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Old 16th November 2012, 01:31 PM   #31
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Thanks so much! I hope I got all the parts correct. I was very careful about picking them. When I placed my order, 2 items are back ordered and not due until January. I'm fine with that since I won't be able to finish this project before then.
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Old 26th November 2012, 08:10 PM   #32
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I got my board from Glen today! yeah!
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Old 27th November 2012, 01:38 PM   #33
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Anyone interested in a single ended to balanced converter? I'm working on one, but I'm open to new ideas.
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Old 27th November 2012, 02:47 PM   #34
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Elliott Sound Products has one. ESP - P87A Purchase Information I've built several of the ESP kits (though not this one) and found them all to be very good.
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Old 27th November 2012, 03:12 PM   #35
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Quote:
Originally Posted by ByronInLawrence View Post
Elliott Sound Products has one. ESP - P87A Purchase Information I've built several of the ESP kits (though not this one) and found them all to be very good.
Well, his version has a problem in that if one of the outputs is grounded, then the opamp driving that will have to drive a 100 ohm load, which is bad.

He talks about the cross feed circuit, developed by HP in 1979, but says that it is too unstable to use in practical circuits. THAT Corp has an IC version of the cross feed circuit:
THAT Corporation 1606/1646 OutSmarts Balanced Line Driver ICs
The 1646, which has some patented new features that make it stable. However, I'm wondering if the noise is too high for a high performance oscillator output. His IC has 10kohm resistors in it, for example. Also, the distortion is not that low (0.0007%).

I've been working with a low impedance version of the cross feed circuit but do not know if it will be stable. In simulation, I'm getting 1.5x10^-5 THD and S/N of 125dB. The four 1K resistors around each of the output opamps could be precision resistor arrays in 8 pin dip format. I'm nervous about this though since others talk about how it is unstable. One problem with working on these high performance circuits is that a purpose built PCB is pretty much mandatory, at least in my experience. I've soldered up several different kinds of designs on regular test boards and things don't go very well sometimes. So, this adds to the expense of developing something really nice. I've gotten pretty good with Pad2Pad software. <g>

On the other hand, I do have a balanced transformer driver circuit that's pretty good, but dependent on the performance of the transformer. I just received some super nice outputs from CineMag. The performance at 20Hz is really great, but it's not going to be in the sub PPM range.
Attached Images
File Type: gif Elliot SE - bal p87-f3a.gif (4.0 KB, 118 views)
File Type: jpg Cross feed bal output.JPG (88.1 KB, 120 views)
File Type: jpg CMOQ-2 mixed feedback balanced 20Hz 0dBu input.JPG (179.9 KB, 117 views)

Last edited by dirkwright; 27th November 2012 at 03:20 PM.
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Old 27th November 2012, 08:38 PM   #36
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Hi dirkwright,

Two boards (a friend wanted one) arrived last Thursday. Glen has done a fine job (as usual). Left is collecting parts. Even that will be easy with BOM and Mouser import file. Thanks! The only thing missing is time so this project will take a while to finish.

I see you are up to a high performance balanced output. I have never dealt with cross feed, interesting nevertheless as performance should be great.

I usually do (did) balanced conversion as shown in the simplified circuit below. Buffers could be added. First OP-amp could be inverting in order to remove some common mode distortion.

unbal_to_bal.JPG
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Last edited by AKN; 27th November 2012 at 09:05 PM. Reason: Complement
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Old 28th November 2012, 12:58 AM   #37
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Tack!

I don't know what to do right now. Maybe I just leave it single ended and be happy.
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Old 28th November 2012, 10:55 AM   #38
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Quote:
Originally Posted by dirkwright View Post
Tack!
Thank you!

Out of curiosity, what sound card do you use? I'm referring to the TruRTA graph in post #35.
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Old 28th November 2012, 01:28 PM   #39
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Quote:
Originally Posted by AKN View Post
Thank you!

Out of curiosity, what sound card do you use? I'm referring to the TruRTA graph in post #35.
Oh, I'm embarrassed to say actually. Right now, it's just a little Behringer UCA202, which TrueRTA recommended. It has cross talk between the two channels, it's only 16 bit, etc.

I bought the TI evaluation board for the PCM4222 ADC that I need to put into a box. It should be 24 bit no problem.
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Old 28th November 2012, 01:45 PM   #40
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Ok, but decent performance, the 16-bits seem to be utilized.

Pic of my unbalanced to balanced converter. Built back in -91 :-)
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File Type: jpg converter.jpg (398.4 KB, 98 views)
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