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Old 7th March 2002, 01:07 AM   #1
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Default Passive Preamp

To dispell the rumor that I am an over priced parts snob, I offer the following project. A passive preamp for under 20 dollars worth of parts from Radio Shack and your local arts and crafts or Container Store. Construction time is a couple of hours. Astonish your friends! Confound your enemies. Give one away at your next audio club meeting. Send it to school with your kid for show and tell. Spend the money you save on your wife for a change. This is a great sounding little project.

2.5 inch signal path with air dielectric
Dual mono
Silver solder and silver plated wire.
Series shunt design that sounds better than pot alone
Neutral tonal balance and transparent imaging

This is not a joke. April Fools day is weeks away. Any interest in construction details?

H.H.
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Old 7th March 2002, 03:35 AM   #2
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lets see the details.....
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Old 7th March 2002, 04:08 AM   #3
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Default Well......................?

always willing to take a look


10kv
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Old 7th March 2002, 09:54 AM   #4
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Is this based on Rod Elliots "Fake Law" passive pre? I have heard some pretty good things about this.

dave
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Old 7th March 2002, 10:53 AM   #5
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Do tell, im interested......
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Old 7th March 2002, 01:35 PM   #6
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Post Passive

No, the one I built has a series resistor (two 22Ks in parallel) connected between the signal pin of the input RCA jack and the wiper (center) leg of the pot. The output RCA jacks center pin is connected to the wiper leg of the pot. Only one of the other legs of the pot is connected to ground. The correct leg is the one that gives a very low resistance between the center leg and it when the pot is rotated fully counter-clockwise. The third leg of the pot is unconnected. So we have an 11K resistor between the signal pins of the input and output RCA jacks and the pot across the
signal to ground of the output RCA jack. The input and output jack are mounted next to each other for each channel. I left split washers out and soldered the ground wires directly to the threaded part of the RCA jacks. You could use the 2% silver solder if you are worried about melting the insulation on the jacks or plastic case. Sounds good, but not not as good as 4% silver solder but is much easier and lower melting point. I often tin the part with the 2% and finish with the 4%with very good results.

The pots are 100K audio taper Alphas from Radio Shack. The wire is 30 guage wire wrap wire with the insulation removed and the solder is 4% silver lead free, both also from R.S. I used box for displaying hockey pucks and cork feet from the Container Store.

I think the clear case adds to the transparency of the design. Okay that part was a joke but the innards are much cooler to look at than a hockey puck! I guess I am an audio voyeur. My audio designer friend and I send each other "Naked Amplifier Pics" from time to time. This is the most audio bang you can get for 20 bucks and is easy to build and find parts for. Sort of like Fast Food for gormet taste. I will be very dissapointed if this doesn't become an audio cult classic. Plug in those soldering irons, head to the Shack,and lets get some feedback on your results. I will try the Rod Elliot Passive for my next project and report on the results.
H.H.



http://sound.westhost.com/project01.htm
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Old 7th March 2002, 02:09 PM   #7
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Lightbulb Passive pre

lets see the details..... Well, I thought that was the point of the clear case. Top to bottem: Right out Right in Left out Left in

A better view of wiring for passive.



H.H.
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Old 7th March 2002, 02:30 PM   #8
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Angry Passive pre

The reds should be on left and whites should be on right for you
that pay attention to such.... Bottom leg not used on pot to left
and top leg not used on pot on right.

H.H.
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Old 7th March 2002, 03:16 PM   #9
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Harry...the volume control arrangement you have...If I understand things correctly, are you are passing the signal to the amplifier through the resistors and controling volume by shunting some of the signal to ground through the pots?

Is this kind of arrangement as good as using a stepped attenuator?

Thanks
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Old 7th March 2002, 03:36 PM   #10
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Thumbs up Passive

Give the man a gold star. You got it.

It depends on the quality of the stepped attenuater contacts and resistors used. I hate the limited resolution for volume steps, the feel, and the COST of most stepped controls. I will put my 10K Caddock resistor/Alps100K Black Beauty Pot control up against any of them.

The pupose of this R.S. project was big bang for small bucks. I will post some other ones for deeper pockets later. Go build this one and you will be amazed at what the price of a good meal will get you. I use it with my hot rodded Aleph 3 and it sounds great. The point is to go try some easy projects and learn by listening. We can talk all day on forums but it won't tell you what something sounds like. This is a more sophisticated design than you might think. I have built more than a dozen passive preamps including a custom for a mainstream reviewer and I am not embassed by this Rad Shack design. Twenty dollars and a couple of hours is not a big investment to find out.....

H.H.
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