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Does damping factor matter?


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    I definitely appreciate the mathematical analysis from Langston and Backfire.  I would agree that horn-loading your woofers adds an order of magnitude more damping force than any amplifier can.  I would also agree that a lot of the difference in what people hear in tube amp bass is a result of the interaction between high output impedance and uneven speaker impedance. 

   For all of these reasons, and many others, I have found active crossovers to be an excellent solution.  Use the big SS amps on your woofers, and then use whatever else you prefer on the mids/highs (tubes, class A SS, etc.).  Seems to offer the best of all worlds. 

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  • 2 weeks later...

I am very happy to see so much expertise shared in this thread. Above all, I admire the clear and understandable language of all the technically savvy participants posting here.

To put what I have learned here it in my layman's terms. The better damping factor (the lower output resistance of the amp) has a significant influence on a more controlled frequency response so that uneven impedances of a speaker should not become a mirror image of the frequency response.

Furthermore (and independently of the linearity issue) the mechanical damping factor in a horn is more significantly effective than a possible high control of an ordinary direct radiating cone by an amp with even a very high damping factor.

Regarding midrange and high frequency horns, I would like to raise another issue. The different material of the diaphragm and the "natural" reverberation, which should be as small as possible. This is a topic beyond the electrical damping factor and that's why it doesn't really belong in this tube amp section. But the ultimate effect is one that possibly cannot be better controlled by the best amplifier in the world. Maybe I'm alone in my perception e.g. for me beryllium has a fabulous attack but a not so good decay. For example, I found the tremolo of an old Fender amp listened with a beryllium horn driver not as intense as with, say, K55V phenolic diaphragms quite apart from other possible advantages of beryllium. I just wanted to mention this because it also has something to do with the control of the sound and damping, which in this case is not in the hands of the amp.


 

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I'm thinking I need one of those K2 amps for my HT subs - 3000 damping factor is really grippin' those woofers......

 

The rounded bass of tubes is quite OK though. Just like the Mac amps with autioformers they have low damping factor too, but yet have a rounded, musical, and authoritative bass in. their own right.

 

I like higher damping for HT and lower for 2 channel.......I think (and yeah, I know, that's a bad idea too).

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  • 4 months later...
On 5/15/2021 at 7:52 PM, Backfire said:

The amplifier is modelled as an ideal (zero impedance) voltage source in series with a resistor R0 (the output impedance).  The loudspeaker is modelled as an ideal (zero DC resistance) inductor in series with a resistor Rs (the DC resistance of the loudspeaker). The ideal inductor has an impedance Z, which is, of course, highly frequency dependent.  The loudspeaker model here is very crude; better models look something like what is depicted in Figure 2.  In each case, it is a resistor Rs in series with the rest of the model.

 

If we stick with the very simplified model in Figure 1, and furthermore if we make the rough approximation that we can just add Rs and Z to get the net "speaker impedance" (i.e. we neglect the complications of the need to take into account the phase angle for the inductive reactance of the speaker coil), then the usual so-called damping factor DF is calculated as

 

               DF = (Rs + Z)/R0,

 

which can get impressively (and deceptively!) large if the output impedance of the amplifier, R0, is very small.

 

But it is clear from the circuit in Figure 1 that R0 and Rs are simply two resistors in series, and they both represent lossy components that tend to prevent the amplifier signal from controlling the speaker cone.  A much more accurate measure of how effectively the amplifier controls the cone is to calculate Z divided by the total series resistance.  Thus we can defined the "true damping factor" TDF as

 

          TDF = Z/(R0 + Rs).

 

The crucial point is that the speaker resistance Rs should be in the denominator in this calculation, not in the numerator!  It is completely misleading to do what is normally done when calculating the damping factor, and adding in the speaker DC resistance (which is typically of order 6 ohms) in the numerator.  The speaker DC resistance totally overwhelms any resistance in the speaker connecting leads, and in a solid state amplifier it totally overwhelms the output impedance of the amplifier.  It even overwhelms the output impedance of many tube amplifiers.

 

DF1.jpg

 

Hello @Backfire,

 

I'm joining the converstaion late but hopefully you'll spot this. I've not got masses of equipment - just a trusty multi-meter. I was curious to work out the Output Impedance of my valve amp so grabbed some resistors and measured the voltage with a couple of different loads. I didn't want to run the amps with no load as I've heard it's not healthy!  So went with:     Zout = ((R1*R2)(V1-V2)) / ((R1*V2)-(R2*V1))

I tried a few combinations of Resistors and got same ballpark but different values. I'm not sure if thats typical but the average of them indicates an output impedance of ~3.6 Ohms.

 

Then I saw your post... and I'm curious to follow your method which seems to make sense in terms of considering the loudspeaker DC load but I'm confused a bit. Figure one shows the load across the loudspeaker terminals as Z + Rs. Presumably that can be measured by disconnecting the speaker wire and measuring resistance across the positive and negative terminals.

 

When it comes to your equation though for true damping factor.... how are you supposed to work out the speaker's seperate values of Z and Rs?

