Newby question: Damping factor

[wg_ski]

If anything scared Wincrest away it was probably the two magic words “Negative Feedback”. Which is, unfortunately, the only way to “improve” DF. You either start with an output impedance you can accept by finding/picking output tubes with a relatively low Ra (which will get you down to single digit ohms, but not lower) or apply that dreaded negative feedback. Paralleling a bunch of tubes only works to a limited extent, because the required transformer primary impedance also drops and the required bias point moves. (What you end up with is a bigger amplifier, with similar output impedance.) Beyond that you just have to apply NFB.

 

[wg_ski]

Output devices are, Nelson Pass exotics excluded, always followers, so output impedance is reciprocal to transconductance.

Not always. There is one widely used class of non-follower SS amp and that’s the transnova (Which uses common emitter output/drivers). It’s more common than you might think - since it was ”discovered” by one maker of pro audio equipment around 1980 as a means to make a minimum-BOM, BOMB-proof amplifier. Everybody and their dog began copying them. Not just using the topology, but making blatant copies, especially in the 90’s and early 2000’s. They use over 80 dB of NFB.

 

[Positron]

I know a few things about damping factor but not enough... I know it makes your bass sound flabby, booooomy, slow, like it's getting in the way of the music that's coming because the damn woofer is still recovering. I know physically, bad damping is the amps inability to control the momentum of the speaker cone, to slow it when it needs slowing, right? So it's kind of like the brakes on a car?

Is bad damping factor caused because tubes are inherently easier to "turn on" than they are to "turn off" (non linear)? If tubes are slower to turn off than turn on I could see how that might make it better at making the speaker cone move than making the speaker cone slow down. Is that right? Or am I wrong on that. Some other fundamental cause?

What is the best method to improve damping factor in an amp design? And, how far can/should you take that method before you start to cause some other problem in the design, because everything is a compromise right.

There are a a few problems one has to deal with when designing an audio system.

1. With SET amplifiers, the damping is not only low due to fairly high output impedance (Z), but varies from low to zero
as the tube approaches, and reaches cutoff.

2. With PP triode operation, matching the speaker to the amplifier's output impedance is still critical. With my PP
triodes and speakers (crossovers included), I had to use the correct speaker wire setup from amp to speakers to obtain
optimum musical reproduction. In my case, I found that for 6 foot long speaker wire runs, I needed:

1. to use 10 parallel 18 gauge wires in parallel in each leg. I had used 11 in one leg and 10 in the other leg but found slightly
better musical reproduction with 10 and 10. In the future I might find a slightly better combination if I find a better crossover
tweak.
2. The total gauge is not only important for deep bass resonance control, but also affects the 150hz +/- minimum Z area
more than the higher Z areas,
3. and lower inductance improves the higher frequencies vs a single conductor.
4. Paralleling wires allows physical separation that minimizes the output capacitance the amp and speaker sees.

The boominess is also caused by an incorrect box design. If the entire Q of the speaker is ~ 0,717 the bass response
will hopefully be what one finds satisfying. I can make my closed box design sound thin or fat at deep bass resonance simply
by altering the amps output Z, the wire's total gauge, or length of a given total gauge.

So in reality, one is mating the amplifier output Z through the correct speaker wire gauge, to the speaker, which includes
the crossover itself. Experimenting is necessary as the confound variables are considerable.

cheers

pos

 

[Windcrest77]

Good job of scaring away the OP guys! I mean, we're not here to help, right, we're here to show off our expertise.
That worked well.

Jan

I really appreciate it guys, learning what I could, I see the complexity of this subject is as much beyond the banana jacks of one's amp as it is inside the amp. Speaker, crossover, cable, expertise has the right to show off! It really seems to be a subject where you need to establish a demarcation point, what can you do up to the banana jacks vs. what you maybe can't control past that or you can just optimize. Negative feedback falls in the Amp side.

 

[bondini]

It really seems to be a subject where you need to establish a demarcation point, what can you do up to the banana jacks vs. what you maybe can't control past that or you can just optimize.

Good point! On the speaker side of that demarcation, Tomcik offers an approach to critically damping a loudspeaker at :

http://dissident-audio.com/Loudspeakers/CriticalLSDamping.pdf

On page 4 he suggests a wee circuit for "pulsing" a loudspeaker and observing the response using an oscilloscope, adjusting a potentiometer to achieve critical damping. I have played with this approach, often with good results.