2. amps-preamps
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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
 
@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.
ramtubes's avatar
ramtubes OP
460 posts
 
@terry9

I see you have jumped on the bandwagon of expensive parts. I have not yet found them to be superior and they are costly and often fragile. Those resistors especially. I dont see the benefit.

As far as dialectric, i understand it well. Let me ask your a question. How much AC voltage (changing voltage) is across a coupling cap in any amplifier?

I am still curious about your falling idle current with rising temp.

How do you drive the capacitance of all those QUADS. Have you done some measurements on what the combined impedance of the whole setup is?

How do you arrange them?
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