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Microphones & op-amps

98.26.91.40
Posted on November 8, 2009 at 06:38:42
emsquare
Audiophile

Posts: 88
Location: North Carolina
Joined: June 14, 2008
 I finally ordered an electronic crossover kit (Marchand XM9L-3) which leads me to a couple of considerations...

Microphone suitable for use with a laptop. Lots of free software out there one can use to measure a speaker. Not so obvious is a finding a reasonable microphone that will work for that purpose. I'm obviously not searching in a useful way. Surely some of you have crossed this bridge before and I wanted to see what solutions you may have found. I'm hoping to get a visual reference for things like frequency balance, impulse response and perhaps driver resonance.

And...

The Marchand that I ordered comes with BB OP2134PA op-amps. I've been reading some interesting opinions about changing those out in favor of National Semiconductor's LM4562NA. I intend to start with the supplied op-amps to become familiar with those. But after that, since it is so easy, try a couple of different types to see for myself. For those that may have tried this before, I'd like to see if you have anything good (or bad) to say about that.


In search of musical bliss...
RE: Microphones & op-amps, posted on November 10, 2009 at 07:35:01
drkielbasa
Audiophile

Posts: 121
Location: Mpls
Joined: February 11, 2005
 The OPA2134's are very good op amps for that application. They are toward the upper echelon of audio op amps. They are neutral and fairly resolving, but a little laid back. You'll find that changing op amps is like rolling tubes - so personal taste will play a major roll in this. Also be aware that the performance of op amps can be circuit dependent. Some are not designed to be used in unity gain, or in high gain factor settings (>10). An OPA2227 is usually superior to an OPA2134 in driving headphones, but the OPA2134 is usually superior to the OPA2227 when used as a buffer. Also, some op amps require higher current source, so it is best to look up the data sheets (google them) to see that the op amp you try does not have significantly higher current requirements and meets or exceeds the gain range limits of the OPA2134.

Other audio op amps of this quality with a bit different sonics (these usually can drop in safely in the same circuit):
OPA2604: slightly more forward upper midrange than the OPA2134.
AD4562: aggressive and resolving. Good PRAT. takes time to break in.
LT1364: similar to the AD4562, but leaner bass.
OPA627: Single channel op amp that requires dual converter adaptor.
This is the king of audio op amps for current conversion and buffer applications. Excellent deep bass drive, excellent texture and the most resolving of all op amps I've used. Few manufactureer uses these as they are spendy. The dual channel adaptor boards can cause issues with physical size. Sometimes, you can get away with adding an extra IC socket in between the board and unit to get more clearance over the caps that are usually mounted next to the op amps.

Prices range from $3 to $11 per op amp, except for the dual adaptor OPA627 boards which are $35-$70/pr depending on where you find them.

RE: Microphones & op-amps, posted on November 11, 2009 at 06:13:37
emsquare
Audiophile

Posts: 88
Location: North Carolina
Joined: June 14, 2008
 Dr Kielbasa; Thank you for what you had to say. The OPA627's x 12 are a bit too expensive for an experiment. But I did acquire a sample batch of three LM4562 opamps. I'll have to compare data sheets before I swap them but don't expect to see anything out of spec. National Semiconductor doesn't seem to publish an equivalency circuit diagram for much of their stuff. Probably a smart thing to do on their part but I'd like to know it the input circuit is similar to the supplied OPA2134's.

gymwear5; Thanks as well. I'm still pondering how to go about looking at a few things for final config once I implement the new XO. Which won't be too soon as I underestimated what level of completion the kit comes in. Whew,... but I've still been enjoying the build process.


In search of musical bliss...
RE: Microphones & op-amps, posted on November 11, 2009 at 07:53:46
Davey
Audiophile

Posts: 1494
Joined: December 26, 2000
 The LM4562 is bipolar input and the OPA2134 FET input. In the Marchand XM-9 topology I think the 2134 would be the better choice. However, the RC combinations of the state-variable integrators could probably be scaled, and the resistors in the output sections decreased in value to optimize the poorer noise performance of the LM4562.

Why not ask Phil Marchand what he thinks?

Cheers,

Dave.
RE: Microphones & op-amps, posted on November 12, 2009 at 14:01:58
emsquare
Audiophile

Posts: 88
Location: North Carolina
Joined: June 14, 2008
 Ya, I agree with that. I think that is where FET's shine is in a differential input circuit. I have to say that I am impressed with the amount of thought that went into this crossover. Liberal use of precision parts and the glass-epoxy circuit board are very well made. I had thought about some initial modifications but there isn't that much there that hasn't already been optimized. I'm still pondering using a pie filter on the power supply and maybe snubber caps on the bridge rectifier. Eh... we will see.

Suggestion noted on asking the designer. Odd that it never occurred to me.


In search of musical bliss...
RE: Microphones & op-amps, posted on November 8, 2009 at 11:35:22
EdG
Audiophile

Posts: 336
Location: California
Joined: June 10, 2001
 Several years ago, I measured the acoustic response of the MG1.6 using a Panasonic WM-61A microphone element and a microphone preamplifer of my own design plugged into the line inputs of a desktop computer soundcard. I wrote software to generate white noise and perform a FFT on the measured data. The results are shown below (green trace is the stock MG1.6). My results were very similar to the Stereophile measurements of the MG1.6.



While the Panasonic WM-61A has ruler-flat frequency response, its weakness is that it has an enormous amount of low-frequency noise (as do all inexpensive condensor microphones) that limits the dynamic range. My low-frequency measurements could go only down to ~40Hz due to microphone noise.

Note that the mic inputs on soundcards have a high-pass filter to remove low-frequency noise. This precludes using the mic inputs for accurate frequency response measurements. Good results are possible using a microphone pre-amplifier and the line inputs.
Ed
RE: Microphones & op-amps, posted on November 8, 2009 at 17:06:20
emsquare
Audiophile

Posts: 88
Location: North Carolina
Joined: June 14, 2008
 Thank you. You bring up a couple of points I had not been aware of. It had not occurred to me that the mic input may have a low frequency filter filter. But it would make sense.


In search of musical bliss...
RE: Microphones & op-amps, posted on November 10, 2009 at 12:32:03
gymwear5@hotmail.com
Manufacturer

Posts: 1218
Joined: April 10, 2002
 You should note that Magnepan uses pink noise as a primary source for measurement and QC Inexpensive mics with higher noise floors like the Behringer are useful for comparative measurements of frequency response at levels of 80dB or higher. You want to get above the noise floor of your home anyway. The threshold of hearing at 20Hz is something like 75dB anyway. The Behringer works just fine - +/- 2dB accross most the range.

If you really want flat and want to measure stuff like distortion, reverberation time, etc. You need a low noise measurement microphone. The best is the B&K Type 4190 or 4191. The 4191 has flat response to 40KHz +/-1db (mostly within 0.25dB in the audible range) and a 15dB noise floor for the 1/3 octave @ 20Hz and The 4190 has a 4dB noise floor @ 20Hz

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