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-   -   Wattage of Pass F5 (http://www.diyaudio.com/forums/pass-labs/205211-wattage-pass-f5.html)

pdhanwada 24th January 2012 02:24 PM

Wattage of Pass F5
 
It is 25 watts max per channel into 8 ohms. Does it mean that x watts in A and 25-x in B mode. What is the x when bias is set at 900mA.

What happens when we double the mosfets as recommended in the second design.

Let me know.

Thanks
Pandu

KatieandDad 24th January 2012 02:53 PM

If you want more power look at the F5-Turbo.

Increasing MOS-FETs on their own will not increase power output.

In order to increase power you do need more output devices but that enables you to increase the power supply rails without stressing the MOS-FETs. Drive circuitry changes might also be necessary.

If you want more power - look at the Aleph 4, it is 100W/Channel in Class A.

pdhanwada 24th January 2012 03:03 PM

Katieanddad,Thanks for your reply. Do you agree with my formula of x in class A and 25-x in B or is it first 25 watts in A and anything greater than 25 in B. Assuming the speakers always run at 8 ohms impedence.Thankspandu

KatieandDad 24th January 2012 03:07 PM

Has anyone compared the Aleph 4 with the F5?

I love my Aleph 4, following a couple of bad reviews of the F5 I am reluctant to try an F5 to compare them.

As my speakers aren't the most efficient (B&W 683s) I'm personally probably better off with the Aleph.

lduarte1973 24th January 2012 03:22 PM

Quote:

Originally Posted by KatieandDad (Post 2876157)
Has anyone compared the Aleph 4 with the F5?

I love my Aleph 4, following a couple of bad reviews of the F5 I am reluctant to try an F5 to compare them.

As my speakers aren't the most efficient (B&W 683s) I'm personally probably better off with the Aleph.

you can bet on that :D

KatieandDad 24th January 2012 03:27 PM

Quote:

Originally Posted by pdhanwada (Post 2876154)
Katieanddad,Thanks for your reply. Do you agree with my formula of x in class A and 25-x in B or is it first 25 watts in A and anything greater than 25 in B. Assuming the speakers always run at 8 ohms impedence.Thankspandu

I'm not familiar with that notation.

However, most of the lesser Pass amps will run in Class A to a certain point and then continue on in Class A/B.

Russellc 24th January 2012 03:35 PM

Quote:

Originally Posted by KatieandDad (Post 2876157)
Has anyone compared the Aleph 4 with the F5?

I love my Aleph 4, following a couple of bad reviews of the F5 I am reluctant to try an F5 to compare them.

As my speakers aren't the most efficient (B&W 683s) I'm personally probably better off with the Aleph.

Yes, the F-5 is a First Watt family member, and while it is a little more powerful than most of its family members, it is still a nitch type amp. You either need fairly efficient speakers, or a pre amp with plenty of gain and fairly efficient speakers, or use it in a near field situation where not much gain is required.

When used with in its capability, it is a stupedous amp. Sometimes its odd what it does and does not drive well. I have a small set of 2 ways, which can really soak up the power. Other than a nearfield situation or one where one has a pre with gain and doesnt need a lot of volume, they are not an ideal match. I have another set of 2 ways, which are not so power hungry and about 91 db efficient which it drives like crazy. Also, my large Econo Waves with 15 inch JBL 2235H and B&C DE 250 compression driver and it will drive them extremely well, VERY powerful.

For your speaks, sounds like The aleph is the way to go....of course the F-5 Turbo will likely change all that.

Russellc

KatieandDad 24th January 2012 03:45 PM

This next explanation is simplistic - it contains errors but will serve to illustrate what I am trying to say.

Lets take a Class AB amplifier.

Some novice thinks it might be possible to convert it to Class A.

It has supply rails of V volts and a bias of I amps.

During operation the ouput transistors nominally carry I bias + 0.5 x I out(rms)

This will generate a small amount of heat ~ I x V which has to be lost by the small heatsink.

Are you familiar with SOAR (Safe Operating Area) ?

A transistor may be rated at 150W , 100V and 15A. However, not all at the same time and only with the transistor kept impossibly cool.

Generally it might take 50V at 1A and then it might take a bit of thermal abuse.

OK, back to our Class AB amplifier.

If we increase the bias we will indeed increase the point at which it stops running at Class A, however, the transistors will now be carrying more current and will be generating more heat.

SOLUTION - bigger heatsinks - keep the things cool.

PROBLEM - are we approaching the SOAR envelope.

SOLUTION - double up on the output transistors so they are carrying less current each.

Eventually we can get to the point where the amp is running in Class A.

OK - you night need to tweek the drive circuitry.

However, once we are in Class A and the outut is swinging from +Vcc to -Vcc. There is nothingthat we can do in order to increase power other than increase the rail voltage.

Whoops - SOAR comes into play again.

More voltage - more power through each transistor, more heat.

This is ONLY a generalisation - DO NOT use this as a lesson to go playing with your Class AB amps.

In the Aleph 4 we have 6 Mos-Fets, each biassed at 400mA, and running with PSU rails of 48V.

They get damn hot even with HUGE heatsinks.

Russellc 24th January 2012 03:56 PM

Quote:

Originally Posted by KatieandDad (Post 2876157)
Has anyone compared the Aleph 4 with the F5?

I love my Aleph 4, following a couple of bad reviews of the F5 I am reluctant to try an F5 to compare them.

As my speakers aren't the most efficient (B&W 683s) I'm personally probably better off with the Aleph.

As to bad reviews, there is no accounting for tastes, but anyone who isnt liking the sound of their F-5 has either Mis-built it, or is mis-applying it, would be my opinion.

Russellc

AndrewT 24th January 2012 03:57 PM

Katie's Dad,
that explanation will do just fine.
It correctly stresses the importance of keeping everything cool.


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