Q About Dying Batteries - Why Do They Cause a Hiss?

[Paul Marossy]

Maybe a dumb question, but why is it that as a 9V battery starts getting low (around 8 volts or so), that it causes a hissing noise in some effects?!

[Paul Marossy]

Would it have something to do with the impedance changing as the voltage drains?

[Stevo]

Never noticed to say!! A dying battery sounds better than new?
Which schemes, and do you use a 47uf cap in parallel to ground on your effects

[Paul Marossy]

The circuit I am talking about specifically is my Maestro Boomerang clone. When the battery gets down around 8V, it starts to hiss pretty loudly. I never thought about it, but it doesn't have a cap on the power supply. Maybe that is the reason why?

[R.G.]

my Maestro Boomerang clone. When the battery gets down around 8V, it starts to hiss pretty loudly. I never thought about it, but it doesn't have a cap on the power supply.

There are two possibilities. First, the internal battery impedance itself may be contributing hiss. The internal resistance is low, so I doubt it adds much hiss. Second - and more likely - is that  your pedal oscillates for one of many reasons when the voltage and/or impedance gets to there. RF oscillation in amps sounds like loud, aggressive hiss. i suspect it does in effects as well. Putting in a powre filter cap may solve it because it stops the hiss. Try a 100uF aluminum, and also try a 0.1uF ceramic. Either may stop it.

[Paul Perry (Frostwave)]

RG is right as usual, but it is true that as batteries age, the power rail noise increases dramatically.
I accidentally came across some "battery stack power supply" noise level CRO pics on a hi-fi site the other day, pretty suprising! No cap there, of course.
(cant remember the site, was looking for stuff on the "amplified zener" regulated power supply).

[Paul Marossy]

Thanks guys! I think RG is correct in the case of Boomerang clone, it will oscillate some with the pot at the most extreme setting possible with an 8.2K for the emitter resistor on "Q2". Actually, it's more like a shrill, shrieking sound as you switch it on/off (this goes away if you use an 82K resistor instead). So... it may be going into an ultrasonic oscillation when the battery gets low, and this is heard as a more aggressive hiss. I could check that out on a scope, but I'm not sure that my 20mHz scope would even pick that up. BTW, the transistors in this case are MPSA18s. I have also used 2N5089s, and they also work fine.

Anyhow, like Paul Perry says, I have noticed this "more hiss with a low battery phenomonon" on some other effects over the years - but nothing specific comes to mind.