Opamp EQ now working but wih oscillating mid range

[zener]

I tried this one in a breadboard


using RDV's preamp


I used a TL072. I hooked the preamp at the first half of the opamp and I took JUST the eq circuit from the first schem and hook it up the other half of the opamp. I took the output and hooked it up to a LM386 amp. I put two weak 9v in series and used it to power the opamp. I also tried powering the opamp with just 9v supply. The eq wasn't working. No matter how I turned all the pot, it just didn't alter the tone. It's like the eq is bypassed.

I noticed that the schem from the datasheet calls for a +15v and -15v. Could that be the reason?

How about this

They are the same right? The difference is just it only used a single opamp and there's no preamp since it's only a single opamp. But I noticed that it doesn't call for a bipolar supply. Is there any difference?

Can I hook up that single opamp eq straight into the amp?

Thanks for any help :wink:

[RDV]

If you don't use a bipolar supply, you'll have to use a voltage divider to bias the opamps.

Gotta get ready for the gig

RDV

[petemoore]

At least soembody's giggin...have a good one!

[RDV]

Try this Mr. Zener!


Regards

RDV

[petemoore]

But thanks RDV, I'll try that ! ! !

[zener]

I tried what RDV suggested (thanks Ricky) and it's working now. Maybe I did some error. I found a new problem, however. The mid control brings oscillation as I turn it. I'm not sure with the orientation of lug1 and 3 but I know it would just work the other way around. With bass and treble at max, I can't even turn mid past 10% or this is what I wll hear:
http://www.angelfire.com/weird2/zener/mid.wav

I turned the pot back and forth, thrice. The guitar wasn't played when it was recorded.

The only sub I did are instead of 3.6K, I used 3.3K and instead of 11k, I used 10K. Not that much to cause the oscillation, isn't it?

Thanks for any help

Zener
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[RDV]

I'd have to say that maybe there's a wiring problem. Make sure that of the wires that cross, only the ones with a dot are connected. I would imagine that the resistor substitutions would change the frequency. I've seen that circuit before, but have never tried it. I think opamps are cool. I've been working with them for a few months and have thoroughly enjoyed it. I don't really understand the 'opamps are bad & sterile sounding' attitude some people have. The Tube Screamer is an OA circuit for christ-sakes. I dig the idea of using OAs and BJTs together.

Also, you're not still using that gain stage that I drew up are you? That would cause some oscillation if you were to set up one of the opamps like that. It is a decent stand-alone booster stage though.

Good Luck

RDV

[petemoore]

Yupp, did this one up on perf.
 Works great. Nice 3 Band EQ.
 Iv'e only tried it Guitar>EQ>Amp so far, does a fine job getting cool tones/wide variety.
 I like my opamps...er the ones I like I like alot.
 I wish they weren't sterile so I could breed them...lol/
 They just sound like opamps to me, there really is no getting around the coloration, I've been using them alot lately too, and really like the way they can be made to sound.
 It's like they just don't clean up completely like a Mosfet can, I like a certain amount of clean dirt usually anyway, and Opamps make nice clean dirt.

[zener]

Hey Ricky,

I just tried your gain stage using 330pf, 10k and 1uf. Made no difference at all :? . I also put a 300ohm in series of each 3.3k to get 3.6K. No difference. :? I swapped pots, still the same. When I replace the cap between lug1 and 3 with a .1uf, the squeal was reduced. It would be just in the middle of the rotation, large middle. I mean 10-20% rotation, no squeal. 80%-100% rotation no squeal. Through there I can hear that the mid is working, only squealing.

I have known the dot connections long ago. The bass and treble is working so, if there's a mistake there, it would be just on the mid side and I can see it right away.

Thanks.

Zener

[zener]

OK, update.

I just put a .1uf instead of the .005uf across the mid pot. I set the treble and bass at max. It squealed but not that much with the .005uf. Then I tried turning down the treble pot JUST A BIT. The squeal went away . The mid control is now working. I just don't know how much difference would there be to the mid between .005uf and .1uf.

because

I have to be honest, I have no idea how mid-range sounds :roll:  :mrgreen: . I mean, I have seen mid controls in many amps and all I knew about it is that it is the characteristic of the tone that is between bass and treble, that's why it is called middle :roll: .

