# MIM Jazz power output - how many milliWatts?

Discussion in 'Effects [BG]' started by Skip, Nov 2, 2001.

1. ### Skip

Mar 22, 2000
Bronxville, NY
I'm buiding a headphone amp and limiter for my MIM Standard Jazz (so I'm completely passive), and I was wondering if anyone knew how many milliWatts runs out of the bass. I'd really like to know the ohms and milliVolts, but I won't push my luck. I just want to make sure I wire in enough gain to the amp.

I don't have a 1/4" plug with bare wires out to test with a multimeter, but I'll have to put one together unless someone here can help.

I appreciate any help, otherwize I'll just wing it.

Chris

2. ### lost_in_space

Nov 16, 2001
France
actually i don't really know the impedance@output of jazz pickups, but i've got some experience in effects design & construction, so i thought i could share some insights.

power really doesn't matter as far as pickups are concerned, and here's why:

1. it is totally insignificant, so low it really means nothing = a fraction of a microwatt. passive pickups have a dc resistance which ranges from 15kohm to 2kohms. I think a jazz pickups is approx 7 kohms but frankly i don't remember exactly. their output voltage varies, but for general design purposes i assume output to be 1 V peak to peak.

2. you cannot just say okay RxI²=Power, because we deal with AC and a pickup has a complex impedance (which involves inductive and capacitive capacitance). it has a resonant frequency, which means that power is purely ohmic for a given frequency only, and reactive for all other frequencies, with a I/V phase
lag that varies according to the frequency. So in order to have the right expression of power you would need to determine watts, frequency, power factor.... tedious to say the least. even the pickup manufacturers don't bother.

3. even for AC signals, usual formulas such as P=VxI are meaningful only for a sine wave. A musical instrument's signal is everything but pure sinewave. there is no easy way to calculate power of a complex waveform (that would involve heavy fourier transform stuff etc.)

4. measuring signal is difficult due to extremely fast transients and ample dynamics. a digital scope is a must. just plugging a DVM is not enough to give meaningful results plus there's the waveform problem - a normal DVM would give false readings.

What really matters for you is the following:

- a pickup should plug into a high impedance load if you want to keep the signal as clean and faithful as possible. For practical purposes, assume high impedance to mean at least 10 times the pickups' impedance. The higher the cleaner, 1 Mohms is standard for stomp boxes and most amps, and you should use that value too. a lower impedance cuts some treble, that is up to you.

- assume signal for a passive pickup to be 1v p to p. Make your calculations based on p to p values, not rms. However, you may need to adjust your design assumption by trial and error. A standard 9v battery usually provides enough headroom, except if you are into sharp Q filters and other special applications.

- a straightforward IC such as LM386 or similar (I know the japanese - toshiba - make some amazing miniature audio output ics) is easy to implement and provides ear splitting volume (put an attenuation pad at the output if necessary).

hope that helps.

3. ### Skip

Mar 22, 2000
Bronxville, NY
Lost_in_Space,

Thanks for the reply, that is what I was looking for. I'm building the amp with a Burr-Brown OPA134 opamp, wired for gain of about 11. It is powered by one 9v battery running +/- 4.5V. The input pot is a 1M ohm audio taper so I should be presenting an appropriate load. And 1V p to p is just right for the limiter so I should be OK there also.

What do you usually build your boxes out of? I'm using a Hammond die-cast alluminum case with a black powder coat finish (cool looking and fairly indestructable).

Thanks again,

Chris

4. ### lost_in_space

Nov 16, 2001
France
hi chris

sounds like what you're putting together is good stuff. OPAs are good. 9v single supply with center tap is ok too, but put liberal amounts of decoupling. x11 is quite a lot of gain, you could get away with less (you would have less noise too). Less gain usually means also better stability.

By the way, check for stray oscillations if you can, sometimes they occur outside the audible range, causing lots of noise, static, excessive power consumption and generally bad sound. Put decoupling caps or an RC network where appropriate.

A funny thing that happens sometimes when op amps overload is a sort of awful farting sound due to bad recovery from overloaded condition. Putting backward biased diodes (1N4148 ok) in parrallel with the supply caps helps.

Regarding casework - good question. Casing is the most tedious part of DIY electronics. I have used all sorts of things : bent sheet metal ala Electroharmonix (a pain in the arse), plastic boxes, Teko enclosures for desktop equipment. I use Teko plain sheet aluminium boxes for making pedals, but they need a lot of work to get a half decent finish, cause they come with some sort of clear enamel on, that I need to strip before I get started with my own stuff. And theres no battery trap, so you have to unscrew the box just like the old MXRs.

The Hammond stuff you talked about seems great but its not available in Europe as far as I know.

I havent come up with a fully satisfactory recipe yet, I need to experiment more on paints, primers, lettering, etc.

So the outside may look like junk, but the inside..., and the sound.... my, my, my....