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Old 3rd May 2007, 07:27 PM   #1
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Default Heater PS alternative / Redux

This horse probably has been ridden into the ground, but here goes:

I'm considering a linear supply for the tube heater circuit of my BottleHead foreplay pre-amp based on the LM7805 linear regulator chip. Under certain conditions a tiny amount of hummm (~120 Htz) comes through the pre-amp, probably generated by my LCD "big" screen TV, which is close to the pre-amp (the humm goes away immediately on TV power off). I am making the assumption that this may be remedied by a little better regulation on the two 12xx7 heaters. Comments?

Suggested circuit ... but I certainly would consider other options:
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Old 3rd May 2007, 07:45 PM   #2
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Default Re: Heater PS alternative / Redux

Quote:
Originally posted by FastEddy
(the humm goes away immediately on TV power off). I am making the assumption that this may be remedied by a little better regulation on the two 12xx7 heaters. Comments?
That's an odd assumption. At any rate, dropout voltage on these things is 2V. Why not just move it a few inches further away from the TV?
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Old 3rd May 2007, 08:00 PM   #3
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" ... Why not just move it a few inches further away from the TV? ..."

Well, actually I would rather place it a little closer to the TV and have the heater voltage better regulated = both or either.

'Sides, if there is any noise coming through the tube pre-amp, I would like to remove it as there is no telling what else is coming through that I might not be able to hear ... like EMF or the occasional police car radio noise, etc. (The BottleHead folks actually have an optional linera regulator kit for the ForePlay that incorporates a similar scenario, but I have all these '7805s and a drawer full of caps = DIYAudio.)

Also the VAC of the secondary tube heater is closer to 8.5 VAC peek to peek, so through the rectifier = 8.5 VDC = 6.5 + 2.0 ... should just be enough to light up the '7805 as shown.
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Old 3rd May 2007, 10:12 PM   #4
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Your tv set is a very rich source of EMI, most of which will be electrostatically coupled (may also be magnetically coupled from the yoke of a picture tube for those of you who like me still have a crt based set) into the circuitry of your pre-amplifier.

Doug is right about moving it a little further away. Alternately you can rebuild your pre-amp in a well shielded box, which would include shield cans on the tubes and a cage. Magnetic fields are difficult to stop and require additional shielding - usually mu metal or one of the newer materials developed for this purpose. Steel is moderately effective but hard to work with, exotic alloys are fragile, expensive and hard to work with. If your problem is magnetic then distance is your friend.

If your pre-amplifier like the other bottle head pre's I've seen was built on a wooden chassis you can add good electrostatic shielding by applying copper tape to the interior of the chassis. Make sure you solder the overlaps here and there and connect it to your star grounding point. The tape is available in most garden centers where it is marketed as a slug/snail repellent.

Incidentally running the filaments of off well regulated dc is not a bad idea (I generally do) and makes noise pick up from the filament wiring a non issue. Note that you only need the pair of diodes on the left. (Anode of one to 7805 gnd pin, cathode of other to filament common.) Given the lack of voltage margin I would use schottky rectifier diodes and possibly consider low drop out regulators like the LT1086-5.0 or the adjustable version set to 6.3V.

I doubt your problem is due to conducted susceptibility so your proposed fix probably won't address the issue, but you can verify this by moving the pre-amplifier far enough away that there is neither form of coupling using an extension cord plugged into the same circuit as the set.
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Old 3rd May 2007, 11:12 PM   #5
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A way to deal with Copper tape overlap that does not need much soldering is the use of 3M brand conductive thru the adhesive Copper tape. Being cheap, I would use the garden center stuff Kevin mentioned for the bulk of the shielding and save the fancy 3M stuff for the joints.
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Old 4th May 2007, 12:38 AM   #6
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kevinkr: " ... Incidentally running the filaments of off well regulated dc is not a bad idea (I generally do) and makes noise pick up from the filament wiring a non issue. Note that you only need the pair of diodes on the left. (Anode of one to 7805 gnd pin, cathode of other to filament common.) Given the lack of voltage margin I would use schottky rectifier diodes and possibly consider low drop out regulators like the LT1086-5.0 or the adjustable version set to 6.3V. ..."

I just measured the volts on the heaters of the tubes after several hours of operation ~ 6.30 VDC with an AC component of ~~ 0.1 VAC = ... So you all are right, there really isn't enough output from the existing PS & filters + current limiting resistor to light up the '7805 = ... so there goes the above PS regulator plan into the dumpster.

I would like to leave the existing kit heater PS in place so I suppose that would mean a '317 regulator or one of those "super regulators" ...

,.,.,. or, any other suggestions ... (even including removal of the current limiting resistor to get voltage from the PS to heaters ^^ 8.x VDC or so ...)

As I write, listening to Jimi Hendrix on this setup = MacBook as music server thru Echo Digital AudioFire4 to BottleHead tube pre-amp (the subject under consideration) ... (Note Samsung flat screen TV does not have very much magnetic radiation, but obviously does have a lot of EMF = the problem looking for a solution.)
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Old 4th May 2007, 02:53 PM   #7
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LM317 typically has drop out in excess of 3V under load, there are newer versions with lower drop out, but they compromise noise performance. You'd be better off with the LT1086. Also you probably don't need a current limiting resistor for the filament supply with an LM7805 in place - I'd remove it, this might give you the margin you need.
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Old 4th May 2007, 05:11 PM   #8
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" ... you probably don't need [the] current limiting resistor for the filament supply with an LM7805 in place - I'd remove it, this might give you the margin you need. ..."

Yes, of course, thanks for the suggestion. This would feed the 8.x voltage directly to the '7805 (or other) without the current restriction = more "headroom" for the '7805. (Meaning, I suppose that I could still consider the original scenario above.)

FYI: The BottleHead upgrade kit includes similar voltage regulation for the individual tube/amps: http://www.bottlehead.com/et/adobesp.../foreplay3.htm = bottom half of this page. Note that this kit does split the heater supply circuits into two separates as well, but still not linear regulated but a "brute force" type = half the current, double the cap filters.

Of interest: this pre-amp also incorporates Sy's LED noise reduction methodology ...
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