Fodera, Sadowsky, aguilar, and other bass preamps

Stompboxes circuits published in magazines, books or on DIY electronics websites.
Ben N
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Post by Ben N »

I don't think anyone has mentioned this. http://www.lynx.bc.ca/~jc/pedals360.html. JC used to have DIY project up at the site. It appears to be gone, but I'm sure someone has it archived.

cerelac
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Post by cerelac »

I built this Sadowsky schematic that was posted, but i dont think it sounds very good. Is it verified? As it is, has way too much gain, and the tone shaping is lame, can anyone confirm the validity of this? thanks

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Post by enakar »

cerelac wrote:I built this Sadowsky schematic that was posted, but i dont think it sounds very good. Is it verified? As it is, has way too much gain, and the tone shaping is lame, can anyone confirm the validity of this? thanks
Hi, check my earlier post about the mods i did.

About the gain issue, you could try to to this:
enakar wrote:- Moved the 500k log Level control to between the tonestack and the second amp stage
I think the result is satisfying, and it sounds good in my opinion, maybe a bit "vintage sounding" if that tells you anything, slightly "warm" and "soft" to use a few completely meaningless words. I havent tried a real sadowsky preamp, though, so I don't know how it's supposed to sound.

I later made another preamp, in which I used a three stack baxandall. I think this works better than the one in the sadowsky schemo, and I might swap it out at some point. The Fender-type tonestack in the sadowsky is simple to build, but not as effective or predictable as the baxandall.

Image
(found in this datasheet: https://www.national.com/ds/LF/LF353.pdf)

/Fredde

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Duckman
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Post by Duckman »

^This is very similar to a 3 band Stingray onboard pre

cerelac
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Post by cerelac »

enakar wrote:
cerelac wrote:I built this Sadowsky schematic that was posted, but i dont think it sounds very good. Is it verified? As it is, has way too much gain, and the tone shaping is lame, can anyone confirm the validity of this? thanks
Hi, check my earlier post about the mods i did.
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/Fredde
Cool stuff, yeah i saw these mods, but even performing mods on it, i felt it crude, it still distorted, and the tonestack was still quite mild and uninspiring, also i found the jfets to add way too much his for my taste

the new preamp you build had the same in and out jfets? cheers

enakar
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Post by enakar »

cerelac wrote:Cool stuff, yeah i saw these mods, but even performing mods on it, i felt it crude, it still distorted, and the tonestack was still quite mild and uninspiring, also i found the jfets to add way too much his for my taste

the new preamp you build had the same in and out jfets? cheers
The new preamp is an opamp design, so it's a completely different animal. It's generally a lot easier to use opamps IMO, as they give more predictable results and rarely any problems with hiss, hum or distortion. And you get low output impedance for free.

One reason to use jfets is their imperfection, they add their own character to the sound. If you want hi-fi and transparency, use opamps :)

/Fredde

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Post by Victor Nery »

Greetings,

I always wanted to build a 3-band EQ but never had found such an easy project like the one posted above. Still I have one doubt about the lack of a capacitor in the end of the circuit. Is it necessary and what should be its value? The fact it‘s missing means we have to put another circuit after it such as a buffer? Does this circuit work with a single 9V battery and a voltage divider? It would be cool to implement this feature in the circuit.

Thanks

enakar
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Post by enakar »

Victor Nery wrote:Greetings,

I always wanted to build a 3-band EQ but never had found such an easy project like the one posted above. Still I have one doubt about the lack of a capacitor in the end of the circuit. Is it necessary and what should be its value? The fact it‘s missing means we have to put another circuit after it such as a buffer? Does this circuit work with a single 9V battery and a voltage divider? It would be cool to implement this feature in the circuit.

Thanks
Hi, the circuit is an example from an opamp datasheet. It's intention is to show the three band tone stack, so most of the extra components you see in a normal opamp application are omitted. But no, you wouldn't have to add a buffer even if you'd add an output capacitor, which is a very good idea. You could use a 10uF good quality electrolytic. Also 100n caps from boths rails to ground close to the opamp itself is always good.

Depending on the opamp it could work with a single 9V and voltage divider, just check the specs and see if your opamp runs on +/-4.5V. At least the OPA2134 works like this in my experience (i've even run one on +/-2.5V powered by USB). But bigger is better concerning supply voltage (as long as you stay within the limits of the opamp), the amount of gain you can produce is limited by the supply voltage. The +-/15V you see in most opamp datasheets is an ideal value, I guess, but not very easy to produce in a stompbox. I rarely use batteries in my builds these days, and my pedal bord power supply can produce 18V, so I can get +/-9V which is enough in most cases.

