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Old 19th August 2010, 04:00 AM   #7241
Ang is offline Ang  United States
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Originally Posted by mayhem13 View Post
An old post to bring up.....i know...but still begs a question. Consider an array of 8 midwoofers and a single, central CD waveguide crossed steep at the directivity limit of the WG (1khz in this example). Would the above effects be mitigated enough?
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.
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Old 19th August 2010, 05:17 AM   #7242
mige0 is offline mige0  Austria
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Originally Posted by john k... View Post
When I am in error I correct it , ASAP.

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But one of the problems with current drive is the possibility thermal runaway..
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

Last edited by mige0; 19th August 2010 at 05:45 AM.
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Old 19th August 2010, 05:40 AM   #7243
mige0 is offline mige0  Austria
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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

Last edited by mige0; 19th August 2010 at 05:47 AM.
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Old 19th August 2010, 11:43 AM   #7244
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Originally Posted by mige0 View Post
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.

Michael
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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Old 19th August 2010, 12:35 PM   #7245
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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.
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Old 19th August 2010, 05:51 PM   #7246
mige0 is offline mige0  Austria
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That is not true Michael.
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
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Last edited by mige0; 19th August 2010 at 06:05 PM.
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Old 19th August 2010, 06:15 PM   #7247
g3dahl is offline g3dahl  United States
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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.
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
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Old 19th August 2010, 07:51 PM   #7248
Elias is offline Elias  Finland
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Hi,

Maybe better design an amplifier with power feedback.


- Elias
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Old 19th August 2010, 07:57 PM   #7249
mige0 is offline mige0  Austria
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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.

Click the image to open in full size.

Click the image to open in full size.



It even could be done in analog with the reverse circuit of my SPICE model



Michael

Last edited by mige0; 19th August 2010 at 08:16 PM.
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Old 19th August 2010, 08:16 PM   #7250
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Originally Posted by mige0 View Post
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
Beyond the Ariel

Michael
Please Micheal,


I am too old to engage in combat. But please read more carfully what I really did say.




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Originally Posted by john k... View Post
But one of the problems with current drive is the possibility of thermal runaway....
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.
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