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Old 8th November 2010, 01:54 AM   #1
marlouv is offline marlouv  Australia
diyAudio Member
Join Date: Mar 2010
Default Adding balancing transformer to preamp, signal loss

I have a nice and fine preamp build for a PVDF pickup, amplifying the signal to more than usable levels.
Click the image to open in full size.

After losing the battle with 60hz hum, I have inserted a passive DI between the preamp (circuit) out and output jack. (no 20dB cut, otherwise identical, without the ground cut switch)
Click the image to open in full size.
that is a 1,000uF cap, isn't it..

After putting it all back together, working fine, except the signal fell down a mineshaft, and all the gain I use to be able to hear it, only just, is silly. Is it normal for passive balancing to cut the level so much?

Is it a case for amplifying the signal again, before or after the transformer?
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Old 8th November 2010, 07:46 PM   #2
amz-fx is offline amz-fx  United States
diyAudio Member
Join Date: Sep 2004
Location: s. Louisiana
The output impedance of the jfet booster is quite high. It is running out of drive when trying to swing the signal through the transformer. You could use a common emitter buffer after the jfet to add some drive capacity... it only adds 2 parts.

Collector to +9v and the emitter is the output; base is the input from the jfet's drain. See attachment.

This should be enough drive...

Attached Images
File Type: gif jfet1.gif (42.6 KB, 74 views)

Last edited by amz-fx; 8th November 2010 at 07:53 PM.
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Old 9th November 2010, 12:54 AM   #3
marlouv is offline marlouv  Australia
diyAudio Member
Join Date: Mar 2010
Made the changes, but still no cigar. Level slightly higher, is it safe or sane to lower the emitter resistor? I'll give that a go, and then strip the circuit back to see if something hasn't gone wrong up the line.
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Old 9th November 2010, 02:00 AM   #4
marlouv is offline marlouv  Australia
diyAudio Member
Join Date: Mar 2010
Came up with a bad TRS jack, only half the balanced signal was getting through, and that's going to be quiet.. All working now, sorry about that, but thanks for your help Jack.

tops off
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