Nice to hear that pete had it working already. Thanks RDV!

Zener

[RDV]

You could try adding 1k resistors to the 10k to make 11k. I think a tone control like this is all about balance between resistances. Getting the values as close as possible might just get you there. Pete didn't mention any oscillations, but he tried to get the values as close as possible. I know, I don't have any 11k resistors either.

RDV

[zener]

I will try that. I'll get back soon.

BTW, I didn't know that pete started another thread about this just a while ago.

[zener]

Ok, update.

I just have it 11k. My previous update is quite inaccurate and therefore to be replaced by this.

I can't get the treble and mid maxed at the same time. It would oscillate. I already have a total of .25uf across the mid pot. With bass and mid at max and treble at 3 o'clock, I can hear a bit of rumbling sound as the notes decay. May be I just won't use the mid at high or just completely take it out.

I'm using a TL072. I will get a TL082 tomorrow and see if there would be any change.

Zener

[spongebob]

In the original schematic pin 5 is tied directly to ground, maybe you should also tied it directly to your virtual ground, without the 1meg resistor. Also, if you raise the input impedance to 1Meg you can reduce the input cap, 100nF should do...

[Rob Strand]

If you look at one of the colored schematic above it has a 270R and 1n cap to the 4.5V point - these can help stop oscillation problems (there's a few other ways as well).

On RDV's schematic I'd remove the 1M resistor on pin 5 (if you want to keep it use 10k or so).   That resistor adds a lot of  noise.

This is a similar sort of thing,

http://www.geocities.com/basspreamp/basspreamp/index.html

The 100p helps prevent oscillation, for guitar perhaps usea smaller value but is has to be large enough to do it's job.

[RDV]

You know all I did was add a voltage divider to a datasheet EQ when I was about half pissed as a newt at about 3:30am after a gig. I ain't designed nothing. I'll change it a bit.

RDV

[petemoore]

I used only one 10k + 1k resistor built.
 All the others were 6k8 + whatever K got me close to 11k...I like using these odd values up...

[zener]

Ok, update.

From what spongebob and Rob said, I took out the 1m to the pin5. I also added the 220ohm + 1n from pin 6 to the Vb. I stayed with Ricky's preamp's gain stage. I just wired a 10ohm for the gain pot. I had a .039uf across the mid pot. All resistor values are exact (3.6k and 11k). Finally, a TL082 instead of the 072.

The mid oscillation is gone. But still, I can't set the treble and/or mid pot beyond 3 o'clock or else I will get some oscillation with the treble pot or the sound will get "scratchy" with the mid pot. The highest setting is just bass max, treble and mid at 3 o'clock. The .039uf is the value I found to be of balance between oscillation and scratchiness of the sound. Lower value, more oscillation, less sratchy; higher value less oscillation, more scratchy.

I found something else, The gain pot value of 100k doesn't seem to be working for me. I put a 27k there and a motorboat began running. I increase the value and the motorboat became bigger. I had no problems when the value is within 1k, but I hardly noticed any change in gain. Maybe it is within 5k, I dunno.

Hey Pete, have you figured out already whether the eq boost or cut the signal? Also, I want to know what opamp you did use and can you maxxed out all the pot with any annoyances?

Thanks.

Zener

[petemoore]

I can't seem to get the HISS out of it. Got worse, at first I didn't even notice it. Unusable...
 I reppaced the 386 [voltage divider] and the Dual opamp...not much change in the noise.
 Having some troubles figuring out what else to try...

[RDV]

I can't seem to get the HISS out of it. Got worse, at first I didn't even notice it. Unusable...
 I reppaced the 386 [voltage divider] and the Dual opamp...not much change in the noise.
 Having some troubles figuring out what else to try...

Replace the 1Meg resistors from Vr with 100K and see if that helps.

RDV