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Old 28th April 2004, 12:02 AM   #21
sss is offline sss  Israel
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Quote:
Originally posted by Stocker
I am beginning not to be surprised at how willing people here are to answer even dumb questions. Very helpful.
thats because its easy to answer dumb questions
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Old 28th April 2004, 12:34 AM   #22
azira is offline azira  United States
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Quote:
Originally posted by tiroth
Well, we know the voltage output is
  • 10V @ 2k (5mA)
  • 5V @ 175R (28.5mA)

deltaI = 28.5 - 5 = 23.5mA
deltaV = 5V
R= V/I = 5V/23.5mA ~ 212R

So 200 in parallel will be about 1 ohm output impedance, or a damping factor of 4.
Interesting, the datasheet shows about 200 ohms of impedance at the output also.
However, the output resistance should be divided by the open loop gain in a feedback amplifier, so you'll have 1/200th of an extremely low output resistance or... extremely low...
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Old 28th April 2004, 01:41 PM   #23
Stocker is offline Stocker  United States
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Quote:
Originally posted by sss
thats because its easy to answer dumb questions

Well golly, I'll just have to keep asking them to he'p other folks look smart!

Quote:
Originally posted by azira
Interesting, the datasheet shows about 200 ohms of impedance at the output also.
However, the output resistance should be divided by the open loop gain in a feedback amplifier, so you'll have 1/200th of an extremely low output resistance or... extremely low...
So then, would the output resistance be...low?
gee, maybe I'll hook up a few subwoofers in parallel for 0.5 ohms load for this bad boy!

but seriously, the open loop or closed loop gain?
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Old 28th April 2004, 01:48 PM   #24
tiroth is offline tiroth  United States
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Azira is correct, the apparant output impedance will be reduced due to feedback. The actual output impedance, though, cannot be altered by feedback and has a profound effect on the amount of current that can be supplied to the load.
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Old 28th April 2004, 03:46 PM   #25
Stocker is offline Stocker  United States
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oooookaaay...

So what use is an apparent output impedance when it is different from the actual output impedance? The bit about the current limiting, I knew.
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Old 28th April 2004, 04:38 PM   #26
azira is offline azira  United States
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Quote:
Originally posted by Stocker
oooookaaay...

So what use is an apparent output impedance when it is different from the actual output impedance? The bit about the current limiting, I knew.
Voltage damping factor...

Think of it as your output impedance and your load are in series to GND. If sourced voltage changes, then it forms a voltage divider between the apparent output impedance and the load.
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Old 28th April 2004, 05:15 PM   #27
hermanv is offline hermanv  United States
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Perhaps a headphone amplifier?

I'm missing something about "all the pots". If each amplifier is configured for a low gain like 1 the worst case output offset is probably a few millivolts.

Since the output offsets will never match, each output will need to be resistor isolated from all the others (no math here just a guess of probably 100 Ohms). This prevents one op-amp from trying to drive all the others to it's own idea of what represents zero. Then, one pot on the input buffer could set the sum of all outputs to as close to zero as you might like.

The biggest problem is that paralleling the Op-amps will probably not provide any improvement in the sound because the mechanisms that make multiple parallel things sound better apply to random events such as noise or D to A non-linearities. These kinds things tend to cancel or even out. With the 074 all the op-amps will have certain gain discontinuities at the same points in the waveform so probably no cancellation will take place. (Hint it wont sound as good as all the work)

The basic idea seems workable. I'll leave it to you to calculate the return on investment of labor.
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Old 28th April 2004, 05:26 PM   #28
tiroth is offline tiroth  United States
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Quote:
Originally posted by hermanv
Since the output offsets will never match, each output will need to be resistor isolated from all the others (no math here just a guess of probably 100 Ohms).
Did you miss the fact that he is trying to get multiple watts out of this?
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Old 28th April 2004, 05:37 PM   #29
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It's been mentioned, but if you just want to build something because you have a free opamp, get a few samples of the LM3875TF from National. They don't output much heat, there's tons of info on them and the sq won't be half bad.
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Old 28th April 2004, 05:46 PM   #30
Stocker is offline Stocker  United States
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The multiple pots bit was referring to a half-joke thread a while back about manually adjusting a(ny) theoretical amplifier's offset voltage manually, plus the mention earlier in the thread about the 30-band eq possibility.

100R does seem a bit high per device to try to get multiple watts out of the whole but how about 1.0 or 0.1?

I *know* this wouldn't be the best-sounding chip-amp around. I know. I know I know I know. That is not the point.

As for return on labor invested, my dad told me one time (about something else, but still valid): "I'ts not wasted if you're enjoying it." And isn't enjoyment the whole point of this hobby?

I will leave the ethics of using "samples" for personal, one-time use to others to argue. I will buy my chips from national or leave it alone. They're only a couple of ducks apiece.
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