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.


128x128ramtubes
ramtubes's avatar
ramtubes OP
460 posts
 
@terry9 

Thanks for the nod ... I guess the takeaway is, "Don't be so lazy. Match HFE at operating temperature and current." Thanks!


Have you measured the idle current of each transistor at operating temp in the amplifier? You of course know just to measure the Emitter drop divided by R. How well is it distributed? Then you could put a scope on them at full power and see how equal those are. 

Dont spread the emitter resistors too much because they might not track and share over full swing. I would make all the emitter resistors the same and do good transistor matching. You might make them a little higher like 0.22 ohm. You won't loose much output.

The concept of varied emitter resistors is a thought I have not had. :)
ramtubes's avatar
ramtubes OP
460 posts
 
@terry9

I would use a dual trace scope, make sure the two channels have identical gain, use one emitter resistor on channel 1, call that the reference. Then use channel 2 to see how the others (one at a time) match up at various power levels. I would find the results interesting.

Have you also looked at the output transistor current driving the speaker at high levels, high frequencies? Trumpet music, thats a killer? Have you measured the impedance of the primary side over frequency? I have found some stepups have so much capacitance that more of the energy goes into the transformer than the panels. FYI, the 57 and 63 transformers, though different, are very cleverly wound. A torroid typically has lots of shunt capacitance that you have to drive. The QUAD transformer does not.

BTW, I am not impressed at all with the Plitron audio transformers. Van Der Veen’s paper seemes flawed to me. He is a great speaker promoting his idea of automatic bias. He believes you can use a gapless tube output transformer to make a practical amplifier. I take great issue with that. I think there are some unasked thus unanswered questions. I would ask him, "what happens when there is a DC offest in the output stage. It will happen when the amp is pushed?"

Similarly if you have any DC across your primary the core flux will be off center. I cant say how many mV is ok, but i think at 50 it could be noticed and at 100 mV it might be a real problem.

Here’s how to set up your test:

Scope ground goes to hot output terminal of course. Im assuming the output stage is emitter follower.

Dont forget to float the AC power on your scope. Othewise you will be shorting your output.

On my bench I only have one piece grounded. Everything else floats. Even my soldering iron floats because sometimes I solder amplifiers while they are on. There are parts of the circuit that dont much mind. Dont do that with SS amps, the are unforgiving.
bdp24's avatar
bdp24
9,445 posts
 

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.

roberjerman
4,319 posts
 
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? 
roberjerman
4,319 posts
 
I have bought a set of 4 Gold Lion (Russian) KT-66's for use in my pair of Heath W5's. $35 a tube! Originals sell for $200+ a tube (UK). They sure do look real nice! I still have the original US Tungsol 5881's in use. I wonder if the Russian ones can stand up long-term to the plate and screen voltages in the W5's?
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