F5 power amplifier

[Tea-Bag]

Originally Posted by fritz39

I've got a channel of the F5 up and running and I noticed my input to the
amplifier isn't sufficient to get to full output power. Unfortunately, my
speakers only have about 82 dB efficiency so I know I won't get to ear bleeding
levels, but I would at least like to maximize the output. Given the output of my
cd player, I need about 2-3 dB more gain. Any suggestion on how to achieve
this without destroying the quality of the F5? Thanks.

fritz,
I beleive others have had success changing the feedback resistors (100 ohm 3 watts) to 150 ohms to get higher gain.

[sangram]

Hi

Do you think a 24" length of this extrusion will suffice for a stereo F5? This means about 6" per transistor, and the charts seem to show that it'll just about make it (~20 degrees/30 watts for a single 6" length). If not, can someone recommend a suitable profile from the Aavid site? I'd like to have a backup in case my first set of heatsinks is not sufficient.

http://www.aavidthermalloy.com/cgi-b...3&airflow=57.2

[jackinnj]

Originally Posted by fritz39

I've got a channel of the F5 up and running and I noticed my input to the
amplifier isn't sufficient to get to full output power. Unfortunately, my
speakers only have about 82 dB efficiency so I know I won't get to ear bleeding
levels, but I would at least like to maximize the output. Given the output of my
cd player, I need about 2-3 dB more gain. Any suggestion on how to achieve
this without destroying the quality of the F5? Thanks.

What's the max output of your source?

For the peak output of the F5 (54W pk, 24W continuous in to 8R) you'll need 3.7V input, assuming that the amp is giving 15dB of gain. Most preamps should be capable of delivering 3.7V

[sangram]

^^Does that mean the F5 will reach maximum output from a source capable of +4dBu output?

[jackinnj]

Originally Posted by sangram

^^Does that mean the F5 will reach maximum output from a source capable of +4dBu output?

Here's a neat little dBu to V calculator
http://www.sengpielaudio.com/calculator-db-volt.htm

[sangram]

I actually have that calc filed away in my bookmarks

When you say 3.7V, is that RMS or peak? Of course I'll be safe if that's peak, but if it's RMS I'll need about 10dB of gain from my sources (I have the 'Balanced Line stage' also from the Pass website that I could build for this).

[jackinnj]

Originally Posted by sangram

I actually have that calc filed away in my bookmarks

When you say 3.7V, is that RMS or peak? Of course I'll be safe if that's peak, but if it's RMS I'll need about 10dB of gain from my sources (I have the 'Balanced Line stage' also from the Pass website that I could build for this).

It's peak.

[fritz39]

Originally Posted by jackinnj

What's the max output of your source?

For the peak output of the F5 (54W pk, 24W continuous in to 8R) you'll need 3.7V input, assuming that the amp is giving 15dB of gain. Most preamps should be capable of delivering 3.7V

My cd source puts out the standard 2Vrms which is about 2.8Vpk. My preamp
has no gain so I need the extra gain to get from 2.8V to 3.7V. I breadboarded
a quick gain stage that added about 10dB and noticed a jump in volume,
but it was too noisy for me to turn it all the way up. I only need about 3dB
but 10 is probably easier to build. Thanks.

[fritz39]

Originally Posted by Tea-Bag

fritz,
I beleive others have had success changing the feedback resistors (100 ohm 3 watts) to 150 ohms to get higher gain.

I will give this a try since I only need a small increase in gain. I assume this
results in less feedback and so maybe a little less stability? Right now it is rock
solid. Thanks for the input Tea-Bag.

[sangram]

I remember Mr. Pass saying there would be a slightly higher THD as a result of doing this. Which is probably as much as an additional gain stage would add anyway I assume...