How To Modify Linkwitz' Stereo Mic For Nearfield Measurement? - diyAudio
Go Back   Home > Forums > Loudspeakers > Multi-Way

Multi-Way Conventional loudspeakers with crossovers

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th July 2006, 12:08 AM   #1
Wizard of Kelts
diyAudio Moderator
 
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Default How To Modify Linkwitz' Stereo Mic For Nearfield Measurement?

Goal: To make an inexpensive microphone suitable for nearfield testing of Ported/TL/Daline systems.

To do this, one channel of the stereo amp must have adjustable attenuation, because in Ported systems, even when the woofer and port are outputting equally, the port will have greater SPL per square inch since it is smaller.

Keele gives the formula on the dB compensation in his article on Nearfield Measurement. It is Article number 6.

Below is the drawing Linkwitz gave from his website, which I reproduced below.

I want to use this with Speaker Workshop, which only accepts one channel at a time. So I need to know how to attenuate one of the channels, combine the two channels into mono, and plug into a sound card-in my case, an old Soundblaster 16.

As there are two R1 and two R2, for convenience sake we can call them Left R1 or R2, and Right R1 or R2.

If possible, I would like to avoid the use of a preamp, and plug directly into the sound card.

I think it would be simple, but I would like the opinion of people more skilled than myself. Thank you.
Attached Images
File Type: gif linkwitz microphone.gif (51.3 KB, 445 views)
__________________
"A friend will help you move. A really good friend will help you move a body."
-Anonymous
  Reply With Quote
Old 9th July 2006, 01:29 PM   #2
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
You can simply connect the two outputs together - they will mix perfectly well. If you know exactly what relative ratios are required, the resistor values can be calculated.
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 10th July 2006, 09:06 PM   #3
Wizard of Kelts
diyAudio Moderator
 
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Thank youi for your answer.

I was afraid that changing the R1/R2 ratio of one channel might cause attenuation in the other.

The R2 number is giving me some trouble, since 3.3k is more than twice than 1.2 k. In fact, it seems to be divided by 2 to the 1.45 power. So that is where I derived my numbers from.

Could you give a look below and tell me if my math is correct for figuring 4 dB attenuation and 16 dB attenuation, both for the right side? I want the left to be unattenuated.

The colored parts are the ones where I changed Linkwitz' diagram.
Attached Images
File Type: gif linkwitz mic amended 2.gif (21.2 KB, 315 views)
__________________
"A friend will help you move. A really good friend will help you move a body."
-Anonymous
  Reply With Quote
Old 11th July 2006, 04:30 AM   #4
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
Yes, you're right, when you connect the two together, one loads the other, so the calculation isn't as simple as when they're kept separate. You can't avoid the left being attenuated, but if you were to set the attenuation to 3dB, then you could have the right attenuation switchable between 7dB and 19dB attenuation, thus keeping the relative attenuation to 4dB and 16dB. Do you know what the input impedance of your soundcard is?
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 11th July 2006, 04:42 AM   #5
Wizard of Kelts
diyAudio Moderator
 
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Let me look it up. Or measure it.

I believe it is 10K ohms or above.

Get back to you tomorrow.
__________________
"A friend will help you move. A really good friend will help you move a body."
-Anonymous
  Reply With Quote
Old 9th October 2006, 11:58 PM   #6
Wizard of Kelts
diyAudio Moderator
 
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
Ummmm.....

"Tomorrow" came and went, as well as a heck of a lot of other days, but I finally got around to measuring the input impedance, for one channel anyway.

It is approximately 40,000 ohms. I assume the other channel is similar.

So any help with those resistor values is appreciated.
__________________
"A friend will help you move. A really good friend will help you move a body."
-Anonymous
  Reply With Quote
Old 10th October 2006, 08:49 AM   #7
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
The move has been achieved and all I have to do now is to work out how to empty the boxes...

Here is what you need. My head wasn't up to unpleasant simultaneous equations this morning, so I did it the lazy (iterative) way with a spreadsheet. I'm assuming you have E24 resistors, but if you have E96, I can plug alternative values in. Note that the switch changes relative attenuations - the absolute attenuations are quite a different thing.
Attached Images
File Type: gif kelticwizard's mixer.gif (5.0 KB, 243 views)
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 19th October 2006, 05:58 AM   #8
Wizard of Kelts
diyAudio Moderator
 
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
EC8010:

Thank you for those resistor values which will enable me to get different values of attenuation.

However, the thought has occurred to me that if I "bite the bullet" and use a separate battery per mic, perhaps I can attain attenuation without any mutual loading between sides.

As this combined output must go onto one channel for Speaker Workshop, please let me know if these designs, based on Linkwitz' drawings, will work.

I substituted the 1/8", (3.55 mm) plug into one channel of a sound card, (40 kilo ohms input impedance) for the op-amp in Linkwitz' drawing;

The purple circle around the mic and fet in the drawing mean both are mounted in the mic capsule;

All colored additions are by me, added on to Linkwitz' drawing.

Here is the one based on Linkwitz' drawing B:
Attached Images
File Type: gif mic pictures linkwitz b.gif (9.9 KB, 191 views)
__________________
"A friend will help you move. A really good friend will help you move a body."
-Anonymous
  Reply With Quote
Old 19th October 2006, 06:04 AM   #9
Wizard of Kelts
diyAudio Moderator
 
Join Date: Sep 2001
Location: Connecticut, The Nutmeg State
And now here is the one based on Linkwitz' drawings, fig C.

Either one of these configurations enable totally independent adjustment of the mic outputs in the combined mono signal?
Attached Images
File Type: gif mic pictures linkwitz c.gif (22.9 KB, 184 views)
__________________
"A friend will help you move. A really good friend will help you move a body."
-Anonymous
  Reply With Quote
Old 19th October 2006, 07:19 AM   #10
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
 
EC8010's Avatar
 
Join Date: Jan 2003
Location: Near London. UK
I don't see that having two batteries makes any difference. It's what happens to the audio that is important, and you said you needed to attenuate one capsule relative to the other. Your new diagrams simply parallel the two signals.
__________________
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Nearfield ribbon? 8675309 Planars & Exotics 5 4th August 2007 05:57 AM
best diy nearfield speakers for <300 shipped thadman Multi-Way 0 25th September 2006 10:28 PM
Nearfield listening xyrium Multi-Way 3 10th July 2006 10:35 AM
Nearfield Stargate Magnetar Multi-Way 18 27th July 2005 02:38 PM
re: nearfield monitors jeantofte Multi-Way 5 12th October 2002 10:27 AM


New To Site? Need Help?

All times are GMT. The time now is 05:41 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2