Re: Importance of Damping Factor
The never ending issue of output impedance / damping rises again, and I want to grab the opportunity of having Mr. Cordell here - worried as much as most of us I believe - with the relative importance of strictly electronic behavior when the electromechanical factor seems to largely overshadow it in the end.
Over a year back there was a discussion in
The many faces of distortion thread which at some point rised a paper by Mr. Hawksford advancing his conviction that current drive was superior to voltage drive once the nonlinear speaker parameters are taken into consideration.
It did not convince me and took some time to work out a sensitivity analysis which I presented in posts 629-30 drawing no obvious criticism on possible mistakes.
To sumarize, voltage mode drive - i.e. zero output impedance or infinite damping factor - seems in principle the best strategy to translate signal waveform to cone velocity to pressure field, even in the presence of transducer nonlinear behaviour for example in terms of Bl dependency with voice coil excursion.
Mr. Cordell is deservedly concerned about the role of speaker introudced distortions, for example with regards with actual vs. ideal step response and its implications. I really look forward to hear his opinion on the output impedance issue, taking into account the whole picture.
Rodolfo
Bob Cordell said:
The never ending issue of output impedance / damping rises again, and I want to grab the opportunity of having Mr. Cordell here - worried as much as most of us I believe - with the relative importance of strictly electronic behavior when the electromechanical factor seems to largely overshadow it in the end.
Over a year back there was a discussion in
The many faces of distortion thread which at some point rised a paper by Mr. Hawksford advancing his conviction that current drive was superior to voltage drive once the nonlinear speaker parameters are taken into consideration.
It did not convince me and took some time to work out a sensitivity analysis which I presented in posts 629-30 drawing no obvious criticism on possible mistakes.
To sumarize, voltage mode drive - i.e. zero output impedance or infinite damping factor - seems in principle the best strategy to translate signal waveform to cone velocity to pressure field, even in the presence of transducer nonlinear behaviour for example in terms of Bl dependency with voice coil excursion.
Mr. Cordell is deservedly concerned about the role of speaker introudced distortions, for example with regards with actual vs. ideal step response and its implications. I really look forward to hear his opinion on the output impedance issue, taking into account the whole picture.
Rodolfo
ingrast said:
OK, first of all, I honestly didn't mean to start up the damping factor controversy again. Keep in mind that my comments on damping factor were in response to a question about what I thought were the main reasons why no-feedback amplifiers (or tube amplifiers) sounded different from solid state amplifiers with feedback. Remember, the question was in terms of different, not better or worse. In regard to difference in sound caused by damping factor differences, I pointed out that, all else remaining equal, a low DF could cause a frequency response difference when the output impedance of the amplifier works against the varying load impedance presented by most speakers. Just a difference, but not necessarily a small difference.
The example I cited was an amp with a DF of 10 drving an 8-ohm speaker, and I got on the order or +/- 1 dB. My KT88 tube amp, and many other classic tube amp designs have a DF of 20, but there are many people driving 4-ohm speakers out there, so we may be back to the +/- 1 dB scenario. SET tube amps have a DF of typically 1-5, so these arguments may even suggest stronger colorations there.
I also agree that for any DF greater than about 100, DF will not matter. I also agree that it would be foolish to compromise other amplifier performance in the pursuit of high DF. In reality, high DF comes fairly easily in most amplifiers without resort to heroics that would compromize other performance - this is at least true with solid state NFB amplifiers. Even with solid state no-FB designs, as pointed out in an earlier post, it seems not terribly difficult to get the output Z below 0.1 ohm. But, as stated earlier, even an amplifier with a DF of only 10 can sound good.
Indeed, even in my super gain clone (SGC) amplifier, I put in a 0.5 ohm output series resistor so that I could compare its sound with my KT88 amp at RMAF on a level playing field; unfortunately, we never had time to do a bunch of listening comparisons between those two amps because the SGC was being used in the measurements workshop.
I definitely agree that voltage drive for a loudspeaker is the way to go, if for no other reason that 99% of them were designed and voiced with a voltage drive amplifier. As a side note, we might ask what kind of an amplifier each speaker manufacturer voiced his speakers with. If he voiced them with a tube amplifier with a DF of only 10, and he's got a wild impedance curve, then those of us listening to those speakers with a high DF amplifier are hearing colorations with respect to what the designer voiced for.
