CCS Design for DHT Filaments

[dsavitsk]

I am looking for a CCS design for a 5V/250mA filament (71A). Does anyone have any suggestions? I'll be using a 6.3V/3A winding with a voltage doubler to get the raw DC supply.

[JosM]

Hello dsavitsk,

You can try this.

[Boris_The_Blade]

With near 16.5 VDC to play with, you could just as soon use a single resistor of (16.5-5)/0.25 = 46 ohms of at least 5 Watts and get pret' near the same CC effect with a lot less parts.

[BL21DE3]

Hello,

Have a look at the following two AudioXpress articles for a variety of CCS circuits designed/proposed by Walt Jung. The first article covers theory/basics and the second one contains the actual CCS circuits.

http://www.audioxpress.com/magsdirx/...a/jung2778.pdf

http://www.audioxpress.com/magsdirx/...a/jung2779.pdf

hope this might be of some help.

Regards,
Ewan

[danlaudionut]

I am not experienced with DHTs at all but
what I have read and heard recommended alot
are CMC before the filaments after the CCSs.
CMC = Common Mode Choke
They are touted as helping the tube being isolated
from the CCS, taking the CCS out of the audio circuit.
AC heaters are supposed to sound better.
DC heaters don't have the hum problem.
CMC with DC heaters get you very close to
the AC sound without the hum.

DanL

[Sheldon]

Originally posted by BL21DE3
Hello,

Have a look at the following two AudioXpress articles for a variety of CCS circuits designed/proposed by Walt Jung. The first article covers theory/basics and the second one contains the actual CCS circuits.

http://www.audioxpress.com/magsdirx/...a/jung2778.pdf

http://www.audioxpress.com/magsdirx/...a/jung2779.pdf

hope this might be of some help.

Regards,
Ewan

A variation on the 1 vbe current source, using a bigger part for the pass element, would probably allow you to use your filament transformer without the doubler. Not the last word in CCS performance, but plenty stout for an output tube filament.


Sheldon

[andyjevans]

I've used quite a few designs for DC filaments. One thing that came up is that a CMC after a voltage reg improved the sound BUT a CMC after a current source degraded the sound.

Overall, current sources sounded better, so I stopped using the CMCs. Best design of all is by Rod Coleman - total overkill but great sound:

New DHT heater

Useful design is the RonanReg - works fine with the LM1084 which takes you up to 5 amps. Handy for bigger filamants. Big reservoir cap is a good idea - like 15,000uF.

I found I needed quite a bit of stepdown voltage with a LM1084 as a current source. e.g. needed at least 12v to step down to 7.5v for a 10Y. So make sure you have adequate margin - seems to be a bit more for comfort than in the datasheet.

Handy tip - for lower current filaments, use cheap DC 12v supplies. Then put the current source in the chassis. the 2.1mm and 2.5mm DC connectors are quite small and useful as filament connectors.

[kevinkr]

Originally posted by dsavitsk
I am looking for a CCS design for a 5V/250mA filament (71A). Does anyone have any suggestions? I'll be using a 6.3V/3A winding with a voltage doubler to get the raw DC supply.

Hi Doug,
I'm just using a single LTC1086 per channel configured as a CCS at 250mA. (w/71A) Current setting resistor is right around 5 ohms. (Trim as necessary to get 5V across filament when warm.) Schottky rectifiers and a 10000uF cap up front, and a 100uF across the output. (Optional - many claim this is not a good thing.) The transformer has 8V secondaries IIRC, and the raw dc supply voltage is around 11V. No audible hum, IMO sounds as good as anything I have heard for dc filament heating..

I no longer use CMC either, but it might provide benefit if placed before the CCS - maybe even between the transformer and rectifiers, particularly if you are using toroids. (I assume you are not using toroids, but thought I would mention this in case you wanted to experiment anyway.)

[dsavitsk]

Originally posted by Boris_The_Blade
you could just as soon use a single resistor

I might look into that, but I'd think the CCS would isolate the filament PS from the signal path a little better.

Originally posted by kevinkr


Hi Doug,
I'm just using a single LTC1086 per channel configured as a CCS at 250mA. (w/71A) Current setting resistor is right around 5 ohms. (Trim as necessary to get 5V across filament when warm.) Schottky rectifiers and a 10000uF cap up front, and a 100uF across the output. (Optional - many claim this is not a good thing.) The transformer has 8V secondaries IIRC, and the raw dc supply voltage is around 11V. No audible hum, IMO sounds as good as anything I have heard for dc filament heating.

Sounds reasonably simple. Since I have a little extra voltage due to the doubler, do you think it is worth pre regulating? This will be (another) headphone amp, so hum needs to be pretty close to 0.

I no longer use CMC either, but it might provide benefit if placed before the CCS - maybe even between the transformer and rectifiers, particularly if you are using toroids. (I assume you are not using toroids, but thought I would mention this in case you wanted to experiment anyway.)

No toroids here -- EP throughout (PT and input choke), except for the outputs which are from Dave Slagle.

[kevinkr]

Hi Doug,
I'm not using any filament pre-regulation in my 71A based headphone amplifier, and the noise floor (without any output attenuation) is so far down I can't reliably measure it with either my Amber 3501A or my pc based fft analyzer.

This is used with a pair of 5842 choke loaded driving a pair of 71A. Dead silent at the outputs. (No hiss, hum or buzz at all.) PSU is tube rectified CLC and is located in a separate chassis.

A pre-regulator won't hurt provided you have enough voltage headroom which I suspect you might be marginal on. You need at least 4.25V across the LTC1086 including the 1.25 across the current sense resistor for really good performance, and slightly more is even better. To work effectively you need about 12.5Vdc under load at low line, regulate down to a hair over 9.25V - if you can provide this then you are golden. A single pre-regulator ought to be fine with a pair of CCS..