#26 pre amp - Page 193

euro21 · 08:41 AM

Quote:

Originally Posted by albertli

the signal generator injects 10v @1k to the preamp.

Hmmm.....
10V RMS input, gain:8, 80V RMS output?
IMHO it's a bit thick.

What is your operating point?

Vinylsavor · 27th July 2012, 08:47 AM

Hi!

You are right 10V RMS in would be too much. I think he applied an input voltage which gave 10V out at 1kHz and then checked the other frequencies

Thomas

Magz · 12:24 PM

Another good choice for shielding is TI-Shield. It is a permalloy core sheet plated on both sides with copper, so it handles both magnetic and electrostatic shielding. I have used it in a few components and it really cuts down on the interferences. It also maintains its shielding properties when bent or soldered, which is an advantage over mu metal. It has a nice copper color if you care about the esthetics. Available at Michael Percy Audio. Here's some info on its construction:

http://www.emsclad.com/examples/emi-rfi-shielding.html

When I breadboarded my 26 preamp I had a lot of hum - what I found was that most of it was coming from nearby electrical devices, such as the lights inside the cabinet the pre was in, the overhead lights, an old tuner that was nearby, etc. unplugging/turning off those sources resulted in a decrease of hum of ~80%.

.

Magz · 12:43 PM

Here are a couple plots showing the background noise of a low frequency EQ unit I worked on. The first is the unit with no shielding around the power supply, the second is with a single layer of TI-shield around the power supply, grounded to the steel chassis. The power supply was in the same box as the audio circuits.

I did make some other mods to the unit that helped, but the majority of the noise reduction came from the TI-shield.

albertli · 27th July 2012, 02:46 PM

Quote:

Originally Posted by Vinylsavor

Hi!

You are right 10V RMS in would be too much. I think he applied an input voltage which gave 10V out at 1kHz and then checked the other frequencies

Thomas

Yes

massimo · 27th July 2012, 06:37 PM

Quote:

Originally Posted by albertli

I cut them out of the legs of the coffee table which people trashed at the yard.

That's recycling!

euro21 · 08:20 PM

My (breadboarded) 26 pre spectrum (and THD) at 6V RMS output.

B+: 220V, filament bias (Rod Coleman supply) -1A with 10R- -10V, Majestic G-26 tubes, 6mA cascode (DN2540) CCS, 1K Riken grid stoppers, Shinkoh resistors on the input and output, 0.22uF russian FT3 teflon capacitor, S&B TX102 TVC.

Power supply on the another board (about 30cm distance): 2*240VAC-AZ1 mesh-CLCLC-(dual)SSHV2; 2*15V-FexFred bridge-C-CMC-CLC raw filament supply.

No audible hum.

Without TVC the bandwith over 300kHz (7Hz-320kHz +/-3dB)!

With different tubes (about a dozen) 50 and 100Hz components changing about 30db (-80..-110dB)!

Salas · 27th July 2012, 08:06 PM

Its the active load it helps your hum a lot anode wise. Inductive load windings pick up.

mogliaa · 29th July 2012, 12:56 PM

Whilst watching the women cycle race (on TV of course otherwise will be down the rain here in London

), I did some first measurements on the Lundahl LL2745 that Thomas Mayer kindly send me as a trial:

http://www.bartola.co.uk/valves/2012...th-ll2745-opt/

looks very promising so far, will now rebuild my 26 preamp and do some listening tests
Cheers,
Ale

gmant123 · 31st July 2012, 08:28 PM

Hi!!

Someone tried choke load and output transformer in parafeed configuration?

Maybe one of these can work?

EDCOR - XSM10K/600
http://www.edcorusa.com/Content/Plots/XSM10K-600.pdf
EDCOR - WSM10K/150
http://www.edcorusa.com/Content/Plots/WSM10K-150.pdf

Can be a more flexible solution...

Alberto.

27th July 2012, 08:47 AM	#1922
Vinylsavor diyAudio Member Join Date: Feb 2002 Location: Lindau	Hi! You are right 10V RMS in would be too much. I think he applied an input voltage which gave 10V out at 1kHz and then checked the other frequencies Thomas __________________ http://vinylsavor.blogspot.com/

27th July 2012, 12:20 PM	#1923
Magz diyAudio Member Join Date: Oct 2009 Location: New Jersey, USA	Another good choice for shielding is TI-Shield. It is a permalloy core sheet plated on both sides with copper, so it handles both magnetic and electrostatic shielding. I have used it in a few components and it really cuts down on the interferences. It also maintains its shielding properties when bent or soldered, which is an advantage over mu metal. It has a nice copper color if you care about the esthetics. Available at Michael Percy Audio. Here's some info on its construction: http://www.emsclad.com/examples/emi-rfi-shielding.html When I breadboarded my 26 preamp I had a lot of hum - what I found was that most of it was coming from nearby electrical devices, such as the lights inside the cabinet the pre was in, the overhead lights, an old tuner that was nearby, etc. unplugging/turning off those sources resulted in a decrease of hum of ~80%. . Last edited by Magz; 27th July 2012 at 12:24 PM.

29th July 2012, 12:56 PM	#1929
mogliaa diyAudio Member Join Date: Dec 2009 Location: London	Whilst watching the women cycle race (on TV of course otherwise will be down the rain here in London ), I did some first measurements on the Lundahl LL2745 that Thomas Mayer kindly send me as a trial: http://www.bartola.co.uk/valves/2012...th-ll2745-opt/ looks very promising so far, will now rebuild my 26 preamp and do some listening tests Cheers, Ale __________________ "A mistake is always forgivable, rarely excusable and always unacceptable. " (Robert Fripp) http://www.bartola.co.uk/valves/

31st July 2012, 08:28 PM	#1930
gmant123 diyAudio Member Join Date: Dec 2009	Hi!! Someone tried choke load and output transformer in parafeed configuration? Maybe one of these can work? EDCOR - XSM10K/600 http://www.edcorusa.com/Content/Plots/XSM10K-600.pdf EDCOR - WSM10K/150 http://www.edcorusa.com/Content/Plots/WSM10K-150.pdf Can be a more flexible solution... Alberto. __________________ Analog as the sound of the nature....

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27th July 2012, 08:06 PM	#1928
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Its the active load it helps your hum a lot anode wise. Inductive load windings pick up.

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