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Old 23rd September 2013, 01:40 AM   #1
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Default F5 Build: How much gain do I need?

I'll be using the F5 with a source that produces only 2 volts. Do I need to put R220 resistors in positions R5-R8 for more gain?
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Old 23rd September 2013, 02:48 AM   #2
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What are you powering with the F5?
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Old 23rd September 2013, 02:50 AM   #3
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Quote:
Originally Posted by KevinHeem View Post
I'll be using the F5 with a source that produces only 2 volts. Do I need to put R220 resistors in positions R5-R8 for more gain?
Most relevant: How efficient are your loudspeakers? And what impedance?
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Old 23rd September 2013, 03:19 AM   #4
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B&W 305 - 8Ω (4.2Ω min.) 91 db
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Old 23rd September 2013, 10:31 AM   #5
AndrewT is offline AndrewT  Scotland
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2Vac with the 6times gain of the F5 results in 12Vac at the output of the F5.
The F5 is capable of a maximum of ~14Vac.
If you want to use that extra 2Vac of output then you need a little extra gain (+1.5dB).

But, will you notice that the amplifier can never clip?
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Old 23rd September 2013, 11:38 AM   #6
flg is offline flg  United States
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The Max output is specified as + & - 20V in my F5 manual. Gain of 15.3db.
If You are talking RMS Volts of 2, into your F5, then what Andrew says is about right. The + & - 20V maximum output spec is peak volts.
Actually I think the gain is only a factor of 5.5
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Last edited by flg; 23rd September 2013 at 11:40 AM.
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Old 23rd September 2013, 12:40 PM   #7
PJN is offline PJN  United States
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You may need to consider using a preamp that can provide sufficient additional gain. With my F5 I found that I couldn't get the volume that I needed with 87 dB spreakers using a typical preamp (gain ~ 10). I ended up building another higher gain preamp (Aikido) and couldn't be happier with the combo.

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Old 23rd September 2013, 12:52 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by flg View Post
The Max output is specified as + & - 20V in my F5 manual. Gain of 15.3db............
that should say:
The max output is specified as + & - 20Vpk.

This is equivalent to a sinewave of 14.28Vac.

Gain of 6times is equal to +15.56dB
From [{100//100}/10] + 1 = 50/10 +1 = 6

Gain of 5.5 times is +14.81dB
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Old 23rd September 2013, 01:08 PM   #9
flg is offline flg  United States
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No argument. But, gain as stated in the original AA article is 15.1db? I'm sure the old "YMMV" applies
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Old 23rd September 2013, 01:14 PM   #10
AndrewT is offline AndrewT  Scotland
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+15.1 is closer to 5.5times, than to 6times.
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