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 » Tue Jun 12, 2018 7:34 pm

Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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

Post by little eddy » Fri Jun 15, 2018 10:46 pm

I was considering what if I were to use the Niobium as a pre-amp to feed a power valve with fixed bias?

I have wired the output as a 'single-ended' output, with the outer rca reference contact and IST winding both connected to 0V.

I then thought why not have both output wires of the IST wired direct to the RCA socket with a simple loading resistor? Also doing it as I have done, wouldn't this be prone to ground loop issues having 2 parallel 0V connections via the earth circuit and signal reference conductor? Having a truly isolated secondary should minimize ground loop risks.

Obviously there would be safety considerations with any interconnects due to the bias voltage being on one of the RCA terminals so might have to use shrouded XLR connectors. Plus when the pre-amp was isolated, there would still be the chance of the bias voltage from the output stage but sure could make this touch proof.

What do people think/recommend?

I have attached a drawing, the alternative output wiring in the top right hand corner.
18-06-15 Niobium Mono PSU Amp Schematic 41MP IT Shunt.gif
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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Nick
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Re: Niobium - Valve pre for SS Power Amp

Post by Nick » Fri Jun 15, 2018 11:49 pm

Why not just add a switch to let the output float if required,
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little eddy
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Re: Niobium - Valve pre for SS Power Amp

Post by little eddy » Fri Jun 15, 2018 11:58 pm

I can fit with a switch but what possible benefits are there connecting to the 0V in the pre-amp? I would have thought most power amps have an earth connection so ground loops here are highly likely.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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Nick
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Re: Niobium - Valve pre for SS Power Amp

Post by Nick » Sat Jun 16, 2018 10:18 am

Well, unless the output float, then you you will always have joined earth connections (people do make cables with the screen only connected at one end to solve the problem). If there is no difference in the ground potential then there is no current and no loop. The difference in a amp is that you are normally passing current via the ground points.
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izzy wizzy
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Re: Niobium - Valve pre for SS Power Amp

Post by izzy wizzy » Sat Jun 16, 2018 11:00 am

Something you might want to try. Remove one of the RC sections in the psu before the regulation if you don't end up with too much volts that becomes another problem.

Cheers,

Stephen

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

Post by little eddy » Sat Jun 16, 2018 8:00 pm

izzy wizzy wrote:
Sat Jun 16, 2018 11:00 am
Remove one of the RC sections in the psu before the regulation if you don't end up with too much volts that becomes another problem.
I am sure I can drop a few more volts across the CCS but what's the logic? Minimising series resistance? Can you have too much reservoir capacitance? At what point does hum become a consideration?

My immediate worry is that my LM3886 power amp won't be up to the quality of the Niobium pre-amp in which case will need to get that sorted or changed before the full benefits of fine tuning can be really appreciated.

Stuck in Sheffield all weekend with the kids swimming is not helping progress but a few jars at Wetherspoons is a slight compensation.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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

Post by izzy wizzy » Sat Jun 16, 2018 8:15 pm

little eddy wrote:
Sat Jun 16, 2018 8:00 pm

I am sure I can drop a few more volts across the CCS but what's the logic? Minimising series resistance? Can you have too much reservoir capacitance? At what point does hum become a consideration?

My immediate worry is that my LM3886 power amp won't be up to the quality of the Niobium pre-amp in which case will need to get that sorted or changed before the full benefits of fine tuning can be really appreciated.

Stuck in Sheffield all weekend with the kids swimming is not helping progress but a few jars at Wetherspoons is a slight compensation.
Here's where I'm coming from. This link is for Mark's phono (Island Pink hereabouts) http://www.izzy-wizzy.com/audio/phono/I ... o_2015.jpg His PSU compared to your line stage doesn't have the two RC sections you have and yet, is good enough for phono never mind a line stage. I recently tried adding an LC section to my line stage with a PSU not too disimilar to Mark's and not only could I barely notice but I'm not sure that what difference there might have been was better. No point in having stuff in there if it isn't doing anything helpful. Can't hurt unless you have other issues at hand. Once one is gone, how about the other :)

cheers,

Stephen

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

Post by little eddy » Sun Jun 17, 2018 9:08 am

Nick wrote:
Sat Jun 16, 2018 10:18 am
Well, unless the output float, then you you will always have joined earth connections (people do make cables with the screen only connected at one end to solve the problem). If there is no difference in the ground potential then there is no current and no loop. The difference in a amp is that you are normally passing current via the ground points.
Thanks for the explanation. I use my own cables and all have a reference conductor terminated at both ends. So in this instance will wire direct from IST to RCA's.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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Nick
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Re: Niobium - Valve pre for SS Power Amp

