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Old 2nd September 2010, 09:14 PM   #1
geminni is offline geminni  Yugoslavia
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Default Peamplifier and active crossover, working project

Hi!
I'm making a new preamplifier and active crossover for my music studio. I have made one that I have for about 8 years. It is standard preamplifier with 4 inputs, volume and balance pot. It is ok but i need something better and with only one input.
Enclosure for preamplifier will be case from Teac cd rom. Active crossover will be in second old Teac cd rom case

For now i have a picture done in Corel Draw, how will it look. Sides will be done with red acrylic, front and rear will be laser engraved with black gravoplast 1,5mm so the letters will be white. Top will be sprayed in black.

The non standard addition will be 12 volt for relay which switches main power supply for all equipment, speakers, amplifiers... This I'm using for many years
and is so much easier to switch only one button I made this years ago when I shared the computer. Back then I already had many components that needed to be switched on every time, which was not problem for me but it was for others, so I made this simple switch that does it all in a same time!

Preamplifier is designed so that everything is on one board, except pots. I used multiple schematics, separate for buffer, for preamplifier, power supply...
I need to draw it so I can post it. All Ic-s are NE5532.

I'm making the board with laser printing and iron. I have made one board, drilled all the holes and then I have discovered that something is wrong. It turns out that the print was smaller! It was PDF, by default it shrinks everything to fit on A4 format. I have told the girl working with laser printer for that, but she said it was ok. It was NOT ok! I was so mad at first. I did all the work for nothing.
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Old 2nd September 2010, 09:17 PM   #2
geminni is offline geminni  Yugoslavia
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Active crossover is variable 12db with frequencies form 50hz to 150hz. Both filters have same value, 50-150hz and are separate for left and right channel. I don't have dual stereo pot, only basic stereo pots so that is why there are 4 pots
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Old 2nd September 2010, 09:50 PM   #3
geminni is offline geminni  Yugoslavia
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Here is the schematic of a preamplifier. Original schematic is Bora's Allegro. Original input cap is 220 nf but i already had 1uf.
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Old 3rd September 2010, 10:24 AM   #4
Artie is offline Artie  United States
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The schematic isn't showing up for me, but the design looks nice. Good luck.
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Old 3rd September 2010, 12:48 PM   #5
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You might want to add a level adjustment on the highs if your amps do not have input level controls.

Any particular reason for your op amp choice? There are lots of alternatives generally said to sound better.
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Old 3rd September 2010, 12:56 PM   #6
AndrewT is online now AndrewT  Scotland
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adjustable crossover frequency is often done using subtractive filters to avoid the complication of trying to get pots to match over their whole resistance range.
But, do some reading on subtractive. I don't like them. They bugger up the slopes in the HP section.

An alternative is to use switched resistances and/or switched capacitances to select different crossover frequencies.
Look up MOX to see extremely detailed design and construction ideas.
There have been a variety of PCBs following this idea, some designed as experimenters toys/design tools and others that are very much simplified stripped down modules linked to give an enormous variety of slopes and Qs and EQs.

And read up on using jFETs as unity gain followers around which crossover filters can be assembled (just like a unity gain stable opamp used as a follower). There are at least two threads discussing successful projects using this solution.
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Old 5th September 2010, 09:19 PM   #7
geminni is offline geminni  Yugoslavia
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One of the goals for this project is to use components that I already have, and to keep it simple. That is why I will use NE5532. Maybe later i can switch to other op
I did not know about those subtractive filters but i get the concept. My crossover is made like many schematics I have found, and I have made many of them over the years. Some 12 years ago I have made my first variable crossover with trip pots that was very hard to manage and set. I want to make something similar but with pots instead of the trim pots.
My system has main speakers, Alesis M1 Mk2 active, and two subwoofers with Quad405 as amplifier.

Here is the schematic for preamplifier
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Old 5th September 2010, 09:50 PM   #8
r0b is offline r0b  Germany
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Hi, I am very interested in this.

Exactly what one would need to build an active Sub-Sat System.
What I would miss is a level adjustment of at least one output (Hi or Lo) relative to the other (I could go without the balance and instead put that in the crossover as a fader between sub and sat) and maybe a summing thingy on the Lo out.

Please go on with your explanations!
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Old 6th September 2010, 09:27 AM   #9
geminni is offline geminni  Yugoslavia
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This setup is not so much intended to be universal, it is for my needs
Preamp has balance pot because sometimes I like to see channel difference manually. Crossovers do not have volume pots because amplifiers already have one.
I had the idea of making one subwoofer with its own filter but I already have two identical passive woofers and one stereo amplifier. Also, I did not liked the sound with just one woofer. I tried placing it all over the room but could not find good place for it. With two woofers sound is more linear, well in this room.
I like full stereo separation, that is why I make crossover and woofers like that.

This week I'll make front and back face plates for preamplifier, black with white letters.
I got my camera back so I can post some progress pictures.
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Old 7th September 2010, 03:30 PM   #10
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I'd lower the resistances on the +inputs to ground to something approaching the resistance seen by the negative input. This will reduce resistive noise and DC offset. At nominal input bias currents you wouldn't think it would matter, but in my experience with NE5532s in active filters, even 10K on the inputs resulted in >30 mV DC offset. I cured that by swapping in OPA2134s.

When designing your filters, I'd recommend sticking with resistances somewhere in the 2-3K range when possible.
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