this is a cut out from the F5 manual:
"For this sort of circuit, a 1.3 amp bias means that the amplifier will operate Class A to 2.6 amps of output current. To understand this, imagine a condition where Q3 and Q4 are idling at 1.3 amps, so that all the current is going from the V+ voltage rail to the V- voltage rail, and none is going through the loudspeaker.
When a positive voltage appears at the Gates of Q1 and Q2, it makes the current through Q1 increase and the current through Q2 decrease. The resulting voltages across R3 and
R4 make the current through Q3 increase and the current through Q4 decrease. This makes the output voltage go positive. As the positive input voltage increases, you approach the point at which Q3 is conducting 2.6 amps and Q4 is conducting 0 amps – and all of the 2.6 amps goes through the loudspeaker.
The power of 2.6 amps into 8 ohms is I^2 * R, or 2.6 * 2.6 * 8 = 54 watts. This is the peak value, and the nature of an undistorted sine wave is that the peak wattage is twice the average, so this circuit would operate 27 watts average Class A into 8 ohms. At currents above 2.6 amps one of the transistors will shut off, leaving the other to continue to increase beyond the 2.6 amps in what is known as Class AB."
So. if you do the math for 4 ohm. you will get: 2.6x2.6x4=27W peak average is half that. So 13.5W class A at 4ohm. beound that current it operates in class A/B.
But if Your point is that you can bias the amp higher to deliver more class A power in 4ohm then in 8 ohm, yes. as long as you have the output devices, heatsinks and PSU for it. but it was stated a STANDAR F5.
"For this sort of circuit, a 1.3 amp bias means that the amplifier will operate Class A to 2.6 amps of output current. To understand this, imagine a condition where Q3 and Q4 are idling at 1.3 amps, so that all the current is going from the V+ voltage rail to the V- voltage rail, and none is going through the loudspeaker.
When a positive voltage appears at the Gates of Q1 and Q2, it makes the current through Q1 increase and the current through Q2 decrease. The resulting voltages across R3 and
R4 make the current through Q3 increase and the current through Q4 decrease. This makes the output voltage go positive. As the positive input voltage increases, you approach the point at which Q3 is conducting 2.6 amps and Q4 is conducting 0 amps – and all of the 2.6 amps goes through the loudspeaker.
The power of 2.6 amps into 8 ohms is I^2 * R, or 2.6 * 2.6 * 8 = 54 watts. This is the peak value, and the nature of an undistorted sine wave is that the peak wattage is twice the average, so this circuit would operate 27 watts average Class A into 8 ohms. At currents above 2.6 amps one of the transistors will shut off, leaving the other to continue to increase beyond the 2.6 amps in what is known as Class AB."
So. if you do the math for 4 ohm. you will get: 2.6x2.6x4=27W peak average is half that. So 13.5W class A at 4ohm. beound that current it operates in class A/B.
But if Your point is that you can bias the amp higher to deliver more class A power in 4ohm then in 8 ohm, yes. as long as you have the output devices, heatsinks and PSU for it. but it was stated a STANDAR F5.
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The increase in power into 4 ohm over 8 ohm is only true at a specific bias point for a specific supply voltage. Increasing the bias will not increase effective 4 ohm power beyond the point the amp runs out of voltage swing.
83dB 4 ohm seems awfully low sensitivity for a standard F5, it would be better to lower the bias to ~1A and increase supply voltage. At even 50W output (5W average), that speaker will only be doing ~90dB/1m, which means you will have to be really close to get decent levels out of the setup. With an amp of bias you will get about 10W of Class A power into a 4 ohm load.
As a reference, my Usher Mini-Dancers are rated at 87dB (they're not, it's the woofer sensitivity - standard hi-fi blindsiding) and the F5 is too quiet for it. The Turbo goes a bit better, and it's not the current, but the voltage that's the limiting factor in my case (as it will be in yours). I strongly suspect 20V is not enough for decent levels out of that speaker.
Clipping will be audible for the most part. You'll know when the transients start sounding funny. The human ear will start compressing beyond about 85dB and I've noticed most audiophiles tend to listen higher than that anyway. If your music does not have much dynamic range and you like low volumes, it should be okay. Remember you have to leave 20dB of acoustic output on the table as a very basic minimum for transients (30dB is better, but probably not possible in this case).
83dB 4 ohm seems awfully low sensitivity for a standard F5, it would be better to lower the bias to ~1A and increase supply voltage. At even 50W output (5W average), that speaker will only be doing ~90dB/1m, which means you will have to be really close to get decent levels out of the setup. With an amp of bias you will get about 10W of Class A power into a 4 ohm load.
As a reference, my Usher Mini-Dancers are rated at 87dB (they're not, it's the woofer sensitivity - standard hi-fi blindsiding) and the F5 is too quiet for it. The Turbo goes a bit better, and it's not the current, but the voltage that's the limiting factor in my case (as it will be in yours). I strongly suspect 20V is not enough for decent levels out of that speaker.
Clipping will be audible for the most part. You'll know when the transients start sounding funny. The human ear will start compressing beyond about 85dB and I've noticed most audiophiles tend to listen higher than that anyway. If your music does not have much dynamic range and you like low volumes, it should be okay. Remember you have to leave 20dB of acoustic output on the table as a very basic minimum for transients (30dB is better, but probably not possible in this case).
sorry
my mind obviously still was in other thread
no , you need to use proper brothers in F5 , no mischmasch
though , I remember our late friend Babowana experimented with not-exactly-brothers solution
F5 v3.0 PSU and Bias Help Needed
my mind obviously still was in other thread
no , you need to use proper brothers in F5 , no mischmasch
though , I remember our late friend Babowana experimented with not-exactly-brothers solution
F5 v3.0 PSU and Bias Help Needed
I'm curious about something. Exactly how hot is it safe to run the outputs on an F5? I've been running it at about 1.1 amp bias and that's putting them at about 75C at idle, maybe 80 or so under hard use. The F5T article suggests not running them over 100C with heatsinks at 55C or so. I'm looking at 82C with heatsinks at about 60C running almost 1.2 amp through them. Ambient temperature in my place is never more than 24C. The output devices are IRP240's and IRFP9240's. Am I safe so long as they never see anything higher than say 95C under the roughest conditions?
There's "safe" and there's "ensured longevity". The data sheet specs the junction temp up to 150ºC, which at 30W dissipation equates to a case temp around 125ºC. It should have a reasonable lifespan at that temp, but it will last much, much longer at a lower temp.
If I understand the push-pull topology correctly (admittedly doubtful), the transistor temp should be constant as long as you stay in class A (whether at idle or under hard use).
If I understand the push-pull topology correctly (admittedly doubtful), the transistor temp should be constant as long as you stay in class A (whether at idle or under hard use).
30W is the maximum allowable dissipation at 125C die temp. Using those two variables with the thermal resistance will yield the minimum possible external resistance for a number of use cases.
It is a specification of limits, not of performance. The designer is left to compute the actual heatsink and thermal material required to not exceed those conditions.