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Old 27th July 2012, 08:27 AM   #1921
euro21 is offline euro21  Hungary
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Quote:
Originally Posted by albertli View Post
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?
Attached Images
File Type: jpg RCA 26.jpg (75.3 KB, 494 views)

Last edited by euro21; 27th July 2012 at 08:41 AM.
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Old 27th July 2012, 08:47 AM   #1922
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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
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Old 27th July 2012, 12:20 PM   #1923
Magz is offline Magz  United States
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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.
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Old 27th July 2012, 12:40 PM   #1924
Magz is offline Magz  United States
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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.
Attached Images
File Type: jpg LFEQ no Shield dBu.JPG (114.2 KB, 468 views)
File Type: jpg LFEQ with shield, background 2.jpg (204.9 KB, 457 views)

Last edited by Magz; 27th July 2012 at 12:43 PM.
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Old 27th July 2012, 02:46 PM   #1925
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Quote:
Originally Posted by Vinylsavor View Post
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
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Old 27th July 2012, 06:37 PM   #1926
massimo is offline massimo  Italy
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Quote:
Originally Posted by albertli View Post
I cut them out of the legs of the coffee table which people trashed at the yard.
That's recycling!
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Old 27th July 2012, 08:02 PM   #1927
euro21 is offline euro21  Hungary
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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)!
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File Type: jpg G-26_1 Spectrum 6V_web.jpg (52.5 KB, 412 views)

Last edited by euro21; 27th July 2012 at 08:20 PM.
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Old 27th July 2012, 08:06 PM   #1928
Salas is offline Salas  Greece
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Its the active load it helps your hum a lot anode wise. Inductive load windings pick up.
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Old 29th July 2012, 12:56 PM   #1929
mogliaa is offline mogliaa  United Kingdom
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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
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Old 31st July 2012, 08:28 PM   #1930
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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.
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