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DIY Compression Driver Experiment

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Posted on March 4, 2015 at 18:10:14
Paul Scearce
Audiophile

Posts: 471
Joined: March 1, 2002
 Problem: My newly built compression drivers had a skewed image, where high frequencies were balanced, but vocals seemed shifted to one side.

Observation: Due to the "folk art" construction tolerances, the voice coils were at slightly different depths in the magnetic gap

Hypothesis: The differences in position in the gaps were causing the skewed imaging. Possibly, they were producing different levels of second harmonics, especially at the lower end of the compression drivers range, making one driver sound louder in that range.

Test: Shiming the diaphragm assembly of one of the drivers to move the voice coil forward in the gap normalized the image.

Conclusion: "Folk art" DIY compression drivers are tricky.

I'll leave this here for peer review. Those of you who are actually knowledgeable about drivers are also welcome to comment if you like.

 

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RE: DIY Compression Driver Experiment, posted on March 5, 2015 at 09:11:56
tomservo
Manufacturer

Posts: 8198
Joined: July 4, 2002
 Building your own drivers is the "final frontier" in DIY, congrats!

You might want to measure them and eq them to the same curve and then see if they are still enough different to notice.
Best,
Tom

 

RE: DIY Compression Driver Experiment, posted on March 5, 2015 at 13:24:47
bwaslo
Manufacturer

Posts: 245
Location: Portland, OR USA
Joined: September 10, 2006
 Paul,

Any photos? I'd like to see how to do compression drivers. Do you use a phase plug?


_

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RE: DIY Compression Driver Experiment, posted on March 5, 2015 at 19:19:36
Paul Scearce
Audiophile

Posts: 471
Joined: March 1, 2002



I didn't take too many construction photos. Sorry. I found one of the front part of the compression chamber/phase plug. The diaphragm is a straight sided cone, suspended at the outer edge and driven from the inner edge. It is formed around the phase plug, and is held about .2mm-.3mm away from it. I wanted to keep the front volume as small as practical.

There is a conical plug glued inside the piece pictured, with the tip at the exit of the driver. All of the sound is directed through the radial slits shown, into the expanding chamber that is formed by this front piece and the conical plug.

 

RE: DIY Compression Driver Experiment, posted on March 5, 2015 at 19:33:52
moray james
Manufacturer

Posts: 1599
Location: Calgary on the Bow
Joined: May 19, 2002
 Joseph Merhaut designed and patented an electrostatic compression driver. I have always found that flat stock and flat things are easier for the DIY'er to build than round things. you might want to track down his patent it is based on very simple but precision flat electrostatic panel and a not too difficult phase plug' Here is just one of Joseph's books. Joseph was Europe's equivalent to Harry Olson. Nice work glad to see you working on such a project sure you will inspire others as well to do so. Best regards Moray James.

books.google.ca/books/about/Theory_of_electroacoustics.html?id=ml7bAAAAIAAJ&edir_esc=y
moray james

 

RE: DIY Compression Driver Experiment, posted on March 6, 2015 at 15:59:55
badman
Reviewer

Posts: 8801
Location: Tustin, CA (Orange County)
Joined: March 10, 2001
 I agree wholeheartedly- they definitely need to be measured carefully to ensure they're doing what you need, in frequency response, distortion, and impedance.



Bass is supposed to sound big. 6.5" is not a woofer size.


 

Update, posted on March 8, 2015 at 13:50:13
Paul Scearce
Audiophile

Posts: 471
Joined: March 1, 2002
 I did some nearfield measurements, and based on those, I will be making a new pair of diaphragms to see if I can get a closer match. In the process, I realized I had hooked up one of the horns out of phase. Now, the image is centered, and stable.

Despite the variations between the two, these things sound quite good. They are clear, clean, and dynamic sounding. They seem ok crossed over second order at 700Hz, and they make it out to about 12kHz. Sensitivity is 90dB at .125w at 10 ft.

 

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