I'm looking to get a pickup for my upright bass and amplify it with my PAS (2 base modules). In researching pickups, I find that different pickups come in different impedances - ranging from 1 meg and up. Further research leads me to believe that one needs to be able to match the pickup impedance to the amp's input load. I read somewhere on this forum that inputs 1 and 2 (of the tone engine) have an input impedance of 10 megs. I understand that this will work for piezos, but I suspect that it may not be optimal number for all pickups. Is there a way to vary this input impedance? Bassist Steve Rodby talks about this and other double bass amplification problems here: http://contrabassconversations.com/guests/episodes/steve-rodby/
Summit Audio makes a preamp with input impedance control and phase reversing. http://www.summitaudio.com/td-100-preamp.htm
Hopefully, I won't need this extra piece of equipment, but if I do, how would I hook this up to the tone engine?
Any guidance is appreciated.
Bewildered,
Dave Little

Hi Dave,

Looks like you are new here. I am glad you joined us.

Here is a link to a discussion about input impedance for the T1®. It's actually in the middle of a really long discussion that deals with all kinds of issues - so watch specifically for exchanges between Hilmar-at-Bose and Tom Munch.

Information about new T1 ToneMatch® Audio Engine.

That should get you started.

Hi Dave,

I had a little more time to revisit this and look at this link:
http://contrabassconversations.com/guests/episodes/steve-rodby/

I'm not sure what you want us to read there, so could you quote the text that is of interest into a message here?

With respect to the impedance matching - From Hilmar's comments (see link in my last post), I think the preamps in Channels 1,2,3 or the T1® are about 1 megOhm impedance, and should be fine for most passive Piezo pickups. Certainly, any instruments with passive Piezo pickups (several) that I have tried have been fine. Just for comparison, they have all sounded much better in T1® Channles 1,2,3 as compared to T1® Channels 4/5 or running straight into inputs designe for typical line level inputs.

I suggest that you go ahead and try your pickup in Channels 1,2,3 and see how it sounds. Definitely do that before spending money on other devices.

Hi Dave,

Short answers: Pickups will not be negatively affected if connected to a higher input impedance. It is not possible to vary the input impedance of an L1 power stand or T1 input without the use of outboard gear. The need to vary the input impedance of these inputs for your application is highly unlikely. As ST said, try it first to see if you like what you are hearing.

Very long answer: (I will attempt to give a technical answer assuming that many readers are less technical than I am and may be curious to know more about input impedance. Readers equally or more technical than I are encouraged to point out any errors I may have made.)

A pickup can be considered to be an electrical generator; a sound wave causes movement between coils and magnets or creates mechanical stress onto crystals or ceramics which in either case generates an electric potential. The input into which the pickup is connected provides an electrical load in which the electrical potential can flow.

Lets consider a 1 kilowatt gas powered generator that, for this example, does not have a load circuit breaker or fuse. It is able to provide 1000 watts at 120V 60 Hertz. If you connect a 1000 watt incandescent lamp (load), the generator is working at capacity. When you connect an additional 1000 watt lamp both lamps light up but will be much dimmer since the generator's capabilities have been exceeded.

Also consider that the lamp filament is a resistor. When the second lamp is connected to the generator, the two filaments are in fact connected in parallel. For example, if you connect two 8 ohm resistors in parallel the resulting load is 4 ohms. A 4 ohm load is considered to be a "heavier" load than 8 ohms. You could say that when you lower the load impedance you create an electrical situation that "encourages" more current (Amperes) to flow. The heaviest load possible is a short circuit which encourages an unlimited amount of current to flow. If you were to measure the resistance of 1000 Watt, 500W, 100W and 40W lamps you would see that the resistance of the 1000W lamp will be the lowest and the 40W lamp's resistance will be the highest.

Now back to pickups... Piezo pickups have a higher impedance than magnetic pickups, 1 megohm and up is typical as you pointed out, and connecting them to an input load of 1 megohm or higher is typically considered an appropriate load. But, if one connected the piezo pickup to a typical 100 kilohm input (DI), then the load would be considered electrically "heavier" and this heavier load creates a situation where more current will be expected from the pickup. The frequency response of the pickup would suffer as the pickup has limited current generating capabilities and a guitar pickup generates electrical potential at many frequencies. The lower frequencies are most noticeably affected. (My theory is that more energy is required to reproduce lower frequencies and thus are most likely to cause the energy limit to be exceeded and this is most noticeable at lower frequencies.).

