API style micpre. Strange gain pot noise...? Schematic included

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jwhmca

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Jan 28, 2009
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when I turn the gain control up I get the  "pufffittttt" noise like a dirty pot or DC on it... then if I turn it up again right away, it's gone. If I wait a bit... there it is again.

It is a CP type pot so I don't think it's dirt, plus it goes away and comes back as described.

I designed this thing my self borrowing from a lot of different designs... I probably borrowed an incomplete circuit? And somehow I am getting DC into my NFB?

http://i907.photobucket.com/albums/ac273/jwhmca/700MICPREB.jpg
 
http://i907.photobucket.com/albums/ac273/jwhmca/700MICPREB.jpg

Here is the schematic.

I have not replaced the op amp... is this something that it could fail like?
 
At first glance:

- Get rid of C8 and C3.
- Unless the opamp you're using has terrible DC offset, get rid of C7 as well.

Also, is your VU meter input buffered?

Good luck,

Mike
 
C8 and C3 are just to build a non-polarized cap... just thought I would see if there was any difference in going with to polarized back to back instead of a standard non-polarized... dumb?
 
I figured that's what you were doing.  It's sort of a process of elimination.  I can tell you from experience that you can use a single polarized capacitor at the tail of the feedback network and your pre won't care a bit.  Just make sure that the cap's voltage rating is fairly high (I'd go for 35v with a +/-15v supply).

What VU meter circuit are you using?   Can you post a link?

 
It's one I scrapped outa an old Yamaha PM700... from what I hear very much like the ones in the PM1000... sorry no link.

** edit** ohh, "circuit"

On the back of the meter there is an input resistor then four diodes then a resister across the DC input into the meter... if that makes sense...?
 
I suggest you follow the signal after R26 to see what the next stage is.  If that's not possible to confirm that your signal is indeed being buffered, I would slap a quick and dirty voltage follower op-amp buffer after R26.

see: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar2.html

P.S.:  A cap or resistor are not buffers.

Good luck!  :)
 
jwhmca said:
It's one I scrapped outa an old Yamaha PM700... from what I hear very much like the ones in the PM1000... sorry no link.

** edit** ohh, "circuit"

On the back of the meter there is an input resistor then four diodes then a resister across the DC input into the meter... if that makes sense...?

That sorta makes sense (the diodes are rectifying your signal) but that also tells me that you need a buffer as mentioned in my post above.  Why don't you disconnect the VU meter for now and see what happens?
 
I will... doing it now will post in a few mins

would you care to elaborate on your thoughts behind this? I would love to learn...
 
I'm no expert.  I just have a feeling about that VU meter interacting with your output.  I may be wrong though.  I don't know enough (nothing actually) about the PM1000 and its components to tell you this.

Maybe someone with more knowledge than me will chime in.
 
the other problem I'm having is a little POP-ish click when I switch from one of the Mic gains to the instrument input... when the 48 phantom is on. Does anybody see a problem with how I have implemented my input/gain switch?
 
One thing that is true about the Layout that is not apparent on the schematic is that my Gain control is mounted on the front panel through about 5 inches of twisted wire... stay C a problem?
 
jwhmca said:
the other problem I'm having is a little POP-ish click when I switch from one of the Mic gains to the instrument input... when the 48 phantom is on. Does anybody see a problem with how I have implemented my input/gain switch?

Now that I look at the input section of your schematic, there is a DC voltage imbalance presented at your primaries when switching from the 'mic gains' to the 'inst input'.  This is not good for your instrument nor for your input TX.

I would re-evaluate the input section completely.  One thing is for sure: you always want the same DC potential at both ends of your input TX!  Also, you do not want a DC potential presented at your unbalanced (instrument) input!
 
Please go ahead and speak Insulting NEWBanese... I'm looking at the schematic... while I wait for you to clarify ;-)
 
Sorry. I didn't mean to sound insulting.  :-[

I will have to take the schemo and highlight what I see as a problem in a graphics software.  Give me till after diner.

Cheers,

Mike
 
Ohh you weren't... I want you to! (like so simple it seems insulting)

Thanks for the effort, I will look forward to it!
 
Yeah, I see that :eek:

The schematic is wrong! Let me fix and re-post... ahhh sorry for the goose chase!
 

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