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Old 17th December 2007, 03:44 PM   #1
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Default Question about "Little Rebel" 9v and 4.5v sources

I'm planning on building the "Little Rebel" to be battery powered (with an optional input for a power adapter), and I have a question about the power source.

I've previously built the Ruby amp from http://runoffgroove.com, which was a simple 1x 9v power source.

The Little Rebel schematic has two sections, the "top" and "bottom" (if you look at the schematic you'll see what I mean). The top section has several "4.5v", "9v", and "9v (2)" inputs. the bottom section appears to have "9v" and "9v (2)" outputs which I'm currently assuming correspond to the inputs in the top section.

However the "4.5v" part of the bottom section isn't listed as an "output" and I'm wondering if somebody could take a guick glance at the schematic and clear up the 4.5v connections. Should all the "4.5v" leads be connected?

Little Rebel homepage: http://rebel.massberg.org
Direct link to schematic: http://rebel.massberg.org/source/schematic.pdf

I'd appreciate it if someone could give me some insight into how to handle the power sources, and what the story is on the power sources in the schematic.

Also, if anyone has any experience with this amp or can tell by the schematic, is a single 9v battery sufficient to power this amp for a significant period of time? It's quite a bit more complex than anything I've built before, so this is all kind of a learning experience.
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Old 17th December 2007, 05:23 PM   #2
gootee is offline gootee  United States
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Hi hollywoodb,

The bottom-most section is the "power supply". On the left, it has the single 9-volt supply input connections for the whole unit.

After the reverse-polarity protection diode, it has a resistive divider (2x 47k) to ground, with the two equal-valued resistors dividing the input voltage by two, and then a 10uF smoothing capacitor for the 4.5v output.

Since this is a single-polarity supply, signals must be referenced to half the supply voltage, to enable them to swing downward as well as upward (since the input signal's voltage would typically be swinging upward and downward from zero volts). The 4.5-volts power supply output is used as that "midpoint reference" voltage (instead of using ground as the midpoint, as would be done in a dual-polarity system).

So the wire labeled "4.5v" in the bottom section is an output from the power supply section, and should be connected to all of the "4.5v" points in the top sections.
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Old 17th December 2007, 05:46 PM   #3
gootee is offline gootee  United States
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Offhand, I don't have a good "feel" for how long this might run with a single 9V battery as the power supply source.

You would at least need to specify the load impedance, to be able to calculate it.

But at least the LM386 datasheet does mention that the it can be used for battery-powered devices.

Looking at the LM386 datasheet (from national.com), the specs say it could typically provide 700 mW with an 8-Ohm load, with a 9V supply. And you'll have 2x LM386, in a bridged configuration. Ignoring the bridging, one LM386 doing 700 mW into 8 Ohms would be pushing a current of i=sqrt(P/R) = sqrt(0.700/8) = 296 mA. I guess that's probably a typical maximum average (or RMS?) current? (Sorry. My brain is not working too well, today.)

I suppose you could do some sort of worst-case current-draw analysis, and add all of the currents consumed, and divide a 9V battery's typical mA-Hours rating by that current, to find a worst-case battery lifespan. But someone else here will probably be able to just give you a reasonable guess.

Alternatively, it should be relatively easy to put the whole circuit into the LTspice simulator (free from linear.com), and get some better data, perhaps even using a WAV file music source for the signal input.
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Old 18th December 2007, 06:54 AM   #4
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thanks for the response, that pretty much clears up everything I was "iffy" on.... much appreciated

I've previously built amps from runoffgroove.com on a 9V battery and on average they seem to last at least 10 hours on a single battery, so if I can get half that on this circuit I'll be happy

My plan is to actually build 6 completely separate and isolated switched circuits into this battery amp, 3 from runoffgroove.com, one variation on the well-known smokey amp, one called the "noisy cricket", and the one I questioned above.... I want each amp circuit completely closed off from the others, and I've priced out the whole setup around $150.... I think it'll be a fun project. I've got the pricing and PCB layout done, and I plan on wiring in 6 speakers in two different 8-ohn configurations, i'm just kind of having fun with this one.
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