn wrote: Mon Jul 12, 2021 10:49 am Isn't saying the type of transformer effects the sound on the same level as saying fuses do?
No, because the differenced between EI transformers and toroidal transformers can be heard and, controversially, measured. The hum from an EI transformer can be obtrusive and, in a domestic setting, that can be important especially if equipment is left powered when not in use.

I have yet to hear anyone measuring any difference between fuses.

I know this is a subjectivist forum but it would be inappropriate to ignore the fact that some differences can be measured. That doesn't mean that we can draw any firm conclusions from those measurements but an absolute absence of measured difference, as is the case with fuses, does leave me wondering whether there might be a placebo effect involved.
most of the measurable differences can be negated by judicial siting of the transformer. I still remain cynical , unlike the numpty who wrote that story kara posted.

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Re: Building a DIY amp with NVA amplifier boards

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Inter winding capacitance has nothing to do with where you site the transformer although imo where you put the transformer is very important (even more important when using an EI).

Lower inter winding capacitance of an EI is due to the construction method, split bobbins. There is less mains crap transfered from primary to secondary due to their relative positions. It acts as a filter but one with no added components, if is just a byproduct and a very nice one if you ask me.

I prefer my 100VA 25-0-25V EI transformers to some very expensive custom wound 300VA 35-0-35V toroids from Canterbury Windings with inter winding capacitance screen and exterior gauss band. I prefer the EI even though they are much smaller VA and voltage and ultimately higher impedance.

In a similar experiment others may prefer the toroids, who knows. I can only speak for myself and I am not alone, others share my preference. Just as with toroids though, not all are created equally.

I expect the only way you will know is to try one for yourself but I wouldn't worry about it too much.

Have a read of this (if you can stomach pfm). Some contribution from Arkless and a few others including me. Jez can be believed about some things but transformers are not one of them imo. When I visited LesW at Avondale (I was there all day) he also was of the opinion that EI are superior, so there is no agreement on this even between electronic engineers. You just do your own comparison and go from there.

https://pinkfishmedia.net/forum/threa ... 82/
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Re: Building a DIY amp with NVA amplifier boards

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karatestu wrote: Sat Sep 25, 2021 7:00 am That transformer has a fair few windings by the look of it. Don't play too much and lose the magic.
It was a class G amp, so the high voltage winding is a little on the high side. Knocking that back with a choke input for the output transistors. The drivers will see the full 54V from a different bridge/cap set (hmm must check the SOA on those), VAS will get an RC filtered version of that and the input pair will get their own ~27V rails. Lots of diodes!

Magic? Not sure i'm a believer. I'm not deaf (yet) so i'll listen then measure and see what is what. Richard gave us these old things to experiment with, it's about time i did.
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karatestu (Sun Sep 26, 2021 7:47 am)

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Re: Building a DIY amp with NVA amplifier boards

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r3xj0hn570n wrote: Sat Sep 25, 2021 8:03 pm
karatestu wrote: Sat Sep 25, 2021 7:00 am That transformer has a fair few windings by the look of it. Don't play too much and lose the magic.
It was a class G amp, so the high voltage winding is a little on the high side. Knocking that back with a choke input for the output transistors. The drivers will see the full 54V from a different bridge/cap set (hmm must check the SOA on those), VAS will get an RC filtered version of that and the input pair will get their own ~27V rails. Lots of diodes!

Magic? Not sure i'm a believer. I'm not deaf (yet) so i'll listen then measure and see what is what. Richard gave us these old things to experiment with, it's about time i did.
What are the output transistors? Are they Darlingtons?
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Re: Building a DIY amp with NVA amplifier boards

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Yep, GP140 & GP145 (pretty much the same as the TIPs). Also might try some 2sb1560 + 2sd2390s, these are purpose built darlington audio output transistors.

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Re: Building a DIY amp with NVA amplifier boards

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Rainy sunday progress
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karatestu (Sun Oct 03, 2021 8:43 am)

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Re: Building a DIY amp with NVA amplifier boards

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Is that 42-0-42V with taps at 21V ? I expect once rectified it will be enough to launch those amp boards into space :grin:

Hope you are having fun.
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Re: Building a DIY amp with NVA amplifier boards

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Choke before caps for the output stage, tweaked to 43vdc. 50+ on the output drivers though. They should be fine...
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karatestu (Sun Oct 03, 2021 9:29 am)

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Re: Building a DIY amp with NVA amplifier boards

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Well, after rebuilding the power supply several times and in different configurations, i'm back to a simple cascading LCRCRC from back to front. Turns out that most of the current draw at idle was in the driver stage, not the output, so keep all the hum and buzz down that needed large caps too, and running it at 50+V produced a bit too much heat for my liking. Oh well, you live and learn. Onto the next phase...
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karatestu (Mon Nov 01, 2021 7:08 am)

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