Another DIY SS Amplifier with NVA Boards

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SteveTheShadow
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Re: Another DIY SS Amplifier with NVA Boards

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Inputs and board wiring done. After the power supply incident, there is now, 6800uF per rail on the front end and 10000uF of slit foils on the power stage. I figured that 20,000uF of caps (output stage) would take fractionally longer to charge than 13,600uF (front end) thereby ensuring that the front end would come up slightly ahead of the power stage. This, I thought, would avoid any nasty noises at power-up. It worked beautifully; it makes far less of a switch-on thump, than the A20.
Last edited by SteveTheShadow on Mon Jan 20, 2020 3:41 pm, edited 5 times in total.
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SteveTheShadow
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Re: Another DIY SS Amplifier with NVA Boards

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It fired up without incident on a pair of sacrificial, cheap speakers...phew!
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38V on the front end, 51V on the power stage. DC offset; there isn’t any. Noise; there isn’t any.
Tomasz’ 0V layout suggestion worked perfectly. 8-)

I’ll leave it soaking in the system for the rest of today, after which I’ll start on the case cosmetics, as it’s pretty darn’ fugly at the moment.
Needless to say it sounds very nice indeed. As a valve enthusiast, this twin power supply amp gives nothing away to valves, in terms of sweetness of delivery. Now Richard’s gone, I feel privileged to have had the opportunity to work with these PCBs.
So glad I didn’t blow them up. :dance:
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Lurcher300b
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Re: Another DIY SS Amplifier with NVA Boards

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38V on the front end, 51V on the power stage.
Of course I don't know the circuit, but what is the point of having lower voltage on the driver than the output stage? Normally the output stage is limited by the headroom of the driver stage. I would have thought that pair of voltages would only give the same output power as 38v on both stages.

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SteveTheShadow
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Re: Another DIY SS Amplifier with NVA Boards

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Lurcher300b wrote: Mon Jan 20, 2020 4:58 pm
38V on the front end, 51V on the power stage.
Of course I don't know the circuit, but what is the point of having lower voltage on the driver than the output stage? Normally the output stage is limited by the headroom of the driver stage. I would have thought that pair of voltages would only give the same output power as 38v on both stages.
Probably, but it came down to the transformers I had available.

The original 25V transformer that pulled the excessive current would have given the same voltages on both stages, but I wasn’t prepared to risk using it again.

Unfortunately, the driver transistors are still lumped in with the input transistors, which I seem to recall, Richard telling me to go easy with. I’ve just had a look and the input transistors look to be BC182L, which have a maximum VCE of 50V, max VCB of 60V and VBE of 6V. So 38V is about right for reliability, I would have thought.

But I’m very new to solid state circuits.
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Re: Another DIY SS Amplifier with NVA Boards

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SteveTheShadow wrote: Mon Jan 20, 2020 5:05 pm
Lurcher300b wrote: Mon Jan 20, 2020 4:58 pm
38V on the front end, 51V on the power stage.
Of course I don't know the circuit, but what is the point of having lower voltage on the driver than the output stage? Normally the output stage is limited by the headroom of the driver stage. I would have thought that pair of voltages would only give the same output power as 38v on both stages.
Probably, but it came down to the transformers I had available.

The original 25V transformer that pulled the excessive current would have given the same voltages on both stages, but I wasn’t prepared to risk using it again.

Unfortunately, the driver transistors are still lumped in with the input transistors, which I seem to recall, Richard telling me to go easy with. I’ve just had a look and the input transistors look to be BC182L, which have a maximum VCE of 50V, max VCB of 60V and VBE of 6V. So 38V is about right for reliability, I would have thought.

But I’m very new to solid state circuits.
Fair enough, though as it is a transistor could see 76v from the front end supply. Having a lower driver voltage avoids the output stage hitting its supply rails. Overload behavior is a challenging time for SS stages anyway.

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Re: Another DIY SS Amplifier with NVA Boards

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I have split the board with the drivers lumped in with the input stage and with the output transistors. Richard never advised anything as regards how to split the power rails so i tried both. The drivers (TIP31 & 32) can take 50 V. Currently i have it the same as you Steve but only because that is the way Richard did it on all the internal pics i have ever seen of NVA amplifiers. He did tell me to go easy on the LTP and VAS transistors which i did.

The way i did it on the Avondale boards i have is what Les advised and that was to lump the drivers in with the output transistors. Two slightly different circuits but i remember him saying that if the input stage has lower voltage then the point at which the amplifier clips will be governed by the input stage psu. Seems to correlate with what Nick is saying :think:
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Re: Another DIY SS Amplifier with NVA Boards

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karatestu wrote: Mon Jan 20, 2020 5:36 pm I have split the board with the drivers lumped in with the input stage and with the output transistors. Richard never advised anything as regards how to split the power rails so i tried both. The drivers (TIP31 & 32) can take 50 V. Currently i have it the same as you Steve but only because that is the way Richard did it on all the internal pics i have ever seen of NVA amplifiers. He did tell me to go easy on the LTP and VAS transistors which i did.

The way i did it on the Avondale boards i have is what Les advised and that was to lump the drivers in with the output transistors. Two slightly different circuits but i remember him saying that if the input stage has lower voltage then the point at which the amplifier clips will be governed by the input stage psu. Seems to correlate with what Nick is saying :think:
Maybe the darlington pair output stage with its high gain, sidesteps the whole issue? I’m only guessing here as I’m a solid state newbie.
Anyway I’ve been blasting for two hours and can’t get either the transformers or the heatsinks above tepid. Running the new output transistors in is going to take years at this rate. The soundstage reaches up to the bloody ceiling. I thought only the OTL could do that.

I’m going to have to be a bit careful, as this thing will now go to ear-bleeding levels without you even noticing until you try to talk. Hearing damage could be only a twist too far of the volume knob away it’s so clean.

My speakers are too efficient. :lol:
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Re: Another DIY SS Amplifier with NVA Boards

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Maybe the darlington pair output stage with its high gain, sidesteps the whole issue?
Don't confuse current gain with voltage gain. Not in valve land now.
drivers (TIP31 & 32) can take 50 V
That may be confusing. if they are C, then they will take 100v, but that's from +ve to -ve, (collector on one rail, emitter driven to the other) so as you say, max rail voltage would be 50v

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SteveTheShadow
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Re: Another DIY SS Amplifier with NVA Boards

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Lurcher300b wrote: Mon Jan 20, 2020 6:24 pm
Maybe the darlington pair output stage with its high gain, sidesteps the whole issue?
Don't confuse current gain with voltage gain. Not in valve land now.
Told you I was a newbie. :lol:

Anyway, thanks Nick, it’s really interesting learning something new. I was getting jaded.
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Re: Another DIY SS Amplifier with NVA Boards

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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. It further gets out of the way of the music.
It’s sweet as a nut, unfatiguing, with an expansive soundstage and is musically, instrumentally texturally and spatially detailed without throwing everything your face. Everything hangs together beautifully, unforced, solid and natural. I love it. It’s my OTL on steroids.
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