MEQ mid-range EQ pultec style (was meq500)

[ioaudio]

NOTE 21.7.2010:
this thread was about the now discontinued meq500 project. i have no parts available anymore.
this thread provides relevant information about the meq equalizer and info will be available for the future.



updated meq5 filter schematics:

prototype @ work , here with two frequencies added at 7 and 10kHz (1nF + 680pF @ 477mH)

two pics showing transformer wiring, inductor orientation and switchboard wiring.
please note that it doesnt show the bypass (switch in a resistor pad instead of the filter network) and the wiring for the switches (simply connect 1-1, 2-2 etc)

watch out when handling with the output transformer - as soon as you take out the screws, the laminations become loose - in case one pokes out simply push it back in. do not over-tighten the screws/nuts for the transformers, i included self-locking nuts.

the orientation of the inductors isnt shown on the pcb, so please refer to the pic below - the pie slice-ish cutout from the black bobbin is crucial.
L = low boost M = mid cut H = high boost

some measured filter curves:

suggested frontpanel

http://twin-x.com/groupdiy/albums/userpics/meq500_3.zip

frontpanel with bandwith switches:

http://twin-x.com/groupdiy/albums/userpics/meq500_3_bandwith.zip

some kind of bom:

9 x 0,08€ resistors metall film 47 Ohm | 270 Ohm | 510 Ohm | 620 Ohm | 1 kOhm | 1,5kOhm | 5,1kOhm* | 22kOhm* | 100 kOhm
3 x 2,50€ gold rotary switches 1x12 (optional lorlin 1,67€/pcs)
2 x 3,03€ potentiometer pipher 16mm (optional alpha 1,28€/pcs) 10k lin +2,2k lin axis >= 30mm
2 x 0,10€ diodes 1N4004
1 x 0,21€ panasonic FC 47µF
2 x 0,49€ panasonic FC 470µF (noname 0,20€/pcs)
1 x 0,89€ panasonic FC 1000µF (noname 0,20€/pcs)
1 x 1,19€ panasonic FC 2200µF (noname 0,46€/pcs)
2 x 0,13€ mkt film 0,1µF
= 17,98€ (10,02€)

CAPACITORS
for filter section - your choice. starting from ~6,85€ for noname films, ~14,65€ for wima, 36,50€ for mallory etc. pcb is single sided for easy replacing/de-soldering and accepts

a lot of footprints.

onboard OPAMP parts (not needed when 3rd party opamp is used)
2 x 0,39€ transistors BD139 / BD140
1 x 0,33€ ne5532p
2 x 0,10€ 3mm led standard red
4 x 0,08€ resistors
= 1,55€

cosmetic:

3 x 0,93€ Eagle knob 17mm knurled pointer
3 x 0,95€ Little Wonder, Black 13mm
= 5,64€

1 x ~ 28,00€ front panel (schaeffer/front panel service )



every frequency band can be extended, or you can add in-between frequencies. i added a solder pad for each inductor value so you can easily add some caps.
here is a calculator which is very handy for this:
http://www.pronine.ca/lcf.htm

e.g. 4,7µf on low boost with 520mH tab = ~100Hz

 

[Denyle Guitars]

Thanks.

I must have missed the bit about bandwidth switches. Do you have any more info on that?

 

[emrr]

The mid cut chart shows the 270 ohm resistor bypassed for minimum bandwidth, which also gives a deeper cut.  The two boost positions do not have a similar series resistor, and are thus minimum bandwidth already.   We will have to experiment with adding a resistor to get a wider bandwidth, and that comes at the cost of maximum available boost.  I don't know that anyone has played with resistor value in either boost position so far.

EDIT - actual bench test results say walk away:

http://www.groupdiy.com/index.php?topic=27964.msg496474#msg496474

 

[ioaudio]

thanks doug - exactly, just switch a resistor in series with the inductor, find out experimental which curve you like best.

-max

 

[Denyle Guitars]

Thanks Doug. I did see the 270R but I wanted to make sure the high & low bands were implemented the same way.

I got my boards today and have already started tearing into them. Great work on the kit.

 

[emrr]

Okay, after several hours of searching I've yet to find a US source for a 2K-2.5K linear pot that will fit.

What has anyone else found so far?

The Alpha 16mm at Mouser are the only ones I'm finding in 2K, and the shafts are too short for the application. By my measurements, the supplied pot (about 28mm/1.0625" shaft extension from circuit board face) is a minimum length. Mouser does have the Piher PC16 series with a 45mm shaft length, but only in the 10K linear.

Starting to look for EQ caps next. None of my lovely vintage types will fit the tiny 500 series footprint, which will at least save my the headache of checking antique tolerances.

 

[Denyle Guitars]

I was planning to use some Relcap RT but I think the larger values won't fit well.

