Balanced F5 question

[EUVL]

A standard F5 has its voltages & current so dimensioned such that it will deliver maximum power at about 8 ohm load.
A bridged F5, as you describe, will then deliver its maximum power at 16 ohm, since now the voltage is doubled (due to balanced operation) and the (max. Class A) current remains the same as before.

So to re-optimise for 6 ohm load (my personal design criterion), the voltage and bias current have to be changed. Other changes are pure personal choices. I like to do distortion cancellation by device choice, hence Toshiba MOSFETs. Matching FETs for left and right halves of the circuit has the same effect - even order harmonic distortion cancellation.

You may also use the standard F5 in bridged mode, and reduce rail voltage to +/-18V, bias current up to 1.75A to optimise for 8 ohm load.


Patrick

 

[Russellc]

You may also use the standard F5 in bridged mode, and reduce rail voltage to +/-18V, bias current up to 1.75A to optimise for 8 ohm load.


Patrick

How is the F-5 bridged? What connects to what?

Thanks in advance,

Russellc

 

[Zen Mod]

How is the F-5 bridged? What connects to what?

Thanks in advance,

Russellc

you need just output grounds connected between two channels ( if they aren't already )

then bring balanced signal to them - positive leg to one , negative leg to other

take just hot outputs to speaker

usual bridging mumbo jumbo

 

[dazed2]

OK still a little confused here. In the F4 manual, Mr, Pass states :
"In mono balanced operation, you can achieve up to 100 watts output into an 8 ohm load. The amplifier is driven by a balanced source, either through the two RCA inputs or the balanced XLR connector."

However,
"In mono parallel operation, you can achieve up to 100 watts output into a 2 ohm load."

Is mono parallel what you mean by bridging then? Where I would have to lower the rail voltage and decrease bias to optimize for 8ohm load to get 100Wpc?

Thanks

 

[EUVL]

Are we talking about the F4 here or the F5 ?
Did Nelson say the same in the F5 manual ?

Perhaps Nelson can explain.


Patrick

 

[dazed2]

Yes it is in the f4 manual, but I'm extrapolating for the f5. If that Is a false assumption then I'm sorry, I'm not sure, that's why I'm asking.

 

[jacco vermeulen]

Mono parallel balanced, with susy, sounds like an interesting challenge.

 

[AndrewT]

Hi,
the F5 is a 27W into 8r0 ClassA amplifier.
It is also a 50W into 4r0 ClassAB amplifier.

When bridged these become 54W into 16r0 ClassA
and
100W into 8r0 ClassAB.

If one parallels a pair of 27W into 8r0 you get 54W into 4r0 ClassA
and
100W into 2r0 ClassAB.

But the bridging and the paralleling must be done in a way that the output devices share the load.

The Balanced F5 is effectively bridged in that the two halves are run out of phase. The Balanced F5 cannot be bridged.

 

[dazed2]

Ahh thank you AndrewT, I think I understand now, almost.

So bridged, would the F5 be 50W class A into 8r0 then switch to class AB up to 100W?

Thanks

 

[Zen Mod]

.......

..... The Balanced F5 cannot be bridged.

meaning what ?

balanced F5 is already bridged ;

you can bridge identical amps to infinity , each one "block" being already bridged or balanced or whatever....... or not .

 

[jacco vermeulen]

So bridged, would the F5 be 50W class A into 8r0 then switch to class AB up to 100W?

Nope.

For bridged mode, think of a train with one locomotive up front and one at the back.
The train moves faster up-hill with 2 locomotives than with a single one (moh powah), but the forces between two carriages do not go up.

Output Power goes up with a bridged setup, one amp is pulling while the other is pushing.
Quiescent current level remains the same, current that exits one amp enters the tag team partner: max Class A output power remains identical.

 

[AndrewT]

Nope.....
Quiescent current level remains the same, current that exits one amp enters the tag team partner: max Class A output power remains identical.

Nope.
the power delivered to the load is defined by any of three formulae depending on which are known and which are unknown.

P = I * V = V^2 / R = I^2 * R

ClassA is an output current limit. We know the ClassA current limit so choose P = I^2 * R.

If the ClassA current limit is fixed, by being equal to twice the quiescent output bias. Then we find that Maximum ClassA output Power is directly proportional to Load resistance.
For bias=1.3A
Maximum ClassA power is 6.76*Rload.
For 8r0 that is an instantaneous peak output power of 54W, or 27W of sinewave power into 8ohms.

Change the load to 16r0 and the ClassA limiting power doubles to 54W into 16ohms. Bridging allows double the voltage drive to allow the amp to deliver 2.6Apk to the 16r0 load.

 

[Zen Mod]

I had impression that Jaccowakoo didn't take in account possibility of sudden doubling of loudspeaker impedance

 

[AndrewT]

But that is precisely what bridging allows for.
A pair of bridged amplifiers can deliver double the power into double the impedance.
Neither the current capability of the pair of amplifiers, nor of the individual amplifiers needs to change.

A pair of 27W into 8ohms amplifiers, when bridged, become 54W into 16ohms.

 

[Zen Mod]

yes ;

sometimes ...... in fact very often I'm wishing that I'm doubled ;

but not for doubled load

 

[dazed2]

Thanks Andrewt I'm almost there.
So how do you calculate when the amp crosses over to class AB?
You said In your post earlier that this amp in bridge mode operates at 100w in class AB into 8r0

 

[jacco vermeulen]

1.3A bias current means the amp will leave Class A when the current through one side (power transistor) reaches 2.6A and through the other 0.

2.6A times 2.6A times 8 Ohm is 54W, peak output.

In bridged mode, one terminal of the loudspeaker sees +20V max, the other terminal has -20V on it.
Totals 40V across the loudspeaker terminals : 40V times 40V divided by 8 Ohm is 200W peak.
200W peak out is 100W continuous, AB.

 

[dazed2]

Ahh ok get it. Class A is current limited and Class AB is voltage limited.
So in the bridge mode Class A leaves at 54W peak into 8r0
or 27W continuous and runs at Class AB to a peak of 200W or 100W continuous into 8r0
Am I finally getting that right?
Then in Balanced mode, we essentially get the same since that the negative and positive terminals of the input are run out of phase.

 

[NYCOne]

In the balanced F5, Patrick is biasing to 2A. This would bring the Class A numbers to 128W peak?

 

[EUVL]

Yes, so 64Wrms.


Happy New Year to all,
Patrick