fixed frequency 3 band stereo eq anyone?

OK, so the "dj mixer" thread seems to be dead-but I do wish to build one (4 stereo channels).

For now I would like to get my head around the eq section:

Most of the eq threads here, if not all, focus on the typical parametric design with selectable frequencies (which I hope to explore in depth in a "sontec build"), and seperate controls. For this project, however, a simple three band fixed frequency design, with combined stereo controls, is all that is required-remember this is part performance tool and simplicity is key, but most DJ mixers sound like crap! Some vintage eq modules (telefunken 395a, siemens 295?) were partly or totally fixed frequency. This is along the lines of what I am thinking. Say 60 or 80hz shelving bass, 700k or 1k ? bell mid, and 12k or 14 k shelving high? Only about +4 or +6 db eq needed, with more like -12db cuts. In the previous thread the Bozak mixer was mentioned, and I believe it has a low and high range eq........any experience with this design?

I apologize in advance if I am re-treading well documented subjects. I just wonder if there is an existing, available, economical design I might use or work from to accomplish this in a compact size. desired price point is ~ $400 in parts/supplies for 4 stereo eq modules. I admit to being too inexperienced to sort this out myself, so if anyone has some ideal laying around it would be a real help.



Erik

> I apologize in advance if I am re-treading well documented subjects.

I remember when boards had no EQ, or 2-band EQ, and some great records were cut on those things. I still love the midrange on the early BiAmp mixers (wish I knew who stole mine).

And 2-band/3-band EQ is just a hi-fi tone control.... the world is full of those, and I expected the Web would be riddled with 3-EQ designs.

Yet I'm not finding much, and what I find is over-designed.

So I'm just kicking this thread along in hopes someone else jumps in and shares.

Personally, I've gone over to VERY gentle wide-band slopes, often 2dB/decade, plus the off 4dB boost or steep cutoff at the extremes. I've grown tired of hyper-EQ. But in performance, it sure can be handy to have some mild choices.

[quote author="PRR"]I remember when boards had no EQ, or 2-band EQ, and some great records were cut on those things. I still love the midrange on the early BiAmp mixers (wish I knew who stole mine).[/quote]

I agree, absolutely. I know most el-cheapo DJ mixers seem to need eq because of their wretched sound.

When mixing bass heavy material it is helpful to be able to cut the low end on the new track being mixed in. That said, with high quality attenuators/faders to mix with (with out any "curve") then a smooth transition seems pretty do-able. I have never really had something like that to try out, however.

Also, since dj/producers insist on such high recorded levels on their vinyl (I know since they comprise most of my vinyl mastering clientele thus far......can't wait to break into more musical circles.....) the HF is often severely filtered, and a little boost can on occasion be nice.

As to the mid-band-maybe it is uneeded...... OK, so this isn't what most on this list are into. My primary focus is my studio gear-but hey even DJ's should have some experience with better sound, no?
(I have to have a DJ TT in my studio to preview how a dubplate, or ref, will play on a "typical" deck- and thought it would be cool to build a mixer with balanced outs to better integrate into my monitoring......)

Well, if nothing comes up I may do a minimalist mixer with no tone control and check that out. Thanks for the consideration.

Erik

I was talking with a friend of mine who uses these things a bit back. I think the poster is referring to basically a line level crossover with a rotary gain with 6 to 10 dB of gain in hand and full attenuation on each band. My friend was pissing and moaning that one or another model wouldn't cut all the way even though it was labelled -infinity. Shouldn't be that hard to cobble something up. Two high pass, two low pass, attenuators and a summing bus. The Rane 2016 seems to have it's eq labelled to "off" so maybe that's a good starting point.

http://www.rane.com/swf/mp2016afp.swf

Schematic here
www.rane.com/pdf/xp216sch.pdf



That Bozak from the other thread was a two band, by the look of it, and more like a "normal" hifi eq. Finding my 1620 schem. lets me say the same for it.

So maybe I need to be chasing a design for high and low pass filters for each stereo channel that have +4db gain?

Sounds reasonable. what kind of curve would the filter present on the "gain" side-same as in cut? (if so that seems to be a pretty radical shelf to me.)

Thanks,

Erik

If that's the sort of eq you're looking for, then that Rane schematic may not be a bad place to start for a basic signal flow. It could probably use a bit of sprucing up.
The gain can be had at the summing amp. Hell, I suppose you could do the filters passive. I'm afraid you'll have to wait for one of our resident genii to to help with the math. I'm more of a "swi", er, "borrow some ideas and poke it with a stick 'til it works" kinda guy.

Dolby made a box, the 740 "spectral processor", that was sort of along these lines. It had sweepable xover points, and upward compression.


Think of a crossover response curve. It just lifts or cuts each band separately. Single pole filter, 6dB/8ve.


Go really nuts and stick one of Kieth's pwm limiters on each band. :wink:

Yes, it's clearly time to commence to drinkin'

*off to bar*

PRR said

I remember when boards had no EQ, or 2-band EQ...

Click to expand...

Last year or so, maybe two, I met a guy with an old brown Quad Eight that he'd gotten in a package deal. Two band, Mowtown style, MM51s I think. The poor thing would have taken a gargantuan effort to really get going again though. I wanted to at least hear some racked up.

