Bartolini active/passive tone control

[ffutterman]

This is probably a dumb question, but we go:

With the Bartolini preamps that have an active/passive switch and 3 (or 4) EQ controls, how do you control the tone in passive mode?

[macribas]

Unfortunately you don't. Some circuits have a passive tone control added for that.

[line6man]

You don't.

But you could add a passive tone control if you want it.

[ffutterman]

So, in theory, would it be possible/reasonable to add a bart MM pickup to a stingray and have the HR-2.4 AP preamp (Volume/AP p-p, Mid/Freq p-p, Treb/Bass stack) controls as well as an additional tone knob?

[line6man]

Originally Posted by ffutterman

So, in theory, would it be possible/reasonable to add a bart MM pickup to a stingray and have the HR-2.4 AP preamp (Volume/AP p-p, Mid/Freq p-p, Treb/Bass stack) controls as well as an additional tone knob?

Sure.

[MoireBass]

Sure! It's possible on any bass with an active/passive setup, and the best way to do it is to wire it so the passive tone control is only in the circuit when the passive option is selected, not all the time. Then put the switch in passive mode for a retro sound and active mode for a more modern sound.

As one example: if you look at Lakland's price list, waay at the bottom is an option to add a passive tone control to an active bass.

[line6man]

Originally Posted by MoireBass

Sure! It's possible on any bass with an active/passive setup, and the best way to do it is to wire it so the passive tone control is only in the circuit when the passive option is selected, not all the time. Then put the switch in passive mode for a retro sound and active mode for a more modern sound.

As one example: if you look at Lakland's price list, waay at the bottom is an option to add a passive tone control to an active bass.

Well, then you can't use a push/pull pot.
It takes a 3PDT mini toggle to do an active/passive switch with a tone control that's only in the circuit in passive mode.

A better option is to just use a high value tone pot so that when you turn it all the way up, it has very little effect.

[MoireBass]

Nope, you can do it with a DPDT and a push-pull pot. Connect the control and capacitor to the short wire that connects from one switch pole to the other pole in the preamp-bypass position.

[line6man]

Originally Posted by MoireBass

Nope, you can do it with a DPDT and a push-pull pot. Connect the control and capacitor to the short wire that connects from one switch pole to the other pole in the preamp-bypass position.

There are only two poles on a DPDT, one to switch the input of the preamp between the volume pots and ground, and the other to switch the output jack between the preamp's output and the volume pots. How are you switching the tone control in and out of the circuit if it would have to be tied either to the output jack or the volume pot's output?

[MoireBass]

Originally Posted by line6man

There are only two poles on a DPDT, one to switch the input of the preamp between the volume pots and ground, and the other to switch the output jack between the preamp's output and the volume pots. How are you switching the tone control in and out of the circuit if it would have to be tied either to the output jack or the volume pot's output?

I put in some ASCII art to show the schematic, but TB apparently strips leading space characters, so that didn't work.

OK, try this, the first pole of the double pole switch has the blend control output connected to the wiper [pin 2] and the preamp input connected to the 'active' end [pin 1]. The second pole has the volume control top connected to the wiper [pin 5], and the preamp output connected to the 'active' end [pin 4]. Then you have a short wire that connects switch pin 3 and pin 6 (the passive end) so that the signal from the blend control gets to the volume control when passive mode is selected. Connect the capacitor and passive tone potentiometer to that short jumper wire. Then, when the switch is set to active, the cap and tone pot are switched out of the circuit and not connected to either the blend or volume control.

[line6man]

Originally Posted by MoireBass

I put in some ASCII art to show the schematic, but TB apparently strips leading space characters, so that didn't work.

OK, try this, the first pole of the double pole switch has the blend control output connected to the wiper [pin 2] and the preamp input connected to the 'active' end [pin 1]. The second pole has the volume control top connected to the wiper [pin 5], and the preamp output connected to the 'active' end [pin 4]. Then you have a short wire that connects switch pin 3 and pin 6 (the passive end) so that the signal from the blend control gets to the volume control when passive mode is selected. Connect the capacitor and passive tone potentiometer to that short jumper wire. Then, when the switch is set to active, the cap and tone pot are switched out of the circuit and not connected to either the blend or volume control.

Draw it on paper and post a pic.

[line6man]

And FWIW, standard active/passive switching...

[line6man]

I suppose you could try it like this, but this would leave an infinite impedance at the input of the preamp.

[MoireBass]

Oh, you almost had it! Just move the jumper from switch pin 2 to pin 3 and you're there!

Maybe this will help:





The 1 Megohm resistors bleed off electrical charges and generally eliminate 'popping' when switching between active and passive. They should be therefore be located at the switch and not at the preamp.

Infinite input impedance generally isn't a concern. While it is certainly possible to design an amplification stage that is unstable when presented with an infinite input impedance, only a very poor circuit designer would select that topology for the first input stage of a preamp design. After all, your home stereo preamp doesn't blow up if you don't connect any input to the aux or tape inputs, does it? A preamp that went unstable when presented w/ an infinite input impedance would cause a lot of units to fail at the manufacturer's final test stage and that would be quite expensive for the manufacturer - and the engineers would very quickly be tasked with coming up with a better design.

Not sure what you mean buy 'standard active/passive switching' - there isn't a standards body (or a reason) to enforce one particular configuration over other possibilities. I can assure you that circuit designers use a switch (or any other component) in whatever way or direction works to solve the problem at hand.

You could choose to view the 1 Mohm resistors as finite impedances to ground, but 4.5 volts across a 1 Mohm resistor yields only 4.5 uA of bias current, well below the needed bias current of most bipolar op amps (op amps with FET front ends inherently have an infinite input impedance anyway), so really they play no role in the circuit except to bleed off the charge voltages.

BTW, the circuit shown above is exactly what my 1992 Warwick Thumb Bass and 2010 Lakland 55-02D use for active/passive switching. And is what Bartolini recommends:
http://www.bartolini.net/instruction...g/ppapsw_n.htm

And note that the passive tone control is not connected when active mode is selected.

[SnakeKappele]

Or just put the tone control after the preamp on a no-load pot. This way you can use the tone with the preamp if you want and totally remove it if you want.

[MoireBass]

Originally Posted by SnakeKappele

Or just put the tone control after the preamp on a no-load pot. This way you can use the tone with the preamp if you want and totally remove it if you want.

Actually, it'd be better before the preamp than after.

Sure, but most people don't want the passive treble rolloff *and* the preamp cut/boost controls affecting the sound at the same time. And if it's a two step-process to switch to active mode and also turn up the passive tone control so that it doesn't affect the sound, instead of just switching to active mode, well, I'm going to forget that second step sometimes (and get a timbre I don't really want). Maybe you're less excitable onstage and will reliably remember though.

[line6man]

Originally Posted by MoireBass

Infinite input impedance generally isn't a concern. While it is certainly possible to design an amplification stage that is unstable when presented with an infinite input impedance, only a very poor circuit designer would select that topology for the first input stage of a preamp design. After all, your home stereo preamp doesn't blow up if you don't connect any input to the aux or tape inputs, does it? A preamp that went unstable when presented w/ an infinite input impedance would cause a lot of units to fail at the manufacturer's final test stage and that would be quite expensive for the manufacturer - and the engineers would very quickly be tasked with coming up with a better design.

My concern was that having infinite impedances on both the input and the output with the leads so close to each other might cause the preamp to start feeding back into itself if you start boosting the gain/EQ too much.

I don't know why I didn't think of adding resistors though (Or doing the switch that way.) that would have taken care of any concerns.