Post by Nick » Sun Jun 17, 2018 11:01 am

Yes, remember there is nothing magical about joining earth’s that creates hum, its only when there is a potential difference between the two. The potential difference (normally AC) creates a current in the joining conductor (as it does), that current creates a voltage (as it does), so the voltage means that the point the signal is referenced too at both ends of the connection is different by the AC voltage (or DC, but we don't notice that), then the side that’s a input sees the AC voltage as added to the input signal.

The AC signal can also be induced in the joining conductor by a AC field. Screens are normally not screened :-)
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Re: Niobium - Valve pre for SS Power Amp

Post by little eddy » Mon Jun 25, 2018 9:48 pm

Well have connected into my system over the last couple of nights with mixed impressions.

First I tried with the LM3886. Some aspects were great in terms of detail and imaging but base was perhaps a bit loud and undefined, but probably the balance just wasn't there.

I then tried with a Technics SU-V700 (rescued from the tip last weekend) and overall balance was much better but doesn't quite grab your attention like the LM3886. Overall the old Technics possibly has the edge but really after the best attributes of both.
System test small.jpg
Won't get it sorted by this weekend but have thought about if I could bypass the Technics gain stage?

Don't know whether anyone will have a good known neutral 'reference' pair of power amps there this weekend but would be really good to understand if the pre-amp is any good.

There's also a bit of hum I presume from the pre. Don't know if it's an issue or not but don't have an earth lift circuit fitted at the moment.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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

Post by little eddy » Thu Jul 12, 2018 10:51 pm

After listening to numerous power amps with the Niobium pre, I am coming to the conclusion that the harshness, or at least a significant proportion of this is coming from the Niobium pre.

Thinking of the individual elements that might contribute to this, I have the IXYS CCS and the LED cathode bias. I have an IXYS CCS in my Rocky amp, and although the sound was clean, it was not harsh like we witnessed at Owston, leading me to think it is the bias method and/or components that I should be focussing on.

So was considering a bypass cap across the LEDs, or conventional R//C that again worked well in Rocky, but then started to think whether perhaps it might be worth considering fixed bias. Have read up on various battery schemes and get put off by the risk of the tubes going pop if the batteries fail so my instinct is that negative grid bias might offer the most neutral solution.

What's the thoughts as to whether this might be suitable or audibly beneficial over R//C for this single valve pre, albeit running at a very healthy 22mA so I consider it more like a driver stage?
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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Re: Niobium - Valve pre for SS Power Amp

Post by izzy wizzy » Fri Jul 13, 2018 8:08 am

To eliminate the CCS as a cause, you could sub it with a resistor.

I like battery bias and use it in my phono. Best place would be in series with the grid so avoiding an input cap. I hate battery bias with a rechargeable in the cathode.

Some LEDs have quite a high Z so might contribute more to the sound the a low Z LED. It's usually a way to get rid of the cathode cap. You could sub in a cap/resistor combo to rule out the LED.

It's also possible your op point might be the reason which a resistor bias would mean it's easier to adjust.

cheers,

Stephen

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Re: Niobium - Valve pre for SS Power Amp

Post by little eddy » Fri Jul 13, 2018 3:15 pm

izzy wizzy wrote:
Fri Jul 13, 2018 8:08 am
Some LEDs have quite a high Z so might contribute more to the sound the a low Z LED. It's usually a way to get rid of the cathode cap. You could sub in a cap/resistor combo to rule out the LED.

cheers,

Stephen
Substituted with an AB 270R carbon and a cheap Panasonic cap. Transformed. Phew. Someone please shoot me if ever I consider LED bias again - done it twice and a disaster both times.
Thorens TD-125 & 'Snail' phono/pre, NAS/SBT, CS4398 DAC, Rocky KT66 amp, still messing with OBs.

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IslandPink
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Re: Niobium - Valve pre for SS Power Amp

Post by IslandPink » Fri Jul 13, 2018 3:51 pm

Interesting result - but I'm not entirely surprised !
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