Finally, it was brought up here something about non-linear effects of the speaker load against the amplifier output impedance. So far, I have merely been discussing linear effects here (e.g., frequency response). The nonlinear part is something that probably needs a closer look. Only recently, I was looking at the distortion of a Hafler DH220, and it was higher than expected. I later discovered that this was due to the nonlinearities in the dummy load resistor I was using. This was not my usual dummy load, and when I replaced it with my good one, the distortions greatly reduced. So, be careful of the linearity of your dummy loads. This does make you wonder a little about the nonlinear nature of the speaker load operating against a finite amplifier output impedance.
Anyway, let's not get too hung up on the DF thing.
Cheers,
Bob
It's an important lesson here. Many have opinion (usually not realized, just stamped in our brain) that an amp is already a perfect "black box", it is a pure signal source, ie : it will not react to any speakers. If there is something wrong with the sound (that doesn't appear with other/previous speakers), usually people look for the answer in the new speaker, not many people will suspect that it could be caused by the interaction of the amp and the new speaker.
I saw it here
http://www.diyaudio.com/forums/showthread.php?postid=1049184#post1049184
and PMA answered here
http://www.diyaudio.com/forums/showthread.php?postid=1049377#post1049377
It seems what Walt Jung wrote (that output node is actually another input for an amp/its feedback system) is too true to give audible effect, not only in measurement equipment
Especially for global feedback amp.
Re: Re: Importance of Damping Factor
Answering my own question: If a copper voice coil had 5.49 Ohms at 20°C(room), it would rise to 6.76 Ohms at 80°(moderately hot). That's a factor of 1.23 or about the impedance change that John sees in his 2 Ohm system with the additional 0.5 Ohm resistor.
But the change would be global for the chassis and not only for a few frequencies around 2 KHz -> much worse.
Cooling the voice coil is a selling point of FerroFluid. I wonder what its viscosity does over temp.
The HIFI world is always great in the search for 2nd and 3rd order problems and then finding glorious solutions -- while ignoring everything else. tunnel vision.
Gerhard
gerhard said:
Answering my own question: If a copper voice coil had 5.49 Ohms at 20°C(room), it would rise to 6.76 Ohms at 80°(moderately hot). That's a factor of 1.23 or about the impedance change that John sees in his 2 Ohm system with the additional 0.5 Ohm resistor.
But the change would be global for the chassis and not only for a few frequencies around 2 KHz -> much worse.
Cooling the voice coil is a selling point of FerroFluid. I wonder what its viscosity does over temp.
The HIFI world is always great in the search for 2nd and 3rd order problems and then finding glorious solutions -- while ignoring everything else. tunnel vision.
Gerhard
Hi Bob,
I read your interview and the following post from you at Asylum :
---My 250 WPC amplifier will occasionally wink at me with its clip LEDs (every amp should have them!) when driving my Morel M3 speakers with well-recorded music at levels that are realistic but certainly not painful.---
http://www.audioasylum.com/scripts/t.pl?f=general&m=457708
---The musical cut with the highest crest factor we have found so far is Rickie Lee Jones' "Ghetto of my Mind" on her Flying Cowboys CD. Play this cut at even remotely realistic levels and monitor it with our meter and you will re-think how much power your amplifier needs to be capable of. On my Morel M3 speakers that have a sensitivity of about 84 dB, I can easily clip an amplifier with enough dynamic headroom to handle 250 wpc for brief intervals at pleasant sound levels.---
I am interested by the sound level used by various people at home and by the dynamic range of records. You seem to listen sometimes at very high levels, probably in a very large room.
Can you give us some numbers about the dynamic range of the above mentionned CD, the impedance curve of the Morel M3 and the maximal demands of current from the amplifier as well as more details about the conditions of the related listening experience ?
I think a Peak-Average Power Display box may be a useful tool for hi-fi enthusiasts not having any technical skills.
