Calculating EQ frequencies in a Yamaha M512

...or similar IC-based EQ. 

Hey, everyone!  There's a Yamaha M512 for sale locally and I'm thinking about picking it up to use as a bit of color & occasional summing amp.  The fixed three-band, IC-based EQ is the major drawback as far as I can see, and I got started thinking about how I might be able to change the frequency points and even Q if possible.  I have some background in tube electronics and do understand very rudimentary guitar-amp style tonestacks, but these 4558-based circuits aren't as intuitive to me since my op-amp knowledge is very, very limited.

I could just buy it and start swapping in different caps, but I'd like to be able to actually do the math if possible. 

Can anyone provide some formulas or links to some reference material? 

Here's the M512 service manual:
http://89.228.128.119/RemoteDownloader/files/20130125/31815a538f949b2962b6956e0d6cf759/M512YAMAHAServiceManual-EN.pdf

Thanks!

Steve

I think the 47uf, 0.056 and 0.022 caps that comes off the pot's will be the one to change frequency.

Adding caps in parallel to these(to increase total capacitance) will shift frequencies down.

However my knowledge is also limited, I would wait for others who know more  ;D

Humner, thanks for chiming in!

Yeah, I figure the same.  I did stumble on a gyrator bandpass filter calculator on muzique that's allowing me to mess with the math for the midband.  I still need to find something similar for the hi- and lo-pass filters. 

Right now I'm seeing that if I adjust the resistor in the feedback loop up, I get a lower frequency AND a lower Q, which I think I want, so I could replace the pot with a concentric pot to avoid carving up the faceplate and add a sweepable mid, albeit with some obvious limitations.  What I don't know is whether there are stability considerations with altering the feedback R that much--say, up to 50K.  I'd guess not, but again, I'm opamp-stupid. 

Same for making the resistor on the pos input a pot--I get higher F and lower Q, but are there limitations on the value of the resistor?

I have a 512 and thought about some of these changes but the panel space is kind of small for adding some of the extra switches but could be done. 

What about adding inserts for using some really strong 51x type eq.  Also the word is to replace the toshiba chips (TA7322) with more modern for a cleaner sound.  The thing is you need something that can handle 24 volt so that puts you into the jensen 990 territory.  There is a tread here about doing that.

I think the  direct out opens up more choices.  Adding something like the discrete circuit used to drive headphones in the schematic could be added to buffer the output.  May also need to add some gain before the buffer if your looking for +4 out.

There is a lot of info on Gyrator in Jung and Lancaster books.  I find you can do a lot by changing to 600hz mid freq.  just lower or dip that level and vocals sound sweet without having to boost top end that can get brittle.  point being the freq choices need to fit your needs per instrument more so than endless possibilities.  I would go 51x for that.

Fazer, thanks for the input!  Do you find the inherent coloration of the board makes it worth having, even if (as I assume) it's not an "all the time" sound?

I have absolutely thought about breadboarding some DOAs with 9-pin adapters to sub into a couple channels, but I'd be afraid about excess current draw if I do more than a few.  Two channels of 990 or 2520 would probably be welcome.  Granted, ultimately I just need to hear it, but for what I'm looking for sound-wise I think it's possible I'll like the slower 7322 thing. 

Direct outs would be great but frankly buffered outs would be as much work as some of the more minor EQ tweaks I'm envisioning; plus, I have no outboard EQ to speak of, and won't have the budget to build out that much for a while.  Certainly if I want to use the preamps in the future and then bring the outs from my DAW back post-EQ to the faders, I'd need inserts anyway, but that's a little more use than I think I'd get out of such a colored mixer. 

Thanks for the tip about Jung & Lancaster.  Forgive my ignorance, but are they co-authors, or two separate authors?


Any thoughts on my specific suggestions about changing that mid band to sweepable?  Can I get away with drastically changing those circuit values?

Hello
Eye-balling the circuit, here's what I see:
The high shelf freq seems to be set mainly by the value if the 0.022 cap.
You could switch to other caps for different freqs.
Low shelf is set more by the RC product of the 0.02 cap 100k resistor. Maybe you could vary the resistor to vary frequency. Keep the pot large, like 1 meg log, maybe some fixed series resistance and/or another cap value to get useful control range.

The middle band is probably tough to sweep without changing gain and bandwidth with pot rotation.. Changing the two caps for other values with similar proportion will give different frequencies. That might be useful.

I see you want to do the math. Here is how I would start:
Full boost middle, hi and low to unity ( or cutting, notice that the cut is 'passive'*, boost is active).

You have the frequency selective network (FSN?) in the lower leg of the feedback network of IC2. The transfer function is approximately 1+6.2k/Z.
Z is the fixed resistance in parallel with the input impedance of the FSN.

ZFSN= Zcs + Zg, where Zcs is the Z of the series 0.056 cap and Zg is the Z of the gyrator.
Zg is approx. Rf *(1+s*R*C), where Rf is the 620 ohm, C is the 2.7 nF, R is the 68k.
(Oh and s = j*2*pi*f, and j*j=-1)

Hope this gets you started!

