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Old 21st May 2012, 05:45 PM   #1
Einric is offline Einric  United States
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Default Tube Amp Gain V.S. RF Gain

I am an RF technician by trade and I am wondering about "Tube Gain".
In RF 100Watts = +50dBm & 1Watt is +30dBm.
My question is, what is the typical dBm level of a source device?
My Aikido has about 9db of gain in it.
I'm planning to build a pair of 98db efficient speakers and want to know in gain value how much amp I need to build if these speakers are 98db efficient at +30dBm of input level.
I don't need a lot of volume maybe only 104db which would tell me I need about +36 dBm of amplifier to drive these speakers.

I know this will have a lot to do with input/output impedance levels and voltage to impedance (microwatts) but I'm looking for an exercise in amplifier math.
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Old 21st May 2012, 05:49 PM   #2
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Depend on the source, naturally. A phono cartridge will let you about few millivolts, and a radio AM detector a couple of volts.
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Old 21st May 2012, 05:56 PM   #3
Einric is offline Einric  United States
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At 2.5V into 5000Ω that is 1.25mW or -9dBm.
My Aikido has 9db of gain, which I believe is about 8dBm of gain.
That would put me to -1dBm level into a power amp.
How much amp do I need? I'm thinking about 5 watts or 37dBm.
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Old 21st May 2012, 06:00 PM   #4
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I donīt understand why you mix audio levels with RF ones.
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Old 21st May 2012, 06:35 PM   #5
DF96 is offline DF96  England
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You need to remember that in RF we normally are interested in power gain, but for audio it is mainly voltage gain. So 9dB for an Aikido is an expression of voltage gain. The actual available power gain will be much greater because the output impedance is much lower than the input impedance. However, in most cases we don't want matched impedances so lots of potential power gain is thrown away.

So when doing audio you should completely forget about dBm and power gain. Think voltage gain. This might seem confusing at first. You have an advantage over some audio designers: you are more likely to build something which does not oscillate at RF!

Last edited by DF96; 21st May 2012 at 06:36 PM. Reason: typo
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Old 21st May 2012, 06:43 PM   #6
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You are confusing too many different things here, speaker sensitvity, amplifier power and source output voltages. dBm measurements are only useful in audio if the impedance is always the same as studio ditribution systems, they have no relevance otherwise.
If you know the sound level you require it is simple to work out the amplifier power you need, this is measured in watts, not dBm.
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Old 21st May 2012, 07:10 PM   #7
DF96 is offline DF96  England
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Quote:
Originally Posted by Einric
98db efficient speakers
98dB (with units omitted) is speaker sensitivity, not efficiency.

I suspect you need to do some more reading on audio, rather than try to continually translate into RF practice.
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Old 21st May 2012, 07:56 PM   #8
Einric is offline Einric  United States
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Thank you all very much, what I was trying to do is convert values between the two measurement standards.

Helpful as a mathematical exercise.
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Old 21st May 2012, 08:10 PM   #9
jcx is offline jcx  United States
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anytime I see "dB" I expect it may require a research project to determine use conditions, reference levels

for example the loudspeaker sensitivity "dB" is dB SPL re 20 uPa for a 1 Wrms electrical power input

additional conditions: anechoic chamber (1/2 space or not? anyone know?), 1 m distance "on axis" (figure out where that is with multidriver towers)

when not otherwise specified we assume dynamic audio measurements are made with 1 kHz, sine wave

loudspeakers have "name plate" impedance, usually of 8 Ohms with factors of 2 above, below sometimes used
but a "well behaved" domestic loudspeaker can vary in impedance by 2x from its rating over frequency
badly behaved speakers may dip 4x below their nominal rated impednce at some audio frequencies

our expectation between amps and loudspeakers is that amps are "good" voltage sources compared to the loudspeaker's impedance, 20:1 "mismatch ratio" is the minimum expected

but not many 100 W audio power amps can put out 400 W into a bad loudspeaker's impedance dip

many mass market commercial 100 W amps don't even put that out into their rated 8 Ohm load continuously - they may be advertizing PEP or "Music Power" to stick a bigger # on the box


then try to sort audio signal level standards - dBV, dBu, consumer, pro, balanced, not (why 600 Ohms instead of 50?)

Last edited by jcx; 21st May 2012 at 08:32 PM.
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Old 21st May 2012, 09:28 PM   #10
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Quote:
Originally Posted by Einric View Post
I am an RF technician by trade and I am wondering about "Tube Gain".
In RF 100Watts = +50dBm & 1Watt is +30dBm.
My question is, what is the typical dBm level of a source device?
My Aikido has about 9db of gain in it.
I'm planning to build a pair of 98db efficient speakers and want to know in gain value how much amp I need to build if these speakers are 98db efficient at +30dBm of input level.
I don't need a lot of volume maybe only 104db which would tell me I need about +36 dBm of amplifier to drive these speakers.

I know this will have a lot to do with input/output impedance levels and voltage to impedance (microwatts) but I'm looking for an exercise in amplifier math.
Nowadays, line level audio inputs are bridging loads, i.e. they don't significantly load the source. This is typically 10K ohms or greater. So the dBm convention doesn't apply. There is however a dBu standard used for bridging inputs, which is equivalent to dBm into 600 ohms. 0dBu = 0.775 vrms.

A related standard which is often used in consumer interfaces is dBV or dB relative to one volt rms.

There are some more or less standard levels for line level sources.

Back when VU meters were used, pro audio gear used +4 dBu (or dBm) as the zero VU calibration level. It was standard to allow peak levels of +18 dBu for a total dynamic headroom of 14 dB above 0 VU. Consumer gear often used -10 dBV as the zero VU calibration level, also with about 14 dB dynamic headroom but there is really no standard. 14 dB above -10 dBV is +4 dBV or about 1.6 vrms. There is also a loose standard for consumer/prosumer line level inputs of 2 vrms. Today it seems to be popular to shoot for about 1 vrms input sensitivity.

dBm is simply a power measure. It can be used for speaker outputs, but typically is not outside of some PA applications. It is correct that with 98 dB/W sensitivity speakers you need +36 dBm or 4 watts to produce peak SPL of 104 dB.

There really isn't a power gain figure that applies due to the bridging input, but what you want is 4 watts output with a line source input level of say 1 vrms. With a 9db preamp, the power amplifier would need an input sensitivity of +9 dBV or about 2.8 vrms for 4 watts output. You may choose to provide for more gain than this to accomodate low level sources and unusually quiet recordings.

Also, working back from the 104 dB peak SPL, the max "0 VU" program level would be 90 dB SPL. You may want to allow for more amplifier power depending on your listening preferences.

Cheers,

Michael

Last edited by Michael Koster; 21st May 2012 at 09:31 PM.
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