Another EL34 SET Amp with Broskie CCDA
I'm a little late seeing this thread but thought i might have something to contribute. I've spent the past 6 years refining a EL34 SET amp design. I use this to drive a single driver (Louther) folded horn, so the load is very simple and efficient -- i haven't tried this amp with any more difficult load. I'm sure its damping factor is lacking. That said, here are some things i've learned after many iterations of this basic design that really make a wonderful difference and are easy to implement.
I originally bought this as an out of production Chinese amp which i was able to pick it up for a song. I liked the chassis and general layout so even if i didn't like the sound of it, i could use this as the basis for experimenting. The original circuit was your run of the mill 12ax7 voltage amp + cathode follower feeding an EL34. It uses a 274b rectifier power supply stage feeding both channels. It sounded ok. Nothing special, pleasant enough.
I won't bore with the 10's of iterations i tried, but will jump to where i am happily today:
- I changed the front end to the Broskie CCDA design CCDA Noval Stereo PCB. I removed all bypass capacitors from the cathode resistor in the power tube stage. I added a little bit of local feedback (300k) from the plate of the EL34 back to the grid of the follower in the CCDA stage. I've rolled tubes here and found 5751 most to my liking.
- My goal was to have just enough gain that my volume control was usually running at 60% or greater. In actual use, i bypass the volume control pot in the amp and use the built-in attenuator in my D/A or TV output. I really dislike mechanical pots for volume control because there is so much detail lost -- even with high quality pots.
There are a few unconventional mods that i got from other sources, which are easy put in place and switch in/out for listening tests. These are the ones i left in as quite beneficial.
- Diode in series with the EL34 screen grid. This has an amazing effect on the 3-dimensionality and realism of the sound of this circuit. I first read about this "stopping diode" here http://www2u.biglobe.ne.jp/~hu_amp/amput3e.htm
- Capacitor between the cathode and suppressor grid of the EL34 (instead of a short). This seems to allow the resolution of minute detail in the music that is otherwise lost. I got this "Hazen Mod" from here DECWARE - The Hazen Mod
- Diode between the OPT and the CCDA power supply. This is my own. I use this as a poorman's "dual power supply". I found that it seems to improve the channel separation a tad. It seems to widen the sound stage beyond the edges of the speakers. (Please excuse the diode between the power supply and the OPT. That was an early test that i just realized is still in my schematic. It's no longer there.)
Last edited by volleynbike; 21st January 2019 at 12:33 AM.