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3-Band EQ


Therezin
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Hi all,

I know there's a bunh of different threads on here about mixers, and I'm thinking of building one for when I get my MBSID going with 4 mono outs. The mixer will be 4-channel mono to 2-channel stereo, with individual track panning, gain control (hopefully) and 3-band EQ. At the moment, it's the EQ that's troubling me. I intend to create something similar to http://www.mitedu.freeserve.co.uk/Circuits/Audio/3band.htm.

Using Eagle Light, I came up with a schematic based on that one, but the connections are confusing me a little. Could anyone shed some ligth on the subject?

I assume that the 2 terminals on the left are the audio input bus, but it's on the right that's confusing me - there appears to be one output terminal, 2 earths and a V+ supply, connected to one of the earth terminals with a 100uF cap. Which of the earth terminals (if not both) do you think would be connected to the output jack?

I've annotated my Eagle schematic and put it on my website - www.danielprice.org.uk/misc/toneControls1.jpg - with what I think might be needed. note that JP1, JP2 and JP3 are the pin headers I will connect my pots to.

Also, have i got the pins of the LF351 right, since the diagram only used 5 pins? I have no experience of op-amps (yet).

My Eagle schematic is also on my site at www.danielprice.org.uk/misc/tone.sch. Feel free to edit it, just let me know!

Many thanks in advance,

Dan.

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Dan,

That diagram you found does look a bit "empty". ;)

If you haven't already visited this, and you're not set on that first circuit, you may want to skim through:

http://www.epanorama.net/links/audiocircuits.html#eq

Their circuit links are usually a bit more friendly. I don't see any "EQ looking" example schematics in that datasheet. Not to say that it might not sound good, or isn't ever used for that, but there are probably other small circuits using more common stuff like 5532's, 741's or 4558's which audio people have tuned up to sound good.

Take Care,

George 

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That diagram you found does look a bit "empty"

Hmm, looking at some of the others on the net, I'd say you're right. I've been looking at the active 3-band Baxendall from http://www.headwize.com/projects/showproj.php?file=equal_prj.htm, and that seems to be more useful for my application - it's active, for one!

Am i right in assuming that the left-hand earth contact (IC1 pin 3) is connected to the earth on the input bus, and the right-hand earth (IC2 pin 3) is connected to the output bus' earth? Or would they be connected to each other as well?

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The baxandall tonestack is well liked for a simple active eq as it's quite effective.

However, you may want to look at some of the values to bring the peak frequencies down slightly.

Since the SID only outputs up to around 4kHZ, the hi-band of the baxandall will be working on harmonics alone, so will only really be effective for square wave type sounds. I'd probably change the center frequencies to something like 60Hz, 500Hz and 4-5kHz. I think that should suit the SID better.

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OK, I haven't done the math with the resistance values yet (thinking of using the same values as in the schematic - I'll only have 2-band EQ on my SID, but I can use the mixer for other stuff if i want), but I have come up with the following circuit. Apologies for only having the board, but I made the mistake of editing the board when I discovered an error, instead of the schematic. I used the OPA2134 instead of a pair of OPA134's.

Board GIF:

3band-OPA2134.gif

http://www.danielprice.org.uk/misc/3band-OPA2134.gif

And the .brd file itself:

http://www.danielprice.org.uk/misc/Baxendall-3band-3-OPA2134.brd

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Man thanks!

Always looking for something "ready to etch". :)

I'm trying to throw together some little stereo to mono adapters, phone amps, and mixer things for convenience in different places. Tone controls might make a cool addition somewhere. Mine wouldn't be for SID's, so I'd probably need alternate values.

I take it that's the "ACTIVE 3-BAND BAXANDALL" from that page you liked?

George

PS- I've been working from the board editor most of the time myself. I'll often use a schematic to get my pin to pin wiring done, but sometimes delete or move it after I start playing with the board routing, to avoid all the "out of sync" errors from Eagle. I also tend to use pin headers and DIP sockets for things that I don't feel like finding library items for, and just wiring them to what I need in the board editor. The schematic view probably wouldn't make much sense there either. 

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I may be a super-noob in these areas, but why does the connection between c3 and c4  run between the legs of the chip? That seems like some auto-routing gone bad! And if you take the "trace" from pin 6 of the chip underneath the chip instead you can do without the wire bridge by bringing pin 5 around the audio output?

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@Jidis: The values in this are just general values as specified in the link, and you're correct it is the "active 3-band Baxandall". And yeah, board editor rules  :P

@Machinate: You're a super-noob? that makes 2 of us then! ;D

You were right about the auto-routing gone crazy... I've updated the board designs with your recommendations, no more bridges!

http://www.danielprice.org.uk/misc/3band-OPA2134-rev2.gif

and

http://www.danielprice.org.uk/misc/Baxendall-3band-3-OPA2134-rev2.brd

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With all due respect, is there a reason you've made the board so big?!

It looks to me like a serious case of 'autoroute gone wrong'  ;D

Maybe it's my roots being in the stompbox field (where everything MUST fit into a hammondBB), but I could live with a much smaller laybout.

Basically i'm just wondering if there's a reason for the size.

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The image is just a screen dump from eagle at full zoom. As for the board itself, well, I'm not the best in the world when it comes to soldering and space in the case I'm gonna build in isn't really an issue. For more confident solderers the board could probably come down to around an inch wide, but I'd rather make a fairly big board I know I can solder than a really efficient one that I can't.

Feel free to miniaturise it, just post your .brd file so I can have a look  ;D

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