F6 Pots
I have a few of questions regarding the pots on the F6.
P2 controls the current setting, so where is the best setting to start adjustment so it's not set above 1.5A or 1.2v on first start up?
P2 has LED2 across it. What resistance value can we expect across P2 for initial startup?
Is there a perferred start setting for P1 for setting the output DC offset?
If building the adjustable 2nd Harmonic version of the F6, how long can the leads be, if one would want to place the 10 Ohm pots on the front or back panel of the amp chassis?
For P3 and P4, is single-turn OK or do they need finer adjustment multi-turn?
What would be a good name to give to the pots? H+ and H-?
Thanks in advance!
Vince
I have a few of questions regarding the pots on the F6.
P2 controls the current setting, so where is the best setting to start adjustment so it's not set above 1.5A or 1.2v on first start up?
P2 has LED2 across it. What resistance value can we expect across P2 for initial startup?
Is there a perferred start setting for P1 for setting the output DC offset?
If building the adjustable 2nd Harmonic version of the F6, how long can the leads be, if one would want to place the 10 Ohm pots on the front or back panel of the amp chassis?
For P3 and P4, is single-turn OK or do they need finer adjustment multi-turn?
What would be a good name to give to the pots? H+ and H-?
Thanks in advance!
Vince
Are you using LEDs(what color?) or Zeners? If red LEDs trim them to the mid-pioint(the come this way from factory). If they are blue you will need to trim down a litle bit. The amp bias will go up slowly because of the caps(Nelson mentioned it), so if you monitor the bias across Rs you will have time to trim it down. For Offset pot I would start from the middle. If you had a variac it would be much easier
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With only 1A bias, you will only get 16 watts class A into 8 ohms. Good enough for testing, but falls short of the potential for easily getting 64 watts or more. Cutting the voltage to 16V would let you go to 1.5A with the same idle power dissipation and get 36 watts out in to 8 ohms.
I have them on 25V rails. 1A bias is most i can go comfortably for long term with current sinks. My new toroid will give me 22V rails, so bias will be increased to 1.2A. I am thinking i have about 60W class A at that point. If i can get a free moment, I will start posting some measurements.
Quick question. Major OT for cascode man
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Here is the calculation for class A push-pull.
Watts(rms) = Imax^2 * Rload/2
where Imax = 2*Ibias.
so for Ibias=1.5A, and Rload=8R, Watts=3^2*8/2 = 36 watts
Watts(rms) = Imax^2 * Rload/2
where Imax = 2*Ibias.
so for Ibias=1.5A, and Rload=8R, Watts=3^2*8/2 = 36 watts
your like an evil school master
Correct. This calculation was for Buzz's balanced F6. It would require at least 14V voltage swing (rails around 16V) for each F6.
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I am asking here if this is the right math:
With balanced you can half the rail voltage because you have an amp on each push and pull with + and - rails and each push and pull sees the full load.
I made an Excel cheat sheet for regular F5.
To solve for power both by current and voltage. So you can see with a known load whether you are voltage starved or current starved to achieve the desired watts in Class A.
Current limit (Peak) P=RxI^2
Voltage limit (Peak)P=E^2/R
So for F5X:
Current limit is the same(Peak): P=RxI^2
Voltage limit (peak): P=(E*2)^2/R
Then just divide by 2 for average power.
For all the rail voltages I have subtracted 4 volts as I've seen this stated as the normal loss regardless of rail voltage. Could be a rule of thumb, I guess.
Pass F5X
Total Amps Ohms
8 ohms 1.50 8.0
4 ohms 1.50 4.0
Current limit P=RxI^2 Peak Watts Average Watts
8 ohm Watts 72.0 36.0
4 ohm Watts 36.0 18.0
Voltage limit P=E^2/R Peak Watts Average Watts
Rail voltage 16
8 ohm Watts 72.0 36.0
4 ohm Watts 144.0 72.0
Number of devises/rail 1
Watts dissipation/device 24.0
Curretn/devise amps 1.5
I hope this is right. If it is verified, I will upload the Excel worksheet, if someone asks for it.
Rush
A note to diyers with F6s with "Zen pots" (See"F6 - Variable 2nd Harmonic" figure in http://firstwatt.com/pdf/art_f6_baf.pdf) and a spectrum analyzer.
Hook up you spectrum analyzer and supply a 1kHz - 5kHz sine wave input for 1 watt - 10 watts output. Starting with the Zen pot wipers at the FET source end, observe H2 and H3 while gradually adjusting the Zen pots away from the source end. This clearly demonstrates how AC source degeneration increased H2 and H3. By carefully balancing the pots you can reduce H2, but you cannot reduce H2 and H3 to a minimum at the same with these pots alone.
In order to minimize both H2 and H3 requires the following:
Hook up you spectrum analyzer and supply a 1kHz - 5kHz sine wave input for 1 watt - 10 watts output. Starting with the Zen pot wipers at the FET source end, observe H2 and H3 while gradually adjusting the Zen pots away from the source end. This clearly demonstrates how AC source degeneration increased H2 and H3. By carefully balancing the pots you can reduce H2, but you cannot reduce H2 and H3 to a minimum at the same with these pots alone.
In order to minimize both H2 and H3 requires the following:
- H3 is minimized with the Zen pot wipers are the source ends.
- H2 can only be minimized at some particular frequency, since it has frequency dependencies due to transformer winding and FET capacitances. At that frequency the open-loop gains of the upper and lower stages of the push-pull amplifier must be balanced. This can be accomplished by placing 1k pots across the transformer secondaries with the wipers connected to the gate resistors. Start with the wipers at the "hot" end, and adjust one or the other pot to reduce H3.
