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Old Today, 03:41 AM   #81
dotneck335 is offline dotneck335  United States
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Join Date: Dec 2012
Quote:
Originally Posted by PRR View Post
Looks near-right to me. On steady Sine the DC current would be ~~320mA. (Remember two current paths working alternately through a Totem Pole.) Even heavy guitar is not as severe, over a full set, as a steady Sine. Apparently they like a 50% fudge factor between test and playing
How did you calculate the ~320mA?
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Old Today, 07:21 AM   #82
Gnobuddy is offline Gnobuddy  Canada
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Join Date: Mar 2016
Quote:
Originally Posted by dotneck335 View Post
Is this an aural observation, or have you measured it?
A fellow by the name of Roly Roper on the Aussie Guitar Gearheads forum found a Microcube in the dumpster, and tried it out with his electronic keyboard. He noticed the very drastic bass roll off, then went looking for a measured frequency response.

I don't recall if he measured it himself, or found a measurement someone had made - either way, he posted a measured frequency response, and it had the drastically attenuated bass response I mentioned. This was measured across the speaker terminals I believe - it was smooth and uniform, and didn't have the jaggies you always see when you measure the acoustic output of a speaker with a microphone.

It's been a couple of years since I saw Roly's post, and I can't remember if the measured response was 3 dB down at 500 Hz, or 1 kHz. (I think - but am not sure - that it was actually 1 kHz. That shocked me at the time.)
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Originally Posted by dotneck335 View Post
I wonder if they do it electronically in the power stage, or if it is just an artifact of the crummy little 5" speaker?
This was definitely electronic, though I don't know at which stage in the chain. My guess is that you're probably right about it being in the power amp.

Quote:
Originally Posted by dotneck335 View Post
My application (C6 lap steel) requires less bass, as the lower strings are tuned ~ a fifth above a regular guitar.
That could be an advantage for battery operation.

In a mix with bass and drums, I find I don't miss the bottom octave of electric guitar (high-pass at around 160 Hz). Sometimes the guitar "sits in the mix" better if I high-pass it as high as 300 Hz, though that sounds quite thin on its own. But it gets along better with the bass guitar, drums, and/or left hand of the keyboard player.

If you're going to be playing along with drums and or bass, you might get away with much less deep bass from your lap steel than you would think.

-Gnobuddy
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Old Today, 08:41 AM   #83
Gnobuddy is offline Gnobuddy  Canada
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Join Date: Mar 2016
Quote:
Originally Posted by dotneck335 View Post
How did you calculate the ~320mA?
If it is a normal push-pull output (and not two amps with the speaker hanging between them in bridge mode), I think we may have been overestimating the (already very limited) capabilities of the microcube.

You are always going to lose a volt or more to output device saturation at each end of the voltage swing, so with a 6 volt supply, it's very unlikely you will ever see a clean sinewave bigger than 4 volts peak to peak across the 4 ohm load. More likely the max is closer to 3 volts peak to peak.

Being generous and assuming 4 volts peak to peak across 4 ohms, we have a peak voltage of only 2 volts.

That means peak speaker current = 2V/4 ohm = 0.5 amps

RMS power to speaker = peak power/2 = 1 watt.

For fun, I threw together a simple push-pull amp in LTSpice; resistance values are tweaked to bias it perfectly in the simulator, but this is not a practical circuit for real-world use where parameter spreads and temperature variations have to be dealt with.

In the sim, as I expected, even with no battery voltage droop, the output starts to clip before you get a full 4 volts peak-to peak to the speaker. Peak speaker current never gets to 500 mA, but tops out at around 380 mA.

The green half-sine curves are the current drawn by one of the two output transistors. The blue ones are the current drawn by the other output transistor. They are slightly unequal because the amp is crude and there is some distortion at full output.

While those peaks get to 380 mA, there is a lot of empty space between the peaks; the average value for a full-wave-rectified sinewave like this is the peak value multiplied by 2, and then divided by pi (3.1416).

In our case, while peak currents are 380 mA, the average current from the battery will be less than two-thirds of that, or around 240 mA. And that's at maximum clean output.

So what we're looking at is a 1 watt amp whose output stage probably draws about a quarter of an amp at full clean output.

I have no idea how much current all the digital stuff in the preamp draws. Considering how puny the output stage is, the preamp might draw a significant proportion of the total power.

It is possible the microcube actually uses a pair of output amps in bridge mode, in which case the output power might approach a thundering 4 watts RMS (clean), rather than the 1 watt I estimated from a single amp.

-Gnobuddy
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