Double Bass Pickups: Optimal loading, not impedance matching

[drurb]

The mention of "impedance matching" with regard to (especially piezo) pickups and pre-amps is rather frequent here. In an effort to help, I offer the following re-write of something I posted earlier:

Impedance matching refers to the adjustment of impedances of two stages (output and input) such that the maximum power is transferred between the two. It's something that's quite relevant for power amps and speaker cabs (although true matching almost never occurs-- rather intentionally, but that's another chapter). Here's a link.

With regard to pickups and pre-amp inputs, the input impedance of the pre-amp is chosen such that the load "seen" by the pickup results in a desirable frequency response. What's being optimized in this case is the load impedance (load optimization, if you will) with no regard for maximizing power transfer from the pickup. The output impedance of the pickup is not even close to "matched" by the input impedance of the pre-amp. That would be quite undesirable. I posted about this in more detail here.

 

[Don Sibley]

I'm not sure this is the right place (because I don't think I'm experiencing an impedance issue) but I am trying to figure out what is going on with my sound. I know we talked about changes in perceived tone in connection with "volume" here (http://www.talkbass.com/forum/f4/ehrlund-blog-sort-909299/index32.html) but I'm pretty sure I'm experiencing something more like what Mike is talking about in post #247 here (http://www.talkbass.com/forum/f4/big-e-4x5-910188/index13.html).

On my iAmp and my GK heads, with the EAP and FC, I seem to get a more even frequency response when I follow the guidelines listed in the Big E thread. The problem is, even with the output gain set to 5 O'Clock, almost maxed, I can only get the input gain to about 10 O'Clock before the tone quality starts to degrade and takes on a brittle high-midrange, emphasis. The problem being that I can't get enough volume before the tone gets "wonky".

The other night I went to see a friend play and I heard the same sound from his set up. He was using a schertler into a doubler with a 12 M-line on a very loud gig. There was no way I was hearing his bass directly. On a set break I mentioned the he could try opening up the output gain and adjusting the input gain down and I swear it seriously reduced that midrange twang and he was, fortunately, able to be just as loud. Now, I understand this was not necessarily scientific but I feel like my experience is pretty consistent.

Is there no empirical evidence that preamp gain has an effect on the frequency response or tonal profile of a piezo?

 

[drurb]

Well, as I said, and as is consistent with Mike's post:

...Unless one is over-driving an amplifier stage, it doesn't matter whether the gain is achieved via the input or the master, although the choice can affect signal-to-noise ratio.

What Mike discussed was basically an overload situation.

Aside from overloading, weird loading effects could, theoretically, occur between an input pot and the source (e.g., a piezo) but I don't think that applies at all to the circuits being discussed.

 

[Don Sibley]

OK, so it's possible that I'm not, at least in this instance, crazy?

I'm just hoping that I can find a head that will allow me to get the volume I expect from my MAS45 without the midrange honk.

This is what worries me. I expect that the 100 watt GK head will run out of juice pretty fast but my iAmp Pro is pushing 600 watts into that cab and I can't keep up with a moderate drummer in a quartet without sacrificing tone. With the Ehrlund and the preamp feeding the Pro and the fDeck S3 in the effects loop, I was hitting the limiter at about 11 O'Clock. Anything at 10 or below sounded fine, but again, not loud enough for reasonable gig.

I ordered the new Clarus+. Even if this head does solve the problem I would still like to understand what is happening for future reference.

 

[drurb]

The mention of "impedance matching" with regard to (especially piezo) pickups and pre-amps is rather frequent here. In an effort to help, I offer the following re-write of something I posted earlier:

Impedance matching refers to the adjustment of impedances of two stages (output and input) such that the maximum power is transferred between the two. It's something that's quite relevant for power amps and speaker cabs (although true matching almost never occurs-- rather intentionally, but that's another chapter). Here's a link.

With regard to pickups and pre-amp inputs, the input impedance of the pre-amp is chosen such that the load "seen" by the pickup results in a desirable frequency response. What's being optimized in this case is the load impedance (load optimization, if you will) with no regard for maximizing power transfer from the pickup. The output impedance of the pickup is not even close to "matched" by the input impedance of the pre-amp. That would be quite undesirable. I posted about this in more detail here.

The last link in this old post was no longer working. Here's a repaired link.