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.


ramtubes
ramtubes's avatar
ramtubes OP
460 posts
 
@terry9 

I was measuring HFE with a handheld, not warm, not at operating current, and only using the result as a first cut to cull outliers.

Emitter resistors are 0R1 +/- 0R02, so not much.

4 x PNP and 4 x NPN.

No VBE multiplier, but then I leave my amps on most of the time, and they stabilize below 50C.

50 C/122F is nicely cool, only 25 C above ambient in a standard room. You either have big heat sinks or not so much idle current.

Do you mean resistors from 0.1 ohms to 0.02 ohms? I like the American system of resistors.  That is a large range. Do your account for this and make the idle currents equal in each transistor?

HFE for a power transistor at low current is rather meaningless. Why not do all of it at operating current?

A VBE multiplier is a very good thing and easy to add. How much does your total idle current vary from cold to hot and how long does it take to get there? What is the idle dissipation?
ramtubes's avatar
ramtubes OP
460 posts
 
@bdp24 

Thanks for the balanced info Roger. 4 resistors > transformer! I've been running the RM-200 with a balanced pre-amp, but if connected to an unbalanced source (First Watt B4 x/o), what is your recommended method of converting the RM-200's XLR jack for use with an RCA plug, or put another way, connecting an RCA plug to the XLR jack? I've seen a cable with XLR's at one end, RCA's at the other. Acceptable?


Yes any proper converter or cable that grounds pin 3 (signal -) is fine. I purposely made the amplifier perform the same either way. Balanced helps in situations where people have ground loops. 


ramtubes's avatar
ramtubes OP
460 posts
 
@assetmgrs Roger; How do you feel about the design characteristics of hybrid amps, say with a tube preamp and a solid state power amp like the Rogue Pharaoh? Does this approach offer advantages of both tube and SS, or does it compromise the best of both sides? Thanks in advance.


So this would be a integrated. I looked at a picture. I would be concerned about tube noise. I have heard their tube phono stage is unusable due to noise. I see the line stage is a 12AU7, thats a poor choice as quiet ones are hard to find. Might be a problem on sensitive speakers.

I dont see a noise or distortion spec on their literature. Thats a little bothersome. The damping factor of >1000 is very hard to verify and will be cut in half with even a heavy speaker cable. So its kind of silly.

I worked on older Rogue amplifiers and was not impressed. Too many things done strange, like DC heaters for the output tubes and running them way too hot. When I see that kind of stuff I have to wonder. There was no way to reduce the idle current (bias) so I had to put in the most expensive output tubes to be able to guarantee any reasonable life.

I hope their new stuff is better and wish them success if it is. Keep in mind it is a value product, not necessarily high end.
cakyol's avatar
cakyol
940 posts
 
Can you please tell me the purpose of resistors R1 -> R8 in the power supply shown below in the URL ?  Is it to reduce the resonant frequency in case the supply oscillates with the stray inductance of the smoothing caps ?

https://www.passdiy.com/project/articles/burning-amplifier-1

Thanks

ramtubes's avatar
ramtubes OP
460 posts
 
@stereo5 

I have a question for you Roger:
What are your thoughts on Class D amps? I have listened to quite a few and always came away unimpressed with my ears aching. I am a Mac guy at heart marrying a Mac solid state amp with a Mac tube preamp. Perhaps I like a more mellower sound?

I heard your tube amps back in the 90’s at Sound II in North Dartmouth, Ma. Leo (RIP) was a huge fan of tubes and carried your line as well as VAC and Audible Illusions. I did a lot of listening and buying in that store. An added plus was there was a fantastic strip Joint right across the street from the store on Route 6. Really fun times!


I was making class D amps in 1990, before they were hardly known. Mac Turner brought me in to consult on his. What disturbes me about them is that they all get their sound from the IC in the front end. Perhaps some are discrete front ends but I have not seen any. 

The biggest problem is the output choke that is necessary to filter the switching waveform. If you look at Stereophile reports you will always seen one impedance that is flat while the others either peak or droop. 

They are going to be popular due to size, low cost, ridiculously high power that hardly anyone needs. I was interested back then as an engineering exercise but no desire to make any. Over 100 watts is only justified by either high listening levels or insensitive speakers or both together. Excess headroom is a myth. 

I have some Mac gear to be quite excellent. I have a post about that here. Just go to my profile to find it.


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