7 band passive eq!

[dano12]

I recently acquired a scanned PDF of an old out of print guitar projects book. Within its pages, I found a schematic for a 7 band passive EQ. The book states that the opamp in front provides around 20db of gain (depending on VR1) to make up for what I assume is a pretty big loss in the tone stack.

Any glaring problems with this before I take it to the breadboard?

Also, any help on figuring out the frequencies of VR2 through VR8 would be greatly appreciated....

[pazuzu]

dude that is fantastic. i have been scouring the internet for a passive eq. i can't wait to get my futurelec order (i know right? ) to try this out.

[frequencycentral]

I may be talking out of my hat, but wouldn't it be an idea to put a unity gain buffer at the output to isolate the tonestack from whatever comes next?

[dano12]

I may be talking out of my hat, but wouldn't it be an idea to put a unity gain buffer at the output to isolate the tonestack from whatever comes next?

Very likely. The book from which this was felched was about building electronics into your guitar, so the author assumed you were going to build this huge monster directly in the guitar.

Will have to play with it a bit when I get the time to see about the  output buffer issue.

[pazuzu]

so the author assumed you were going to build this huge monster directly in the guitar.

....no. i could see 7 mini toggles, but 7 pots plus volumes and pup select? good lord. it would look look a franken@#$%job.

[Mark Hammer]

Now THIS is something which can benefit from a big supply voltage.  Somehow a maximum gain of 11 doesn't strike me as the sort of thing that could survive all that potential passive loss and still provide a decent output level.  And asking the front end to provide a clean gain of 20 or 30 with a measley 9v battery seems unfair.  Ladies and gents, haul out your charge-pumps!

[dano12]

Now THIS is something which can benefit from a big supply voltage.  Somehow a maximum gain of 11 doesn't strike me as the sort of thing that could survive all that potential passive loss and still provide a decent output level.  And asking the front end to provide a gain of 20 or 30 with a measley 9v battery seems unfair.  Ladies and gents, haul out your charge-pumps!

Or your 18vDC adapters?

[CynicalMan]

I may be talking out of my hat, but wouldn't it be an idea to put a unity gain buffer at the output to isolate the tonestack from whatever comes next?

Very likely. The book from which this was felched was about building electronics into your guitar, so the author assumed you were going to build this huge monster directly in the guitar.

Will have to play with it a bit when I get the time to see about the  output buffer issue.

Built directly into a guitar? 
Firstly, that's crazy. Secondly, that thing only has a 47k input impedance, it would make your guitar sound like crap unless you had an input buffer too (or cranked up the highs on the passive EQ). The input stage should be changed into a high input impedance non-inverting stage, even in you weren't planning to use it in a guitar.

Now THIS is something which can benefit from a big supply voltage.  Somehow a maximum gain of 11 doesn't strike me as the sort of thing that could survive all that potential passive loss and still provide a decent output level.  And asking the front end to provide a gain of 20 or 30 with a measley 9v battery seems unfair.  Ladies and gents, haul out your charge-pumps!

Since it's already been decided that an output buffer is needed, why not use an output gain stage and have some gain on the input and recovery gain on the output. That might let you get away without a charge pump.

[liquids]

Hey Dano...neat find!  

You may have seen and experimented with this already, but if not, after you try the passive, you could probably think clearly enough to integrate it into something like this: http://freecircuitdiagram.com/wp-content/uploads/2008/11/3-band-graphic-equalizer-circuit.jpg

Which would give you active control.  Actually, on that alone you could probably replicate the concept, but just add a few more bands.

Now THIS is something which can benefit from a big supply voltage.  Somehow a maximum gain of 11 doesn't strike me as the sort of thing that could survive all that potential passive loss and still provide a decent output level.  And asking the front end to provide a gain of 20 or 30 with a measley 9v battery seems unfair.  Ladies and gents, haul out your charge-pumps!

Definitely!  I'd agree that buffer before, and a recovery stage AFTER would make more sense for cleanness, since you've already an 8 pin op amp involved, as they say,  "make it a double!"

But all that still doesn't take away the joy of having a neat starting point to tinker with.  

[Mark Hammer]

Excellent ideas, all.  Yeah, when I looked at it and saw the inverting input stage, I kinda wondered.  First, the input impedance would be lower, and second, the output phase would be inverted.  So, it's a handy circuit, to be sure, but not the best exemplar of what it tries to do.  A recovery stage would be in order.  I mean, if a BMP tone control needs it then this thing certainly does.

[dano12]

As I read more through this book/PDF, I'm finding that it is a veritable treasure-trove of information about pickups, wiring, schematics for piezo/microphone pickup mixers, tone controls, a whole lot of cool stuff.

Unfortunately, the only copy I could find for sale was on Amazon. The seller is attempting to extort $241.11 for it Lol.

[ianmgull]

Unfortunately, the only copy I could find for sale was on Amazon. The seller is attempting to extort $241.11 for it Lol.

Oh. THAT book. 


Anyone notice that VR2 and VR8 look different from the bands between them? I wonder why this is. Possibly a High/Low Shelf?

[Hupla]

Unfortunately, the only copy I could find for sale was on Amazon. The seller is attempting to extort $241.11 for it Lol.

Anyone notice that VR2 and VR8 look different from the bands between them? I wonder why this is. Possibly a High/Low Shelf?

Yeah I was wondering why they were different?

[dano12]

Unfortunately, the only copy I could find for sale was on Amazon. The seller is attempting to extort $241.11 for it Lol.

Oh. THAT book. 


Anyone notice that VR2 and VR8 look different from the bands between them? I wonder why this is. Possibly a High/Low Shelf?

I was used to seeing Borlesconi's Stompbox Cookbook for 300 bucks but not unknown books like this. Fortunately, the Stompbox Cookbook and almost all these other books are floating around for download...

[WGTP]

IIRC SHURE had a passive 7 band EQ for use with PA's many years ago.  Never saw the schematic, but might be worth a search.  

If this is placed in the feedback loop does it become a 7 band boost?

[puretube]

IMHO,
the 7 bands would interact even less,
if the node @ C16 wouldn`t go to a 100k-pot,
but to the "virtual ground" (= inv. input of a standard well-known circuit...)
of an again (Mark:   ) phase-inverting stage...

OTOH: where will all those phases go, after all them (band-) passes...      

[puretube]

Mark: the losses aren`t that big, IMHO:
they happen individually at different frequency bands,
for that band;
not: arithmetically added...

and: the first opamp`s output is supposed to be of very low Z,
so we`re not looking at a "voltage divider" dividing by 7...

[puretube]

Dano: "Berlusconi" is walkin`on thin ice  ,
and we knowit....

[dano12]

Dano: "Berlusconi" is walkin`on thin ice  ,
and we knowit....

Yeah, but he sure gets a lot of tail.

Sorry, meant Boscorelli.

Been reading too much news lately...

[puretube]

Corleone, Mascarpone, Moskovskaya, Borreliosis... ?