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.
.
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
451 posts
 
@uberwaltz 

 Now a novice question.

i have zero tube amp experience.

a lot of my equipment runs xlr connections

i have not seen many tube integrated amps sporting xlr inputs.

is there a good reason for this?

Balanced inputs take a lot of extra stuff. Most integrateds are going to turn the balanced into unbalanced right off because of volume control issues. The extra stuff in SS amps is a 50 cent op amp. In tube amps it would another tube or transformer. Much more expensive.

Balanced makes more sense on power amps where it is actually easy to do because of the absence of a volume control. 

Keep in mind just because a unit has a XLR input that is no assurance that the input is balanced.


bdp24's avatar
bdp24
4,908 posts
 
Roger, I assume the balanced/XLR-only input on your RM-200 is accomplished with a transformer. Do you wire it in accordance with AES File 48 (pin 1: ground, pin 2: non-inverted signal, pin 3: inverted signal)? Thanks---Eric. 
ramtubes's avatar
ramtubes OP
451 posts
 
@rego Where do I begin when considering an Amplifiers Characteristics?

The first thing people are hearing, whether they believe it or not, is the difference in amplifier output impedance that will affect some speakers more than others. An amplifier with an output impedance greater than 1/10 the tap impedance will start to modify the frequency response of a speaker. The frequency response modification will simply follow the impedance curve of the speaker. 

There are some tube amps out there, that people love, that have an output impedance greater than the tap impedance and those will make very obvious changes in frequency response. The bass will always have a peak, the mids may have a dip, the highs may be accentuated or reduced, all dependent on the impedance curve. 

I purposely did not use the term "damping" because that term implies controlling the woofer which is actually not what is going on. However 1/10 the tap impedance or 1/10 the speaker impedance is a damping factor of 10.  I firmly believe that we should retire the term damping factor or get a better understanding of what is really going on. One has to remember that an 8 ohms speaker has typically 6 ohms of DC wire resistance in the voice coil. This makes the output impedance rather unimportant.

I expect some flack on this. So just be kind and logical and I will blow your mind about damping.


ramtubes's avatar
ramtubes OP
451 posts
 
@bdp24 

Roger, I assume the balanced/XLR-only input on your RM-200 is accomplished with a transformer. Do you wire it in accordance with AES File 48 (pin 1: ground, pin 2: non-inverted signal, pin 3: inverted signal)? Thanks---Eric.


Actually it does not though I do use th AES standard for the pins. 

The balanced input is a differential pair with feedback right to the input so that the characteristics of the amplfier are determined largely by 4 high quality resistors.. and of course the rest of tube magic.

The CMRR is very high at 90 db and the amps works and sounds the same if driven unbalanced (single ended)
ramtubes's avatar
ramtubes OP
451 posts
 
@terry9  I build solid state amplifiers, and like to match output transistors. At first I matched for HFE, then VBE at constant current similar to operating spec. Only later did I realize how much this parameter drifts over the first hour of warmup.

However, my latest amps are Class A, and I suspect that a more realistic match is obtained by culling outliers by HFE, then match from VBE using the bias at constant potential and sufficient to generate the operating current. Finally, instead of using matched emitter resistors, I use emitter resistors tailored to the output devices, so that each emitter resistor sees the same potential drop.
Your thoughts? Any advice appreciated.


Kudos to you for building your own gear. I have often said that the best amplifier is the one someone builds for himself. It brings along with it the joy that it is your creation not someone elses.

Of course you match HFE at operating current?

What is the spread of emitter resistor values? That part concerns me a little.

How many transistors in parallel?

Yes transistors are very temperature sensitive. VBE varies at -2 mV per degree C. Thus a 40 degree rise will be 80 MV and that is a lot to deal with. Do you have a VBE multiplier in the bias circuit?
More to discover