What's the Idle Current of the F5 Power Amp

[NV&H]

In his AudioXpress article, Nelson wrote that a power supply with a 6 amp continuous rating is needed. Does that mean that when the thing is powered up with no input signal, it's drawing that much?

Perhaps it draws less than that at idle with the 6 amp specified to ensure reliability?

 

[jacco vermeulen]

1.3A, with +/-24Vdc rails makes ~62.5W dissipation per channel.

It's a Class A power amp, so you want the PS to have a nominal power rating a couple of times as much.
The F5 is also a Push-Pull design with an output stage that is strong enough to deliver a lot of peak current, another reason for desiring a bulky powersupply.
Read the first 15 to 20 pages of the F5 thread for continuous and peak current rating of the PS.

 

[Nelson Pass]

The rating refers to the capacity of the secondary windings.

The draw of the amplifier is about 80 watts per channel at idle,
which results in an (rms) draw of about 1.5A from the wall
at 120V for 2 channels. In Paris, that would be more like 0.75A.

 

[jacco vermeulen]

180W as in 2 x 62.5W x sqrt2.

 

[MEH]

Where'd the sqrt2 come from?

 

[jacco vermeulen]

Smoking Californian oak ?

The F5 manual reads 1.3A per leg, 25Vdc rails max, 62W dissipation per channel, makes 124W for stereo.
But also : power consumption = 180W and slow fused for 300W Max.

The amp is supposed to do 10A peak, 10A times 18V rms= 180W.
(if you add the bridge rectifier dissipation it becomes 133W/stereo)

 

[Nelson Pass]

You're right, 60 watts. I had 1.6A stuck in my head.

 

[jacco vermeulen]

If only,

none of the F5 scavengers noticed that the mains fuse is rated at 2.5A.
The production F5 has 1 transformer and 1 fuse, means the power supply is sized at 180VA to 225VA Max for both channels.

Basically means the F5 is dimensioned for 4 Ohm use : 20V rails /(4+0.47)~ 4.5A peak aka ~3.15A continuous.
3.15A times 25Vdc per channel times 2 comes to ~160VA, with a little cherry on top makes 180VA.

For 2 Ohm use the peak output current would be around 8A, makes ~5 3/4A continuous, results in a single 300VA transformer for both channels.
Some already indicated to use a 800VA biggy for the F5 they're assembling, they'll probably laugh at a 50W Accuphase P102 Class A amp that runs on a chicken-leg 160VA transformer per channel.

 

[Babowana]

Thanks, Jacco.
Helpful info!

 

[mpmarino]

I had 1.6A stuck in my head.

Yeah.... I hate it when the dang Grado short circuits.

 

[umut1001]

Hi.My transformer is 600VA when regulated with capacitors it is +-22 volts.And after 0.47R resistors it makes about 21.3 volts.Idle current is about 1.5amps.I am using 4ohm nokia 18cm 90dB woofers and westra 94db tweeters.I think i should decrease the V supply by resistors RCRC filter to +-17volts and increase bias to about 2 amps to get more class A sound.I will use 2sk1530/2j201..I should change some component values I think.My redesigning will be like this picture below.what do you think about my idea?

 

[umut1001]

woowwww i increased bias to 1.9a and decreased the supply voltage to +-18volts and sound is much much great now ! bass is great treble is cleaner.I think it is much better to increase bias for my 4ohms speakers!

 

[tinitus]

I think it is much better to increase bias for my 4ohms speakers!

Sure is, thats expected, low impedance means more current

 

[umut1001]

What do you think about feedback resistors ? should i increase them because of lower voltage ? I do not know how to calculate feedback resistors

 

[bobodioulasso]

I think i should decrease the V supply by resistors RCRC filter to +-17volts

This is not the right way to do it. Your rails will fluctuate following current demands.
Use a capacitance multiplier or a regulator instead.

 

[a.wayne]

woowwww i increased bias to 1.9a and decreased the supply voltage to +-18volts and sound is much much great now ! bass is great treble is cleaner.I think it is much better to increase bias for my 4ohms speakers!

Increase the Bias, does this mean you are running less class -a ?

 

[tinitus]

Increase the Bias, does this mean you are running less class -a ?

Maybe look at this way
You either have a classA 25watt that does 12watt classA into 4ohm
Or you have maybe classA 20watt that stays 20watt classA into 4ohm, or just a little less
Into a normal 6ohm load they may be about the same

I dont know how much bias it would take to make a classA 20watt(8ohm) do at least 30watt class into 4ohm

 

[a.wayne]

exactly , this is what I'm trying to determine from his statement ...

 

[umut1001]

Maybe look at this way
You either have a classA 25watt that does 12watt classA into 4ohm
Or you have maybe classA 20watt that stays 20watt classA into 4ohm, or just a little less
Into a normal 6ohm load they may be about the same

I dont know how much bias it would take to make a classA 20watt(8ohm) do at least 30watt class into 4ohm

I think i should increase bias to 2amps to get more class A.I want to increase more but transistor power area does not allow more dissipation.I think 2amps +-18volts are ideal.

 

[jacco vermeulen]

I dont know how much bias it would take to make a classA 20watt(8ohm) do at least 30watt class into 4ohm

30W Class A in 4 Ohm is 2.75A current, peak current is sqrt(2) times higher, ~ 3.9A.
In a push-pull output stage that's 1.95A quiescent current, regardless of rail voltage level.

(the 20W/8 Lang MOSFET amp design from the early 1980s was set up to run Full Class A in 4 Ohm, 2A bias and output stage rails of +/-20Vdc, for a single pair of Hitachi TO3 Mosfets)