F5 Build: How much gain do I need?

[KevinHeem]

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?

 

[toxic ingestion]

What are you powering with the F5?

 

[CanAm Man]

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?

 

[KevinHeem]

B&W 305 - 8Ω (4.2Ω min.) 91 db

 

[AndrewT]

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?

 

[flg]

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

 

[PJN]

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.

PJN

 

[AndrewT]

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

 

[flg]

No argument. But, gain as stated in the original AA article is 15.1db? I'm sure the old "YMMV" applies

 

[AndrewT]

+15.1 is closer to 5.5times, than to 6times.

 

[12B4A]

The gain change is not going to make a meaningful difference. If you feel it doesn't have enough volume as is then you're going to want an amp with 2 to 10 times more power.

 

[flg]

I'm not even sure where I got that from but, I think it was a real figure people were getting some time in the past

 

[AndrewT]

To get 14.28Vac instead of 12Vac output from the F5 you need to change the four 100r to 123r. You could try 130r or 120r and see if you hear any difference.

 

[AndrewT]

The overall gain through the INPUT Filter and the amplifier will be a bit lower than the amp's basic gain.
The attenuation of the RF Input Filter will depend on the Rin (series) used in the RF filter.

That would account for lower reported overall gain values and would also account for different reported values.

The basic gain of the amp is 6times.
A 1k series resistor feeding into 100k for Rin, reduces the overall gain to 15.48dB, cf. basic gain = 15.56dB

 

[Hikari1]

My f5 w/91db 8ohm speakers, Cd player and passive pre had enough volume w/200ohm feedback resistors. Not enough volume for use w/my vinyl setup though.

Switched to a Jfet BOZ and 100 ohm feedback resistors. It was the right choice I think.

Even though the F5/200ohm resistor/passive setup had enough gain and volume it seemed to lack punch and dynamics. All was cured when I went to the 100ohm feedback resistors and a preamp. You gain weight and dynamics and lose a little mid-range purity and detail. But it's worth it.

 

[tinitus]

a little preamp gain will probably make it sound 'harder'
you might hear that as being more dynamic
right or not, some multiway speaker may need that little exstra to sound 'more alive'

but in that case, maybe tone controls or some form of EQ would be even better