Series/parallel and level matching

[dincz]

I'm using a Seymour Duncan SMB-4A with a series/parallel switch wired direct to the output jack. As you'd expect, there's an enormous level difference between the two settings. I'm thinking of wiring in a simple L-pad to reduce the level in series mode. Is the extra pickup loading likely to be a problem? I know I could just try it but thought I'd ask first - I'm lazy.

 

[Jazz Ad]

The difference in output is usually the main reasons you want the switch.

 

[dincz]

The difference in output is usually the main reasons you want the switch.

The switch is there for the tonal difference i.e. more or less inductance. If it was only a level difference I'd just wind the volume up or down on the amp.

 

[dincz]

On second thoughts maybe a series resistor would be better, although I guess it would lower the Q of the pickup and broaden the resonant peak.

 

[RobbieK]

I've wired this sort of thing many times. A L-pad is fine. I've posted diagrams on this board several times for this as well. Just use a series-parallel switch with an extra pole or two that kicks in the attenuator in series mode. A couple of 100K resistors is a good place to start.

Of course it will darken the series tone a little but that's fine for me as it makes the series sound even more of a contrast to the parallel sound.

I presume your bass is passive...

 

[bassbenj]

On second thoughts maybe a series resistor would be better, although I guess it would lower the Q of the pickup and broaden the resonant peak.

I have done this on my G&L L2500 and it helps but is not the end-all solution. The reason for the volume change is that when pickups are in parallel the two coils interact giving a certain output. they sort of cancel so two pickups is not louder than one pickup. However when you put two pickups in series the voltages in each add and that creates twice the voltage which means it's 6 dB louder.

If you put a pad after the pickup it changes the impedance of the pickup as seen by the tone control and that changes the tone as well as the way the tone control reacts to adjustement. Ideal would be an active bass where you can switch in a resistor to change the gain 6 dB without changing the pickup loading. An Idea I had (but have not tried yet to see how works) would be to put an L pad on the OUTPUT to the jack you switch in and out. That way it doesn't interact with the pickup and is sort of just like turning down the volume. The problem is that it may take more poles on the switch than the simple series R above.

What I have tried is the above circuit. And as I said it sort of changes the tone]. So I rather compromise. I tried resistors until I got a tone change I could live with and at the same time it toned down the level change so it was easier to live with too.

 

[dincz]

Thanks for the feedback. The bass (a Thingray ) is passive at the moment but I've made a Stingray 2 band preamp on stripboard. Haven't got around to wiring it in yet because the bass just sounds so good as it is. It seems that evening out the level difference in passive mode is going to be a compromise, so I guess I'll go for switchable gain in the preamp or an output pad as suggested.

 

[RobbieK]

An Idea I had (but have not tried yet to see how works) would be to put an L pad on the OUTPUT to the jack you switch in and out. That way it doesn't interact with the pickup and is sort of just like turning down the volume.

I guess I'll go for switchable gain in the preamp or an output pad as suggested.

Actually, what I've found, especially with a single pickup MM style active bass, is that an L-pad on the input is preferable to the output, or any other gain-set adjustment that happens after the input buffer.

The reason is that the pickup is buffered directly by typically 1meg, with no volume or blend pots between it and the pre, and I think you'd be disappointed by just how similar the series and parallel sounds are when their volumes are the same.

The advantage of having the L-pad on the input of the preamp for series mode, is that you are both attenuating the pickup and also lowering the input Z of the pre. This gives you a much more recognisable series tone. IOW, warmer. Therefore making the switch far more useful. As I said, I've had this type of wiring in many of my basses over the years. It works especially well as a slap switch.

If you do go ahead and put a preamp in, you may want to think about a switch that goes from passive-series to active-parallel. This is great as a slap switch.

 

[dincz]

Actually, what I've found, especially with a single pickup MM style active bass, is that an L-pad on the input is preferable to the output, or any other gain-set adjustment that happens after the input buffer.

The reason is that the pickup is buffered directly by typically 1meg, with no volume or blend pots between it and the pre, and I think you'd be disappointed by just how similar the series and parallel sounds are when their volumes are the same.

The advantage of having the L-pad on the input of the preamp for series mode, is that you are both attenuating the pickup and also lowering the input Z of the pre. This gives you a much more recognisable series tone. IOW, warmer. Therefore making the switch far more useful. As I said, I've had this type of wiring in many of my basses over the years. It works especially well as a slap switch.

If you do go ahead and put a preamp in, you may want to think about a switch that goes from passive-series to active-parallel. This is great as a slap switch.

Food for thought, thanks. It's hard to make a direct comparison because of the level difference but, as you say, there doesn't seem to be a great difference in tone between series and parallel modes. Looks like more tinkering may be required.

