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seth

Ruby Baby

Posts: 5,572 Member Since: 26/01/2011

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Apr 22 17 6:11 PM

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I have a mic pre I built from a kit that's misbehaving. The gain goes up til the knob is halfway, then the distortion goes up but no more gain. I assume it's something with an active device, and I really haven't had time to look carefully. The one thing I did notice is that, when I run a 1 kHz tone through it, the voltage of the negative signal at pin three on the output is double what the voltage of the positive signal is. This is a Class A device - shouldn't the voltages be the same? Am I jumping to an unwarranted conclusion? Thanks.
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Mike Rivers

Aqua Marine

Posts: 2,503 Member Since:13/10/2012

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Apr 22 17 6:22 PM

Most of the time there's a probelm with one or more electrolytic capacitors. Get out your shotgun and start replacing them.

As far as the voltage difference between ground and pins 2 and 3, actually there's a differential output circuit topology where this can occur. Sometimes they adjust bias to trim it so that the inverted and non-inverted pins are the same voltage, and sometimes they don't. What matters is the voltage between pins 2 and 3. With this sort of output stage, if it's feeding a single-ended input, if you tie one of the hot output pins to ground, the voltage, referenced to ground, goes up by the amount of voltage that the other pin measured to ground, and is now zero. You can tell if it's that kind of output circuit by measuring the voltage between pins 1-2 with pin 3 floating, and then ground pin 3. If the voltage between 1-2 goes up, then it's working like it's supposed to.

But maybe it's not that kind of output, and it's broken.



For a good time, call mikeriversaudio.wordpress.com

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zmix

Aqua Marine

Posts: 4,060 Member Since:20/01/2011

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Apr 22 17 6:45 PM

Lefora won't allow my text to be entered outside of the quote bubble..

How are you determining that the voltage at pit 2 and pin 3 are different on a transformer coupled output?

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Mike Rivers

Aqua Marine

Posts: 2,503 Member Since:13/10/2012

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Apr 22 17 8:19 PM

seth wrote:
Thanks Mike. This is actually a transformer balanced output that's feeding a balanced input.


Oops!!!!  Well, then I have another explanation, but my first one was more fun. Unless the transformer secondary has a center tap that's connected to ground, then neither pin 2 or pin 3 are referenced to ground - one of those two pins is referenced to the other pin. Anything that you measure between pin 2 or pin 3 to ground is just capacitive coupling, and that's both frequency dependent and doesn't necessarily have to be the same for both pins.

So just look at what's between pins 2 and 3. If that starts clipping on one half of the cycle much sooner than it starts clipping on the other half of the cycle, then you have an electronic problem.



For a good time, call mikeriversaudio.wordpress.com

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Mike Rivers

Aqua Marine

Posts: 2,503 Member Since:13/10/2012

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Apr 22 17 8:22 PM

zmix wrote:
Lefora won't allow my text to be entered outside of the quote bubble..
 


If you click in the area below the quote bubble and it still doesn't let you type, move your mouse cursor down to the bottom of the typing area and you'll see a dashed red line with a left-angled arrow in the lower right hand corner. Click on that arrow and you should be able to type. That's some kind of paragraph dividing thingy.



For a good time, call mikeriversaudio.wordpress.com

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seth

Ruby Baby

Posts: 5,572 Member Since:26/01/2011

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Apr 22 17 9:27 PM

I was measuring between pin 2 or pin 3 and ground, and there's no grounded center tap on the secondary of the output transformer. Live and learn.

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zmix

Aqua Marine

Posts: 4,060 Member Since:20/01/2011

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Apr 23 17 7:13 PM

seth wrote:
I was measuring between pin 2 or pin 3 and ground, and there's no grounded center tap on the secondary of the output transformer. Live and learn.

Pretty common (no pun intended) misapprehension about transformer balanced ins and outs..

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