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Duckman
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Post by Duckman »

Here you got a complete similar proyect, single rail version (easlily founded googling "3 band eq" :thumbsup )
Planned to use TL072 or NE5532, but remember that 072 is a FET and 5532 is a BJT, wich basically means, much higher input impedance with the 072 than the tipically 100k of the 5532.

http://users.otenet.gr/~athsam/3_band_eq.htm

enakar
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Post by enakar »

Duckman wrote:Here you got a complete similar proyect, single rail version (easlily founded googling "3 band eq" :thumbsup )
Planned to use TL072 or NE5532, but remember that 072 is a FET and 5532 is a BJT, wich basically means, much higher input impedance with the 072 than the tipically 100k of the 5532.

http://users.otenet.gr/~athsam/3_band_eq.htm
Yes this is a more complete version. But how is this a single rail circuit?

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Duckman
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Post by Duckman »

enakar wrote:But how is this a single rail circuit?
Because you have +15v/0v instead of +15v/-15v, so your bias point is around 7.5v; your "swing" will be +/- 7.5v... if you go double rail, your bias point is around 0v, hence, your "swing" will be +/-15v, just the double of headroom.
Any way, you can run it @ 9v with a 4.5v bias point without problem; just less headroom.

enakar
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Post by enakar »

Duckman wrote:
enakar wrote:But how is this a single rail circuit?
Because you have +15v/0v instead of +15v/-15v, so your bias point is around 7.5v; your "swing" will be +/- 7.5v... if you go double rail, your bias point is around 0v, hence, your "swing" will be +/-15v, just the double of headroom.
Any way, you can run it @ 9v with a 4.5v bias point without problem; just less headroom.
Ok, sorry i just looked at the schematic, didn't read the explanation text (don't know if i still understand it). Why not just make a simple voltage divider to get +/-4.5V and connect both supply pins?
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Post by Duckman »

That's exactly what they do, but @ 15v. Just change it to 9v and that´s it.
I forgot to say "click in the single supply version link", sorry, my fault.
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enakar
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Post by enakar »

Duckman wrote:That's exactly what they do, but @ 15v. Just change it to 9v and that´s it.
I forgot to say "click in the single supply version link", sorry, my fault.
Haha, ok now I see, totally missed that link. Thanks for posting, yes this looks more like what I expected :D

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Post by bajaman »

has anyone traced the opamp 3 band preamp fitted in the Fender Deluxe jazz or Precision basses :?:
just curious :wink:
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lietuvis
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Post by lietuvis »

bajaman wrote:has anyone traced the opamp 3 band preamp fitted in the Fender Deluxe jazz or Precision basses :?:
just curious :wink:
bajaman
I have done for Fender Precision Deluxe Lite, this model has been made in Japan only. Its on second page in this thread. If this is what you looking for.

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Post by bajaman »

thanks lietuvis :wink:
i saw that one - it is probably the same circuit, but the one I am interested in is the smd version in the American deluxe models of jazz bass and precision bass.
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Post by bajaman »

okay folks
I downloaded the pdf (http://support.fender.com/service_diagr ... A_SISD.pdf) for the American Deluxe Jazz bass - there are quite a few diagrams but this one shows the most detail of the smd board used.
It is similar to the MIJ precision light preamp as traced by lietuvis BUT not the same :wink:
I have no idea what value the components are, but from what i have deduced by tracing it from the picture in the pdf, it has a non inverting gain stage for the first section NOT a buffer.
There are also limiting resistors fitted at the cw and ccw ends of all three tone pots - the tone pots are all 100k linear though.
There is an additional capacitor fitted in series with resistor connected to the wiper of the bass control and an additional resistor fitted in series with the capacitor connected to the mid control pot wiper -there is also an additional capacitor connected between the ends of the limiting resistors on the treble pot (like the bass control).
The protection diode in series with the +9v (looks like +18v on the Fender pdf) feeds the voltage divider biasing network only - the +ve supply is connected directly to pin 8 of the opamp (not through the diode).
The output level control is 25k log not 100k.
The treble and bass controls are a dual concentric 100k linear pot.
The midrange is a single 100k linear pot.
The pan control is a dual ganged 250k linear pot.

It seems that this circuit has increased gain - always handy for those that want to overload the input to their amplifier, the limiting resistors are probably there to prevent harsh clipping of the LF422 dual opamp when full boost on the tone controls is used.
Hope this useful - would love to know the values :wink:
cheers
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lietuvis
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Post by lietuvis »

found this on web if it can help to trace. it seems that all bands are peaking rather than shelving(for last picture of PCB). and it uses different IC. even the PCB are populated slightly different.
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bajaman
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Post by bajaman »

wow :shock: :shock:
where did you find them - just kidding :wink:
Thanks so much - that makes life a lot easier for me :thumbsup
The opamp is the smd version of the low current TL062 (TI equivalent of the NS LF442).
Looks like a few of those "limiting" resistors are in fact just 0 ohm links.
Good stuff mate :hug:
cheers
bajaman
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