Yamaha Revstar "Boost" Control

BillDL · April 2023

I've watched a few videos about the Yamaha Revstar guitars, but in one by Phil McKnight he shows an old style small transformer in the control cavity and explains and demonstrates how this works to boost the signal when the relevant control is used.

I don't know of any other manufacturer that incorporates a transformer in a passively wired guitar in this way. I don't know enough about inductance, resistance, impedance, etc to know how an old style transformer used as a "step-up" might affect the signal. Phil McKnight's video gives a reasonable idea of the end results of the boost control when used even though I don't know from an electronic point of view the signal is being affected by the transformer wire coils, but I would have wondered about an EMF (or whatever the resultant interference would be called in this case) that might be problematic having a transformer in such close proximity to the other wired components.

I'm also curious to know if there have been any other production guitars using a step-up transformer this way.

Neilybob · April 2023

I bought a pedal from a guy who had one of those 100 Gibson Les Paul. It had only 3 knobs and a second switch in place of the bottom tone pot. The switch when on bypasses all the pots and enables the bridge pickup on max volume so like a boost.

https://reverb.com/item/5296200-gibson-les-paul-100-deluxe-2012-gold-top?utm_source=rev-ios-app&utm_medium=ios-share&utm_campaign=listing&utm_content=5296200

BillDL · April 2023

Yeah, I recall some guitars with a bypass button that worked as a boost because it retained more of the pickup signal rather than pots being able to bleed part of the signal to ground. Having a transformer in a circuit wired to step up voltages is only going to add a lot more impedance to the signal and, I would have thought, lose a lot of high frequencies and reduce the overall signal again. The only thing I can see it really doing is boosting the signal but with only mids and lows retained.

JohnCordy · April 2023

@BillDL that's a good description of it. It basically boosts but also rolls of high end basically like rolling off your tone a bit.
Works ok for Brian May or jazz type tones, but isn't super useful in my experience

BillDL · April 2023

Thanks for that @JohnCordy ; So it's not really a "boost" as such. A bit of a misnomer if it's only boosting the overall level of the signal with certain frequencies retained to compensate for the loss of some frequencies in the process. I wasn't intending buying a Revstar, but I had been wondering about whether it might be a useful mod to do to a guitar I already have if I knew what transformer was used.

thermionic · April 2023

You can’t boost a signal without an external source of power. Unless you’re exploiting special relativity and converting mass to energy, which I doubt Yamaha are doing!

roberty · April 2023

thermionic said:

You can’t boost a signal without an external source of power. Unless you’re exploiting special relativity and converting mass to energy, which I doubt Yamaha are doing!

I have a passive step up transformer for my turntable. It steps up the voltage and steps down something else (I'm not sure). Essentially the volume is boosted though. Such things exist

ICBM · April 2023

thermionic said:

You can’t boost a signal without an external source of power. Unless you’re exploiting special relativity and converting mass to energy, which I doubt Yamaha are doing!

It depends on what you mean by boost. Using a step-up transformer will increase the signal output voltage, and if very little current is being drawn - as it is if the load is a typical 1M input impedance of a valve amp etc - then the signal will effectively be boosted. The power won't be, but it's voltage not power which is detected by the amp.

Also, an LC circuit - which this could also be if it contains a choke rather than a transformer - can boost a signal by up to 6dB at the resonant frequency, without any external power source. The energy is drawn from narrowing the bandwidth so other frequencies are reduced - which this could also be doing if it's rolling off top-end.

So it's not contravening the laws of physics.

BillDL · April 2023

Thank you for the input. @ICBM ; When you say "choke", what would that look like? The photos I saw of the component in the Revstar resembled an old style transformer with what looked like 2 separate coils. Could a choke look like that?

ICBM · April 2023

BillDL said:

Thank you for the input. @ICBM ; When you say "choke", what would that look like? The photos I saw of the component in the Revstar resembled an old style transformer with what looked like 2 separate coils. Could a choke look like that?

If there are two coils it's either a transformer or a humbucking choke, depending on how it's wired.

BillDL · April 2023

Thanks @ICBM.

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Yamaha Revstar "Boost" Control

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