bass eq

[SirPoonga]

http://www.albertkreuzer.com/pics/el/pre/schem/preamp_sch21.gif

Is the points between B and C the EQ?  Could I just use that to make a
EQ stompbox, B is input, C is output?  Would it work fine on 9V?

Out of curiosity, would it make a good onboard preamp?

I tried emailing Albert directly, but the email bounced.

[Mike Burgundy]

That would work as a stompbox, on 9V, just fine with a few adjustments.
Since a TL072 wants a dual voltage supply, you should use a voltage devider to get 4.5V (Vref) and fake the opamp into believing it's running on +/-4.5V.
(see a tubescreamer for this trick)
run R14 to Vref instead of ground, and do the same for the + input of the second opamp.
Maybe include a volume pot instead of the 2M2 resistor, add a stompswitch, a pull-down resistor on the input, done. Maybe you should decrease the 47k input resitor or even omit it totally as levels for a stompbox are lower than in this schematic.

[rubberlips]

just finished building it.
Yes the EQ is between B and C, the circuits fairly standard for preamp tone shaping although values vary depending on the application
Just trying to work out a way of getting a but more grunt from it to drive the power amp

Pete

[Mike Burgundy]

as long as no filtering is involved, decrease in-series resistors. Increase feedback resistors on opamps to get more gain, too.
This circuit has enough gain, it's just what you want from it (some things it just CAN'T do, some things can be tuned in) and at what level you want it.
If the circuit doen't have the grunt you want, does that mean you want more output, or a bit of raunch? If it's the latter, try simply adding a booster to the front-end. If it's the first, some tweaking of feedback resistors or even adding your own booster should solve that.
Tweaking rules

[SirPoonga]

as long as no filtering is involved, decrease in-series resistors. Increase feedback resistors on opamps to get more gain, too.

No clue what you are talking about.

[Mike Burgundy]

Um, okay..If you use an in-line cap, that's a high-pass filter. The actual frequencies it filters depend on both the value of the cap, and the resistance (impedance, even...) around it.
a resistor in-line with a cap to mass is low-pass (the cap shunts highs to ground), a resistor to mass with an inline cap is high-pass.
So, as long as there's no cap/resistor combination, all that's happening is attenuation of *everything*, without cap-induced preference for certain ranges. In this case, changing resistors wont change harmonic content (or at least it shouldn't...but we're talking theory. real world stuff has all kinds of not-intended effects and values however.  It's just that way.)

If there is an "RC" combination, you might be safe with tweaks, but you might not. Understanding what RC combo's do gets you a long way into tuning pedals - you don't need to know the math. Just understand that a larger cap lets through lower frequencies, and just how much and even at exactly what frequency depends on the  resistance around it.
If you want, I could adjust the schem for you when I get off from the studio. It helps to understand this though, and not build it as-is (esp. since I'm also winging it )

[SirPoonga]

Right, I get that.  It's how the tone control on the bass works.  

It's the terms" in-series resistors" and "feedback resistors".  I can read the schematic to make it, but why it is that way I don't really have a clue.

[rubberlips]

as long as no filtering is involved, decrease in-series resistors. Increase feedback resistors on opamps to get more gain, too.
This circuit has enough gain, it's just what you want from it (some things it just CAN'T do, some things can be tuned in) and at what level you want it.
If the circuit doen't have the grunt you want, does that mean you want more output, or a bit of raunch? If it's the latter, try simply adding a booster to the front-end. If it's the first, some tweaking of feedback resistors or even adding your own booster should solve that.
Tweaking rules

Yes more output.
I've been looking at other preamp designs and noticed that most of them have an opamp buffer at the output to drive a poweramp (now whether it has some gain to it, it still buffers). Looking at Alberts preamp design, it has the same type of modules and gains as other preamps I've seen, but for some reason it might be loading down the input of my poweramp

Pete

[Albert Kreuzer]

http://www.albertkreuzer.com/pics/el/pre/schem/preamp_sch21.gif

Is the points between B and C the EQ?  Could I just use that to make a
EQ stompbox, B is input, C is output?  Would it work fine on 9V?

