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Old 1st March 2005, 04:35 PM   #1
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Default Heater supply recommendation for 6922?

I was hoping to get some ideas/recommendations on the DC heater supply for the 6922 preamp I'm building.

I was originally set upon the technique Joe Rasmussen uses in the tube buffered GC, an LM7808 feeding a series 5R6 resistor. Low parts count, addresses turn on surge, doesn't limit current (which some say is bad). But I have 4 tubes, so 300mA x 4 is more than 1 7808 can handle.

So, any favourite techniques out there? I MUCH prefer sand for this.
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Old 1st March 2005, 04:39 PM   #2
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CCS feeding a series string. That way, you can get by with a single 317 or similar chip.
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Old 1st March 2005, 10:55 PM   #3
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Thanks SY.

I don't think I will go that route though, I have a transformer already and its winding is too low in voltage to put them in series.

So I guess you don't buy the current-limiting-is-bad argument. Any pitfalls to just using an LM338 to regulate to 8VDC and using 4 resistors, 1 to each heater?
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Old 1st March 2005, 11:28 PM   #4
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It'll work fine, albeit without the slower warmup of the CC version.
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Old 2nd March 2005, 01:52 AM   #5
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leadbelly,

You can use the trafo you have and a series heater string. Rig up a "full wave" doubler from 2X Schottky diodes. You'll do fine, as long as the caps. in the doubler stack are good sized. TANSTAFL, you get 2X the Volts and 0.5X the Amps. The RMS current rating of the trafo should be 4X the DC draw at the doubled voltage.

BTW, Schottky diodes in the filament rectifier are a good idea no matter what topology you use. Stamp out switching noise.
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Old 2nd March 2005, 03:20 AM   #6
sijosae is offline sijosae  South Korea
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Please check my CCS method,
http://www.headphoneamp.co.kr/bbs/zb...sijosae&no=217
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Old 2nd March 2005, 04:22 AM   #7
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Use LM350T, heatsink it, adjust the right voltage with two fixed resistors and use two big electrolytics on the input and on the output of the regulator (>20000uF) and pair of protection diodes over the chip.

But the truth is AC heating - more musical, more 3D and less compressed sound - especially for 6922 tube.
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Old 2nd March 2005, 09:36 AM   #8
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Hi,

Quote:
But the truth is AC heating - more musical, more 3D and less compressed sound - especially for 6922 tube.
With indirectly heated tubes?
Could it be there was something wrong with the implementation of that DC feed?
Why the 6922 in particular?

Cheers,
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Old 2nd March 2005, 10:00 AM   #9
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I have built about 10 preamps and headphone amps with 6922 and 5687 tubes and all they sound better with AC heating and center grounded pot. I don't know why, but the difference is huge:

- AC - musical sound and 3D soundstage
- filtered DC - dull sound and flat soundstage (with >30000uF caps used the soundstage is getting better but not much to like it)
- regulated DC - dull sound and compressed dynamics

I'll be very grateful if somebody explains this phenomenon.
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Old 2nd March 2005, 10:02 AM   #10
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Have you tried current-regulated DC?
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