 

Thanks,

Alex

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Damping factor has been misunderstood by many, mostly that the bigger the damping factor the better. Technically that is not the truth.

 

The optimal goal is to be "critically damped" which means you reach 0 amplitude fastest with no residual oscillations. Over damped which is many amplifiers on the market will have a slower time to 0 amplitude but will have no oscillations. Under damped is when you reach 0 amplitude fastest BUT there is a continuing oscillation for a period of time.

 

Not all speakers are the same obviously but they all will have a resonance down in the bass region. This shows itself as a large impedance spike on an impedance vs frequency plot. An amp with a huge damping factor will dampen this resonance without residual oscillation but it is not necessarily best because the time to get the motor to actually stop (0 amplitude) will be slightly longer than when it is critically damped. Over damped is without a doubt better than under damped but to critically damp is tougher to achieve. A good way to get best speaker to amplifier relationship is to have an adjustable damping control not so popular today. This control is to be able to dial in the best control over this nasty bass resonance. Typically current feedback is used from the speaker that allows both positive and negative feedback adjustment. Yes I said positive feedback (negative resistance) which is how you can actually achieve better damping with a tube amp vs SS amp. BUT this can be implemented to SS amps too so the technology is not mutually exclusive.

 

 

Many tube amps I have tried will have what I call one note bass, this is that resonance frequency being louder than other frequencies and mistaken as "good bass". Another HUGE issue with tube amps is the fact that the output transformer core does not have enough flux to be able to maintain the low frequency waveform, this shows itself at the trailing edge of the waveform where it falls apart. This non-linearity rears it's head as second harmonic distortion which will reinforce the fundamental frequency as extra information. Hifi we really do not want extra information (distortion) that wasn't  present in the original program material. So this extra information/distortion sounds like enhanced bass or "good bass" but it is really just distortion. Not saying this is good or bad, I am saying it accounts for many claiming great bass from tube amps.

 

I would like to see more offerings from manufacturers of amplifiers with a damping control to allow the user to dial in the best control between amplifier and speaker. You do not need a huge DF, anything over a DF of 8 is useless and what you really need is an adjustment to critically dampen the nasty bass resonance present in almost all speakers.

 

 

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21 hours ago, captainbeefheart said:

Damping factor has been misunderstood by many.

 

After reading all these good replies, no one should be confused about damping factor. Bottom line... Does it matter? Yes, but as soon as you insert the speaker wire and speaker into the output circuit of your amp (tube or solid state), the DF comes down to single digits (low double digits if you are lucky). 

 

 

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49 minutes ago, Curious_George said:

 

After reading all these good replies, no one should be confused about damping factor. Bottom line... Does it matter? Yes, but as soon as you insert the speaker wire and speaker into the output circuit of your amp (tube or solid state), the DF comes down to single digits (low double digits if you are lucky). 

 

 

 

Yes very good point!! The speaker wire will bring down these ridiculous DF claims from some manufacturers so take them with a grain of salt. Does it matter? Like you said yes but you do not need as much as many claim and more importantly can the amp dampen the resonance (my La Scala is around 50Hz) well? I like current feedback from speaker because as impedance changes so does the current through the speaker, current creates the movement not the voltage so current feedback in my opinion keeps things in order far better than voltage feedback which the error correction does not include any information from the speaker.

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19 minutes ago, Curious_George said:

My impedance plot has a peak at 50Hz too, but the entire range is below 10 Ohms, so I'm not too worried about what amp I use to drive my LaScala's. I use both tube & SS. 

 

LaScala with Swamping Resistor.jpg

 

 

I agree my La Scala's seem fine with any good amp in both SS or tube camps.

 

Possibly because I keep my 14awg solid copper speaker wire as short as possible. I found in the old days doing pro audio when we didn't have self powered sub-woofers I had some Peavey power amps that had variable damping control to make up for variables in speaker wire impedance on long runs and different speakers. It actually worked pretty darn good, I did notice that with long runs the bass would sound off if I didn't adjust the variable damping. Other power amps that didn't have the feature seemed to be hit or miss with long cable runs and bass performance.

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On 5/3/2021 at 12:41 PM, NOSValves said:

I have to ask what are you mega watt SS guys doing posting in the Tube section 🤷‍♂️... Us tube guys all know you have long ago burned all the hairs out of your ears to know what sounds good :) 

 

 

 

ewwwwww......  so what I'm hearing you say is you have hairy ears.  

 

TMI

 

:)

 

(yeah, I know this is 45 years after your post, I just saw the thread after it popped up  I guess that simply means your ear hair is now longer  EWWWWWW)

 

 

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3 hours ago, captainbeefheart said:

 

Solid single conductor like Romex.

Does a single conductor make a positive difference for you? From what I've read the current flow through a conductor induces a magnetic field which produces eddy currents forcing the flow of electrons to the surface of the conductor in what is known as the "Skin Effect" therefore stranded wire provides more surface area for electons to flow. Frequency plays a part in this but at the moment I can't recall the specifics.

