Double Bass Input Impedance

[Jared Houseman]

Could anyone point me twords a comprehensive guide to pickup impedance, why it's important, and how to correctly select products?

In a recent thread I asked about using a mixer to mix a mic and pick up and was told I need an input with the correct impedance.

In the past I have always used a pickup (k&k bass max) into a standard di box or occasionally straight into the the mixer. I've always have had great results but maybe I just don't know what I'm missing.

 

[jlmorgan84]

Short non-techincal answer: Input Impedance less than 1 megOhm will result in all the bass being cut out of your pickup signal (especially a piezo) and will make your pickup sound strident, thin, and generally bad. Mixer line inputs are usually much lower than 1 megOhm, although there are some mixers that offer an instrument input and those should generally work fine.

Only matters what the pickup sees immediately following it, so if you're plugging into a DI and the DI has a suitable input impedance you can plug it into whatever just fine.

 

[DoubleMIDI]

Never directly plug gour piezo into a passive DI. This will bring the input impedance down a lot.
Some active DIs might work. But keep in mind that the input impedance of the DI is in parallel with the input impedance of the amp/following device which results in a reduced input impedance.
The good thing is that in solid state electronics (no valves) the input impedance is set by a resistor and if there is a FET or FET-OpAmp (usually when the device has a MegOhms input impedance), the resistor could be changed to a higher value (i.e. 10 MegOhms instead of 1 MegOhm).
It is easy to bring the impedance down by a resistor in an adapter cable from hot to shield, but even then this resistor acts in parallel with the internal one of the following device.

 

[Ric Vice]

Could anyone point me twords a comprehensive guide to pickup impedance, why it's important, and how to correctly select products?

In a recent thread I asked about using a mixer to mix a mic and pick up and was told I need an input with the correct impedance.

In the past I have always used a pickup (k&k bass max) into a standard di box or occasionally straight into the the mixer. I've always have had great results but maybe I just don't know what I'm missing.

Jared,
Check your Private Messages I left you some information.

 

[Pat Harris]

Never directly plug gour piezo into a passive DI. This will bring the input impedance down a lot.

To clarify, NOTHING BAD will happen by plugging your pickup into a passive DI. The tone might not be "optimal", but you won't hurt any gear.

FWIW, I've done many festival gigs with backline and plugged directly into a passive Radial J48 and had very usable tone. Many times, the impedance matching is going to introduce more "open" or "full" low end to your signal which will just get cut in the context of a full mix anyway.

 

[Ric Vice]

I remember when I got my first Walter Woods MI-100-8 and how much better it sounded with an Underwood Pickup then other amplifiers did.

I called Don Underwood and we chatted for a bit, he pointed out that when Walter was building some of his first MI-100-8 amplifiers, he had Don send him a pickup and

he set the input impedance of his amps to achieve the best sound with that pickup. The result was a 1 Meg Ohm load on the inputs of his amplifiers. Ampeg did the same

with there B-15N's in the 1960's. The input impedances of the B-15N are Channel 1, 5.6 Meg Ohms, Channel 1, 1 Meg Ohm and Channel 2 2.2 Meg Ohms. Ahthough,

Piezo pickups were not avaliable commercially until around 1975, Ampeg set their amps so that they could be used with all kind of pickups that were around at the time.

So impedance loading does help the bass sound better. Just my take,

 

[lz4005]

Could anyone point me twords a comprehensive guide to pickup impedance, why it's important, and how to correctly select products?

In a recent thread I asked about using a mixer to mix a mic and pick up and was told I need an input with the correct impedance.

In the past I have always used a pickup (k&k bass max) into a standard di box or occasionally straight into the the mixer. I've always have had great results but maybe I just don't know what I'm missing.

Another way to think about is that magnetic pickups usually have an output in the tens of thousands of ohms range. Many, maybe even most, amplifiers and mixers that are designed with the assumption that they'll only be receiving magnetic pickup signals.

Unbuffered piezo signals are in the million ohms range.

Sending a million ohm signal into something designed to receive tens of thousands of ohms results in a scratchy, no low end, bad sound.

That's why if you're going to plug a piezo pickup into most amps you need to have a preamp in between to buffer the signal from millions to tens of thousands so it sounds better. Or, of course, an amp designed to receive that type of signal. This applies to any piezo pickup signal from any instrument, not just bass.

 

[Dr. Love]

It is easy to bring the impedance down by a resistor in an adapter cable from hot to shield, but even then this resistor acts in parallel with the internal one of the following device.

Could you expand on this like is there any specific resistor value=ohm ratio? Also, would using a potentiometer instead of a fixed resistor provide for a variable impedance where you could "tune in" to an optimum setting?

 

[jthisdell]

Invalid Link Removed

Mark has a good explanation on his website.

 

[DoubleMIDI]

Could you expand on this like is there any specific resistor value=ohm ratio? Also, would using a potentiometer instead of a fixed resistor provide for a variable impedance where you could "tune in" to an optimum setting?

Rtotal = (R1 x R2) / (R1 + R2)

With a potentiometer you can get variable impedance, but potentiometers in this range are more sensitive to contact failure than potentiometers below the MegOhms range. Also the resulting resistances of parallel potentiometers with a fixed resistor is no longer linear.

In a preamp it might make sense to have a potentiometer in series with a 1 MegOhms resistor. That way you still have linearity and extend the fixed resistor value.

A switch to short the potentiometer can be used to get to the 1 MegOhms range quickly and in case of a noisy potentiometer can get to a clean sound again.

In a existing circuit you need to break a connection to either insert a larger fixed or variable resistor in series. (In series the resistances simply add up.)