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After my happy sonic experience with a stereo SE PX-25 amplifier I designed and built 2 years ago for a Canadian customer, I hankered for something with more power that would approach the same excellent sonic performance. So I designed the circuit shown. It was expertly built by Trent Weaver on two chassis as monoblocks. The design will deliver 25w/ch into a 4 ohm load with the secondary wired for 4 ohms.
We got the second amp working yesterday and listened and listened. It turned out, as designed, very well. The sound is very full with very good bass response and an extended-sounding high end. Without being broken-in yet, the level of information conveyed is very satisfying, so I expect more with time. Right now I'm listening to Disc 1 of the Crosby, Stills and Nash collection and the harmonies are really lovely.
There is much work to do yet "tuning" the power supply and operating points for best sound. We'll see how that unfolds.
Kevin Carter
K&K Audio
www.kandkaudio.com
Follow Ups:
Hi Kevin,
interesting approach with that power supply. Have you considered fixed bias? It may give you a little more headroom with the HT and more punch.
I'm in the process of building a GM70 amp. I'm going fixed bias (and a small unbypassed cathode resistor). It's probably going to be CCS loaded 5687 nput stage, cap coupled to a 45, IT coupled to the GM70.
Looking forward to hear more about your findings with this amp.
I could hardly ask for more punch than this amp has now!! I don't really need the extra power that would come from getting the 70v back, either. Each of us has to take the path that makes the most sense to us as individuals in our own listening context. I look forward to hearing how yours goes.
Since the original post I have really only made 3 changes:
Changed cathode bypass caps to "L cancelling pair" Black Gate NX (unobtainium now)
Changed interstage to LL1692A_AM/18mA
Changed GM-70 to copper plate type
All of these changes were very worthwhile from a sonic perspective. The thickness that troubled me in the lower midrange/upper bass is now gone and the overall transparency is improved considerably. I will probably next work on getting the noise level down. It's fine for 90dB sensitivity speakers, but not for 96+dB speakers.
Kevin Carter
K&K Audio
www.kandkaudio.com
I guess you're not a big fan of fixed bias? Sorry if I came across as a wiseguy. I'm going to try both and let you know how it turns out.
Can you put to words the difference between a copper and graphite plate GM70?
Arend-Jan
No, you've been around here long enough that I don't think you are a wiseguy. You're just a passionate DIYer, like me.
It's not that I dislike fixed bias, but in the original conception of the GM-70 amp I was trying to reduce complexity and a bias supply didn't help me achieve that goal, so it was abandoned. It might still be a good idea, but there are things ahead of it in the line to be explored.
The copper plate GM-70 has a decided advantage over the graphite plate tube in overall transparency, but is otherwise tonally similar.
Kevin Carter
K&K Audio
www.kandkaudio.com
That is one way cool amp! The power supply is too cool! WOW T Willman
Kevin,
Is a better (more legible) schematic for your GM-70 amp availalbe? It is difficulty reading the values of the components on the current one (or do I need new glasses?)
Thanks,
Francois
interested in your experiences with the GM70.
Regards,
Bas
To infinity and beyond!!!
We've waited until the amps were totally broken in, which it appears they are. So now we are ready to proceed. They have improved a lot during the break in and now have considerably more life and dynamics to go with the big, full sound present at the beginning. This design is a keeper that will be well worth spending some time (and money) improving. I have ordered iron amorphous core interstage transformers already. I'll keep you all current on progress here on the forum.
Kevin Carter
K&K Audio
www.kandkaudio.com
gm70's?
To infinity and beyond!!!
So far, graphite.
Kevin Carter
K&K Audio
www.kandkaudio.com
Never seen that before.
Have output transformers and GM70's but haven't gotten around to building anything in a long time now. But everytime I read about someone building a GM70 I get a little more inspired.
To infinity and beyond!!!
That's a choke loaded voltage doubler. It's a pretty useful circuit given the transformer available. It works really well.
Kevin Carter
K&K Audio
www.kandkaudio.com
.
To infinity and beyond!!!
Here they are in the foreground with my Art Audio Vivo prototype in the background.
Kevin Carter
K&K Audio
www.kandkaudio.com
Is that a prototype of the same amp used at RMAF last year? I was very, very impressed with the sound of that system.
Best,
Mike
The one in the background is, yes. It is my main listening amp here.
Kevin Carter
K&K Audio
www.kandkaudio.com
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