Re: What a Parrot Brained Fuckwit

On 6 Aug 2005 15:13:04 -0400, mrivers@xxxxxxxxxxx (Mike Rivers) wrote:

>
>In article <42fbcbe6.23887718@xxxxxxxxxxxxxxxxxxxx> donald@xxxxxxxxxxxxx writes:
>
>> >If I get more volts out than
>> >I put in, and volts is what I care about, I say it has gain - voltage
>> >gain. Are you suggesting that gain is only defined if you define it in
>> >terms of power?
>
>> Absolutely.
>
>Well, I'm completely floored. You might have just as well told me that
>the sky is green. Are you telling me that the 20*log(out/in) formula
>that we've all learned is bogus, or useless, because it doesn't
>apply to "real" dB? How can I ever correct all of my teaching and
>writing sins? I guess I'll just have to die.
>
>> That is why you can't simply put a transformer between a
>> speaker and a microphone and expect it to amplify. A transformer is
>> simply an impedance converter. It has no gain.
>
>Why bring transformers and loudspeakers and microphones into the
>picture? But since you did, what do you call the phenomenon that if
>you connect a 1V source to one side of the transformer and measure
>10V on the other side of the transformer? You can't relate that to
>impedance.
>
I bring transformers in because they are devices that, by your
definition of the dB, have gain. And of course you can relate it to
impedance - that is exactly what this is all about. A transformer
simultaneously transforms voltage and impedance. It can only do the
former because it does the latter.

>> >I think you're trying to get me to say that such an amplifier has
>> >power gain, and would agree with you. One could also express the
>> >voltage gain for a particular load impedance. That would change as the
>> >load changes, but (assuming the amplifier doesn't crap out) the power
>> >gain would be relatively constant.
>
>> No - we can define this amplifier to have as low an output impedance
>> as necessary. It could then drive an arbitrarily low load impedance.
>> The voltage wouldn't change, but the current would. So the gain would
>> depend on the load impedance - which is where we came in.
>
>Well, maybe we could DEFINE it, but if we build an amplifier, it has
>whatever output impedance it has. This (as well as the load impedance)
>would determine the voltage delivered to the load. The difference
>between a transformer and an amplifer (as well as a whole bunch of
>other things) is that it has an external source of power. The output
>power is in essence controlled by the input signal. In a transformer,
>there is no external source of power. The only thing that "powers" the
>output is the input, and you don't get something for nothing. So there
>can be no power gain in a transformer. But you can get more volts out
>than you put in if you look at open-circuit voltage (another "in
>theory" thing).
>
No, what powers the output in a power amplifier is not the input, but
the power supply. That is how it can be an amplifier. If all it had
was the power of the input, it couldn't amplify.

>> No - I never mentioned a "toy audio industry" at any point. The
>> problem has never been the industry - and this isn't even a problem,
>> of course; just a very minor niggle.
>
>Well, somebody in this thread did, or at least implied that engineers
>working with audio equipment weren't real engineers because they
>didn't understand things like the true meaning of dB and how it cannot
>be used to express voltage ratios.
>
What I said was that most engineers in studios were not actually
engineers, but artists. Obviously I don't know about you, but most
will possess no electrical engineering degree. This is not a put down
- merely an acknowledgement that the term engineer if not actually
wrong, means something different.

>> When you are designing for low noise, power transfer is very
>> important, so matters like impedance matter greatly.
>
>Not all designs are about low noise. I'm sure that this is considered
>when low noise is part of the design criteria. It's fairly typical for
>a microphone preamp to have an input impedance in the range of 300 to
>2500 ohms, but this isn't simply to match the source impedance of the
>microphone. It's just as often a choice of the designer trying to load
>a transformer at the output of the microphone to give the flattest
>frequency response out of the transformer. There's at least one really
>low noise mic preamp, the Gordon, that has an input impedance of a
>couple of megohms. Different designer, different choices.
>
>> No, I specifically said that within the confines of the music
>> industry, you can use or misuse the dB any way you like, as long as it
>> suits you. Just don't get the idea that what you are doing with it has
>> anything to do with the real world.
>
>This is the attitude that offends me. The suggestion that the audio
>(or as you call it, "music") industry is in some way not connected
>with your "real" world.

I call it the music industry, because the audio industry is about the
design of equipment, an area that must be well populated with people
who do understand the dB properly.

d

Pearce Consulting
http://www.pearce.uk.com
.