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Old 27th October 2004, 01:10 PM   #1
jesper is offline jesper  Sweden
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Default Input transfomer on P3A

Hi,
I'm using 4 P3A's (home made PCB's) in an active configuration and I have serious ground loop problems due to active filters being in a separate box.
I got hold of a bunch of BeyerDynamic input 10k transformers and intend to have those in my next P3A layout.

My question is, how do I do this? I'd like to remove the input DC-blocking cap. I've tried just removing the cap, connecting the transformer secondary windings from input to ground but this the P3A started to distort.

Any inputs or links to other similar input transformer coupled amp would be greatly appreciated.

/Jesper
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Old 27th October 2004, 06:51 PM   #2
moamps is offline moamps  Croatia
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Old 27th October 2004, 07:11 PM   #3
jesper is offline jesper  Sweden
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Yes, that works offcause.
But I'd like to remove the 4,7uF input cap. When I do so I get distortion.

/Jesper
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Old 27th October 2004, 08:35 PM   #4
moamps is offline moamps  Croatia
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Quote:
Originally posted by jesper
Yes, that works offcause.
But I'd like to remove the 4,7uF input cap. When I do so I get distortion.
/Jesper
Hi,
there are two possibilities:
First, P3a isn't very happy with DC coupled input.
Second, the transformer is saturated with DC current that flows thru the secondary winding from GND to the input transistor's base. Best keep the input cap, IMO.

Regards,
Milan
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Old 27th October 2004, 09:26 PM   #5
sam9 is offline sam9  United States
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Quote:
Best keep the input cap, IMO.
However, increasing the value of that cap can get you much of the benefits a eliminating it. If space is an issue, consider a pair of oppossed (in series with the negative poles facing each other) 47uF tantalums. Or even a single non(bi)-polar electro of 47uF.
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Old 27th October 2004, 10:40 PM   #6
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What if you placed the secondary between the base resistor and ground?
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Old 28th October 2004, 05:56 AM   #7
jesper is offline jesper  Sweden
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I've tried tying the secondary to the base of Q1, both directly and by via the 2k2 base resistor, neither worked at all.
However, I've not made any thorough testing. I'll bring a oscilloscope home tonight to see what's really going on.

/Jesper
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Old 28th October 2004, 09:56 AM   #8
djk is offline djk
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"BeyerDynamic input 10k transformers "

Are those rated for line level, or just mic level?
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Old 28th October 2004, 10:26 AM   #9
moamps is offline moamps  Croatia
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Quote:
Originally posted by djk
"BeyerDynamic input 10k transformers "

Are those rated for line level, or just mic level?

Beyerdynamic 10k linelevel transfomer
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Old 30th October 2004, 07:39 PM   #10
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Quote:
Originally posted by jesper
I've tried tying the secondary to the base of Q1, both directly and by via the 2k2 base resistor, neither worked at all.
However, I've not made any thorough testing. I'll bring a oscilloscope home tonight to see what's really going on.

/Jesper

When you connected the transformer coil from the 2k2 resistor to ground, as far as DC bias, you just shorted out the 22K resistor to ground. DC resistance of the coil is very low. 10K refers to the impeadence at 1KHz. Measure the resistance with an Ohmmeter and you will see what I mean. What I ment before is to break the circuit between the 22K (R2) and ground. Place the secondary coil there so that it is in series with the 22K (R2) and ground. This should not adjust the DC offset too much---if you have DC offset adjustment. You may need to add a loading resistor to the secondary, then again maybe not....experiment and see. I have made circuits using this method and they are very stable as far as DC offset and drift.
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