Rack preamps for bass: why designed with low output?

Far too often I find that rackmount preamps for bass guitar are designed with main (non DI) outputs that are unbalanced and that put out fairly low levels (under 1V RMS).  Yet the PA-style power amps that people expect to drive them with tend to require relatively high levels of input signal (over 1V RMS) for optimal operation, and very often the signal at those inputs is dropped by a further 6 dB if the input is unbalanced.

There are certainly exceptions: some power amps have low input sensitivity ratings, some preamps have sufficiently high output levels, and some combinations of them are ideal (or at least adequate) whether balanced or not.

But the majority trend is out of whack.  As a result, many bassists have gotten far less efficient (powerful and toneful) performance from their rig than they could have gotten.  And there has been a great deal of misunderstanding about why one preamp or power amp will sound wimpy (weak or toneless) compared to another, when almost every time the real problem was a mismatch of levels.

I once raised this complaint on a bass forum, and most the replies fell into one of two categories:  engineers who agreed, and non-technical types who said “you’re crazy, professional rockers have been doing it this way for decades, nobody ever had a problem before!”  The thing is though, just because large numbers of people have made do with what they had, doesn’t mean they couldn’t have done better. Face it, most musicians are not trained in the more technical aspects of their equipment.  And when confronted with technical issues, many of them say “quit listening with your calculator–music is about feeling and intuition, not science.”  But what needs to be brought forward is the understanding that knowledgeable application of science can make your feelings and intuitive artistry come out louder, clearer, and better!

After all, what makes an amp/cab rig sound great is how well it was designed, and how well the components work together.  Most people wouldn’t hook a 10 watt solid-state amp up to an 8×10 fridge cab, or hook a 1,000 watt PA amp up to a single 10″ speaker.  Those are obviously extreme examples, but the exact same idea applies matching the in/output levels and un/balanced connections between preamps and power amps.

Again, the handful of engineers that I’ve talked to all agree, and consider it basic obvious information.  So why do the engineers that actually designed the preamps not understand this?  Why is there such a blatant disconnect between preamp designers and power amp designers?  More particularly, since PA power amps are comparatively large-scale commodities, and bass preamps are very small productions, why don’t preamp designers make it a point to ensure their products will operate ideally with the large majority of commercially-available power amps?

What’s especially weird about this disconnect is that mic preamps and channel strips designed for recording almost always have the correct type and level of main output: +4 dBu, balanced.  So the ridiculous failure to communicate is almost entirely on the part of bass guitar preamp designers specifically.

Note to industry: open your eyes!

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  1. Sebastian Storholm said,

    December 24, 2009 @ 10:23 am

    Have you checked what kind of signals the inputs on “bass power amps” need? Like the big ones from SWR for example?

  2. Cyrus said,

    December 25, 2009 @ 5:37 pm

    I haven’t checked all of them, but they are just as much of a problem (in terms of random, thoughtless design) as PA amps. In fact the input sensitivity of the SWR Power 750 is almost 1.6 V, which is way “worse” than most PA amps!

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