Baja Music Man guitar and bass onboard preamps

Andy_ · Post by **Andy_** » 19 Apr 2017, 23:23

Hallo Manfred,

... those were the days

Maybe we should mention that this modification will not change the sound in general...

Viele Grüße
Andy

JayGunn · Post by **JayGunn** » 21 Apr 2017, 22:59

About this SMD version of the MM preamp, has anyone bought and populated this circuit board?

https://www.freestompboxes.org/download/ ... &mode=view

If you look at the silk screening you can see that it is missing a number of components, like multiple resistors and multiple capacitors from the bill of materials. Maybe these are added to the back side? Otherwise it couldn't really be the MM preamp this thread is about...

John

deltafred · Post by **deltafred** » 23 Apr 2017, 21:57

I've just built a perf board version of the preamp with a FET input buffer and fitted it in an EUB (with piezo pickups). It sounds pretty good but it does need a HPF after it because of all the lows that the EUB puts out.

I don't think the SMD board above is for this preamp, there are the wrong number of resistors and capacitors and nowhere to connect the pots. It also has + and - inputs which looks like it could be for a differential signal, like an XLR mic or line signal to single ended output.

JayGunn · Post by **JayGunn** » 23 Apr 2017, 22:42

Thanks for confirming my suspicions!

Andy_ · Post by **Andy_** » 02 May 2017, 17:43

deltafred wrote:I've just built a perf board version of the preamp with a FET input buffer and fitted it in an EUB (with piezo pickups). It sounds pretty good but it does need a HPF after it because of all the lows that the EUB puts out.

You should omit the fet buffer and wire the pickup directy to the circuit

Otherwise the treble control will not work as it should be. You pickup parameters should be in the range of the original (a resistance of about 2k points to a suitable pickup). Unfortunately there are many replacement pickups with too much output available. Although the resistance is not the problem it comes along with an improper inductance will will lower the treble frequency peak to somewhere in the midrange...

deltafred · Post by **deltafred** » 02 May 2017, 21:39

Andy_ wrote:You should omit the fet buffer and wire the pickup directy to the circuit Otherwise the treble control will not work as it should be. You pickup parameters should be in the range of the original (a resistance of about 2k points to a suitable pickup).

The buffer is staying, piezo pickups need a high impedance buffer to work properly.

The output impedance of the buffer is low and the treble control works very well, I have a genuine MM with a 2 band pre so I know exactly how the treble control should work.

I also have one fitted in a bass with a resistor in series with the pickup when the coils are in series (to match the series and parallel volumes) and the treble control does not work properly unless you shunt the resistor with a suitable value capacitor.

marsbel · Post by **marsbel** » 03 May 2017, 07:49

Could you post the circuit of the buffer? I'd be interested in trying this out on my bass. I've got plans to add a piezo bridge.

Andy_ · Post by **Andy_** » 08 May 2017, 14:07

deltafred wrote: (...)
I also have one fitted in a bass with a resistor in series with the pickup when the coils are in series (to match the series and parallel volumes) and the treble control does not work properly unless you shunt the resistor with a suitable value capacitor.

That's exactly what I wrote some posts ago

I can imagine that there is some kind of treble functionality even with the buffer but this would only be "half of the circuit".
The treble rolloff caused by the cap (that one wired to the output) should be hearable.

To say it again (of course this is not directed to you because you use a piezo): the "magic" of the circuit is the combination of the pickup's inductance and the series cap at the input. Any additional resistor (or balance pot) will dampen the circuit more or less...

bscur · Post by **bscur** » 04 Jul 2017, 06:17

Hi, I wired up one of these and put it in a Stingray. It sounds great, but the treble is working in reverse. I used a 1megC reverse log pot and double checked the wires, but the more you turn it on the more it cuts the treble. If I roll it off all the way, it boosts the treble. It sounds great, but the pot is working backwards. I tried swapping the outside lug wires, but that sounded horrible. Any suggestions?

deltafred · Post by **deltafred** » 04 Jul 2017, 09:31

bscur wrote:Hi, I wired up one of these and put it in a Stingray. It sounds great, but the treble is working in reverse. I used a 1megC reverse log pot and double checked the wires, but the more you turn it on the more it cuts the treble. If I roll it off all the way, it boosts the treble. It sounds great, but the pot is working backwards. I tried swapping the outside lug wires, but that sounded horrible. Any suggestions?

If you like the sound of a reversed C slope log pot then use a non reversed A slope pot.

I seem to recall in one preamp build I didn't have a C slope pot handy so used a linear (B slope). I must be happy with it because it has been like that for about 10 years and hasn't bothered me.

bscur · Post by **bscur** » 04 Jul 2017, 14:18

Hi,
My problem is that it works backwards. If I crank the treble knob to ten it is all the way off/cut/mellow. If I roll it back all the way it is brightest. I just want it to work like a normal tone or volume knob- the more you turn it clockwise, the more intense it gets. Somehow, it is working backwards.
Brad

PaulBass · Post by **PaulBass** » 05 Jul 2017, 02:21

the pot is wired wrong. thats the most common mistake. either that or you made a mistake in the circuit somewhere

av500 · Post by **av500** » 16 Aug 2017, 16:21

TI is stopping the LM4250 production, stock up while you can

https://tech.ebv.com/PCN/pcns_show.php?pcn=1502911127

PaulBass · Post by **PaulBass** » 17 Aug 2017, 21:04

I stopped using LM4250's when I found better opamps

Jarno · Post by **Jarno** » 06 Sep 2017, 17:21

It does add a bit of "je ne sais quoi" in this preamp though.
Somewhat less treble I suppose, and then there's the 50 fold (or so) reduced power drain when compared to tl072.....

Jarno · Post by **Jarno** » 15 May 2018, 08:02

To follow up, I did a layout for the Stingray preamp, which has a bit more room for film capacitors and bigger electrolytics.
Slightly bigger but still not too large:

Uses SMT resistors:

Let me know if someone wants a board.

bajaman · Post by **bajaman** » 15 May 2018, 08:39

thanks man
bajaman

temol · Post by **temol** » 06 Nov 2018, 11:52

Assume I'd like to mix piezo and magnetic pickup with a bass preamp. Is it just a matter of putting summing op-amp in front of 1u cap?

T.

tommymariotti · Post by **tommymariotti** » 01 Apr 2019, 20:21

Hi guys,
I finished this preamp now and I mounted it in my MM Stingray.
It work well but when the bass potentiometer is at maximum it's doesn't sound.
What could it be?

thetragichero · Post by **thetragichero** » 01 Apr 2019, 23:50

my first thought is something contacting ground that shouldn't be (I usually don't find this out until the laaaaast pickguard screw is fastened...)

Baja Music Man guitar and bass onboard preamps [documentation]