Connecting a magnetic pickup to a 1 megohm input would not have a negative effect on it's frequency response since it is in fact connected to a lighter load. Most quality guitar pedals have an input impedance of 1 megohm or more.

I don't know if there is a minimum pickup impedance/load impedance ratio stated anywhere. I do know that know that if someone connects a piezo violin pickup directly into an input of 100 kilohm or less the usual results are an unpleasantly scratchy sound (especially when connecting directly into a typical mixer where the line level input is usually 10kohm and 2kohm for a microphone input).

Notes: Ohm is used for both resistance and impedance but they are not the same. The filament resistance of an incandescent lamp varies with temperature. The impedance of a speaker varies with frequency. Incandescent lamps are often used to protect tweeters because of the aforementioned varying resistance characteristic of the filament. Speakers are not usually used to protect light bulbs. This last paragraph should be ignored...

ST,
You will have to listen to the audio, as I don't see a written transcript of Steve Rodby's interview. The one I am referring to is at the top of the page and is entitled "CBC 85: bass gear with Steve Rodby". It lasts about 30 minutes and he gets to the meat of the issue about 1/2 way through. Its a good listen, even if you're not a double bass player, because his comments can be useful to anyone using pickups -particularly piezos. He talks about the impedance thing as well as phase and envelope -in layman's terms.

Le5,
Wow! Thanks for the lesson in impedance 101. This is an area that has always baffled me, but hasn't been a cause for concern until lately. You've cleared things up considerably. If you have the time, could you listen to the Rodby interview and comment here?

I will certainly try going straight to "the Bose" before putting other stuff in the chain. I'm confident that it will sound great (the amplification that is, not my playing). First I must spend some time "fretting" over which pickup to buy?

BTW,
Over at http://www.talkbass.com/forum/forumdisplay.php?f=44
I ran across this gadget that might be useful when you're at a jam session or some other gig where you have to use the house system: http://personalpages.tds.net/~fdeck/bass/hpfpre.htm
Its a little impedance matching gizmo.

Thanks
Dave Little

Hi Dave,
I've heard that interview as well. It's a great interview and very informative. Definitely worth a listen for anyone who struggles with amplifying an acoustic instrument - especially bass.

I'll just add that I've been using L1 Model 2 and B1s plus a T1 as my sole acoustic bass rig for quite a while now, and I've got no trouble finding a good solid tone. So before you add any outboard gear, take a listen to how it sounds direct -- you may find that with the bass presets and tone controls in the T1, you don't need anything else.

-Jesse

Hi Dave,

I'm glad the information is helping you deepen your understanding. I read a post roughly a year ago in either the Guitars and Other Strings or Bass Guitar forum that inspired me to research the relationship between pickups and input impedance. The information I posted is a snapshot of how my mind understands the subject.

I listened to 22 minutes of "CBC 85: bass gear with Steve Rodby" until my somewhat high speed connection ground to a hault! I believe that I likely heard most of what you wanted me to listen to. It was very interesting and helped me better understand why you started this tread. I particularly liked his idea of varying the input impedance in order to change the tone of an instrument/pickup. There is at least one DI on the market with that capability. I think it is made by Radial, but there are likely others.

I disagree with a few statements that Steve made, which to me, seemed to imply that most or all inputs of mixers or instrument amplifiers have low input impedance. I believe that many devices are guilty of having been designed with unnecessarily low input impedance but there is also a growing number of equipment, old and new, that can provide adequate and even excellent pickup loading characteristics. But, it can be very difficult to figure out which ones would be better suited for base guitar pickups. Piezo pickups are the most likely pickups to suffer tone loss from the effects of low input impedance.

I agree with Steve that there are many factors that affect the selection process of a pickup for an instrument. I wish you a pleasant experience on your pickup selection journey.

Remember that the simplest setups are the ones that most often offer the greatest satisfaction. For example, I use a Dean Markley Pro Mag sound hole pickup in my 12 string acoustic guitar. Although I am very happy with the sound, I still wonder if a different pickup would bring possible improvements. I had an opportunity to test drive an LR Baggs M1 active after reading positive comments here in the forums. It is indeed a great pickup. It allowed me to discover that because of the way I play, my little and ring fingers often hit the guitar pick guard. This is not audible with the Pro Mag but annoyingly noticeable with the M1. (I am not ready for a discussion on choices of pickups for my guitar at this time, I will eventually start a thread in the appropriate forum. I mention it here only to raise the point that if a setup sounds good it is not necessary to keep searching.)

The simplest setup is the L1/B1s that you already own. The addition of the T1 may likely offer you greater possibilities as it is a wonderful tool.