I haven't look yet for the pots/switches. That's coming next.

How about this one? http://www.mouser.com/Search/ProductDetail.aspx?qs=RE5i0Vhyk30e0Eg1a1Fxvw%3d%3d

 

[emrr]

[quote author="Denyle Guitars"]
How about this one? http://www.mouser.com/Search/ProductDetail.aspx?qs=RE5i0Vhyk30e0Eg1a1Fxvw%3d%3d[/quote]

The diameter of the back appears too big for starters. The pots will have to not extend past the edge of the circuit board at all, if going in a 500 rack.

Clearly there are some European options, but I'd like to find them here if possible.

 

[ioaudio]

hi everybody i´m back
so, if there´s any questions let me know.
cannot answer the question where to find the pots in the us, but if you use the 16mm alpha you probably use a 4mm-->6mm adapter with which you can extend the axis. i didnt try that though, dont know if it fits.

-max

 

[emrr]

Max, where have you sourced your pots? Is there a single shop that sells both of the required types? I looked at www.banzaieffects.com, but couldn't see any real details about shaft length. At a glance those 4mm-->6mm adapters look like they are for shaft diameter expansion only, rather than extension. The shaft extenders I have are all too long to fit in the available space.

 

[ioaudio]

yeah, maybe the adapter is nonsense.
i got mine from somewhere in GB , the brand is radiohm.
i´m afraid the 16mm alphas are too short
this piher from banzai fits:
http://www.banzaieffects.com/Piher-4mm-Pot-Mono-2-5k-lin-pr-18830.html

 

[emrr]

Some capacitor questions, for clarification and/or correction.

Running the numbers to find the precise values to really nail the frequencies (in theory), I am coming up with the following bits which are radically different from both the circuit board and the Gyraf schematic.

target freq. - actual C - mH - specified C=actual freq.
mid cut
7k - 0.0152041 - 34mH - 0.01=8631Hz
5k - 0.0298 - 34mH - 0.015=7047Hz
4k - 0.0465625 - 34mH - 0.03=4983Hz

Anyone have comments or observations?

 

[Denyle Guitars]

What would be the effect of using a 10k potentiometer for the low boost? Or a 1k? Those are all I can find.

 

[Rob Flinn]

Max

Can you post the diagram for the make up amplifier ?

 

[ioaudio]

wow - i have missed some of the questions - sry!

emrr, you are right - but it would be very interesting to have a filter sweep from the original - frequencies on the frontpanel sometimes do not match with the calculated/measured lc- resonant points. thanks for finding out!

denyle, i didnt try different pots - better find the specified.

rob,

the make up amp was born on the bench, when i found out that there´s some space left from the output transformer cutout - so i´ve never drawn a schematics. but it should be clear from the little pcb - if needed, i can draw it.

-max

 

[Rob Flinn]

[quote author="ioaudio"]
rob,

the make up amp was born on the bench, when i found out that there´s some space left from the output transformer cutout - so i´ve never drawn a schematics. but it should be clear from the little pcb - if needed, i can draw it.

-max[/quote]

Max

I didn't mean the amp itself, I wanted the caps& stuff around it so I could work out the voltage ratings.

 

[ioaudio]

25v electrolytics are fine.

 

[Denyle Guitars]

This calls for a bump and a drink.

 

[emrr]

Awesome.  What pots did you finally source?  I've still not found any in the USA that will fit.

Is the FPE panel design unmodified from the original?  Did it require any changes?

Comments on the wide/narrow switches?  Worthwhile?  On all bands?

Please post any other changes or suggestions you recommend.

 

[Denyle Guitars]

Yes, FPE file, unmodified. I'd change the lettering from black to white and rotate the In & Out so they're above and below the switch (the switch only fits vertically). I had to massage the L bracket holes about a 1/4 inch to get everything lined up. The color is bronze, which turned out darker than expected. You might have to slot the four holes for the pcb also. Mine fit the 500hr rack that I keep in the shop for testing but it bottomed out (just barely) in my BAE racks. OSA & API racks are also slightly different, as I discovered with a S&M EQ.

Regarding the Q switches, it depends on the source and settings, whether they are noticeable or not. I used 270R on each and haven't experimented with other values yet.

I used a 10k Piher for bass, treble boost and changed the 510R, but I bought a 1k also that might work better. Those were the only values I could find. I mis-ordered the pots and was sent the ones with switches. I decided to wire them as a bypass for the high and low bands but I couldn't hear any difference.

As you can probably see, I used mostly Wima, with polystyrene in the high bands. I used a 2520 opamp. It sounds nice but I haven't done any testing beyond running a guitar through it and turning knobs. Everything worked as expected on the very first try, which is always a pleasant surprise for a hack like me.

I miscalculated the bypass resistor so there's at least one thing to change.