Parmetric eq with instrument input

[James HE]

I've been looking around and researching parametric eq circuits. The main purpose will be for amplified acoustic guitar, I want something that I can control before the board, so basically a guitar preamp with many eq bands. I'm a newbie to electronics, this will be my first build.

For the Eq section I think I'll try this from ESP.

http://sound.westhost.com/project28.htm

This design seems very compact compared to others I've seen. I'm thinking this will be in a larger stompbox housing as long as I can fit all the knobs on there. If it ends up rackmount that's ok. For a pedal version I might skip the low band and just do a hi pass filter via a switch.

I have a lot of questions at this point.

Since this will need a high impedance instrument input, I need to change the input stage. Do i get away with just using a different opamp for more gain? or should I take a different approach with a different gain stage all together? One of my acoustics has an active preamp, another dosen't, Hi and low inputs would be ideal here, or maybe just something that can handle both.

Opamps:
The author recomends Tl072 for the simulated inductor stages, NE5534 for the output stage. Any different suggestions in light of it's intended use?

the power rails - this is more of a general question. I'm confused about powering opamps with double vs. single ended supplys. Some scematics I've seen have the Vcc- pin to ground, some to the V- rail. Is there an advantage either way? From what I've been able to dig up, opamps are designed to run on double ended supplies, but that single ended is fine in some circumstances. I stumbled upon a TI paper on this subject, but really it confused me a little more (not understanding the vitual ground concept)
I just wanna know should this have a + and - rail or just + and then to ground. I'm pretty lost of the whole power situation, Battery power seem's easy enough though.

 

[henk]

Look in this forum for a nice eq from Mr Bary Porter called portereq or neteq , look by rogy and you find all the nice stuff you are looking for.

 

[Svart]

search shows one thread. this thread.

 

[clintrubber]

TC 1140 ?

http://www.groupdiy.com/index.php?topic=669&postdays=0&postorder=asc&start=0

http://img56.photobucket.com/albums/v172/gyraf/TC1140_schematic.gif

 

[James HE]

I've clamoured together a few different schematics from the net and I've come up with this... THING

keep in mind I just started learning about electronics about 10 days ago a so... I'm sure this is just wrong in some places.

*edit* fixed switch.

There are 2 inputs. One has Tillman's FET preamp. The idea is to have hi and lo impedance inputs.

The parametric eq section is from http://www.fortunecity.com/tinpan/humperdinck/223/stma.gif (that is a little easier to read) I am trying to keep things compact to possibly fit this in a pedal. (with the pots likely panel mounted) The EQ design looks to be not so many parts. just 2 opamps. 2 sweepable bands. If I wasen't aiming for such a small footprint I would do an eq with more features. But if this one is crap I'll look elsewhere.

My "power supply" is from http://www.geofex.com/circuits/+9_to_-9.htm I even added in the battery switching detailed on that page and also some supply bypassing (needed?) that i just lifted from the "Designing with opamps" on ESP's site. http://sound.westhost.com/dwopa.htm . I have no idea if one 9v will be enough. I could do two for 18v.(15v?) And/or a wall wart supply, but I haven't really researched that yet.

The input buffer, the hi-pass, and the output section are really just stabs in the dark. The output "gain" pot is just an assumption. Having R22 be a variable resistor to change the relationship to R23 and therefore change the gain of the non-inverting opamp. Right? :? I'm not sure how much gain I'll nedd so I'll have to tweak it.

The hi pass I want to be around 80Hz. Just to kill some rumble.

I'm using Express PCB's schematic editor. Seems easy, but looks funky to me. Had to cram things in there to fit it all in. Now I'm going to try and figure out if this will fit on a reasonably small circuit board or perf board.

to clarify I labeled the dual gang pot DR1. not sure what the convetional way of labeling dual pots are.

OK bombs away. What'd I mess up? I'll be surprised if someone says it'll even pass a signal :green:

 

[Guest]

[quote author="Svart"]search shows one thread. this thread.[/quote]
Or another thread with this URL
http://www.collinsaudio.com/Porter%20EQ/portereq.pdf

 

[Kit]

Since this will need a high impedance instrument input, I need to change the input stage. Do i get away with just using a different opamp for more gain?

Do you want gain or high Z input?

A high Z input is easily realised with a non inverting TLOXX fet-input-opamp. If 100K is not enough, just increase the value of that R.

Hi and low inputs would be ideal here,

Again, are we talking Z or gain?

If were talkin Z, you could make HI input to non-inv. and LO to inv. input of the same opamp.

 

[James HE]

[quote author="kubi"]Please check the bypass switch!
When bypassing the EQ section, both oamps around the switch are shorted together (Pin 1 Pin 8). Use a double switch or switch in and out the output of the EQ section, not the input.[/quote]


And that's a bad thing right? lol

Thanks for pointing that out! I see it now. I will make that change soon and post a revision.

 

[James HE]

[quote author="Kit"]

A high Z input is easily realised with a non inverting TLOXX fet-input-opamp. If 100K is not enough, just increase the value of that R.



If were talkin Z, you could make HI input to non-inv. and LO to inv. input of the same opamp.[/quote]


Yes Z. I've used The FET preamp for the high impedance here. I'm not sure of the specific impedances i'll need here yet. I will want to tweak the inputs for two specific guitars actually. One has just a magnetic pickup. (the FET input for this one) And the other a magnetic pickup and onboard preamp. (After a bridge reglue, the stock undersaddle piezo was trashed, and no replacement fits. I hacked a magnetic pickup to use the onboard preamp, so the guitar itself has some stange mismatches) The guitar with the onboard preamp seems to like to see a mic input, but the gain is very low.

Donald Tillman says the FET preamp has an input impedance of 3.0M and an output impedance of 6.0k. The output impedance of my frankenstein pickup? who knows. How can I measure? The mic inputs on my board are 1.8k and the line inputs 10k. (just looked it up now) The guitar with the onboard preamp seems to like to see the mic input, I get a little more gain and less noise (or just slightly less noise with the same gain) and it sounds better. Either way I'm having to really crank the gain. So I'm inferring that I need a rather low impedance for that input. maybe, or maybe I'm all mixed up and overthinking it. If I lower R5 to suit the lo input better, it affects the hi input - what if i parallel another resistor before the switch on the FET side of the switch?

I'm trying to fix and consider as much as i can. I only want to make one order from mouser, so I can just finish it once I start. :grin: No local supplier that I can find.

So if anything else seems wrong, let me know! Thanks.

 

[James HE]

[quote author="Kit"]

If were talkin Z, you could make HI input to non-inv. and LO to inv. input of the same opamp.[/quote]

Just to clarify do you mean a single opamp or two halves of a dual op amp?

 

[Kit]

I meant a single, but you could use a dual.
No big price difference between a single and dual.