Valley Audio 401 mic pre, parts questions

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redrabbit

Member
2004-10-28 4:25 am
BACKGROUND:
Sourced from ebay, this "Valley Audio" 401 mic preamp has 2 damaged (50k) pots that need to be replaced. Schematics are unobtainable.
(the plastic backs of the pots have been blown open from blunt force).
No worries i think, because it looks like I can prob just remove them, and glue the backs back on just enough to test them.......and determine if the taper is Linear, Audio, or Reverse Log. (there are 4 of them, so I'll test and replace all of them).
The only markings on the pots say:
Piher, 50k, 421N

QUESTION:
But while I'm in there, I might as well replace/upgrade the electrolytic PSU caps, and possibly the mic input 48v phantom power blocking caps....which are currently 47uf/63v standard polarized Panasonic types.
About these input phantom blocking caps.....What qualities (ESR, etc..) would best for these (2) polarized caps ?
The principle mic chip inside this mic pre is a SSM2017.

Thanks for reading my whole post :)

=R=
 
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redrabbit

Member
2004-10-28 4:25 am
Here is an interesting article on "Mic preamp designs", from That Corp, but it does not mention the qualities of the 48v phantom blocking caps....
http://www.thatcorp.com/datashts/AES129_Designing_Mic_Preamps.pdf

....and here is a text provided by someone who has done upgrades on this type of unit...
I have done the following:
1. Replaced the four problematic German brand pots with four high quality sealed pots that fit perfectly on to the board and have the correct shaft size and value. This returns normal operation and adds high reliability.
2. Tested all of the electrolytic capacitors with an ESR meter and replaced any that were bad or even close to marginal. All of the surface mount electrolytic capacitors were replaced with quality 105 degree centigrade rated capacitors. Done to lower distortion and increase reliability and extend the life.
3. Upgraded the microphone input coupling capacitors to the highest quality Nichicon "Muse" audio grade capacitors for lower distortion.
4. One of the voltage regulators had a top and bottom heatsink and they were joined with faulty nylon and hardware. That was replaced with a combination that will not break. For high reliability.
5. Added a resistor/capacitor network to bridge the rear ground reference binding posts since the noise floor is lower with it than with the typical short wire jumper between the chassis ground and signal ground posts. This done for a lower noise floor.
6. Increased the filter capacitance amount on the phantom power power supply for lower noise.
7. Tested and after a burn in and full functional evaluation it passed the critical listening tests.The reason for the attention to detail is that I am a hypersensitive listener. I'm a recording engineer, an audiophile, a recording studio maintenance tech and a broadcast engineer.
Source...... Valley Audio 401 Microphone Processor Refurbished Re-capped & New Pots Excellent | #1896464515
 
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