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: Mon Nov 01, 2021 2:18 am amp.jpg

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...
Looking good Rex.

What values of inductor and resistor did you use in the psu ? I have psu's with very small inductor values between the caps just to eat a bit of high frequency crap. I also have psu's with resistors between the caps on some psu's but always less than 1 ohm. Combined with the very large capacitance involved in the psu I think they still can do something useful.

Have you ever tried isolating an input stage from an output stage which shares a psu ? There is a trick with a series IN4004 diode and 200R resistor in the power rail between the stages like below . I have heard the effect and it does help to isolate the two stages.

Image
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r3xj0hn570n
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Re: Building a DIY amp with NVA amplifier boards

Unread post by r3xj0hn570n »

The inductor is a 12V lighting transformer (x4). Primary measures around 70Ohm and 3H. This inductance is adjusted down by a loading resistor on the secondary. This drops around 59V down to around 34.5V across the 2x4700uF first bank.

Next, a 27Ohm resistor drops into a 8600uF cap for the driver stage, 34V. As mentioned, direct from it's own rectifier hanging off the main secondary was just too noisy.

An 82Ohm resistor drops just over another volt to the pre-driver, mirror stage at 32.7V.

Finally, the input pair power hangs off the driver stage through a diode like in your picture for 31.8V.

I can measure about 4mV ripple (at idle) in the first cap bank. It's difficult measure any ripple in any of the others.

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Re: Building a DIY amp with NVA amplifier boards

Unread post by karatestu »

I have never tried resistors between caps any bigger than 4R7 and that was only for the LTP. Do you find this CRC method squashes dynamics in the music ? Just asking because I have some transformers with too high voltage that would be good to use.
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Re: Building a DIY amp with NVA amplifier boards

Unread post by r3xj0hn570n »

You need to use an initial L to knock off lots of voltage. Theoretical perfect LC supply returns ~0.9xVrms.

I have another home-made amplifier (newer NVA boards) with a similar layout. In the context of directly driving a high efficiency speaker, the LCRC type of power supply doesn't noticeably squash dynamics (and i'm fairly sensitive to that).
Mind you it does have stupidly large (75x150mm siemens) main caps in it.

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Re: Building a DIY amp with NVA amplifier boards

Unread post by r3xj0hn570n »

Aww crap. Extended testing as shown the large transformer gets too hot. Exactly the opposite of what i was after with this amp. Time to try some others.

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Re: Building a DIY amp with NVA amplifier boards

Unread post by karatestu »

r3xj0hn570n wrote: Wed Nov 03, 2021 7:28 pm Aww crap. Extended testing as shown the large transformer gets too hot. Exactly the opposite of what i was after with this amp. Time to try some others.
That's bad luck Rex. My EI transformers have been humming the last few days and that hasn't happened before with these. Something is going on with the mains :roll:
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Re: Building a DIY amp with NVA amplifier boards

Unread post by karatestu »

Hey Rex.

RD had problems running four power supplies on the amp boards (LTP, VAS, drivers, output). Kept blowing the boards. That is why I never bothered trying more than two psu's per board.

Can you think of any reason why it would happen ?
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Re: Building a DIY amp with NVA amplifier boards

Unread post by r3xj0hn570n »

Hmm, i had problems keeping it quiet.

As for why they kept blowing up, what fails?

The boards i'd got given typically had destroyed bias diodes and base resistors, pointing to catastrophic failure of the output devices. This can happen if the driver clips, sending a HF pulse into those output devices (which is just one of the reasons why running the drivers at a higher voltage than the last rail helps).

I've given up on the LC power supply. The rails are just too soggy. Rebuilding it again.

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Re: Building a DIY amp with NVA amplifier boards

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r3xj0hn570n wrote: Sat Nov 06, 2021 8:36 pm Hmm, i had problems keeping it quiet.

As for why they kept blowing up, what fails?

The boards i'd got given typically had destroyed bias diodes and base resistors, pointing to catastrophic failure of the output devices. This can happen if the driver clips, sending a HF pulse into those output devices (which is just one of the reasons why running the drivers at a higher voltage than the last rail helps).

I've given up on the LC power supply. The rails are just too soggy. Rebuilding it again.
I have no idea what failed but I do know that apparently once they have blown some of the small transistors are weakened and even if you manage to repair it the board fails again a little time later, RD used to just scrap the old boards and put a new one in.

Sorry, not much help. I think the statement nva amps ended up with 3 power supplies but I have no idea how the rails were split.
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Re: Building a DIY amp with NVA amplifier boards

Unread post by r3xj0hn570n »

Weakened transistors? I made an amp out of a couple of blown up boards about 20 years ago. The have been permanemtly turned on the last 5 years or so. 40v rails. N=1 i know, perhaps part sensitivity was an issue?

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