unless I'm missing something your woofer+waveguide array will only have one optimal listening position as the woofers are falling off @ 3dB/m and the tweeter falls off at 6.
I know John !
Thats actually not quite true and kinda misleading if you emphasis on.
There always is a thermal equilibrium for any given input – and thus output – for a given SPL requirement.
I noticed that miss-concept also in other discussions at the topic – so I thought I correct it here.
Simply, there is no such thing as „thermal run away“ with current feedback systems in the sense that we burn a speaker once we ask for a SPL within its capabilities how long ever we like to hold that SPL.
Michael
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Lynn I am with you to the most part of what you say but you should also not forget that there is variation of output impedance - you are so concerned with - anytime the transformer-rectifier system is charging versus switched off.
Any output device (no matter sand or valve) is highly dependent on the former stage it gets its voltage / current from - which in the case of the output stage is not only the signal driver stage but *also* the PS as a whole on the drain leg.
Also – to go even a little more OT - the whole discussion about current charge peaks leaking into signal output is more a matter of what I said above – plus – the common bad design practice to use a single ground wire towards ground of charging capacitors and *from there* to the point of transformer ground – this is especially a bad thing for two rail designs most guys are not aware of
Michael
Any output device (no matter sand or valve) is highly dependent on the former stage it gets its voltage / current from - which in the case of the output stage is not only the signal driver stage but *also* the PS as a whole on the drain leg.
Also – to go even a little more OT - the whole discussion about current charge peaks leaking into signal output is more a matter of what I said above – plus – the common bad design practice to use a single ground wire towards ground of charging capacitors and *from there* to the point of transformer ground – this is especially a bad thing for two rail designs most guys are not aware of
Michael
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That is not true Michael. If you look at a model of the process it shows that the rate of increase in temperature is dependent on the rate of heat generation minus the rate of heat rejection. Assuming that the VC won't melt, in the voltage driven case the heat generation term will decrease as the VC heats up because Re increases and the head generation is I^2 X Re = V^2 /Re. Since V^2 is constant and Re increases, the generation decreases and the current decreases since I = V/Re. At the same time, regardless of the mechanism of heat rejection; conduction, convection, radiation, the rate of heat rejection to the surroundings will increase as the VC temp increases. Thus, at some point the generation and rejection rate will be equally and an equilibrium temperature reached.
In the current driven case the heat rejection rate behaves the same way but the generation rate increases as VC temp increases (G = I^2 Re(T) = Const x Re(T). Thus there is the possibility that, if the generation rate increases faster than the rejection rate that the temperature will ultimately increase exponentially without limit.
Of course in the real world the VC will melt at some point and in either case we can burn out the VC with sufficiently high input drive, be it a voltage source or current source.
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Hey Gary, have you tried just a parallel resistor on the Radian? Or an L-Pad? Either of those certainly flatten the impedance curve.
You are going to use an autoformer to reduce voltage, but you have to be careful there, the impedance of the driver gets multiplied. GaryP and I had that discussion some time back.
You are going to use an autoformer to reduce voltage, but you have to be careful there, the impedance of the driver gets multiplied. GaryP and I had that discussion some time back.
Before going on the battle field with you 🙂 - possibly at a different place - let's clarify if what you say really is:
1.) A (any) speaker - when driven from a current amp - shows "thermal run away"
2.) this "thermal run away" will end into destroying the speaker anytime a (any) certain SPL is asked from that speaker long enough (up to infinity)
Above is what could be read out of your posting IMO
http://www.diyaudio.com/forums/multi-way/100392-beyond-ariel-145.html#post2276827
Michael
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Haven't tried just a parallel resistor or an L-pad on the Radian, but I expect to get a chance to do that later today.
I normally use a parallel ("swamping") resistor along with an autoformer, which as I understand it, locks down the impedance. There are some good discussions on the subject of autotransformer attenuators for compression driverson dave slagle's forum at intact audio -- click into the "Attenuators" topic heading for some interesting threads.
My UT-3619 autoformers don't have quite enough attenuation for the present purpose, so I will have to make do with a resistive L-pad until I can get the goodies from dave slagle.
Gary Dahl
A feed forward plug in for compensating Thermal Distortion should not be that difficult to code, though I have not seen any (nor one for compensating Doppler IM 🙁 ) yet.
Given the ease of capturing the main Thermal Distortion spec's of a single speaker and the increasing use of DSP and PC solutions among DIY'ers this is already heavily overdue IMO.
It even could be done in analog with the reverse circuit of my SPICE model
Michael
Given the ease of capturing the main Thermal Distortion spec's of a single speaker and the increasing use of DSP and PC solutions among DIY'ers this is already heavily overdue IMO.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
It even could be done in analog with the reverse circuit of my SPICE model
Michael
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Please Micheal,
I am too old to engage in combat. 🙂 But please read more carfully what I really did say.
