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Old 1st November 2005, 07:34 PM   #1
MPW is offline MPW  United Kingdom
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Default The Simplest Pre-Amp

Hello everyone, this is my first post and i'm sure i will offend someone, but can i assure you that i have searched for an answer to this question for about an hour (im on dial up!). All i want to know is - is the most basic of all pre-amps just a log pot with an input and output? Or am i missing something here. If so can someone recommend a good pot? Apologies in advance for my lack of understanding and inability to ever find anything, especially keys.
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Old 1st November 2005, 07:40 PM   #2
dhaen is offline dhaen  Europe
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Hi, welcome to the forum
You are right, a pot with input and output is the simplest. Conventional audio wisdom says a log pot. Some will try to convince you that a linear pot is better; my ears have a log loudness characteristic...
Whether such a simple arangement is satisfactory will depend on a number of factors including impedance, lead length and whether any gain is required. Choice of pot value is important too.
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Old 1st November 2005, 07:52 PM   #3
adason is offline adason  United States
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Quote:
i want to know is - is the most basic of all pre-amps
hi MPW,

the pot itself can not classify as pre-amp
the word "amp" assumes amplification
amplification happens when signal comming out has higher voltage than signal comming in
the pot can only attenuate the signal, hence classifies as pre-attenuator
properly set up transformer could classify as pre-amp, passive preamp that is
there are quite a few passive preamps out there based on trafo's
some pretty expensive
ed
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Old 1st November 2005, 08:21 PM   #4
dhaen is offline dhaen  Europe
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Quote:
the pot itself can not classify as pre-amp
the word "amp" assumes amplification
Indeed there is a sematic inacuracy, but it has got(ten) into our general language, like using the word "battery" for one cell.
Most of us use the term preamp for the control unit prior to the amplifer that is used for controlling it, whether it has gain or not.
Imagine a box with a volume control on it which said "I'm a preamp when my control is at 60 degrees or more, but an attenuator below"

Anyway, don't get confused, just enjoy the banter of the forum.
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Old 1st November 2005, 08:42 PM   #5
adason is offline adason  United States
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i did not mean any criticism, i was just thinking out loud

anyway, MPW, best pot is no pot, just stay away
seriously
make it "switching attenuator"
linear or log, does not matter much, just decide input resistance and number of steps
there is ton of web pages about it, just google it
enjoy the music
ed
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Old 1st November 2005, 10:51 PM   #6
anatech is offline anatech  Canada
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Quote:
The Simplest Pre-Amp
In this spirit I have to say, use a decent log volume control. You can put a gain stage in front of it if you want (op amp). Use an op amp, or buffer after the volume control to drive the cable properly. The gain should be around 1 (no gain).

The hard part is the power supply and grounding. An input switch will make this much more useful. After you get this working, you can expand or improve it as your needs dictate.

-Chris
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Old 2nd November 2005, 04:42 PM   #7
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You can make a simple preamp using an LM324 quad opamp chip or similar.

I made a mono preamp that has a variable freq filter, so it can filter out voice and treble to drive a sub, and is switchable to full range. Only uses a single chip, and two POTS.
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Old 2nd November 2005, 05:50 PM   #8
anatech is offline anatech  Canada
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At least use a TL074!

Your point is well made.

-Chris
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Old 2nd November 2005, 06:25 PM   #9
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This hit a nerve. I get very annoyed when I hear people that ought to know better refer to a non-active electronic device like a pot a "passive preamp" when "adjustable attenuator" is the correct term. They may argue that any device installed in front of the amplifier may be called a preamp. OK, so a turntable has now somehow become a preamp. In this line of casual dumbed down thinking a resistive attenuator between a power amp and a speaker must apparently be called a "passive power amp". I guess its time for another snake oil outfit to make a bundle marketing simple speaker cables as passive power amps! I mean, you'd expect to pay more for an amp than a piece of wire right!? Backing the car out of the garage on the way to buy an amplifier at a stereo store could also be called a preamp. Rant off.

Glad to see there were some who posted above who also see the stupidity in this.
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Old 2nd November 2005, 06:29 PM   #10
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There have been threads about this before. Yes, the term "passive pre amp" is strupid and technically wrong. However, the term has been around and used for at least 15 years, so I am afraid it is well established whether we like it or not.
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