Has anyone here built his or her own EQ/Preamp circuit? [schematic] - created 08-12-2002

Novetti, Julio - 08/12/2002.21:21:27

Hi guys

I want to build a homebrew preamp\eq circuit to fit in my first bass using a Basslines\musicman pickup.

Basslines also sells the circuit ,but i couldn't find it here in Brazil.

I was seaching the net for a schematic,diagram to a easy to build eq\preamp.Does anyone here tried this before? I have experience with eletronics(tube amps) so if i get the schem, i could build it i think.

Any suggestions? recomendations?

Thank's...

I found in the internet a schematic for a 2 band active tone circuit in the Tobias site.It's the stock circuit on their basses.

Charlie did you built any similar circuit?

This Tobias schematic needs a operation amplifier sold by JRC (japanese fabricant)code (NJM 062) ...i found the similar Operational Amplifier from National Semiconductor code(TL082)three band active tone control.But here in the TL082 manual says it needs a simetric power suply. Is that correct?How it would be? 2 batteries with reversed polarity beetween then on the circuit?

Even in the National op. amp. manual is there a example of a 3 band active tone control simplier than the Tobias one.The boost\Cut are on the 100 Hz , 1KHz and 10 KHz.....it's the same on the tobias schem.

Would worth a try to build experimentally this too?

Thank's

There's a circuit in an old National Semiconductor Linear data book from 1982 for a three band active EQ. I built it sounds pretty good, but it required two batteries. I can't post the circuit because of forum rules, but try to locate it in the library.

You can use a biased single-ended supply (with one battery) rather than a bipolar supply, but you wouldn't have as much headroom as with two batteries.

Vlad Ardelean is the forum member who designed much of the preamp stuff in the library. He has a schematic for a preamp with active tone circuit but I'm not sure if anyone has built it yet, or if it's in our library.

You can find it on Vlads' preamp webpage.

Thank's guys...

I bought a broadbroad(here in Brazil is called (protoboard)) yesterday....very easy and fast do try any circuit...i will do some tests....if i have any progress i'll report it.

My bass is just setting it's intonation....i just planning and waiting to hear it...I will post some pictures of it soon.

Hey Jamie, i think we're talking about the same National data book circuit...i will try it.

I will check Vlad's homepage too.

Jamie...

The TL082 will work fine, a TL072 is better- lower noise than the TL082. Actually, just about any dual opamp IC will probably work just fine.

Build the circuit in the National Semiconductor opamp book. In that book, there's probably a section on running the opamps off of a single power supply. You'll have plenty of headroom.

I'll post an EQ schematic later.

Here's a quick schematic of a 3 band EQ, with single-ended power supply. Note that the values in the tone circuit may not be to your liking, this was lifted from another project. Use a TL072 for the opamp.

There are an endless variety of circuits that'll do the job. IIRC, old MusicMan preamps used the LM4250, a VERY low power single op amp. Using these, it's quite possible to make a preamp that draws around 10µA! A 9V alkaline battery will last almost it's shelf life even if the circuit is never turned off! However, if you need gains greater than about 5, You could probably find a better audio performer at the expense of power consumption.

With op amps, I find real world differences in noise specs to be fairly small when used at modest gains. Many commercially available onboard preamps don't seem terribly concerned with noise so much as power consumption. Something like the TL06x series is ideal for battery power. The LM442 (dual) is comparable with lower noise specs IIRC, though it may not make much of an audible difference.

One circuit I'm liking these days is the Utility Boost.

A nice preamp/EQ could be made using two of these in series, with a tone control circuit inserted between the two stages, and gain adjusted for least distortion. Several nice tone circuits could be found and simulated using free software available on the Duncanamps website, the Tone Stack Calculator. Such a circuit would draw less than 1mA.

Are the tone pots linear or audio?

Actually, an 'S' taper works the best, it gives a very even response from the controls. These are pretty much impossible to find in small quantities, so a linear taper would be the next best choice.

Thank's Charlie....thank's everybody here....

I tested this circuit in the past week....it worked great...

it's incredible how a EQ\preamp circuit can wide the range of tone in a bass.In mine it sounded from deep bass to a mid rangy sound and after to a trebly slap sound...just dealing with the knobs...very versatile....

I with take some pictures of my bass this week, scan then and add it in the Forum.

Looking at the schematic again, there are some parts that can be removed. There's a .47uf cap and a 4.7k resistor connected in series between ground and pin 2 of U1A. Since we're bringing the signal into the inverting input (pin 2), we don't really need these parts. Like I said earlier, I grabbed this circuit from something else I did, made some quick changes, and posted it here.

That schem does have some peculiarities about it. As far as I can tell, that .47uf cap and a 4.7k resistor seem to be intended as a simple high end roll off tone control, but I wonder if they have much effect at all. The 200k and 220k resistors on U1A could both probably reasonably be raised to 1Meg - 2.2 Meg ohm for higher input impedance. And the 2.2k resistors comprising the biasing network in the lower left corner could probably be raised by a factor of ten, maybe as much as a factor of 100, for less of a drain on the battery, and still perform just as well.

If you raise the value of the 2.2k resistors too high, there is a good possibility that the circuit will 'motor boat'. You could probably go 10x, but 100x is too high.

If you increase the feedback resistor to 1-2M, you need to reduce the value of the feedback cap to maintain the frequency response. It will also probably be more noisy. Or, you could connect the input signal to the non-inverting input, and raise the value of the input resistor without changing the gain too much. However, you then have to consider the voltage divider created with the input resistor and the 1M bias resistor.

Originally, I was using the non-inverting input of that stage, and the RC network was used to set the gain and frequency response (along with the feedback resistor and cap). When I modified the drawing for the forum, I moved the input to the inverting pin, to keep the signal in phase from input to output. I simply forgot to remove those parts.