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Double Bass Impedence question about SWR California Blonde

J

JoeB

Dec 19, 1999
31
0
Toronto, ON, Canada
#1
I am currently looking into buying an SWR California Blonde because of it's great sound and because it has a stereo input (TRS) that I can use with a two transducer K&K pickup I have.

In my current set up, I use a Raven Labs PMB to blend the two signals from my Pickups into my Ampeg B50-R. I would love to cut my setup time (and reduce the gear I haul) by using an amp that does not require a preamp to handle a high impedence piezo pickup.

What I would like to know from you Blonde users out there is this: are you able to go straight into the amp without using a preamp and are you able to get a good sound in doing so?

Thanks,
Joe
 
#2
I use a California Blonde, but have never used the stereo input. The inputs on this unit will sense the impedance, and adjust accordingly. I don't know the figures that they adjust from/to. I have run the Underwood, BP-100 and Fischman Full Circle straight into it with good results. I usually run thru a Fishman Bass Blender mounted on my stand, but I only do this for easy access to EQ and volume controls. I've been very happy with the Blonde.

Jim Lownds
 
#5
JoeB said:
... I would love to cut my setup time (and reduce the gear I haul) by using an amp that does not require a preamp to handle a high impedence piezo pickup...
Click to expand...

What you're asking is kind of an impossible task. Here's why:

A coaxial cable plugged into a multi-megohm input looks like a big capacitor, which means not only will you get a tremendous amount of high frequency loss, but the cable will act like a big condensor microphone. As you flex the cable it'll generate noise.

My AI amp has inputs labeled low (1M) and high(10M) impedance, but I still find that the fishman performs far better when run into a preamp mounted on the bass tailpiece. Good audio practice says that ideally any element in the chain should have infinite input impedance, so it doesn't load down the device feeding it, and zero output impedance, so it can handle any load.

You can't in practice get ideal conditions, but you *can* get close. A good preamp will present a very high impedance- and hence negligible load- to the pickup. This is important, since a piezo generates high voltage but can supply almost no current. The high impedance means that the piezo isn't being asked to do something it can't.

The output of the preamp is very low impedance, whcih means it can supply all the current needed to drive the cables and the amp without running out of power and distorting in some way.

A good compromise would be a simple, one-FET preamp mounted as close as possible to the pickup that would draw very little current but would isolate the pickup from the amplifier. I'm working on prototytping Don Tillman's popular circuit (found here:)

http://www.till.com/articles/GuitarPreamp/

into a compact package. When I get it finished I'll report back.
 
#7
mje said:
A good compromise would be a simple, one-FET preamp mounted as close as possible to the pickup that would draw very little current but would isolate the pickup from the amplifier. I'm working on prototytping Don Tillman's popular circuit (found here:)

http://www.till.com/articles/GuitarPreamp/

into a compact package. When I get it finished I'll report back.
Click to expand...
I'm very interested!
Please report back!
 
#8
mje said:
A good compromise would be a simple, one-FET preamp mounted as close as possible to the pickup that would draw very little current but would isolate the pickup from the amplifier. I'm working on prototytping Don Tillman's popular circuit (found here:)

http://www.till.com/articles/GuitarPreamp/

into a compact package. When I get it finished I'll report back.
Click to expand...
I'm interested in this as well. What about the little line driver that PUTW makes (and sells separately) - would this be similar to Tillman's?

[Invalid or Expired Link Removed]

(Here's me not knowing squat about electronics).
 
#9
mje said:
What you're asking is kind of an impossible task. Here's why:

A coaxial cable plugged into a multi-megohm input looks like a big capacitor, which means not only will you get a tremendous amount of high frequency loss, but the cable will act like a big condensor microphone. As you flex the cable it'll generate noise.

My AI amp has inputs labeled low (1M) and high(10M) impedance, but I still find that the fishman performs far better when run into a preamp mounted on the bass tailpiece. Good audio practice says that ideally any element in the chain should have infinite input impedance, so it doesn't load down the device feeding it, and zero output impedance, so it can handle any load.

You can't in practice get ideal conditions, but you *can* get close. A good preamp will present a very high impedance- and hence negligible load- to the pickup. This is important, since a piezo generates high voltage but can supply almost no current. The high impedance means that the piezo isn't being asked to do something it can't.

The output of the preamp is very low impedance, whcih means it can supply all the current needed to drive the cables and the amp without running out of power and distorting in some way.

A good compromise would be a simple, one-FET preamp mounted as close as possible to the pickup that would draw very little current but would isolate the pickup from the amplifier. I'm working on prototytping Don Tillman's popular circuit (found here:)

http://www.till.com/articles/GuitarPreamp/

into a compact package. When I get it finished I'll report back.
Click to expand...

While this is true, in practice some cable shunt capacitance may actually be desirable to roll off some high end . Without this effect, some piezo pickups may sound way too bright. Then you just end up rolling off the top end with EQ. In my experience, this is often the case anyway.

An FET buffer stage at the source (pickup) could be a good idea. I would suggest however using a unity-gain buffer stage like the one in the circuit I've attached here**. It presents the source with a very Hi-Z input and has a very Lo-Z output but provides no gain. It also does not invert the signal by 180 degrees like the preamp on Don Tillman's web site does.

Usually, it is considered good engineering practice to maintain the absolute polarity of the signal path. I know people who would readily hear that the sound from their bass and from their amp were "out of phase" -- kind of like a pair of stereo speakers being out of phase with each other.

If you do need some gain, you could connect 2 stages like the circuit shown on Tillman's site in series -- possibly with a 50 to 100K ohm audio taper pot in between them (in place of R4 on the 1st stage) to adjust gain. BTW two inverting stages in series would provide a non-inverted output (output in phase with the input).

(**Most any low-noise, general-purpose, N-channel FET, like the 2N5458 shown, can be used for the buffer circuit. Also the Drain and Source connections are usually interchangeable.)

-bob
 

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#12
robgrow said:
...
Usually, it is considered good engineering practice to maintain the absolute polarity of the signal path. I know people who would readily hear that the sound from their bass and from their amp were "out of phase" -- kind of like a pair of stereo speakers being out of phase with each other.
Click to expand...

That's a very good point- I'm used to making electronics for solid-body instruments, and forgot that my DB has an acoustic output, too! ;-)

I think I'll build your circuit instead.
 
#13
Just wanted to point out that the FET buffer is not my design. It's really just a standard source follower circuit with a couple of things added to make it a more practical circuit. The 10K resistor in series with the input and the FET's gate and the 10pf capacitor from gate to ground form an RF filter network, so you don't pickup channel 13 on your next gig. Anyway this basic circuit is found in the front end of many devices including some condenser mics with FET impedance converters.

If you do want to build an FET buffer, I would highly recommend that, at minimum, you read and take heed all the information about FET's on Don Tillman's site. When you buy your FET's, it is indeed a good idea to get a few extra. While it's a simple circuit, reasonable soldering and circuit layout and building skills are required for it to work properly. (Sorry for the disclaimer!)

-bob
 
#14
I referred to it as "your design" to differentiate it from Don's. I have enough parts in the junk box to build a few examples of both, so I may just do that. Just need to find some perfboard I can cut into 3/4" or 1/2" square pieces.

I noticed the other day that a guitar company is offering a built in *tube* based preamp in some of their guitars- I think they they run reduced plate and filiment voltage, and run the tube in a non-linear region to get some "tube sound". Maybe I'll build a clip-on tube preamp for the hard core audiophile bassists ;-)
 

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