The BSM AP-WA Treble Booster is a medium gain treble booster from the German maker BSM, designed mainly for guitars fitted with medium or low output PAF style humbuckers, mini humbuckers or P90 pickups. BSM describes it as "almost a clean booster with a special frequency response", overdriving a tad with the guitar volume at full and staying perfectly clean below a setting of 9 on the gain knob1. The pedal is a tribute to the live tone of Andy Powell of Wishbone Ash, and its two-way tone switch also reaches toward Michael Schenker and Gary Moore1.
What it does to your sound
BSM's manual says the AP-WA dramatically increases the dynamics of the signal chain, and that muddy sounding neck pickups can be cleared up with transparency1. Because it is close to a clean booster, most of the character comes from how it shapes frequencies and drives the amp's input rather than from heavy clipping of its own. BSM states the average output level is 5 dBm, with a maximum output voltage of 4V when the strings are struck really hard1.
Gain knob and tone switch
The AP-WA keeps its controls simple: one gain knob and a two-way mini toggle for tone matching12.
| Control | What it does |
|---|---|
| Gain | Works counter clockwise for technical reasons: the most gain is at the full left position12 |
| Tone switch, fatter position | BSM says it achieves Gary Moore's famous Les Paul with Marshall tone1 |
| Tone switch, more slender position | Enables copying Michael Schenker's Flying V live tone on UFO's "Strangers in the Night"12 |
The gain knob turns the opposite way to most pedals. BSM notes that "for technical reasons the gain control works counter clockwise, gives you the most gain on the full left"2. If the pedal seems to have no gain as you turn it up, you are turning it the wrong way.
Power and placement
A 9V battery powers the unit, with a current consumption of approximately 260 uA, and the pedal can also be powered with 9 volts through its external DC jack1. BSM notes that the negative pole of the battery is ground1.
Placement matters with this kind of pedal. BSM's manual is direct: "The AP-WA Booster has to be inserted between guitar and amplifier, not into the FX loop"1. Like other treble boosters, it is meant to hit the front end of the amplifier, not the effects loop.
Players and tones behind the design
BSM states the first intention for the AP-WA was to pay tribute to the pearly-sweet and creamy live tone of the British guitarist Andy Powell with Wishbone Ash1. The manual also points to Rainer Baumann's highly dynamic and creamy sound with the German band Frumpy, for example on "How the gypsy was born", when using an ES-335 type guitar12. The tone switch then covers two more named sounds: the fatter position for Gary Moore's Les Paul with Marshall, the more slender position for Michael Schenker's Flying V on UFO's "Strangers in the Night"1.
Videos
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Dirk Gentz Rocker
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Questions
Is the BSM AP-WA Treble Booster true bypass?
Yes. According to BSM's manual, all BSM boosters work true bypass when switched off, and the AP-WA is one of them. That means the pedal's circuit is taken out of the signal path entirely when it is off, so your guitar's signal reaches the amplifier unchanged until you switch the booster on.
Does the BSM AP-WA run on a battery?
Yes. BSM's manual says a 9V battery powers the unit, with a current consumption of approximately 260 uA. Because the draw is so low, a battery lasts a long time. The pedal can also be powered with 9 volts through its external DC jack, and the negative pole of the battery is ground.
Which way do you turn the gain knob on the AP-WA?
Backwards compared with most pedals. BSM notes that for technical reasons the gain control works counter clockwise, so the most gain is delivered with the knob turned fully to the left. If the pedal seems quiet when you turn it up, turn it the other way.
Can the BSM AP-WA get a Michael Schenker tone?
BSM says so. With the mini tone switch set to the more slender tone position, the manual states you can copy Michael Schenker's Flying V live tone on UFO's live classic "Strangers in the Night". The fatter position of the same switch is aimed instead at Gary Moore's Les Paul with Marshall sound.
Does the AP-WA go before the amp or in the effects loop?
Before the amp. BSM's manual is explicit that the AP-WA booster has to be inserted between guitar and amplifier, not into the FX loop. Like most treble boosters, it is designed to drive the front end of a tube amplifier, where the amp's own input stage does part of the work.
Sources (2)
- untitled, BSM, 2025
- AP-WA Treble Booster, BSM (treblebooster.net), 2025