Redd 47 Mic Preamp

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ruffrecords

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mixguy said:
Ok, I have removed the boost switch and input xlr.  Noise is still steady. I have a sample of the noise but mp3 is not an acceptable attachment format. I can email it to anyone who is interested.
Thanks

OK so you are saying with no NFB is still squeals?

Why did you remove the input XLR? Try a test noise with the transformer input shorted.

Cheers

Ian
 

mixguy

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Thanks Guavatone. I will check those components. I have to step away from this for a couple of days and go to work. I will report back on what I find.
 

dbonin

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Hes using the DRIP PCB. 
I just finished two of them but they are in the box for shipping tomorrow, so I no longer have my hands on them! 
I didn't add the phantom power to them, so can't speak to that.

Are you using the sowter xformers?

Double check all your ground connections for continuity to the case, triple check your wiring etc  - basic stuff, but  these things are extremely sensitive to stray flux bouncing around the case.   

When you can, take a few pics and post the links here.
 

mixguy

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Been looking it over and haven't found any obvious mistakes. I asked Drip Electronics if they had a picture of an unstuffed PCB  so I could check component placement but no reply yet.
 

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rock soderstrom

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I see some problematic areas. The unshielded wires from the  XLR and DI Ins are very suspicious for crosstalk with the  output (and the feedback/ gain selector part) of the amp, especially if you keep these cables so close together. This can lead to oscillation...and your cable management of the VU input wires is suspect number one for that. Don't forget, this is your (XLR) output and you tie it together with your inputs, all of them unshielded.

Nice build BTW!
 

mixguy

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Thanks for the compliment. I unhooked all the inputs and gain selector switch. Only thing I left was the output hooked up and the noise is still present at the same level.
 

rock soderstrom

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mixguy said:
Thanks for the compliment. I unhooked all the inputs and gain selector switch. Only thing I left was the output hooked up and the noise is still present at the same level.

I am not sure if this is a good strategy. I would leave the basic circuit as it is. If your components on the PCB are the right ones than the problem is in the harness. Did you unhook those yellow-red VU meter cables at the XLR  in/out? Use shielded wire for the DI in and other sensetive parts of your amp. Seperate Ins from Outs.
 

mixguy

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Ok, I checked some voltages at the test points and every thing is almost double of what it should be. Not sure what is causing this.
 

dbonin

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Did you test your mains?  You should be able to tell if your meter is reading close to your expected mains voltage.

I used diodes instead of the 6x4 power tube, and my voltages we a tad high, but not doubled.

Did you test this with your phantom power supply completely disconnected?   
 

mixguy

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Phantom totally disconnected. Mains voltage is normal but I put the meter on the max range 600V to check 380v check point and meter reads over 700v. Not home right now so I can't read my notes but I think it was around 793v. I think my meter should have read OL if this is the true voltage
 

guavatone

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Read this or similar

http://www.learnabout-electronics.org/PSU/psu11.php

Chances are your transformer is wired for half wave but you are using 4 diode bridge.
 

vmanj

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Greetings to all.

/// dbonin ///
- Reply #76 - you wrote that you use diodes instead of 6x4.

I have a question for you.

I also used diodes instead of 6x4, most likely it was in the original (judging by the documentation for REDD.47) and 6x4 is someone's imagination .. :)

When I turn on the power, the oa2 flash too bright.
Then, when EF86 and E88CC heat up, the brightness of the oa2 is normalized and everything works fine.
I made a separate switch for anode voltage and served with a slight delay.
I have some concern for oa2 ..

- Do you switch on heating and anode voltage at the same time?

 
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