Paul Stamler (new guy here) published a very simple 3 band eq that I think is exactly what you are looking for in the November 1998 Recording magazine. It is part of his solid state mic preamp that appeared in the October, November and December issues that year. It is a simple 2 op amp design that uses OPA604s whose outputs are biased into class A. I've built the circuit several times. It's affect is very subtle and not aggressive at all. Perhaps if you PM him he can be lured to copy you on the circuit.

[quote author="Carl_Huff"]Paul Stamler (new guy here) published a very simple 3 band eq that I think is exactly what you are looking for in the November 1998 Recording magazine. It is part of his solid state mic preamp that appeared in the October, November and December issues that year. It is a simple 2 op amp design that uses OPA604s whose outputs are biased into class A. I've built the circuit several times. It's affect is very subtle and not aggressive at all. Perhaps if you PM him he can be lured to copy you on the circuit.[/quote]

I cribbed most of the design from Walter Jung's Audio IC Op-Amp Applications; I used one opamp for bass and treble shelving, a separate one for mid bell-curve. Since each stage inverts, using two means I can switch the combination in and out without changing signal polarity.

I had the LF hinge at 200 Hz and the HF at 2kHz, the same as Jung, and I find those work for my somewhat limited applications. Unusually, I put the mid EQ at 1.8kHz, for the express purpose of pulling down a squawky quality in Neumann KM-84s, which I find sound a lot nicer with 1.8kHz pulled down a dB or two. All of the frequencies can of course be changed by scaling the capacitors accordingly; in the case of the mid EQ you should scale both capacitors by the same factor.

The output transistors are biased at 2mA apiece. If you're planning to drive lower impedance loads, you may want to juice up the bias on the second amplifier. Bias current ~= 0.6 / Re, where Re is the resistor on the emitter of the transistor at the output of the opamp that Jack built. You can do this with a single OPA2604 package, but if you run the second amplifier hot you should probably use separate single-amplifier packages to avoid cooking the chips.

Here's a GIF of the circuit:

http://members.aol.com/pstamler/EQ.gif

Peace,
Paul

I take it the OPA604 is an approved opamp?
Stephen

i'm a nobody but i put my stamp of approval on them!

i love the 2604 for duals, or I go wtih 627 for singles..or even two 627s on a small board for a dual opamp, but that's just overkill..

Well, I seem to have 10 OPA627s and a few 604s so I'm up and running!
Stephen

My second desk is an old soundcraft 200b, which has 4 band fixed freq eq. Many people seem to think this is a limiting feature, but I find it mostly does a good job. I think that the frequencies chosen are quite musical, & if I need scalpel type precision its easy to insert a more flexible eq. For most tracks you might just be rolling off a bit of top or adding bass etc & you don`t need too much control.

If you`re interested in looking at the diagrams PM me with your email and I can send you them.

My first decent mixer was made from junked 200b channel boards which were given to me by a guy who used to work for Soundcraft - it's all in bits now though.
Stephen

Unfortunately my 200b is in a flaky condition. I have to slap it about to get some of the channels working. I think much of it is to do with the shite switches they used. I need to give it the once over but it not too high ton the list of priorities !

[quote author="aurt"]

Think of a crossover response curve. It just lifts or cuts each band separately. Single pole filter, 6dB/8ve.

*[/quote]

I'm interested in this idea. It seems that it might provide the "infinite cut" that some dj's like, and a mellow gentle boost on the other end. Any idea where I might get a peak at the general design? If I could limit the control so that only a little boost is allowed then, hmmm maybe a go.

Otherwise, thanks to everyone you piped in with 3 and 4 band eq schemo's! I admit to being limited in my knowledge to adapt them, but if I stare at them long enough...........I may just get something.

I am so pleased to have found this resource. Perhaps I may be able to contribute more someday soon....................

Cheers,

Erik

Here's the relevant part of the Rane schemo I linked.



you could set the gain of the summing amp up to provide 3 or 6 dB of gain with all pots maxed out and then mark where unity falls on the chassis.

I plan to build a pair of fixed frequency eq's based around Fabio's 2520 boards (or maybe the Melcor 1731). My main purpose for these will be buss eq's. Just to do a small amount of shaping to a drum kit, or guitars. It's a little ways off, but it will definitely be a top priority this summer.

Hi Everyone,

sorry to dig this old thread up, but was reluctant to start a new one, as the topic is quite the same...

So i am working on a Fabric Astronaut clone of Bozak rotary mixer (you can find the project and all files over here : https://fabricastronautblog.wordpress.com/2016/07/24/pathos-dj-mixer-pt-1/), and i would like to tweak the EQ section to make it more intense (i dont need more boost, but more cut.)

I attach the schematic of the EQ section, and would like to ask you :

Do you guys think it is possible to modify the cut level in this design?
Which components should i focus on?

I just checked the docs and it seems to be +/-10dB at 50Hz and 10kHz.
So maybe i'd actually be also interested to know how to change the frequencies

(i guess its the left part of the schematics... with capacitors and resistors... just want to be on the right way and not try anything stupid!)

Thanks in advance!