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Passive Pre
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Old 11th October 2007, 10:52 PM   #1
lpd is offline lpd  Canada
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Join Date: Nov 2003
Location: BC
Default Passive Pre

Ok so I build the passive pre from one of the members on this site. Granted I'm new to electronics, so bear with me. I built it inside an aluminum guitar foot pedal that was broken. I used silver wire, and 2 100K log pots ie: dual mono. I tried to use a 10K resistor but the output was too loud. I tried a 20K and even a 50K but still too much volume (I use a Aleph 30 as an amp). I also tested to make sure the ground was on the right post as well. On the wrong post it is loud right away. On the other it gradually gets louder, but still has way too much gain.

I have another passive made with a Alps Black Beauty 100K no shunt resistor just in and out the pot and it sounds better and is smoother than the one here. Am I missing something or should I use a different value pot? There is some buzz as well, with the shunt pre, but not with the single Alps Black Beauty Pot. I didn't use shielded wire.

Any help is appreciated.

Shunt Passive Pre
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Old 11th October 2007, 11:51 PM   #2
Conrad Hoffman is offline Conrad Hoffman  United States
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Join Date: Mar 2007
Location: Canandaigua, NY USA
It almost certainly isn't wired right. Look at the diagrams in the thread again and be sure your pot is shunting signal to ground. Can you take a photo of it that shows the wiring?
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Old 12th October 2007, 12:33 AM   #3
vynuhl.addict is offline vynuhl.addict  Canada
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Location: Vancouver, BC,Canada

Like Conrad said it is definately not wired right from the sounds of it. the series resistance should go from Iput directly to output for each channel, from the output node after the series resitor should be your adjustable shunt to GND. Sounds like you may have wired the pot from pins 1 and 3 the fixed resitive element rather than the wiper (middle pin typically) and pin 1 ground side, use a multimeter and measure the pot so that when turned fully counterclockwise should read 0 ohms and is a direct short to ground fully off. I currently use this idea for a preamp using a 100k Alps Blue and a 15k 1/2w resistor and it sounds bloody great, cannot be compared to using a single carbon tracking pot which sounds inferior to even a cheap metal film resistor. at loud listening level output impedence is around 4k, just make sure your output cables are short and low capacitance. The higher you make the series element the higher you output impedence will be to maintain good listening levels which will be detrimental in possibly cutting into high frequencies with capacitive cables, its a balancing act the needs to be looked at system by system, but doing this will reap huge sonic benfits that will make most actives sound bloated and muffled, you will also be very surprised at the amount of low level detail that gets missed by utilizing more active circuitry..

Best Wishes
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Old 12th October 2007, 04:33 AM   #4
lpd is offline lpd  Canada
diyAudio Member
Join Date: Nov 2003
Location: BC
Ok here's what I have. From input to output I have a 50K resistor wired. This goes to the center of the pot. I have the grounds tied together on the input/output jacks then to pin 1 on the pot. Buzzy and too much gain. Pots are older Alps 100K A pots. I can post a pic later. Thanks for the help.
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