Bass Block In Depth

A word on the technology behind the Bass Block 800 from Pat Quilter

Unlike our guitar amps, which have an intentionally low damping factor for greater warmth and tone, the Bass Block uses conventional voltage feedback to achieve a "stiff" output impedance, to maintain tighter control on large bass speakers. The actual damping factor for the Bass Block is approximately 40, which may not seem very high, but it is sufficient to reduce load-dependent frequency variations to less than 0.2dB, while leaving just a bit of "warmth" to avoid an overly sterile sound.

There's a lot more to effective bass performance than damping factor however. Amps that "don't deliver" probably suffer more from lame pre-amp voicings, inadequate power supplies, poorly behaved clipping, and lack of thermal capacity - all possibly acceptable in consumer gear but not to be compared to standards in the pro-audio field, like the many reliable QSC amps that we have made over many decades.

The Bass Block starts with a solid pro-audio power platform, derived from the very successful QSC K-series speaker series, but with higher current FETs to meet the needs of high current bass speakers. The Bass Block has a well-regulated power supply that supports 90V peak outputs, and the output current limit is sufficent to drive full voltage down to about 2.5 ohms, to ensure robust performance into real-world 4 ohm cabinets. We combine this with an artfully designed preamp with bass-specific EQ points and a very effective multi-section "bottomless" limiting system that prevents obtrusive clipping without limiting the dynamics or smooth surge of power.

Getting back to damping factor, the term expresses the ratio between the amp's internal impedance, and the impedance of the load, which is mostly the resistance of the voice coil. The presence of the voice coil resistance will always put a limit on how much "control" the amp can put on the cone motion; once the amp resistance is well below the voice coil resistance there is very little change even for "infinite" damping. It would be more enlightening to look at the inverse term, which is called "regulation" - the percentage of voltage drop from no-load to full-load. A damping factor of 1:1 (very low) equates to a regulation of 50%, which would allow a highly audible 6dB change in frequency response vs speaker impedance peaks and dips. 10:1 equates to a regulation of 9%, yielding less than 0.8dB frequency loading effect, which is already "pretty good". At 40:1, we're at 2.5% regulation, or 0.2dB - and 1000:1 or 10000:1 will only cut this negligible effect closer to zero. (In contrast, our guitar amps are designed with a high output impedance, causing a very audible frequency vs impedance response of up to 12dB, which brings out the full tone of guitar speakers).

So bottom line: good bass performance starts with a foundation of a reasonably "stiff" amplifier, with the all-important high voltage and current reserves to cope with speaker impedance changes, and a power supply having adequate capacity to support heavy low frequency transients. Using this foundation, we add a good-sounding preamp that has been artfully voiced to bring out the best qualities of the modern bass guitar, plus a nice set of real-world convenience features, all in a 4-pound package.

It's true that modern bass speakers are much closer to conventional pro-audio practice than guitar speakers, and are tuned to perform well with stiff amplifiers; we have found however that chasing that last couple of percent of regulation has no benefit in transient response, and if anything, it's better to leave a small amount of warmth in the response.

Those readers seeking an old-school tube-amp bass sound will like our 200-watt Tone Block series, which play louder than you would think due to the “inflationary” properties of our guitar-amp output impedance. These are well suited for bass players who like to exploit overdrive. The Bass Block will “growl” a bit when pushed, but never really bottoms out.

Best regards

Pat Quilter

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