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Amplifier output characteristics vs acoustic feedback in horn loaded cone drivers
Amplifier output characteristics vs acoustic feedback in horn loaded cone drivers
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Old 4th December 2015, 06:26 AM   #1
AllenB is offline AllenB  Australia
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Default Amplifier output characteristics vs acoustic feedback in horn loaded cone drivers

Any thoughts on this from the corner horn FAQ? Link

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Amplifiers that exhibit high output impedance have the effect of providing a "reverb effect" in-room, especially if the room is small and relatively live acoustically. What kind of amplifiers have relatively high output impedance? Tube or valve-type amplifiers.

"Why is This an Issue...What is Happening?"

The reverb effect is due to strong room reflections back to the horns/drivers themselves, which are much more efficient than direct-radiator speakers at converting electrical energy into acoustic energy - and back again (...i.e., they are acting like microphones)
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Old 4th December 2015, 06:46 AM   #2
PMA is offline PMA  Europe
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Output impedance results in modulated frequency response at amplifier output terminals. See the measured influence of real 3-way speaker to amplitude response of the amp with only 0.25 Ohm output impedance. You can easily imagine what happens with tube amp with >1 Ohm output imp.
Blue .... card loopback
Red .... amplitude response into 6.8 Ohm load
Green ... amplitude response with 3-way speaker

The rest are speculations.
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Old 4th December 2015, 07:39 AM   #3
Cask05 is offline Cask05  United States
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See https://community.klipsch.com/index....everb-effects/

Chris
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Old 4th December 2015, 08:37 AM   #4
Cask05 is offline Cask05  United States
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That FAQ is also on this forum: Corner Horn Imaging FAQ - diyAudio
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Old 4th December 2015, 09:04 AM   #5
AllenB is offline AllenB  Australia
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Originally Posted by Cask05 View Post
What are these tests supposed to be showing? It would seem that one channel is driving a signal into the room. Another idle channel is being watched as the speaker connected to it picks this up. There's no surprise in that.

If the amp were to make a contribution I could hardly see it being greater than modifying a reflection picked up at the speaker, one which would be reflected acoustically at any rate.

If there were to be reverb, wouldn't that require a delay loop somewhere?
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Old 4th December 2015, 11:52 AM   #6
Cask05 is offline Cask05  United States
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The loop delay comes from the room itself: acoustic delay of the sound entering the room, being reflected locally, and returning via the loudspeakers' microphonic effect.

Mike (mikebse2a3 on the K-forum) tried to recreate the effect that Bob Carver described using two different amplifiers that he had on hand and a Jubilee bass bin, which has a minimum impedance of about 3 ohms at 60 and 120 Hz. The HF horn/driver combination of that loudspeaker has a minimum Z of 11 ohms at about 600 Hz, so its contribution to the effect was ruled out due to its high "damping factor" (including the amplifiers chosen) of at least 1. I don't believe that Mike showed the smoking gun levels of reverb, but he did verify that it is occurring, and it can be measured.

Carver described a reverb effect that was at least 20 dB higher in the quoted Enjoy the Music article, which would be audible during the decays. He claims that his amplifiers are in fact designed to amplify that effect--how he did that, he didn't discuss (as is usual for Mr. Carver). Perhaps he is assuming a crest factor of 20 (i.e., dynamic range of the recordings used) during transients to arrive at his -30 dB reverberation levels due to loudspeaker microphonics and high output Z amplifiers.

The reason for the tests by Mike was due to the effect that I put into that FAQ. It is the only effect that I can find that explains the differences in sound heard--typically called "tube magic" which is a function of its output impedance relative to the loudspeaker's input impedance with very efficient loudspeakers, i.e., horn loaded loudspeakers located in the corners of a room where the room acoustic reinforcement is maximized. This effect typically adds to the depth of the soundstage, hence the "reverb effect", like dipole loudspeakers produce in-room to create an artificial depth of soundstage image even when there is none in the recording.

The last of my responses to Mike is that there is a freeware utility provided by Bill Waslo (Liberty Instruments) called "Audio Diffmaker" that could be used to capture the differences between amplifiers - with and without high output impedance using horn-loaded loudspeakers located in corners of the room, especially for the transients, where the effect of reverberations induced would be most audible. Neither he nor I have run those tests yet.

You could--if you're running horn loaded with tube/valve amplifiers like a SET or OTL without any kind of feedback to lower its output impedance.

Chris

Last edited by Cask05; 4th December 2015 at 12:16 PM.
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Old 4th December 2015, 01:28 PM   #7
PLB is offline PLB  United Kingdom
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Hi Allen,

If I understand Bob Carver's description of the "reverb effect" correctly, he's saying that the loudspeaker is providing a signal to the feedback loop of an amplifier that then gets amplified at its output. My problem with this is that the feedback network is applied to the inverting input and the resulting inverted output will appose the applied input signal !
So instead of the applied signal increasing in amplitude, surely it must get smaller, because the signal has gone through an inversion. Sounds like emperors new cloths to me!

Peter
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Old 4th December 2015, 01:38 PM   #8
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That's exactly why Mike B. did the tests, so that "pundits" can accept the fact that what Carver is describing is actually happening. I don't know about you, but I believe Carver, and not the pundits that say that it "can't happen". Triodes have internal feedback that respond to the effects of changing load conditions.

Testing is the reason to believe - not mental models and naysayers that don't want to believe it because it's inconvenient that it's happening with their favorite low power SETs, and that it isn't some "magic" that is occurring that makes the sound "better than recorded".

YMMV.

Chris

Last edited by Cask05; 4th December 2015 at 02:02 PM.
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Old 4th December 2015, 07:08 PM   #9
Pano is offline Pano  United States
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Amplifier output characteristics vs acoustic feedback in horn loaded cone drivers
Where are the measurements? Didn't see them.
I can understand how this might happen in corner horn woofers with a very low damping factor amp, but don't know how it could happen with typical mid or high horns. They usually have a resistive attenuation network that would isolate the amp from the microphonics.
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Old 4th December 2015, 07:36 PM   #10
Cask05 is offline Cask05  United States
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Quote:
Originally Posted by Pano View Post
Where are the measurements? Didn't see them.
You didn't see Mike's measurements in this thread?

https://community.klipsch.com/index....everb-effects/

"Big as Dallas" as they say. The bass bin that was used is typically crossed over at about 425 Hz. I'm not sure, but I don't believe that Mike low-passed the bass bin in his measurements.

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Originally Posted by Pano View Post
I can understand how this might happen in corner horn woofers with a very low damping factor amp, but don't know how it could happen with typical mid or high horns. They usually have a resistive attenuation network that would isolate the amp from the microphonics.
To some degree, you're right, but not by more than the relative efficiency of the midrange horn/driver or tweeter vs. the bass bin. Usually there is about a 3-6 dB difference with the fully horn-loaded loudspeakers that I've dealt with (the subject of the FAQ).

If you're taking issue with only a part of the argument--as it appears that you are--I recommend that you use your Altec A5s to measure it yourself. I'm not the one that disbelieves.

Chris
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