6688, whether you speak Mandarin, or Cantonese, has connotations of “easy” and “prosperous”. So a tube that bears this number, naturally attracts the Chinese in me to view it as more than “a vacuum tube”. If all tubes are equal, clearly this fella is more equal than others.
I actually played with this little fella a longgggg time ago… via the Grounded Grid Gain Stage, back then using it as an I/V convertor for the then Monica2… Looking back, that was likes ages ago…
Anyway, here in 2017, we have moved far ahead and the GG stage was sitting there collecting dust, so I gutted everything there, and added in some other components to finally make a headphone amplifier worthy of our time and attention.
The circuit is really simple.
Nothing special here.
You know I’m a fan of the Cree Schottkies at the tube cathodes.
You have seen parallel feed as well.
This is the first time I’m using this rubber wood chassis, as well as 2-chassis construction.
Also, first time I’m using Cinemag iron.
Cinemag CML-2394 420H (!) @20mA
Cinemag CM-13107 10k : 600/300/150/32 *50% Nickel
Rat’s nest, as usual. I have a reputation to maintain!
The 600 ohm taps go to the Output RCA while the 32 ohm taps go to the headphone jacks. Well, if I have headphones of various impedances, I’ll wire in a switch but no need here.
The tuned RC is optional. Panasonic metallized polypropylene capacitors for power supply here.
After the initial run in period, I have modified the schematic few times before arriving at current iteration. Schottkies still better than a few RC combos I tried at the cathode.
There shall be more modifications…
Okay, how’s the sound?
Marvelous! Even via audio output from my Mac, the spaciousness, the airiness… makes you want to listen and listen some more. So far don’t even detect any fatigue-ness at all.
All in, highly recommend you get the Cinemag irons. First time I’m using this irons and I believe the sonic quality is from the irons alone. The other tricks, I have played many times before.
And it’s so versatile. Could be used as a line amplifier (which I haven’t tried yet).
Good to see that you are back again!
By lack of extra monica’s I got a Project DAC which too uses tda 1545, and I/V over a resistor, and yes it does sound great again (better half complained about the huge DAC/Phono pre amp so everything needed to get smaller). Always interested in new creations from your side. Myself I am in the process of making a slot loaded open baffle using BD15, supravox rtf64 and tiny supertweeter.
thanks for writing! how’s business?
do update me on your project. we have limited resources to work on every interesting projects, so if you could share your experience with me, i could put up an article too.
I saw your reply just now! Open baffle now sounds great and (is going to) look good. Small OB for supravox/tweet + N-frame sub woofer. All rather airy, sounding great as an OB should and with nice bass too (thanks to sub-amp!).
I would post a picture if possible