Out of curiosity, would it make a good onboard preamp?

I tried emailing Albert directly, but the email bounced.

Yes to all of the above questions, but you have to provide a reference voltage instead of the opamp ground (voltage divider, R.G. has a good description at www.geofex.com).
Another thing: The 47k input resistor sets the input impedance which is rather low for a guitar or bass pickup. You can
a) raise the value of both 47k resistors (may result in more noise)
b) change the first opamp stage to a non-inverting amp with high input impedance (will invert the overall phase of the signal)
c) put a buffer stage in front of the whole thing (opamp or FET source follower)

Email: Try info "at" albertkreuzer.com, maybe you had the old address.

Cheers,
Albert

[Rob Strand]

Out of curiosity, would it make a good onboard preamp?

I think the standard nat-semi circuit has too much boost for an on-board preamp.  The trend in recent times is to keep the boost/cut to a minimum-so you don't run into headroom problems on a 9V battery.

[Albert Kreuzer]

Hi Rob,

doesn't the original application have unity gain? I pretty much stuck to the original values (I know you have tinkered with this schem too).

Pete: It's not the output loading down your power amp in, but the input of the eq loading down your guitar signal.

If you want more "grunt", try the whole preamp, not just the eq section  :wink:

Cheers,
Albert

[Rob Strand]

doesn't the original application have unity gain? I pretty much stuck to the original values (I know you have tinkered with this schem too).

Yes, it's unity gain when the controls are flat.  It's the amount of boost when you max out the controls, pushes the 9V rails a bit.

[rubberlips]

Pete: It's not the output loading down your power amp in, but the input of the eq loading down your guitar signal.

If you want more "grunt", try the whole preamp, not just the eq section  :wink: t

Albert, I did - still didn't have enough grunt. I started up a new thread called Opamp booster problem/question maybe you could have a look at it
Pete

[SirPoonga]

Interesting.  I am starting to figure it out.

Let me give some background to this question.  I was browsing through some luthier sites checking out basses.  My dad saw this and wants to make me a bass now that he picked up a whole bunch of new tools that he needs to try  So I looked into it.  My idea would be to get a prebuilt neck (like from warmoth.com) and make the body.  So, there's decisions to make.  I am thining either a P/J pickup setup or duel humbuckers.  Well, all of the wiring schematics I have found for humbuckers is for an active bass.  So I started researching active electronics.  It seems they are just a 2 or 3 band EQ for the most part.  

Now, the question on 9v vs 18v.  Would it be a matter of just replacing the opamp from the jfet preamp eq with one that handles 18V such as a TL072 or LT1115?

Like I said, I'm just researching right now.  So i'd make a stompbox version of this to try it out first.

Now, looking at the free information society, there's a E-H and Ibanez EQ.  It uses an opamp per freq to boost.  I like the eq from the jfet preamp since it uses a single dual opamp.  To me that seems like it would take up less space and less power consumption.  All I need is a 3 band.

Is there a formula to determine how to boost frequencies?  Like what caps and resistors to use?  While developing thise if Ic ould just socket caps and resistors so I can fine tune what frequencies get boosted would be perfect.

[SirPoonga]

I can see there is a pattern but I can not find any info on making an eq with just one dual opamp.

[SirPoonga]

http://www.ethanwiner.com/spectrum.html

reading to do

[Ardric]

I can see there is a pattern but I can not find any info on making an eq with just one dual opamp.

How about the Tonemender?

http://www.diystompboxes.com/sboxforum/viewtopic.php?t=31057
http://www.runoffgroove.com/tonemender.html

You could change the tonestack parts to taste.  I found Duncan's Tone Stack Calculator to be really useful for that.

http://www.duncanamps.com/tsc/

In the TSC try selecting the Fender stack, set the source impedence (Zsrc) to be 100, and the load resistor (usually R5) to 1 megohm.  Then change the rest of the values to match the Tonemender schematic and simulate what it does.  This way you can try variations and see the resulting EQ curves before building it.

[SirPoonga]

Ahhh, that's what I was looking for, something that shows the calculations of the tone stack.  Thanks.