MXR 1970 6 band EQ

[George Giblet]

FYI,  the later Jim Dunlop stuff is often referred to by the "M" number.   However, the old stuff was actually coded in this way too,  the numbers appeared on the PCB and weren't used in conversation as they are with the JD models.   I think it's clearer to refer to the later models as Jim Dunlop versions.  On the whole they tend to be close to the original I don't know what has changed on the 6-band.   A possible difference would be the opamps or opamp brand, which can affect the tone in this circuit.

[jakehop]

Adding to what George said, the capacitors in the old models also seem to be "above chinese mass production quality". Like good 100V MKT's.

Jake

[tcio]

I wondered how much those older caps might affect the tone. As far as I can tell, except for the caps used for the 6 bands they're all old color coded caps which are next to impossible to purchase these days. You may find one here or there if you get lucky. Every "old" MXR EQ I have opened up so far has these in them.

[wokeupfalling]

I am curious about those caps also...  I love the old MXR EQ, it's one of my most beloved pedals!

[Mark Hammer]

At the time the pedal was produced, if I wanted to order plastic caps from one of Canada's largest component distributors, that's what I got.  They were made by Philips.  Still have a bunch of them in my parts bins.  Never get around to using them because they end up being too big for the layouts posted.

They were better than ceramic.  Are they "better" than contemporary polyester formats?  Probably not.  Just more colourful.  I'll tell you this:  you did NOT want to struggle with the leads on those.  I had plenty that failed on me because the entire side of the cap crumbled when you wiggled the lead too much.

[tcio]

I seem to be getting somewhere here with the M109 schematics (thanks MUCH George!). I am only using a couple of bands for now and they seem to work fine but the response is minimal. I am a little confused on the boost/cut db part. I am definitely not getting anywhere near 18 +/- cut or boost. Any help would surely be appreciated.

I realize I could always throw a boost/gain section in but I am trying to capture the old MXR 6 band EQ original tone as much as possible. When I turn the pot for the specific band/frequency I can hear the frequency rise and fall like it should but without the 18 +/- DB boost/cut. There doesn't seem to be any boost or cut, just a frequency change. It looks like I am on the right track so far, just need to figure out why there is no 18 =/- db boost/cut 

[tcio]

This might help. This is what I put together so far:

[tcio]

For the life of me I cannot figure out what happened but I plugged into this thing today (M109 DIY) and it worked like it should. I lined it up with the old blue 1970s MXR 6 band EQ and the gain/potency was VERY similar although hard to tell if it was exact as I currently only have to bands/frequencies added onto the circuit. I am aiming to either just add tone controls/stack seeing I mainly only use the mid range frequencies. Hopefully I will get the same results as the original pedal for the most part, perhaps even better. I will post my result whenever I feel I have accomplished my goal. Thanks all for your comments and thanks again George for posting the schematics (still can't figure out how I missed those on geocities.com).

[George Giblet]

When you pull bands off the response doesn't quite match.  However,  what you can do is connect one 12k resistor from U2B pin 5 to N, and another 12k resistor from U2B pin 6 to N.  This (dramatically) improves the match.  What those added parts do is try to emulate the effect of the components in the removed bands   With that type of equalizer the bands set to 0dB still affect the other bands.   If you pull off less bands then you have to increase the resistor a tad.

You can make further improvements to the match if you deviate the part values of the band circuits from the original values.  This might seem counterintuitive but again it is because the parts of the circuit interact.

> thanks again George for posting the schematics (still can't figure out how I missed those on geocities.com).

No problem.  Because I have raw files on geocities the only way you can find them is from old links on this forum.  I'm sure I put it up in the past but quite often the forum search is a bit fussy and misses a lot of stuff.   To make matters worse that particular file got deleted from my site, so you would have to PM me even if you found the old posts!

[John Lyons]

I'm glad you are getting this worked out tcio.
I know you have been working on this for a while.
DId you use linear pots?

George comes through with the key info a few places around here. 
Many thanks.

john

[tcio]

DId you use linear pots?

I don't know for sure but I believe so. I use cermet pots (http://elexp.com/cmp_sts.htm) for all of my breadboarding. Probably not the most accurate way to go about it but it sure makes things easier and I usually get very close results when I compile the pedal and put in the correct pots (at least what I have put together so far).

I figured out what I did to get more output on this circuit. I increased the 10K (R3). I don't know if this is the best approach but it definitely gave it a nice boost in volume and gain. Otherwise it was neutral. No boost, no cut. Just a frequency change when turning the pots