I read your interview and the following post from you at Asylum :
---My 250 WPC amplifier will occasionally wink at me with its clip LEDs (every amp should have them!) when driving my Morel M3 speakers with well-recorded music at levels that are realistic but certainly not painful.---
http://www.audioasylum.com/scripts/t.pl?f=general&m=457708
---The musical cut with the highest crest factor we have found so far is Rickie Lee Jones' "Ghetto of my Mind" on her Flying Cowboys CD. Play this cut at even remotely realistic levels and monitor it with our meter and you will re-think how much power your amplifier needs to be capable of. On my Morel M3 speakers that have a sensitivity of about 84 dB, I can easily clip an amplifier with enough dynamic headroom to handle 250 wpc for brief intervals at pleasant sound levels.---
I am interested by the sound level used by various people at home and by the dynamic range of records. You seem to listen sometimes at very high levels, probably in a very large room.
Can you give us some numbers about the dynamic range of the above mentionned CD, the impedance curve of the Morel M3 and the maximal demands of current from the amplifier as well as more details about the conditions of the related listening experience ?
I think a Peak-Average Power Display box may be a useful tool for hi-fi enthusiasts not having any technical skills.
Further thoughts on PIM
I wonder how much Phase Intermodulation (PIM) distortion a tweeter might create.
Consider that the speed of sound is approximately 0.33 mm/us.
Assume that the tweeter has a very small cone excursion of 0.1 mm p-p when the complete loudspeaker is playing at realsitic levels. This could be the result of excitation by lower frequencies near the tweeter's resonance that are not completely yet attenuated by the crossover.
Merely looking at the phase modulation from the tweeter's dome moving back and forth by 0.1 mm, we get a change in delay to the ear of about 300 ns p-p, or about 107 ns r.m.s. if I've done my math right.
Is it fair to compare this with the 1-10 ns of PIM that the power amplifier might generate?
I'm not trying to drag us onto a loudspeaker topic, but rather pose the question to see if it may put the amplifier PIM numbers in context. I know that for most loudspeaker and amplifier distortions, the former are much larger, and we normally don't see it as appropriate to compare speaker distortions to amplifier distortions (perhaps because their nature is different in some ways). However, the PIM mechanism seems more analogous, that's why I pose the question.
Food for thought...
Bob
I wonder how much Phase Intermodulation (PIM) distortion a tweeter might create.
Consider that the speed of sound is approximately 0.33 mm/us.
Assume that the tweeter has a very small cone excursion of 0.1 mm p-p when the complete loudspeaker is playing at realsitic levels. This could be the result of excitation by lower frequencies near the tweeter's resonance that are not completely yet attenuated by the crossover.
Merely looking at the phase modulation from the tweeter's dome moving back and forth by 0.1 mm, we get a change in delay to the ear of about 300 ns p-p, or about 107 ns r.m.s. if I've done my math right.
Is it fair to compare this with the 1-10 ns of PIM that the power amplifier might generate?
I'm not trying to drag us onto a loudspeaker topic, but rather pose the question to see if it may put the amplifier PIM numbers in context. I know that for most loudspeaker and amplifier distortions, the former are much larger, and we normally don't see it as appropriate to compare speaker distortions to amplifier distortions (perhaps because their nature is different in some ways). However, the PIM mechanism seems more analogous, that's why I pose the question.
Food for thought...
Bob
Crest factor of music
Yes, the Peak-Average display box was a very big hit at the Rocky Mountain Audio Fest. A number of people wanted one. I'd be interested in knowing how much interest there is out there.
What I described in the quote above was my home experience, but we pretty much repeated it at RMAF in Workshop #5. The details of that workshop can be found on my web site at www.cordellaudio.com.
What is really interesting is that I was not playing it "really loud" at home, and we were also not playing it all that loud at the RMAF demonstration. I would describe it as satisfyingly realistic levels. Peter Smith, who ran the demo at RMAF, routinely asked the participants if he was playing it too loud, and they never said he was. In my home environment, the room is about 12 X 15. The hotel room was probably of a similar to slightly smaller size. The loudspeaker we used at RMAF was a two-way that Peter built, with a sensitivity of probably 89 dB, yet during the demos we would see long-term average power levels of 1-2 watts, while at times the average power on musical peaks would read as high as 200-275 watts from the big Denon amplifier driving the speakers.
