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How is gain defined here?

GeorgeBynum

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I'm a little confused by gain figures as used in the HiFi audio (particularly amplifier) industry.

I __BELIEVE__ I have an excellent understanding of gain and log measurements (Bel and deciBel) as an electrical/electronic engineer involved, working in college 50 years ago, in the rf communications transceiver design/manufacturing/testing industry.

Let me explain my confusion.

Say I have an amplifier with purely resistive input Z of 10k ohms, not unusual. It is driving a perfect speaker with purely resistive load Z of 4 ohms.

I input 1V RMS (2.83 P-P, but that's not the issue, just acknowledging the 2*root2 relationship). The input power is 0.1 mW.

ASSUME the output to the speaker is also 1V RMS. The output power is 250 mW.

The gain of this amplifier is 2500 (250 mW/0.1 mW) or 34 dB.

I get the impression that the HiFi industry defines this amplifier gain as 0 dB based only on input and output signals.

Will some smart person explain the mistakes in my logic, or explain what someone like NCore means when they advertise gain figures? Do input and output power relationships get any consideration?
 

solderdude

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Gain is simply output voltage / input voltage assuming attenuators are always set at 'maximum' (so least attenuation) when present.

The voltage amplification factor can also be converted to dB (20log of gain)

It is not about input power versus output power in audio (unlike RF)
 
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sergeauckland

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Amplifier gain is normally voltage gain, so 20log (volts out/volts in). Impedance and power is not considered relevant as amplifiers can have any value of input impedance and hence power, and similarly the load can be of any impedance, and it's not matched to an output impedance, so power in to power out isn't used in audio.

RF where impedances are standard and matched to source and load, power gain is relevant and transmitter gain is normally power.

S
 
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GeorgeBynum

GeorgeBynum

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So I see the audio industry has chosen to redefine physics ... Bel is DEFINED as a power ratio. This does not sound like something that an industry would condone ... but since it does, I see it can make its own facts.

I can live with whatever the rules are now that I know what they are. What does NIST know?
 

March Audio

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I'm a little confused by gain figures as used in the HiFi audio (particularly amplifier) industry.

I __BELIEVE__ I have an excellent understanding of gain and log measurements (Bel and deciBel) as an electrical/electronic engineer involved, working in college 50 years ago, in the rf communications transceiver design/manufacturing/testing industry.

Let me explain my confusion.

Say I have an amplifier with purely resistive input Z of 10k ohms, not unusual. It is driving a perfect speaker with purely resistive load Z of 4 ohms.

I input 1V RMS (2.83 P-P, but that's not the issue, just acknowledging the 2*root2 relationship). The input power is 0.1 mW.

ASSUME the output to the speaker is also 1V RMS. The output power is 250 mW.

The gain of this amplifier is 2500 (250 mW/0.1 mW) or 34 dB.

I get the impression that the HiFi industry defines this amplifier gain as 0 dB based only on input and output signals.

Will some smart person explain the mistakes in my logic, or explain what someone like NCore means when they advertise gain figures? Do input and output power relationships get any consideration?
You are confusing power and voltage gain.

dB is just a ratio. Perfectly normal and universally accepted usage. Physics has not been redefined.
 
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solderdude

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So I see the audio industry has chosen to redefine physics ... Bel is DEFINED as a power ratio. This does not sound like something that an industry would condone ... but since it does, I see it can make its own facts.

I can live with whatever the rules are now that I know what they are. What does NIST know?

As has been mentioned Bell is just a ratio.
20log for voltage and current, 10log for power (optical and electrical) it has always been this way not only 'here' but everywhere.
 
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waynel

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I’m an EE and have worked in RF and analog circuits. There are many different ways of defining gain , voltage gain, current gain , power gain , transconductance (voltage in current out) etc. In audio amps we are almost always talking about voltage gain so in dB it’s 20logbase10(vout/vin)
 
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GeorgeBynum

GeorgeBynum

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As I said, what is, is. I accept it (even though I feel it is a mistake).

When, 1969-1972, working for a manufacturer of commercial 2-way, we, of course, did our RF at a constant 50 ohm so whether voltages were compared using 20logbase10(vout/vin) or power using 10logbase10(pout/pin), all is as now. We designed and tested internal and PA amplifiers. For those, we defined our gain numbers including impedance, typically 47k in, 2 ohm out (no transformers, and we needed more power than 12.0V P-P could give). If we could use 14.4 like some less scrupulous companies did, it would have been easier. We wanted to honestly claim >10W at 10% THD for our transceivers.

Ah, those cavities with 250W transmitters and 0.25 microvolt receivers 600 kHz apart sharing an antenna ... we made those also, but lost our shirt.
 

Pluto

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So I see the audio industry has chosen to redefine physics
Not at all. Back in the day (say <1950) serious audio circuits typically had a 600Ω output impedance and a 600Ω input impedance (based upon the natural impedance of a standard telephone twisted pair). In such an environment comparisons of power levels were entirely appropriate and valid. Part of the reason for this was the sheer scale of things. A control console (such as it was then) could easily be on the third floor while the associated amplifier racks might be in the basement, so every element had to be built on the assumption that it might be connected to things some distance away.

With the changing nature of electronic technology the practice developed of a low-ish output impedance feeding a high (typically 10×) input impedance. A DIN standard of the time also specified the idea of a high output impedance feeding a low input impedance i.e. current driven as opposed to the former approach which was voltage driven. As soon as the in/out impedances are not identical, the idea of measuring the power gain of an audio amplifier becomes irrelevant for most purposes, hence the adoption of the voltage gain as a useful, more applicable substitute.
 
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