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19 hours ago, RickD said:

Does a single conductor make a positive difference for you? From what I've read the current flow through a conductor induces a magnetic field which produces eddy currents forcing the flow of electrons to the surface of the conductor in what is known as the "Skin Effect" therefore stranded wire provides more surface area for electons to flow. Frequency plays a part in this but at the moment I can't recall the specifics.

 

Skin effect is real and we pay close attention to it with things like radio frequency equipment and microwaves but for audio frequencies it really isn't anything to worry about. I do see some exotic speaker wires bringing attention to skin effect and why their wire is superior causing many audio enthusiasts to lose sleep over it. Let's all pretend we are 14 years old again and can hear up to 20kHz, yes at the higher frequency the skin depth has decreased (I won't get into the math) but we need to keep it all into perspective. Easiest way to think of the skin effect is similar to using smaller gauge wire with higher currents. The resistance of the wire at smaller gauges is increased and depending on the length and the current you will get a loss across the wire. With skin effect the skin depth decreases as frequency increases which means the signal uses a smaller portion of the total conductor. What this does is increases the impedance effectively giving a loss across the wire. 

 

Why this effect does not worry me? First of all the increase in resistance at 20kHz is not enough to notice especially by ear, the losses are very small and inconsequential. Why inconsequential? Have a look at the size of the tweeter on our speakers. The driver is tiny compared to the woofer and the power rating of the tweeter is much much smaller because in music there is really not much content up there in frequency. The tweeter does not have to dissipate 100 watts like the woofer. For example the output of your amp may be at 100mW for the tweeter while the tweeter sees 99mW from the loss of the conductor. Worrying about losses at bass frequencies will trump losses from skin effect at high frequencies every time. But that doesn't stop some companies from making you worry about the detrimental skin effect, yes it is real and yes it is measurable, barely but when put into perspective isn't nearly as troublesome as some companies lead you to believe. I tend to focus on things that are a magnitude greater importance to improve my sound quality. So for the grand scheme of things solid wire seems to perform better than stranded wire so I go with that as it has more pros vs cons.

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I myself haven't used any solid core wiring or pumped a 100 watts into my woofer (if I did I would no longer need a hifi). My speakers are Khorns  and the watt meters on my McIntosh rarely have seen over .02 watts even though I have 200 per channel. My  point is that speaker wires don't play a big part in the end result. The dealer I bought my speakers from lent me some very expensive cables and I or anyone who listened to them could tell a difference between the high end cable and monster cable. One day I'll connect some Cat 6 wire to my system and see if it sounds any different than the 12ga wiring I currently use.

 

Regarding Skin Effect it seems logical that there would be less resistance with multi strand wire of say 12ga vs solid core of 12ga due to the increase of available skin.

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For us Klipsch users with sensitive heritage series speakers wires don't matter as much because we are not using 100's of watts of power and the speaker cable does not see large currents. With very low currents we do not see the losses across the wire. Same for the "Skin Effect", you will not notice anything different between stranded or solid wire because at audible frequencies the skin depth is still deep enough to not need anything special, especially since we push very little signal current from the amp to the speaker because of their efficiency.

 

 

So for people probably thinking I am crazy about being able to increase damping performance and actually overall low frequency performance with variable damping control here is a link about the subject. The cool thing is they are referencing the Klipschorn for the reading material. The article actually references "Variable Damping" written by Paul Klipsch October of 1956. I am still looking for the actual article about variable damping that Paul himself wrote so stay tuned as I will post it as soon as I can find it.

 

http://www.rfcafe.com/references/radio-news/positive-curent-feedback-november-1957-radio-tv-news.htm

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10 minutes ago, captainbeefheart said:

The article actually references "Variable Damping" written by Paul Klipsch October of 1956. I am still looking for the actual article about variable damping that Paul himself wrote so stay tuned as I will post it as soon as I can find it.

 

http://www.rfcafe.com/references/radio-news/positive-curent-feedback-november-1957-radio-tv-news.htm

 

I recall seeing an amplifier in the Audio Anthology series with variable damping. I'll have to look through all my volumes to locate it. 

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16 minutes ago, Curious_George said:

 

I recall seeing an amplifier in the Audio Anthology series with variable damping. I'll have to look through all my volumes to locate it. 

 

Nelson Pass uses positive feedback for damping control in his amps like the F7. There are many vintage tube amps with variable damping.

 

From memory PWK said you want the highest damping factor in 1956 and setting anywhere else made the sound worse. He was referencing the type that has both positive and negative current feedback and so negative current feedback would reduce damping so he preferred the Klipshorn with the most damping so he would only need positive current feedback to increase damping factor. Remember this was the 50's and since they really only had tube amps with output transformers you couldn't really use more than -30db negative voltage feedback before stability starts to become an issue, so output impedance was somewhat limited unless you were also using positive feedback to increase damping.

 

I like to have the option to have both +/- current feedback but the point of all this is discussing damping of tube amps and that people believe we are limited to a minimum DF. This is untrue because with positive feedback we can achieve better damping control vs solid state, but modern tube amp builders are not using this technology to improve performance as I feel it is very misunderstood.

Edited by captainbeefheart
grammar
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