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Old 20th February 2015, 04:13 PM   #1
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Default Layman's question for the pros

I have Adcom GFA-565 mono blocks that are rated 300w/8-ohms. They have a 27db gain. How many mv from my amplifiers RCA pre outs do I need to reach their full potentials?
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Old 20th February 2015, 04:28 PM   #2
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Here you are;
20*log(x) = 27dB X= voltage ratio output/input Therefore X = 31,7
Voltage required for 300W RMS into 8R ( P/R = I2 = approx 6.2A
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Old 20th February 2015, 04:32 PM   #3
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Is half the calculation missing?

I make it 1.5VRMS for peak output BTW.

Last edited by billshurv; 20th February 2015 at 04:34 PM.
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Old 20th February 2015, 05:16 PM   #4
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1.5V is correct. What does BTW mean?
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Old 20th February 2015, 05:27 PM   #5
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Yes, please. I need calculations to the mv level. Though Jon's clarification is much appreciated, it's a long time since Mr. Planks maths class (also from England) . I require something at the mv level. I'm looking at the Onkyo NR838 with an RCA line out of 200mv but a Max line out of 480?
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Old 20th February 2015, 05:29 PM   #6
DrDyna is offline DrDyna  United States
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Quote:
Originally Posted by Lockport View Post
Yes, please. I need calculations to the mv level. Though Jon's clarification is much appreciated, it's a long time since Mr. Planks maths class (also from England) . I require something at the mv level. I'm looking at the Onkyo NR838 with an RCA line out of 200mv but a Max line out of 480?
That seems really low for that unit.

Oh, I see, the specs say "rated" and then "maximum" ...it's a little bizarre that the two numbers they give are so far apart..guessing though, I'd say you won't have any problem driving a 565 with it.

Edit: More strange specs on that sheet, evidently the tone controls operate at 20 hz and 20 khz.
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Last edited by DrDyna; 20th February 2015 at 05:34 PM.
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Old 20th February 2015, 06:01 PM   #7
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.BTW= by the way

Ok,

Rated RCA output and impedance 200mV/470 ohms
Maximum RCA output level and impedance 4.6V/470 ohms

Will I obtain my 300w rms at 8ohms with this amp?
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Old 20th February 2015, 09:01 PM   #8
DrDyna is offline DrDyna  United States
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I would say "probably", but the way they've worded the specs is very odd to me. 4.5V seems like a reasonable maximum output, but it's odd that they included a "rated output" that's what, like, 25x lower?

So, the answer is, if the maximum unclipped output is 200mV, no.
If the maximum unclipped output is 4.6V, yes.
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Old 20th February 2015, 09:20 PM   #9
sreten is online now sreten  United Kingdom
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Hi,

My best guess is input sensistivity for line inputs is 200mV.
And at full gain gives 4.6V. Output = 200mV = nonsense.

rgds, sreten.
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Old 20th February 2015, 09:36 PM   #10
cbdb is offline cbdb  Canada
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They may be saying that the diference is the head room of the amp, a little strange but makes sense.
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