Redd 47 Mic Preamp

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dbonin

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Any anecdotal responses to this,  using the DRIP u47  with and without the 200 ohm output attenuator? 600 ohm T-PAD  with no mods? 
Cheers all,
Don
 

dbonin

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So this is what I'm thinking,  I have some 200 ohm stepped attenuators  that I can use on the front end. 
But to make it just a bit more versatile I was thinking of wiring in a bypass switch that will  remove the attenuator completely.
Best of both worlds, I'm thinking. 
 

JonnyP

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dbonin said:
So this is what I'm thinking,  I have some 200 ohm stepped attenuators  that I can use on the front end. 
But to make it just a bit more versatile I was thinking of wiring in a bypass switch that will  remove the attenuator completely.
Best of both worlds, I'm thinking.

I am curious about this.  I have a Dizengoff D4 (redd47) pre and it would be really nice to have more pad options.
 

dbonin

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The 200 ohm, continuous impedance attenuator  I used  has  "zero"  as the first  position, which, as you may have guessed, effectively bypasses the attenuator.  Works great and no need for a switch to bypass.

Signal is perfectly silent with a both mic and DI until I engage the low cut switch, which i a simple 2uf poly cap across  switch...  with it engaged there is a barely perceptible low frequency hum.  The switch is on the power supply side on the front panel and the longish lead dressing for the cap is likely the "antenna" picking up the power supply flux... 

I'm going to try a physically smaller film cap to get the leads as short as possible and try some kind  of shielding to see if that will eliminate the hum.
Otherwise, this project was a great success!
 

dbonin

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The hum on the low cut switch was of course 60 cycle power supply noise.  I cut a piece of  aluminium sheeting and made a small shield around the switch and cap - problem solved.
Next time I would simply put the low cut switch on the other side of box, away from the power supply.

Cheers all!!
 

mixguy

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I've finished a Redd 47 but it has a high frequency whine on the output. It does not vary in level with the gain knob but varies with the boost switch. Any ideas on what could be causing this?
 

ruffrecords

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mixguy said:
I've finished a Redd 47 but it has a high frequency whine on the output. It does not vary in level with the gain knob but varies with the boost switch. Any ideas on what could be causing this?

Sounds like the negative feedback is not working right.

Cheers

ian
 

mixguy

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Thanks Ian,
Any ideas on which components in the circuit could be the culprits?  Perhaps changing some values will remedy the problem.  The circuit works like it should other than the low level whine.
 

mixguy

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It is the 34, 40, 46 db boost switch. I only found a copy of the original schematic which I can't attach because I get an error message that says the upload folder is full.
 

guavatone

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Ian's probably right. but check the 100K anode resistor for bad solder joint too and check the boost switch solder points.  Are the caps correct on the switch?
 

mixguy

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Thanks Guavatone,
I am going to look into those issues. Gotta get this solved. Love the tone of this preamp just got to get rid of this noise.
 

mixguy

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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
 

guavatone

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Check your 1M6 and 330K resistors. I suspect the 1M6 being way too low which would make V2 scream unpleasantly.  You can just measure the resistance between V2  pins  1 or 6 and the junction of the 330K and the 1M6.  Also check the 8K2 resistor on V2 for correct value.

If that’s not it. Check all voltages on tubes and post them. Anode cathode and V1 screen.
 
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