Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: DummyLoad on May 01, 2026, 06:39:51 pm

Title: Tube Preamp SMPS
Post by: DummyLoad on May 01, 2026, 06:39:51 pm

Nifty little DC-DC converter - 9V > 250V + 6.3V - Rated at 250V 15mA / 6.3V @ 1.5A. Works really well. Have a 12mA R load on the HT (22KΩ 6.5W R), and about 1A on the 6.3V output. Pics in link. Performs well, about 100-150mV ripple on the HT output. link to part on Amazon. Beware, that the power source (5.5mm x 2.5mm jack) the tip is (+), and not tip (-) like for pedals. May build a guitar pre with it. Use a polarity inverting cable to use pedal wall-wart. 

Link to pics. (https://www.dropbox.com/scl/fo/rmzflfupnypsvlfmkf356/ANgwawUZY7OHdEIC3GTzUxg?rlkey=fi3fbxzzcrhqxjile31oqh2cl&st=o101c142&dl=0)

Link to SMPS on Amazon. (https://www.amazon.com/dp/B09ZV31X69?ref=ppx_yo2ov_dt_b_fed_asin_title)


--Pete

Title: Re: Tube Preamp SMPS
Post by: Jonas on May 03, 2026, 08:08:52 am

THANKS PETE!!! I HAVE A COUPLE ON ORDER

Title: Re: Tube Preamp SMPS
Post by: DummyLoad on May 03, 2026, 02:17:54 pm

Quote from: Jonas on May 03, 2026, 08:08:52 am

THANKS PETE!!! I HAVE A COUPLE ON ORDER

 :smiley:

Title: Re: Tube Preamp SMPS
Post by: Jonas on June 06, 2026, 11:48:05 am

Pete, what was your approach to eliminate the switching noise? Do you have any recommendations of things to try?

thanks!!!

Title: Re: Tube Preamp SMPS
Post by: DummyLoad on June 08, 2026, 03:08:09 am

Tamed it with a with a pair of Pi filters - The first iteration I used a single 20H 10uF Pi filter, was ok, but still had some pop/buzz - Added a second Pi filter with 100uH 4.7uF, fixed most of the noise. Not perfect but tolerable and usable. Has about 23-30mV of ripple now, down from about 150mV average. I wound the toroid myself - Had some ferrite cores from a lab that was shutdown, so wound a  Micrometals T68-3 with 25 turns of 24AWG, salvaged and stripped wire from an old ethernet cable. No, you don't need 1W Rs in HV string, that's what I had in the bins. 

Schema attached.

Pic of ripple measurement of B+ line - Link below.

https://www.dropbox.com/scl/fi/z9kjf10i7owtmwhq1g5ot/IMG_20260608_021259634_HDR.jpg?rlkey=xgynz75541ry58kwv7kcrz7vc&st=fjdakbg0&dl=0 (https://www.dropbox.com/scl/fi/z9kjf10i7owtmwhq1g5ot/IMG_20260608_021259634_HDR.jpg?rlkey=xgynz75541ry58kwv7kcrz7vc&st=fjdakbg0&dl=0)

--Pete

EDIT: The 1W resistors in my application are overkill, my circuit draws about 3mA total. YMMV - Depends on the PS current load.

Title: Re: Tube Preamp SMPS
Post by: Jonas on June 09, 2026, 08:47:46 am

test

Title: Re: Tube Preamp SMPS
Post by: Jonas on June 09, 2026, 08:51:17 am

thanks Pete and nice job! - I will give that a try

Are you getting any noise form the 6.3v heaters?

I an not using the on-board dc jack, wiring direct (tried to upload a pic no success - attachment options is not working for me???)

Title: Re: Tube Preamp SMPS
Post by: Jonas on June 16, 2026, 04:57:55 pm

I didn't have room for large chokes inside my design so I'm going to try a different approach. I read that a darlington pair configuration could be used to create a buffer for the HT. Has anyone tried this?