Possibility is the key word here. I didn't say "any speaker", nor did I say "will result in destruction".
What I am pointing out here is that with current drive there is a different dynamic at work. A voltage drive system is potentially self limiting where as a current drive system is potentially unstable.
Thermal runaway exists in lots of areas of since.
Ok, ok - and what *if* we combat about the key word of "possibly" - for any real world speaker 😉
I "guess", you can't even hold that ...
C'mon, you are not *that* old !?
😀
If you prefer, I'd agree on rephrasing "combating" with "teamwork investigation" 🙂
Michael
I "guess", you can't even hold that ...
C'mon, you are not *that* old !?
😀
If you prefer, I'd agree on rephrasing "combating" with "teamwork investigation" 🙂
Michael
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Progress !
Sorry to interrupt the research ....
Just thought Gary and Lynn might like the attached pic of my first Jensen bass unit for the combination . I know it took me a barely finite amount of time to do this, but I'm quite proud of the woodwork !
I got some autoformers for the 288-H's from Dave a while back, and it was nice to find the horns sounded the same with or without the autoformers . Good effort Dave !
I'm off on hols to Austria now before tackling the final work on speaker no.2 ....
Mark
Sorry to interrupt the research ....
Just thought Gary and Lynn might like the attached pic of my first Jensen bass unit for the combination . I know it took me a barely finite amount of time to do this, but I'm quite proud of the woodwork !
I got some autoformers for the 288-H's from Dave a while back, and it was nice to find the horns sounded the same with or without the autoformers . Good effort Dave !
I'm off on hols to Austria now before tackling the final work on speaker no.2 ....
Mark
Attachments
Beautiful work, Mark!!
I noticed your discussion thread on dave slagle's forum. Glad to hear that you got good results.
Gary Dahl
I noticed your discussion thread on dave slagle's forum. Glad to hear that you got good results.
Gary Dahl
Yes indeed. Thanks for the links to Dave's forum. Lot's of info there.
I'm curious about the difference in sound between and autoformer and parallel resistor vs series + parallel resistor. Basically swapping the series resistor for the autoformer.
I think John is just pointing out that the points to consider rather than drawing conclusion.
Given that John had done quite some investigation into Thermal Distortion - both on his page as well as almost for a whole thread - he probably should already *know* better.
If he has some ideas though, that thermal instability (run away) with current amps is a realistic scenario then I'd definitely would like to hear about.
To all of *my* knowledge / investigation on the topic of Thermal Distortion and thermal speaker behaviour, this is practically impossible - but lets see...
There IMO is an underlaying mis-concept/mis-interpretation about "thermodynamics" in heating/cooling mechanism that "possibly" 🙂 leads to such conclusions.
But as said - there is always a good chance that I may have overlooked something. Being clearly in the second half of a full century but possibly not as old as John - I nevertheless would like to exercise in "getting gracefully old" by a teamwork investigation about thermal instability.
Michael
If he has some ideas though, that thermal instability (run away) with current amps is a realistic scenario then I'd definitely would like to hear about.
To all of *my* knowledge / investigation on the topic of Thermal Distortion and thermal speaker behaviour, this is practically impossible - but lets see...
There IMO is an underlaying mis-concept/mis-interpretation about "thermodynamics" in heating/cooling mechanism that "possibly" 🙂 leads to such conclusions.
But as said - there is always a good chance that I may have overlooked something. Being clearly in the second half of a full century but possibly not as old as John - I nevertheless would like to exercise in "getting gracefully old" by a teamwork investigation about thermal instability.
Michael
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Hi John, my apologies. Thankyou for attributing me with a sense of humour, however in this case my comment was due to my poor understanding and even poorer attention span. Must try harder.
Would you mind explaining why an engine is not an LTI system whereas a speaker is?
Best,
Mike
If anyone is really interested, putting together a simple Howland current pump using a chip amp might not be that difficult.
Never heard of, and not studied it yet.
Maybe this would be similar (in some way, at least): Variable Amplifier Impedance
It's a mixed voltage and current feeadback circuit. Because it's quite simple to mod an existed ordinary SS amp to that kind of operation, I gave it a try.
Eventually I got 20 Ohm (or so) output impedance for my amp, and it works very well with my OB bass with low-Q woofers. (I posted a thread here before.)
Days ago, I happened to get a chance to listen to a CD with helluva bass, at a very satisfying level - pipe organ version of [Pictures at an Exhibition] (Dorian record). The woofers seemed fine, there was no sign of thermal problem, or any abnormal sound I could hear. They just kept blowing me away. 😀