Back to reality: very few CDs have as much dynamic range as the RLJ CD, but the exercise is still a wake-up call. Incidentally, the peaks on the RLJ cut come when there are thwacks to the snare drum. It is also notable that the RLJ CD is recorded at a lower average level to make room for the peaks. Unfortunately, most CDs out there suffer from more compression wielded at the studio.
The Morel M3 speakers have an unusually uniform impedance curve (apart from the usual double-hump that comes with a vented system), since they employ first-order crossovers. So they look fairly resistive at just below 8 ohms over most of the frequency range. They are also described on my web site.
Cheers,
Bob
forr said:
Yes, the Peak-Average display box was a very big hit at the Rocky Mountain Audio Fest. A number of people wanted one. I'd be interested in knowing how much interest there is out there.
What I described in the quote above was my home experience, but we pretty much repeated it at RMAF in Workshop #5. The details of that workshop can be found on my web site at www.cordellaudio.com.
What is really interesting is that I was not playing it "really loud" at home, and we were also not playing it all that loud at the RMAF demonstration. I would describe it as satisfyingly realistic levels. Peter Smith, who ran the demo at RMAF, routinely asked the participants if he was playing it too loud, and they never said he was. In my home environment, the room is about 12 X 15. The hotel room was probably of a similar to slightly smaller size. The loudspeaker we used at RMAF was a two-way that Peter built, with a sensitivity of probably 89 dB, yet during the demos we would see long-term average power levels of 1-2 watts, while at times the average power on musical peaks would read as high as 200-275 watts from the big Denon amplifier driving the speakers.
Back to reality: very few CDs have as much dynamic range as the RLJ CD, but the exercise is still a wake-up call. Incidentally, the peaks on the RLJ cut come when there are thwacks to the snare drum. It is also notable that the RLJ CD is recorded at a lower average level to make room for the peaks. Unfortunately, most CDs out there suffer from more compression wielded at the studio.
The Morel M3 speakers have an unusually uniform impedance curve (apart from the usual double-hump that comes with a vented system), since they employ first-order crossovers. So they look fairly resistive at just below 8 ohms over most of the frequency range. They are also described on my web site.
Cheers,
Bob
Re: Further thoughts on PIM
Doppler effect is a natural thing, though... Leslie effect, for example, sounds very pleasant to our ears, turning boring sound from multivibrators and triggers to something more alive.
Long time ago I had an idea of implementing PIM device for a bass guitar, I thought it may sound interesting.
After a brief experiment I gave up the idea, but my friend implemented it, built a device, and tried to sell it to bass guitarists, nobody bought it.
But a bit later, listening to one Deep Purple record, I heard a sound that was like a comb-effect, with many rejector filters. I did not know yet what it was (it was a tape - recording trick), so I tried with rejector filters using the same voltage controlled phase shifters, but more of them (pair for one rejected frequency) controlled by a infrasound oscillator instead of intermodulation, actually re-inventing a phaser that was already available then, but unknown yet to me, and the effect sounded brilliant. Unlike that earlier phase shift inter-modulator.
The point is, similar effects may be both pleasant and unpleasant, and it depends on do they resemble natural phenomena, or not.
Bob Cordell said:
Doppler effect is a natural thing, though... Leslie effect, for example, sounds very pleasant to our ears, turning boring sound from multivibrators and triggers to something more alive.
Long time ago I had an idea of implementing PIM device for a bass guitar, I thought it may sound interesting.
After a brief experiment I gave up the idea, but my friend implemented it, built a device, and tried to sell it to bass guitarists, nobody bought it.
But a bit later, listening to one Deep Purple record, I heard a sound that was like a comb-effect, with many rejector filters. I did not know yet what it was (it was a tape - recording trick), so I tried with rejector filters using the same voltage controlled phase shifters, but more of them (pair for one rejected frequency) controlled by a infrasound oscillator instead of intermodulation, actually re-inventing a phaser that was already available then, but unknown yet to me, and the effect sounded brilliant. Unlike that earlier phase shift inter-modulator.
The point is, similar effects may be both pleasant and unpleasant, and it depends on do they resemble natural phenomena, or not.
