Doede's Audio Tweaks...
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Why did I want to change the 300BXLS amplifiers (Golidav..) you might ask? Weren't they already very good ? Yes they were, but as always, you keep thinking and small aspects over time are getting your attention.... I was never 100% satisfied with the AC heating, as always some rest HUMM was left over..... And a quiet tube amplifier is just a great kick .... Also I had the feeling I could use a bit more transparency. Without publishing I built a regulated tube power supply, which made the sound more direct and dynamic. This aspect I liked very much !! However it also brought some undefined "distortion" to the stereo image I could not explain..... I left it for a while and decide to switch back to normal C-L-C filtering and tube rectifying. The sound was smooth again, but I lacked the dynamics and wider sound stage ..... In the meantime TriodeDick had made some good experience with the, unfortunately very expensive, BG WKZ capacitors and fixed negative grid bias. He claimed similar gain as I had with the regulated supply, but without the "extras" :-) so I decided to redo things.....
After ample planning and preparations, I did a major work up on the KR 300BXLS amplifiers and was more than happy on the results !! I did the following:
Replaced the last filter Oil Capacitors with 100uF BG WKZ ones (see circuit)
Also removed the Rk including the necessary decouple Oil C. I left of course the current meter and just to go sure, bypassed it with a 12uF Auricap, which happened to be on hand
Implemented fixed bias voltage of my own design (happened to be on hand as well for another project which never came to life :-)
Installed a separate transformer, feeding the Tent Heater Supply which delivers 5V at 2A (combination of voltage and current source, depending if you look DC or AC...)
Improved Power Supply Filters:
Not much to tell I am afraid, In the 300B filter section I removed the old oil capacitor next to the 300B tube and replaced with one half of the BG WKZ (which is actually a 2x 100uF version). Than in the MU Stage filter section I added the 30uF which by passed the RK, so also the filtering of the MU Stage improved. Also here replaced the final C with the BG WKZ. This has become now a very rigid power source which is coming close to a regulated version. I did some testing with load jumps and it looks great; on a 100Hz full power swing I lose only 3 volt on 500 Volt supply.....
I have been going a somewhat different way as what you normally see. In stead of using a simple RC filter and a potentiometer to adjust the bias, I choose for a regulated version. I wanted to have the opportunity to keep the bias as much as possible out of the signal path, as it is basically seen as a load for the MU stage.... If you look at the circuits it is just a cascaded LM317 solution, so really nothing special, but it gives a nice fixed voltage and keeps the whole voltage source more or les outside the signal path.
An image of the small PCB I used. The single version for the SE amplifier is on top. The other ones are for a push pull amplifier with 4 tubes...
I will not go into a lot of detail as on the Tent labs website this is very comprehensively described. I just installed the 2 units (thanks to TriodeDick who let me test these...). Only thing I like to say, is that this a great solution, just plug and play :-) Of course I removed the potentiometer for de-humming purpose.
How does it SOUND ????
All this made the amplifier more dynamic, more direct, more transparent and for all dead quiet.... By having more "insight" also the stereo image opens up and that was exactly what I missed a little bit- End good, all good !! I think it fits great with the NOS DAC and SACD player with Transformer output. Some may like it to be to fresh and want the somewhat browner tube sound, but that might be personal. I prefer the sound as it is now, extremely transparent, without the "typical" analytical flat sound from a transistor amplifier....
Happy Listening and Building !!!!
IMPORTANT: The information provided on this page is intended as guide for DIY activities and therefore free to copy and or publish. If any one wishes to use any of the information from my WEB site, please make sure to refer and footnote to my URL Link as source! Doede Douma