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voodooless  
#1 Posted : Monday, July 2, 2007 2:00:05 PM(UTC)
voodooless

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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)  | Reason: Not specified

Russ White  
#2 Posted : Tuesday, July 3, 2007 7:39:38 AM(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)  | Reason: Not specified

voodooless  
#3 Posted : Tuesday, July 3, 2007 8:01:38 AM(UTC)
voodooless

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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)  | Reason: Not specified

Russ White  
#4 Posted : Tuesday, July 3, 2007 8:37:38 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)  | Reason: Not specified

voodooless  
#5 Posted : Tuesday, July 3, 2007 8:57:24 AM(UTC)
voodooless

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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?
Russ White  
#6 Posted : Tuesday, July 3, 2007 9:03:35 AM(UTC)
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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.
voodooless  
#7 Posted : Tuesday, July 3, 2007 9:09:52 AM(UTC)
voodooless

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'Il do that when I'm at home again. Thanks for the tip.
voodooless  
#8 Posted : Tuesday, July 3, 2007 10:18:11 AM(UTC)
voodooless

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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)  | Reason: Not specified

Russ White  
#9 Posted : Tuesday, July 3, 2007 10:46:42 AM(UTC)
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Send pics when you can. It could be something simple.
voodooless  
#10 Posted : Tuesday, July 3, 2007 11:01:07 AM(UTC)
voodooless

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UserPostedImage

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)  | Reason: Not specified

Brian Donegan  
#11 Posted : Tuesday, July 3, 2007 11:15:03 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)  | Reason: Not specified

voodooless  
#12 Posted : Tuesday, July 3, 2007 11:47:06 AM(UTC)
voodooless

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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 Applause 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)  | Reason: Not specified

Russ White  
#13 Posted : Tuesday, July 3, 2007 12:13:03 PM(UTC)
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Excellent. :)
voodooless  
#14 Posted : Tuesday, July 3, 2007 1:19:27 PM(UTC)
voodooless

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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)  | Reason: Not specified

Russ White  
#15 Posted : Tuesday, July 3, 2007 2:06:37 PM(UTC)
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Thanks! I am very glad it all worked out. :) Enjoy!
voodooless  
#16 Posted : Wednesday, July 18, 2007 1:41:20 AM(UTC)
voodooless

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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 Applause

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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