2. amps-preamps
  3. 140029-80294

Does anyone care to ask an amplifier designer a technical question? My door is open.

I closed the cable and fuse thread because the trolls were making a mess of things. I hope they dont find me here.

I design Tube and Solid State power amps and preamps for Music Reference. I have a degree in Electrical Engineering, have trained my ears keenly to hear frequency response differences, distortion and pretty good at guessing SPL. Ive spent 40 years doing that as a tech, store owner, and designer.
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Perhaps someone would like to ask a question about how one designs a successfull amplifier? What determines damping factor and what damping factor does besides damping the woofer. There is an entirely different, I feel better way to look at damping and call it Regulation , which is 1/damping.

I like to tell true stories of my experience with others in this industry.

I have started a school which you can visit at http://berkeleyhifischool.com/ There you can see some of my presentations.

On YouTube go to the Music Reference channel to see how to design and build your own tube linestage. The series has over 200,000 views. You have to hit the video tab to see all.

I am not here to advertise for MR. Soon I will be making and posting more videos on YouTube. I don’t make any money off the videos, I just want to share knowledge and I hope others will share knowledge. Asking a good question is actually a display of your knowledge because you know enough to formulate a decent question.

Starting in January I plan to make these videos and post them on the HiFi school site and hosted on a new YouTube channel belonging to the school.


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ramtubes's avatar
ramtubes OP
460 posts
 
@bdp24 

Roger, here’s a question about light loading with the RM-200 Mk.2 amp: With a loudspeaker given a nominal 8 ohm load rating by it’s designer/manufacturer (it measures between 10 and 20 ohms from 20Hz to 20kHz save for the 60Hz-180Hz band, where it dips down to 7 ohms, centered at 80Hz), the RM-200 will of course put out at least it’s rated 100w/ch when connected to the amp’s 8 ohm taps. If it is instead connected to the 4 ohm taps (light loading), what will the amp’s output wattage be?

Because of the speaker’s relatively high and even impedance curve, it is not for the flattest frequency response I am interested, but rather the lowest distortion and longest tube life. I realize that with light loading those will be achieved at the cost of less power output from the amp. I have a modestly-sized room, and don’t listen at very high SPL, so am willing and able to accept that.



As I tell everyone. Light loading can be done on any tube amplifier. If you dont play above 80 dB  go to the lowest tap. If you play 90 go one higher, if you play more than 90 you may have to use the high tap to get the volume you desire. This change should be far more apparent than a cable change. 7 ohms on the 8 ohm tap is no problem for the amp but lower taps always perform better technically though how it sounds to you is more important.

4 ohms on the 4 ohm tap produces 100 watts. 8 ohms on the 4 ohm tap around 60 watts. As you go down in taps you spend more time in the class A region. The distortion and damping are much improved.

Would be nice if you had a way to measure the peak voltage you require, then much better advice can be given. You can also get at that with a SPL meter at one meter at listening level. Then with speaker sensitivity we can compute it

Everyone needs an SPL meter. For $50 its an execellent investment. People spend more than that on a poorly designed high end fuse. Please do not buy premium fuses, TuningFuses are the worst and the others I have not dissected but the people who sell them should be dissected. 

ramtubes's avatar
ramtubes OP
460 posts
 
@terry9 @atmasphere 

Actually this is pretty common with bipolars. They can get into a phenomena known as 'thermal runaway' if this is not well controlled
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Ralph, did you perhaps mis-read this. He said the idle goes down not up as the amp warms. This is the opposite of thermal runaway.

Terry, do you still hold your idle current goes down as it warms?

If so I want to see your circuit. Hey, I'd like to see it anyway. This is unusual. 

ramtubes's avatar
ramtubes OP
460 posts
 
I still prefer US, European or Japanese vintage production over the new stuff!


Is a 1980s RM-9 in good working condition vintage enough? :) They just going like the Energizer Bunny
atmasphere's avatar
atmasphere
11,861 posts
 
Ralph, did you perhaps mis-read this. He said the idle goes down not up as the amp warms. This is the opposite of thermal runaway.
Yes- you are correct- totally misread that one. It is pretty weird. I’m wondering how the thermal feedback is accomplished. If its done with an active device, that device might be seeing its hfe drift up as it heats up- thus causing the outputs to shut down more than expected. A schematic would be interesting.

Nelson Pass has designed an interesting amplifier based on the old VFET/SIT devices. As solid state goes one of the most musical I’ve heard.
https://www.diyaudio.com/forums/pass-labs/276711-sony-vfet-amplifier-2-a.html
It also employs low voltage rails, about 25-28 volts.
ramtubes's avatar
ramtubes OP
460 posts
 
@roberjerman
So Roger: new production tubes from our old Cold War rivals. Can the Russians really make and sell quality tubes at such low prices? A quartet of 12AX7's for $18 (shipping free!). We did that ourselves back 50-60 years ago. But that was when US tube production was in the millions for the radio/television market! Do Russian mfgr's have to compromise on material quality and less vacuum- pumping to sell at a low cost?


They are doing better and better. Sometimes I send back a whole batch but not so much lately. Low noise 12AX7 are hard to make. Where do you get 4 for $18? Im sure they are not tested for noise or microphonics. Luck of the draw like poker.

Sadly the USA makers never got the noise as low as the Europeans. However USA makers were very good at power tubes.

Vacuum pumping is not a problem, cathode coating is. They told me when I visited the EI factory. "All we know is we mix up the cathode coating in a tin pail with whatever water and let it sit overnight, its better that way. However we don't know if thats because it sits overnight or perhaps the night watchman takes a leak in the bucket."

Im not kidding and that came via Dragona, our translator, right from the head of production. I spent 10 days at that factory. Sad its gone, they were on to something and I wanted to help by offering noise testing which they could not do. I passed on their offer to make me $100,000 worth of tubes, prepaid. they were broke, Good luck seeing those.
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