Niobium - Valve pre for SS Power Amp

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little eddy
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Re: Niobium - Valve pre for SS Power Amp

Post by little eddy » Wed Feb 13, 2019 1:34 pm

So I am now up and running with my Niobium pre-amp and MoFo power amp, and results are very promising after changing pre-amp bias RC to Takman/Cerafine.

But I have a hum, potentially the summation of three sources:
1. There is a slight hum when I switch on the power amp. This can only be heard when you put your ear next to the speaker. Hopefully when I move to a CLC power supply from the current CRC that should help.
2. As soon as I switch on the pre-amp, (even with the inputs grounded), there is an audible hum through the speakers.
3. When the VR tubes kick in, the level of hum increases (and 'fills out').

With the sum of the 3, the hum is audible when nothing is playing but mostly lost when playing music at reasonable levels.

If item 3 remains an issue then Nick's MJ Statistical Regulators might do the job. Plus will the large 10uF filter cap help attenuate the inherent hum? But that's not really solving my problem.

Should the VR tubes introduce any hum; should I try without the parallel cap?

My thoughts are that the grounding scheme I have used, (from Rankin's book), is not working or I have implemented incorrectly. Earth lift circuit added but didn't help.

Aside I will try and move filter components and wires as far away from the mains transform in case the hum is induced.

Thanks for any advice/ideas in advance.
19-02-03 Niobium Mono PSU Amp Schematic 41MP IT Shunt small.jpg
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Nick
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Re: Niobium - Valve pre for SS Power Amp

Post by Nick » Wed Feb 13, 2019 1:51 pm

Should the VR tubes introduce any hum; should I try without the parallel cap?
You may be assuming that because the hum increases when the vr tubes light, the vr tubes are generating the hum. More likely the arrival of B= which will cause the vr tubes to light is causing the hum.

If it hums a bit before B+ I would find out why first, may be coupling from mains TX to output transformer.
Resistance isn't futile it's V / I.

little eddy
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Re: Niobium - Valve pre for SS Power Amp

Post by little eddy » Wed Feb 13, 2019 2:09 pm

Are you suggesting inductive coupling between the power and output tx's?

They are as far apart as I can get them - the power tx at the front on the bottom level, the output txs right at the back on the upper deck. So about 5 to 6 inches.

Can you use something like a Faraday screen with an inductive sheet/mesh? And if so I was presuming would need connected to the chassis earth.

I can try to separate them more physically as a test as it should reduce with distance.

The power transformer has an 'ES' connection that I assumed was a screen so have connected it to the chassis earth. Isn't an electrostatic screen for high frequencies so may not help in this instance.

Should I ground the output transformer frames?

You maybe right in that the hum kicks in as soon as I switch on and is not increased as the thing powers up.

I was not suggesting that all of the hum is attributable to the VR tubes, but when they ignite and I assume start to conduct, the hum 'fills out', appearing to increase in level. So thought this might be an additional source of hum. But by getting the 'base' hum sorted would prove this.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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Nick
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Re: Niobium - Valve pre for SS Power Amp

Post by Nick » Wed Feb 13, 2019 3:14 pm

The electrostatic screen is to prevent capacitive coupling between the primary and secondary. Doesn't affect hum field.

Just try rotating one of the transformers and see if the hum changes.

Is it hum (50Hz) or does it have edges? (100Hz+charging)
Resistance isn't futile it's V / I.

little eddy
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Re: Niobium - Valve pre for SS Power Amp

Post by little eddy » Sun Feb 24, 2019 11:40 am

While trying to reduce the hum, I am also trying to tidy up the wiring.

I have 2 parts to the removal top plate. The front half contains the VR tubes, and the back half the triodes.

I am thinking of having some form of links or jumpers on the top, but being exposed needs to be safe (180Vdc) and also neat.

Anyone any ideas? I am currently considering these types and just making up some 4mm banana plug jumpers. Or just having single sockets and flying leads from the triode section with right angled shielded banana plugs keeping it quite low profile:
s17-1400p01wm.jpg
s17-1400p01wm.jpg (11.58 KiB) Viewed 643 times
s17-2330p01wm.jpg
s17-2330p01wm.jpg (6.38 KiB) Viewed 643 times
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

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pre65
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Re: Niobium - Valve pre for SS Power Amp

Post by pre65 » Sun Feb 24, 2019 12:29 pm

The blue one is a type I have been using for ages now, I like them. :)
Life is what happens to you while you're busy making other plans. John Lennon

G-Popz THE easy listening connoisseur. (Philip)

little eddy
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Re: Niobium - Valve pre for SS Power Amp

Post by little eddy » Thu May 16, 2019 3:56 pm

Still got some changes to the psu planned, tidying up and rearranging to do. But just fitted one of these pots that I think Phil found on the WD Forum. With only 23 steps, I've fitted a switch to select the output tx windings between 100 and 30% to give finer volume control:

https://www.ebay.co.uk/itm/SMT-Resistor ... 2749.l2649

I think that's about it before Owston.

Mike.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

little eddy
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Re: Niobium - Valve pre for SS Power Amp

Post by little eddy » Thu May 16, 2019 4:00 pm

Nick wrote:
Wed Feb 13, 2019 3:14 pm
Is it hum (50Hz) or does it have edges? (100Hz+charging)
I think it's 100Hz and it starts immediately upon power-up with the inputs grounded. So I think it is induced but wouldn't mind if someone could have a look at Owston and advise before I embark on some psu changes.
TD-125/RB250/MC25FL & 'Snail' phono, NAS/SBT with CS4398 DAC, 41MP pre & MoFo Power, still messing with OBs.

izzy wizzy
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Re: Niobium - Valve pre for SS Power Amp

Post by izzy wizzy » Thu May 16, 2019 4:11 pm

little eddy wrote:
Sun Feb 24, 2019 11:40 am
While trying to reduce the hum, I am also trying to tidy up the wiring.

I have 2 parts to the removal top plate. The front half contains the VR tubes, and the back half the triodes.

I am thinking of having some form of links or jumpers on the top, but being exposed needs to be safe (180Vdc) and also neat.

Anyone any ideas? I am currently considering these types and just making up some 4mm banana plug jumpers. Or just having single sockets and flying leads from the triode section with right angled shielded banana plugs keeping it quite low profile:

s17-1400p01wm.jpgs17-2330p01wm.jpg
Only just noticed this. I'm doing something similar inside my amp as it splits in two. Using 1000V rated 2mm test sockets and their mating plugs. Very similar to what you're planning outside.

Cheers,
Stephen

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