#26 pre amp - Page 220

Rod Coleman · 21st October 2012, 09:35 AM

If you use filament bias with my regulators, there is a strong rejection of cable noise anyway.

The digram shows the influences. Provided the cables run parallel, ambient noise is picked up as common-mode.

The regulator rejects common-mode noise (=grid-to-cathode noise) because Fil- is directly at ground. The regulator rejects differential noise by means of the current regulator-action.

With cathode bias, common mode noise appears across the cathode resistor, and so produces a current that runs through the cathode capacitor. This may well add to problems presented to electrolytic capacitors, with their high parasitic inductance, and effective resistance. But I imagine that no-one here is using cathode bias with a capacitor now, anyway.

andyjevans · 21st October 2012, 09:45 AM

Is there any point in twisting the pairs of wires in the cable?

andyjevans · 21st October 2012, 09:48 AM

Another question, what's the optimum size cap to put in the signal chassis at the input of the cable? Should you put 10,000uF here for example, or less or more?

Rod Coleman · 21st October 2012, 10:00 AM

Quote:

Originally Posted by andyjevans

Another question, what's the optimum size cap to put in the signal chassis at the input of the cable? Should you put 10,000uF here for example, or less or more?

With any of my PCB-based Regulators, reducing the ripple to 80-140mV should be enough, even for line amps. So LC with 10mH or more and 10000uF is easily good enough.

Of course, this all has to do with differential mode ripple and noise.

When you use a high-capacitance trafo (eg Toroid) the noise that gets in is common-mode. The regulator will reject this, but not completely. Hence the recommendation for split-bobbin trafos.

The old 2004 Regulator circuit rejects CM and DM noise much less effectively.

Rod Coleman · 21st October 2012, 10:01 AM

Quote:

Originally Posted by andyjevans

Is there any point in twisting the pairs of wires in the cable?

Only if the cables pass by a strong noise source, like a power trafo. Running them parallel & close is enough.

mogliaa · 22nd October 2012, 07:31 PM

Replaced the HT filtering stage with a salas SSHV2 regulator (185V output) and then the CCS with the glow tubes (150V and 15mA each channel).
When the salas board was under test hanging with a couple of test leads and not fitted in the preamp (probably at 15 cm with some test leads) the output noise was very low (-85dBV). Happy with the results, fitted the Salas SSHV2 board into the preamp really close to the glow tubes, soldered final cables and when tested again surprisingly the noise went up again to -70dBV....
Are the glow tubes responsible for this due to being so close to the salas PCB or shall I look to the noise source anywhere else?
Ale

iko · 22nd October 2012, 07:47 PM

You mean the VR tubes after the shunt reg? The Salas shunt reg should be quieter and have lower output impedance than the VR tubes.

mogliaa · 22nd October 2012, 08:02 PM

Yes but with a CCS in between. So: SSHV2 -> CCS -> VR
They look really cool! Strange thing was that when did first test over existing preamp it was dead quiet, when fitted the SSHV board in the preamp, noise went back.

I will try to replicate situation by removing physically the board from the preamp....
Ale

iko · 22nd October 2012, 08:12 PM

I said it before. The VR tubes generate their own noise, higher than the noise of the Salas shunt reg. I think it completely defeats the purpose of the shunt regulator to put the VR tube last. The CCS has no effect. I too like the look of the VR tubes.

As for the higher noise after you installed the shunt reg in the box; where did you measure the noise before and after, on the tube anode?

mogliaa · 23rd October 2012, 07:59 AM

Iko,
found the issue! It was a ground loop introducing noise. The problem was that I omitted grounding the output as I'm not using the differential connection now so return was through amplifier's earth. I grounded with a pair of test clips the return of the output cables and noise is completely gone!
Will do proper measures again to reflect this, am a happy man at last.

BTW, the CCS has a good noise rejection capability as Morgan Jones clearly showed in his statistical regulator. I did the same but replaced zeners with the nice glow tubes. My tests were very promising:

http://www.bartola.co.uk/valves/2012...egulator-test/
Ale

22nd October 2012, 07:31 PM	#2196
mogliaa diyAudio Member Join Date: Dec 2009 Location: London	Replaced the HT filtering stage with a salas SSHV2 regulator (185V output) and then the CCS with the glow tubes (150V and 15mA each channel). When the salas board was under test hanging with a couple of test leads and not fitted in the preamp (probably at 15 cm with some test leads) the output noise was very low (-85dBV). Happy with the results, fitted the Salas SSHV2 board into the preamp really close to the glow tubes, soldered final cables and when tested again surprisingly the noise went up again to -70dBV.... Are the glow tubes responsible for this due to being so close to the salas PCB or shall I look to the noise source anywhere else? Ale __________________ "A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp) http://www.bartola.co.uk/valves/

22nd October 2012, 08:02 PM	#2198
mogliaa diyAudio Member Join Date: Dec 2009 Location: London	Yes but with a CCS in between. So: SSHV2 -> CCS -> VR They look really cool! Strange thing was that when did first test over existing preamp it was dead quiet, when fitted the SSHV board in the preamp, noise went back. I will try to replicate situation by removing physically the board from the preamp.... Ale __________________ "A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp) http://www.bartola.co.uk/valves/

23rd October 2012, 07:59 AM	#2200
mogliaa diyAudio Member Join Date: Dec 2009 Location: London	Iko, found the issue! It was a ground loop introducing noise. The problem was that I omitted grounding the output as I'm not using the differential connection now so return was through amplifier's earth. I grounded with a pair of test clips the return of the output cables and noise is completely gone! Will do proper measures again to reflect this, am a happy man at last. BTW, the CCS has a good noise rejection capability as Morgan Jones clearly showed in his statistical regulator. I did the same but replaced zeners with the nice glow tubes. My tests were very promising: http://www.bartola.co.uk/valves/2012...egulator-test/ Ale __________________ "A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp) http://www.bartola.co.uk/valves/

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21st October 2012, 09:45 AM	#2192
andyjevans diyAudio Member Join Date: Sep 2003 Location: London	Is there any point in twisting the pairs of wires in the cable?

21st October 2012, 09:48 AM	#2193
andyjevans diyAudio Member Join Date: Sep 2003 Location: London	Another question, what's the optimum size cap to put in the signal chassis at the input of the cable? Should you put 10,000uF here for example, or less or more?

22nd October 2012, 07:47 PM	#2197
iko diyAudio Moderator Join Date: May 2008 Location: Toronto	You mean the VR tubes after the shunt reg? The Salas shunt reg should be quieter and have lower output impedance than the VR tubes.

22nd October 2012, 08:12 PM	#2199
iko diyAudio Moderator Join Date: May 2008 Location: Toronto	I said it before. The VR tubes generate their own noise, higher than the noise of the Salas shunt reg. I think it completely defeats the purpose of the shunt regulator to put the VR tube last. The CCS has no effect. I too like the look of the VR tubes. As for the higher noise after you installed the shunt reg in the box; where did you measure the noise before and after, on the tube anode?