 

[RobbieK]

there doesn't seem to be a great difference in tone between series and parallel modes

Umm, that's not really what I said. If the pickup is buffered directly as is usually the case with a single pickup active bass, then yeah, if you match the outputs without altering that (high) input Z, then the tonal difference is often barley noticable.

In just about all other cases, when the pickup is loaded differently in each mode, or if it's loaded by a pot or two, or if it's a passive bass, then yeah you can hear the difference easily.

Years ago, I modded a pot to have three gangs. This was for a series-parallel blend pot that also attenuates the series sound as you sweep the pot. I really heard this problem when I went to put it in my MM pickup P bass. I ended up using a filter cap to roll off some treble for the series sound to sound more realistic.

 

[dincz]

Umm, that's not really what I said. If the pickup is buffered directly as is usually the case with a single pickup active bass, then yeah, if you match the outputs without altering that (high) input Z, then the tonal difference is often barley noticable.

OK. As it's running passive at the moment with no controls, the only loading on the pickup is the cable capacitance and the amp's input impedance. That's a fairly light load and probably close to the buffered situation (except for the extra C of the cable). Would that account for the lack of difference?

 

[RobbieK]

wired direct to the output jack

Apologies! I didn't see this in your original post.

Would that account for the lack of difference?

yes, for sure.

Again, sorry to confuse the issue.

If all you have is a single switch on your bass, then yes, I'd definitely go for an l-pad. A single in-line resistor as an attenuator will act differently for every amp, pedal and DI you plug in to, depending on their input Z.

If it's just a mini toggle, then a 3-pole one is cheap and easy to find.

Here's a diagram for an L-pad attenuator in series mode.

Two equal resistors will give 6dB of attenuation and this will usually do the trick. With a good lead, and a modern amp, a couple of 68K or 82K resistors should sound about right. For 7dB attenuation, use a 68K for the resistor that goes to earth, and an 82K for the resistor that's mounted on the switch. For 5dB, swap them around.

Note that the L-pad is true bypass, and will have no effect in parallel mode.

 

[dincz]

Thanks for the tips. I think this thread is done

 

[RobbieK]

cool. I'm glad it worked out for ya.

 

[tiglon]

I ended up using a filter cap to roll off some treble for the series sound to sound more realistic.

Thanks RobbieK! This was driving me nuts! I used two 82k resistors in my MM with the diagram above and it worked! Now, I'm wondering what cap did you used to give "series" a little bit more treble. Thanks again!

 

[RobbieK]

Cool. Glad it worked out for you!

wondering what cap did you used to give "series" a little bit more treble. Thanks again!

Actually I was talking about another thing there- a series-parallel blend pot instead of a switch. Plus I was talking about taking treble away from the series setting not adding it.

Honestly, I can't remember the value of the cap, and it was in a different context anyway. But if you put a cap in parallel with the resistor that goes to earth, then that would roll a little treble from the series sound. Plus it would be out of the circuit when in parallel mode. Off the top of my head, I'd think something in the region of 1nF would be a good place to start.

If you want to brighten the series sound, you will have to raise both resistors. Maybe try a pair of 220k resistors.

Or use an active buffer I guess. But if you go that route, you are better buffering and boosting the parallel sound, than attenuating the series sound. I've wired many basses with a simple buffer pre that has 6dB of boost in conjunction with a series-parallel switch so that you have active-parallel in one setting and passive-series in the other. I call it a "night and day" switch.

 

[tiglon]

Cool. Glad it worked out for you!

If you want to brighten the series sound, you will have to raise both resistors. Maybe try a pair of 220k resistors.

Thanks! I guess the volume will go down a little bit, but I'll try it.

 

[RobbieK]

I guess the volume will go down a little bit

If you want to make the series sound louder, (IOW you want less attenuation), you have to change the ratio of the resistor values. Maybe go for a 270K resistor to earth, and a 220K resistor on the switch. This will equate to about 5dB of attenuation. Two equal resistors is 6dB. Well, really, that's in isolation. With a passive system, everything is interactive, even your guitar cable.

I'm wondering why you want the series sound brighter? why not just switch to parallel?

Oh and try a no-load tone pot. This will take the tone pot out of the circuit when it's turned right up. This will make both settings slightly brighter. So will new strings, so will a good cable that's no longer than it needs to be, so will a good speaker etc etc etc blah blah blah...

 

[tiglon]

I'm wondering why you want the series sound brighter? why not just switch to parallel?

With the 82k resistors, I have to turn all way up the treble pot to hear a little brightness. I just want a little bit more to achieve slap sound with series mode (like Flea on Aeroplane).

 

[Will_White]

With the 82k resistors, I have to turn all way up the treble pot to hear a little brightness. I just want a little bit more to achieve slap sound with series mode (like Flea on Aeroplane).

That was probably a parallel MM pickup not series, I could be wrong tho.