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Switching power supply for tube filaments possible???
Switching power supply for tube filaments possible???
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Old 7th November 2017, 12:17 PM   #31
Sunsun22 is offline Sunsun22  Hong Kong
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I have tried to use a 317 module with adjustable output voltage for DHT filament supply. The sound is very dull, no low and no high at all. Bass is very muddy.

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The next trial is to use the buck dc to dc converter. Since it does not have a rectification module, I am using the same 317 module as rectification to supply DC to the buck converter. The result is a little bit improvement on the high but no improvement on the bass.

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I believe there is no result without some extra work therefore I build a rectification module with a simple bridge and cap. Now the highs and lows are there and it is the best setup among the three that I tested.

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It is still not comparable to dedicated DHT filament heater and I would say about 80% performance only. Yet, if you compare the price and performance ratio, it worth! As a remark, I discovered the buck converter is using a 317 as current limiter and I think the sound quality could be improved by removing the current limiter. Um..... I don't think I will try to do this by myself. I rather ask my friend to send me two more buck converter without current limiter - of course, same as before, FOC.

Last edited by Sunsun22; 7th November 2017 at 12:23 PM.
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Old 8th November 2017, 12:45 PM   #32
Sunsun22 is offline Sunsun22  Hong Kong
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The new buck dc-to-dc converter arrived this afternoon. This is great to live here since there are so many toys around you that you can order and receive them within overnight at extremely low cost. The modules arrived with a courier fee of US$0.70 to my Shenzhen location; yes 70 US cents within same city (but still 2 hours drive apart).

Actually, I received two simple modules. One of them is very primitive while the other one has a display showing the input and output voltage. The costs are only peanuts - US$2.5 for the simple module and US$3.5 for the module with voltage display.

The simple module does not have connectors therefore I have to solder the power and output lines on it. Besides, I also changed the caps from 220uF solid caps to Panasonic FC 1800uF 35V (I don't have larger capacity with lower voltage on hand). I further changed the trimmer to fix resistor to give a consistent output voltage. They look like this after modification: -

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The US$3.5 fancy model look like this without any modification: -

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I did some measurement on this fancy module, the display does not show accurate voltage at all with a deviation of about 0.08V (at 1.4V output)

After installing the modified simple module to the DHT preamp; without burning in, I would say there is improvement to the module that has current limiter. The bass is getting clear and more while the high produces crispy "dings and dongs". If I have to give a comparison to the dedicated DHT filament heater supply, I would say the bass is now 90% and the high is 85% of it.

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I have also ordered some 10000uF from mouser this evening and look forward if there could be further improvement by changing the cap after the bridge rectifier to this value.

Last edited by Sunsun22; 8th November 2017 at 12:51 PM.
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Old 8th November 2017, 10:42 PM   #33
vt4c is offline vt4c
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I bought something similar; a different switching buck converter. The unit I'm having can't be used without adding lots filtering capacitor and inductor at the output. Lots of hissing and high frequency noises when powering (filament bias) the 4p1L and 10Y DHT tubes.
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Old 9th November 2017, 01:14 AM   #34
Sunsun22 is offline Sunsun22  Hong Kong
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I don't have issue on noise and the system is very quiet and I think the quietness is about 95% of Herron Audio's. Can you post a picture of the unit that you used? May be I can ask my friend to comment on your module.
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Old 9th November 2017, 12:54 PM   #35
vt4c is offline vt4c
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This is what I bought...

DC-DC Step Down Adjustable Constant Voltage Current Power Supply Module | eBay

For the price I paid, it's a bargain. Need some additional work to get it optimise.

Last edited by vt4c; 9th November 2017 at 01:00 PM.
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Old 9th November 2017, 02:01 PM   #36
artosalo is offline artosalo  Finland
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Quote:
Originally Posted by vt4c View Post
I bought something similar; a different switching buck converter. The unit I'm having can't be used without adding lots filtering capacitor and inductor at the output. Lots of hissing and high frequency noises when powering (filament bias) the 4p1L and 10Y DHT tubes.
Do you have a hum canceling resistors (trimmer) across the filament, similar that is used when having AC-filament voltage ?
Attached Images
File Type: gif Filament hum can..gif (42.4 KB, 90 views)

Last edited by artosalo; 9th November 2017 at 02:04 PM.
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Old 9th November 2017, 03:13 PM   #37
v4lve lover is offline v4lve lover  Netherlands
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Switching power supply for tube filaments possible???
Just increasing the output capacitance on the switcher might not be the best approach.

Look at the datasheet for most buck converters E.G. 2575 or LT1076 and you will see that the manufacturer has given recommendations with regard to optional output filters.

Using them as a pre reg for a decent CC/CV linear supply is doable.
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Old 9th November 2017, 10:45 PM   #38
vt4c is offline vt4c
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What I did was to add some low ESR reservoir capacitor along with some decoupling caps. More of a C-L-C configuration at the output. This tame quite a significant HF noises that I was previously able to hear at the output of the DHT driver tube. I'm happy with the latest configuration since the noise at the speaker (100dB) is barely audible.

I guess the quality of those reservoir caps on the board are in question. I'm still trying various approach powering the filament bias. Might rip-up all those original caps on the buck converter eventually and replace it with some better ones. Still experimenting.

Another switching buck converter board that I'm bought poses more challenges to get the noise down.

Last edited by vt4c; 9th November 2017 at 11:00 PM.
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Old 9th November 2017, 10:52 PM   #39
vt4c is offline vt4c
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Quote:
Originally Posted by artosalo View Post
Do you have a hum canceling resistors (trimmer) across the filament, similar that is used when having AC-filament voltage ?
Thank you for the suggestion

I did use hum cancelling when supplying a.c. to the filament. For HF switching PSU or buck converter, it is less effective.

With filament bias, the ground return has to be on one end of the filament connection.

Last edited by vt4c; 9th November 2017 at 10:57 PM.
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Old 12th November 2017, 02:06 PM   #40
Sunsun22 is offline Sunsun22  Hong Kong
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Hi Vt4c

I passed your link to my friend running an electronic factory in China. His comment is your module is a very standard module and there should be no hissing at all unless the factory is using some inferior parts. No further comment.
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