* only the high cut is truly passive, mid and lo use gyrators...

VictorQ: whoa.  I love a post that I have to read 6 or 8 times!  I'm going to keep working on that over the next few days.

Also, your tip on varying the lo shelf is super helpful as well.  Thank you. 

Just wanted to hit on one thing you said and ask a follow-up question:
"The middle band is probably tough to sweep without changing gain and bandwidth with pot rotation."

Using the calculator I can see how drastically Q is affected by changes in either the 68K or 620 ohm resistor in that middle stage, but I'm not as clear about how the gain is affected because frankly I don't understand that much about gyrators.  I can parrot back that the opamp is a buffer, but isn't that just because R1 is so much bigger than R2?  If I were to lower R1 all the way to, let's say, 4.7K, to achieve a frequency of 7.5Kish and a Q of .6, wouldn't the gain of the stage rise from ~1 (1.009, I think) to ~1.13?

Honestly, that's not at all a deal breaker in a color piece, I'm just trying to figure this out. 


Now all I need to do is figure out whether there's room in there for some concentric pots.  ;D

Sorry, 'nother follow up question:

"Low shelf is set more by the RC product of the 0.02 cap 100k resistor. Maybe you could vary the resistor to vary frequency. Keep the pot large, like 1 meg log, maybe some fixed series resistance and/or another cap value to get useful control range."

Won't raising the value of that resistor raise the Q significantly?

Humner said:

I think the 47uf, 0.056 and 0.022 caps that comes off the pot's will be the one to change frequency.

Adding caps in parallel to these(to increase total capacitance) will shift frequencies down.

However my knowledge is also limited, I would wait for others who know more  ;D

Click to expand...

For the hi and lo shelf, you can get a total of 4 frequencies with 2 mini toggle switches that engage parallel caps of 2 different values. Use dual pole toggles for the mid band. It'll sound nicer than a sweep mid, which typically uses a state variable circuit with 3 opamps.

You could convert the mid band to an inductor EQ with a multi-tap inductor. That usually requires a dual-ganged rotary switch, rather than toggles. I'm not sure what happens if you connect 2 inductor taps at the same time.

gnsdy,  the books are Don Lancaster's active filter cook book has info on Eq/filter circuits and I was also thinking of Walter G Jungs Op Amp cook books and audio opamp books. Both are easy reads for quick info.

  Fenris has some ideas on changes have you tried those?    I looked in the Jung book and could not find a circuit like what is in the yamaha.  but you can tell Its a single amp filter circuit with a eq summing amp above with cut boost hung between + - inputs of IC2.  I've looked at books that suggest  2 opamps for a gyrator the yamaha is using 1 , IC4 a for bass and IC4b for mid and highs.  That is an resonate LC equalizer opamp filters instead of an Inductor / Cap.  Sorry I can't be more helpful on the EQ Mod.

Check out this tread if you haven't seen it.  M508 modded:

http://www.groupdiy.com/index.php?topic=50521.msg640465#msg640465

There is info on some other chips to substitute for the TA7322's.  That said,  I like the sound of this mixer for a lot of things because of the Headroom it has.  In the above thread they talk about fixing or changing the sum amps to a newer chip that runs on 24volt.   

Also I added inserts on the L R mix between the summing board and the Line out amps.  It worked great with some ADR compressors I have.  There was no buzz or frequency problems.  all runs unbalanced.  cost me 2 TRS Jacks for insert combo returns. (Get a pair of 160x dbx for dirt cheap) 

gnsdg said:

Won't raising the value of that resistor raise the Q significantly?

Click to expand...

The shelf circuits are essentially first order... No Q at all.
The thing to avoid is too low a resistor in place of the 100k. That will boost the highs a bit as well..

A 500k log pot and 22k in series would give a pretty good range..
EDIT:
Here's a picture (done with LTspice):

gnsdg said:

Using the calculator I can see how....

Click to expand...

Band pass (edit: hi pass) filters aren't really applicable, since it is not used as such, but rather as an impedance synthesizer.. A virtual RLC series combination. It's really more of a graphic eq mindset than parametric.

The problem with sweeping is that the absolute impedance level matters, so you'd have to change the C and the L simultaneously.. You can do that with switches, as discussed above.

Guys, thanks so much for all the thoughts and insight.  Lots to think about.  I am going to pick up the board, so as long as it's working as well as the seller says (I'll find out tomorrow!), these questions and considerations will become much less hypothetical.

Fenris: that's a good idea.  I was thinking about trying to stay away from drilling extra holes, but I'm over it.  That would be a really flexible way to get a bunch more frequencies... even 600 ohms (good call, fazer), 2K, 4K, or whatever would be a LOT more useful than 2K only. 

fazer, I'm SURE I will get to adding direct outs, so that is all good info.  Don't think I've read that thread. 

VictorQ: thanks for the sim!  That's looks actually VERY useful.  I did get a little confused about the last comment... you're saying I can NOT sweep the mid band, or the hi band?  I assume you mean mid, because that band does look pretty graphic-ish (I think the Q is like 2.3 or something).  The hi I could just use a switch to swap the cap, as you suggest, correct?