how much damage could an 18v booster do?

[darron]

hey guys. a friend of mine bragged to me about a boutique booster that's just a 9v opamp challenging me to make something better for him. i was thinking of making a 17v opamp hifi booster using the geofex voltage pump. this would give double the clean headroom. but with a potential 17volt signal output, what if anything could that actually end up damaging? obviously you wouldn't plug it into the mic input of a home stereo cranked to the max. of course you'd distort a lot of effects later in the line, and even preamps, but would it have too much potential to reach damaging point for many devices?

what do you guys think?

thanks (:

darron

[alanlan]

I dare say that there are some which could be damaged - the problem is there are so many different designs out there you can't possible account for them all.

If you take the LM741 for example, it has an absolute maximum input voltage rating equal to the supply (http://www.national.com/ds/LM/LM741.pdf) so if an input signal of +/- 7V or so is connected directly via a coupling cap to the non-inverting input of one of these with a single 9V rail, will it cause damage?  Well National Semiconductor don't guarantee it won't.

If your amplifier has an LM741 on it's input, you would probably expect that the supply rails would be +/- 12-15V, so normally they would be OK and/or there may be some extra added protection like series resistors or clamp diodes etc. but not all equipment is well designed. 

Valve pre-amps are pretty robust so I don't think you need worry about these.

(edit)
I should also add that there are probably loads of designs out there capable of chucking out more than double your average 9V battery supplied pedal and I don't know if these generally cause a problem ( I was thinking of stuff like Boss pedal boards and the like - I can't imagine these use a single 9V supply rail internally).  Perhaps someone else can chip in?

[darron]

thanks for the input (:

if national say that the absolute max input threshold is equal to supply, perhaps they just mean this is the case for correct and intended operation (ie that is here you loose headroom and start clipping)? that's the sort of question that crosses my mind. there's a a geofex article that goes on about how to protect the inputs of semiconductors, but it's not really the sort of thing that i see in schematics or pedals except for the series resistors. i think that was a point that you were making also.

i'm still interested to know if i shouldn't be building a 17v booster. it'd be cool for driving an amp, but it'd be an amp in itself and who wouldn't at one stage just turn the gain knob all the way clockwise? in my hands that sounds great but maybe not to give away.

any other experiences?

[Mark Hammer]

You know, it IS possible to stick a volume control on the output of these things.  And it IS possible to stick a resistor in series with the input of the volume pot so that the max volume is never reachable (a kind of safety-resistor).

But here's the thing.  To achieve the maximum possible signal output with that supply and whatever device/s used, given the amplitude of the input signal, how much gain would have to be applied?  And when all that gain is applied, how much of what comes out the output jack is amplified hiss?  Humungous clean boost to a clean signal is wonderful - that's what gets done with voice mics in a studio every day of the week - but that input signal has to be clean, and most guitar signals aren't.  At least not enough that you'd feel comfortable applying a gain of 100x to them and sending them off unfiltered to the amp.

And here's the other thing.  At what point does additional clean headroom in a booster disappear when it hits an amp input stage with significantly less headroom?  We had a thread here last week form someone who was dismayed that a booster into the dirty channel of their amp produced no discernible volume hike, and that was the problem - no more headroom left at the inn/in.  Your emphasis should be on heard headroom, not isolated hypothetical headroom.

Now, is there some value to running a device intended to be clean at a higher supply voltage?  Yes.  Do such devices by default aim for the maximum possible output?  Rarely.

[alanlan]

if national say that the absolute max input threshold is equal to supply, perhaps they just mean this is the case for correct and intended operation (ie that is here you loose headroom and start clipping)?

quote from datasheet:
"Note 2: "Absolute Maximum Ratings" indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits."

I don't think you shouldn't be building a 17V booster.  I used to use a micro-amp into a JCM800 and it sometimes fell a little short i.e. the micro-amp was clipping before it hit the amp (especially if the battery was a bit worn), so I say go ahead and try it but just be aware of what you are driving.

Also, most guitars are more than capable of producing a good 800mV pk-pk peak output (mine certainly is and it's just a stock Telecaster) so it doesn't take that much gain to get a lot more unclipped signal than you can achieve with a single 9V battery.  If we're talking about high gain and noise etc. then most distortion pedals produce a hell of a lot of gain internally which is why they can be so noisy.

I would try it out and let us know what you think.

[aron]

I never had a problem with the Shaka HV run on 18V or even higher.

[andrew_k]

Maybe it only uses its voltage for clean output and not pure boost, but the MI Audio Boost n' Buff runs at up to 24v and puts out 35dB... Is that the level of boost and voltage you're considering?

[the_random_hero]

if national say that the absolute max input threshold is equal to supply, perhaps they just mean this is the case for correct and intended operation (ie that is here you loose headroom and start clipping)?

quote from datasheet:
"Note 2: "Absolute Maximum Ratings" indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits."

I don't think you shouldn't be building a 17V booster.  I used to use a micro-amp into a JCM800 and it sometimes fell a little short i.e. the micro-amp was clipping before it hit the amp (especially if the battery was a bit worn), so I say go ahead and try it but just be aware of what you are driving.

Also, most guitars are more than capable of producing a good 800mV pk-pk peak output (mine certainly is and it's just a stock Telecaster) so it doesn't take that much gain to get a lot more unclipped signal than you can achieve with a single 9V battery.  If we're talking about high gain and noise etc. then most distortion pedals produce a hell of a lot of gain internally which is why they can be so noisy.

I would try it out and let us know what you think.

800mV? My guitar has twin humbuckers and with both pickups selected at once it puts out around 380mV for a hard strummed chord 

[darron]

Mark, I'd very much expect that it would distort the preamps pretty quickly, but that's what he actually uses boosts for. At least you'd get more of his amp flavour rather than an overdriven silicon opamp. It would no be used so much as an actual volume boost pedal, but for lead and increased distortion. i think he uses it before pedals too for more gain.

alanlan, thanks for finding that quote. looks like hitting a lot of pedals with a 17v peak to peak signal could in fact do some damage, as you might expect ):

with the microamp (which would be almost what i had in mind anyway), did you increase the gain potential? my theory is that with double headroom you could double gain. like you said mentioning output signal, i allocated 1v output and while trying to reach pretty much the limits of the booster (in response to mark's comment) i figured i could set gain at about 20, which supposedly is about 26db

i expect more than 380mV from any of my humbuckers since i can get them to clip schottky diodes pretty strong and easilly.




so considering all of the feedback that i've gotten, a 17v amplifier might be a good idea if it still only gives out a similar output to a 9v one, with about 20db gain, but i almost don't see that point in that since it would be sitting among it's other 9v pedal friends. a 17v amplifier could be a very good idea, but then it may as well just have a higher voltage input instead of the voltage pump. a 17v pedal to drive a tube amp could be a very good idea indeed, but put in the wrong hands it could make someone unhappy when (when....) it gets missused in the wrong place.



thanks for your input guys. i'll refile the idea in the back of my head somewhere for later.





(: