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In the Forum: Audio Discussions
In the Thread: Small SET’s bass, besides everything- is it about power?

Post Subject: Weak SET bassPosted by nl on: 6/17/2009

The weak bass of low-power SET amps is well known. I don't think anyone would disagree.
Some reasons:
Simple low power, especially with 93db speakers (try with 106db like Yorkville LS1208 horn subwoofer it might be different).SE output transformers have narrow bandwidth, typically optimized for midrange not bass. A -1db rolloff at 30hz is noticeable.SE amps have high output impedance, typically around 2 ohms, compared to <0.5 ohms for solid state. This leads to poor woofer control, and woofers have the greatest back-EMF.SE amps tend to have rather flabby power supplies, typically passive with smallish capacitance. This is especially apparent in the bass because output Z of a powersupply capacitor rises with lower frequencies.SE amps often have rather flabby driver stages like 6SN7. These are chosen for their midrange sound, not for "maximum control."Lastly, all tube amps are a little looser in the bass than solid-state, even when they have lots of power and negative feedback.
To maximize the bass response of for example 300B SET, used full-range (ie without compromising midrange and treble), a typical strategy would be active tube-regulated powersupply and 5K OPT transformer (lower output impedance), and maybe a more burly driver stage like 6BL7.

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