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I just received my boards and build the first one. Then I hooked it onto my scope, and checked if it was working. As in Input, I used my soundcard with a 5V (p-p) sinewave (1 Khz). At the output of the buffer I only measured 1V (p-p). The same on FBK. I used a single ended input and shorted - to ground. Strangely, both FBK signals have the 1V sinus, as do the outputs (so they are not out of phase as one might expect from a balanced signal). Reistors used (as included): R1 1.1k R2: 2.21k RF: 1.1k RG: 2.21K This should give a gain of 2.5. I omited C7/C8 and set J1 and J2. The THS4131 gets a bit hot. What could be wrong here? I checked all values and connections, but everything looks just fine.. Then again: 2.5 * 2 = 5... The numbers seem to fit, but the gains seems to be no gain, but a decrease in signal strength. Edited by user Monday, July 2, 2007 2:12:07 PM(UTC)
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Hello, It sound like something is amiss. :) Please post a picture if you can. The gain is actually correct for a single ended input and each output compared to GND. With SE input the gain with the resistors mentioned will be approximately unity. And the output for each signal should be -6b measured to GND and in opposite phase. It will be unity gain when compared differentially, each output compared to the other. Cheers! Russ Edited by user Tuesday, July 3, 2007 7:49:57 AM(UTC)
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Russ White wrote:Hello,
It sound like something is amiss. :) Please post a picture if you can. I will do that later on today. Quote:
The gain is actually correct for a single ended input and each output compared to GND.
With SE input the gain with the resistors mentioned will be approximately unity. And the output for each signal should be -6b in opposite phases or unity when compared differentially.
Well, I'm guessing the phase is actually correct, since my scope is a bit simple and has only one channel.I's really hard to see ;) Just to get things right: - So gain is (or should be) 2.5 - Input is 5V single ended p-p. - Output is of each channel 6 db less than 5 * 2.5, so (5 * 2.5)/2 = 6.25V - I Actualy measured: 1V p-p per signal, that would be (5/2.5)/ 2 ... Am I correct? Edited by user Tuesday, July 3, 2007 8:09:26 AM(UTC)
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G = ( RF / RG ) * ( 1 + (2R2 / R1) ) G = ( 1100 / 2210) * (1 + (4420 / 1100)) = ~2.5 G = ~2.5 So with differential(balanced) inputs and outputs gain will be 2.5. Since you are referring one input to GND the differential output will be -6db or G = 1.25 measured between the outputs. Measured to GND each output Vout will be at ~.625 X Vin. Cheers! Russ Edited by user Tuesday, July 3, 2007 8:38:44 AM(UTC)
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Russ White wrote:
Measured to GND each output Vout will be at ~.625 X Vin.
Okay, with 5V single ended input, I should measure about 3.125 V on the output of each signal?
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Thats what I would expect yes. You may need to measure and be certain you have 5V on input.
You could actually do this. Take a single AAA battery (actually any ~1.5V battery) and measure its actual voltage. Then connect it to the +IN and GND with GND connected to -IN. Measure your output both between each individual output and GND, and between both outputs.
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'Il do that when I'm at home again. Thanks for the tip.
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Okay, AAA connected to + and GND, - to GND. input: * between + and GND I measure 1.42V output: * between + and GND: 247 mV, * between - and GND: -247mV, * between + and - : 486 mV After the lm's (before RG): * + and GND: 10.95V * - and GND: 1.8V * - and +: So somthing is definantly wrong... Measured again: + and - 5V on the outputs... measured aganin: back to 0.25V.. I really don't get it.. And now,I also have a DC offset on the output. This is not going very well :( Edited by user Tuesday, July 3, 2007 10:44:10 AM(UTC)
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Send pics when you can. It could be something simple.
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 With both inputs to GND I measure + and - 4.7V DC on the outputs... +7 and -11V on the instrumentation amp...supply +/- 12V (simple 7x12 and 10.000 uF for testing). Edited by user Tuesday, July 3, 2007 11:16:51 AM(UTC)
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One of your 47uF caps is backwards (top in your pic). Have a picture of the bottom? Edited by user Tuesday, July 3, 2007 11:19:33 AM(UTC)
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Brian Donegan wrote:One of your 47uF caps is backwards (top in your pic). Have a picture of the bottom? Damn, you are correct  But that did not do the trick. I also resoldered the LM... Now with 1.4V input I get 1.8V output. That should be correct :) Trying a real signal now Edit: signal is fine! no DC anymore! Edited by user Tuesday, July 3, 2007 11:49:46 AM(UTC)
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The second one is also ready and working. I hooked them up! Oh men, what a soundstage :d/ This is really a dramatic improvement as an addition my passive relay volume contol (not yours ;) ).Great work! I still hear some fizzing though my speakers though (volume independent), but everything is lying around without a box and fairly chaotic, so that might just be the problem. I's to dark and to take a look at it now Edited by user Tuesday, July 3, 2007 1:26:57 PM(UTC)
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Thanks! I am very glad it all worked out. :) Enjoy!
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I redid the wiring yesterday and now I used nice shielded coax cable between attenuator and buffer. Nasty surprice: it didn't do a thing.... Stilll the same bzz, bzz, bzz in half a second intervals. So I took a sheet of aluminum to investigate the source. Held it above the boards, on te side, below, until at some point the fizzing was gone. When i removed the sheet, it started again. Following the direction of the sheet I found the source of the problem: It was my DECT base station, apparently sending out bursts of energy that are picked up by the driver. So I moved the base station away, an there you have it: silence at last Well, if the final setup is complete, I will put it in a nice aluminum case anyway, so I won't have these kinds of nasty surprises anymore.
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