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#26 pre amp
#26 pre amp
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Old 19th March 2012, 10:14 AM   #1771
andyjevans is offline andyjevans  United Kingdom
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I came across this power supply from John Swensen who has recommended snubbers of 22nF and 330 ohms in his experiments. It uses a choke in the PSU - something like I've been using and as Thomas recommends. The supply is low power but could be scaled up. Chokes can be simple Hammonds - for the 26 filament supply the model is 159ZC, 60mH 2A. There is a 159ZA, 300mH 1A. A bit close for a 26, but maybe OK in starved filament. This might precede a Rod Coleman board - I don't know how much Rod would find this necessary. I use choke input with a small cap just like this circuit for filament bias. I don't use the LM1084 which may be redundant here with Rod's supply.
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File Type: gif swensen snubberPCPS.gif (10.8 KB, 685 views)

Last edited by andyjevans; 19th March 2012 at 10:17 AM.
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Old 19th March 2012, 10:31 AM   #1772
Rod Coleman is offline Rod Coleman  United Kingdom
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#26 pre amp
Yes, the snubber is recommended. The details are in my application note, where 47R & 100nF are stated - but the values are not critical.

The purpose is to introduce electrical damping to the tuned circuit formed by the (leakage) inductance of the transformer, and the total of stray capacitances around the transformer-rectifier loop.

For perfection, you can make a little current-probe (toroidal core made from inductor-type ferrite, with a few turns). Slide the toroid over a wire in the rectifier circuit & scope the turns on the probe, and adjust the R and C to obtain the smallest & smoothest turn-OFF pulse.

But just using the suggested values should be good enough for almost all designs.
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Old 19th March 2012, 06:09 PM   #1773
Magz is offline Magz  United States
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So I've begun work on the 26 preamp, and am building the first version on a plywood board so that I can tweak to my heart's content before making a nice chassis.

I'm isolating the 26 tube on a sub-assembly in order to control microphonics. I've attached a few pictures of how I've assembled this in case you're curious. It starts with a 4mm aluminum plate (courtesy of andyjevans' fire sale a few weeks ago), into which I've drilled and punched the appropriate holes for the sockets and mounting bushings (picture 1).

To this is added a layer of self-adhesive Soundcoat absorbent material (sheets purchased from Parts Connexion) - picture 2.

On top of that goes another 4mm aluminum plate, which is glued and bolted down. (picture 3)

Connex teflon tube sockets (economy grade) from PCX are added - picture 4. The Connex economy teflon sockets are perfect here as they provide some damping of their own and because the body of the socket is quite long, allowing the socket to stick up through the outer chassis which will eventually be added over the mounting plate.

The mounting is through Deflex bushings on each corner; I made them from Deflex vibration absorbing washers (also PCX) by sandwiching a cored out center of one Deflex washer between two full size Deflex washers.

The assembly is very dead sonically, rapping on it produced a soft thunk, as opposed to the ringing of the plain aluminum plate.

Overkill? Perhaps, but I think I can be confident that I've dealt with chassis-borne vibrations, anyway...
Attached Images
File Type: jpg 26 Mount 4.jpg (24.3 KB, 650 views)
File Type: jpg 26 mount 3.jpg (23.2 KB, 620 views)
File Type: jpg 26 mount 1.jpg (26.7 KB, 609 views)
File Type: jpg 26 Mount 2.jpg (24.3 KB, 564 views)

Last edited by Magz; 19th March 2012 at 06:26 PM.
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Old 19th March 2012, 06:46 PM   #1774
dhtrob is offline dhtrob  Netherlands
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Looking neat and clever!
I got good results using these stand-offs.. It is soft natural rubber in between two threaded rods.. They are supposed for vibration damping.. These are M4 but also available for M3 and with one threaded rod and nut/insert.
When used, you can move the valve almost 45 degrees to each side and tapping on the chassis is barely heard through my speakers (only a very soft thunk).
These allow me even to use a stepped attenuator...

Rob
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File Type: jpg 100_3847.JPG (235.6 KB, 232 views)
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Old 19th March 2012, 06:48 PM   #1775
Bas Horneman is offline Bas Horneman  Netherlands
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Quote:
I got good results using these stand-offs..
Where do you buy them Rob?
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Old 19th March 2012, 06:50 PM   #1776
dhtrob is offline dhtrob  Netherlands
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Hi Bas,
Here: Verpas - Groothandel in doppen en kunststofproducten
Lots of different sizes etc etc..
Rob
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Old 19th March 2012, 06:52 PM   #1777
Bas Horneman is offline Bas Horneman  Netherlands
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Thanks!
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Old 21st March 2012, 09:38 PM   #1778
jamesdb is offline jamesdb  United States
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Default 26 Filament Chokes

I found Signal Transformers sell directly and have a choke slightly larger than the Hammond 159ZC (60mH 2A), Signal model CH2 that is 70mH 2A .9 ohms DCR for only a few dollars more than the Hammond.
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Old 22nd March 2012, 08:51 AM   #1779
Vinylsavor is online now Vinylsavor  Germany
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Hi!

Although the Lundahl LL1660 works very well with the 26, a transformer with a higher step down ratio would be better for the 26 if a output impedance is needed to drive real 600 Ohm loads like a LCR RIAA EQ. For that a ratio closer to 6:1 would be better. I convinced Lundahl to add such a transformer to his line up. See my blog post about it:

VinylSavor: New Lundahl Line Output / Interstage Transformer

I will build a 26 preamp around this transformer soon

Best regards

Thomas
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Old 22nd March 2012, 11:46 AM   #1780
andyjevans is offline andyjevans  United Kingdom
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Cool stuff. This could offer some useful ratios for SE to PP stepdown. I haven't calculated those - what would they be?

andy
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