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F5 Turbo Builders Thread
F5 Turbo Builders Thread
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Old 5th August 2019, 03:29 PM   #5691
rhthatcher is offline rhthatcher  United States
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Any advice on what to get for F5 Turbo V3 JFets?
Will I need 1 matched pair of 2SJ74 + 2SK170 per Monoblock (2 matched pairs total)? Or will I need parallel 2SJ74 + 2SK170's per monoblock (two matched Quads total)?
I'm looking at Punkydawgs offerings, what should target Idss be? He has some 10mA and 7-8mA range?
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Old 5th August 2019, 05:26 PM   #5692
johnhenryharris is offline johnhenryharris  United States
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One pair per Mono Block, preferable matched.
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Old 6th August 2019, 05:56 PM   #5693
BigE is offline BigE  Canada
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Toshiba BL range. I would think the high side of the range is better.... someone might correct that opinion.
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Old 6th August 2019, 08:50 PM   #5694
Bfpca is offline Bfpca  Canada
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F5 Turbo Builders Thread
Mine were at the very bottom of the range at 6ma IIRC. That meant I had to change out a resistor in order to get enough adjustment range to get the bias current I want. 8 -10ma should be about right.

I built V3 monoblocks with separate power CLC power supplies. They are fantastic amps and have been trouble free for 2 years now. I’m running about 46 volt supplies and about 2,1 amps total bias. I recently measured the power out and it was just over 400w at 2 ohms. That is without the output stage diodes in the circuit.
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Old 10th August 2019, 02:56 AM   #5695
rjm121012 is offline rjm121012
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Default Max Class A bias watts

This has probably been explained previously here somewhere but I could not find.
How do I figure out the maximum output class A power output based on the source resistor bias voltage?
I understand that I have set the bias at .350 V across .5 ohms. 700ma.
I also know that I have 2 pair of output devices in parallel.
Ideally both pairs will be biased the same at 700ma ea for a total of 1.4 amps bias with a 32 V power supply. Now what?
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Old 10th August 2019, 11:36 AM   #5696
AudioSan is offline AudioSan  Norway
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1.4A bias *2 = 2.8A peak.
2.8*2.8*8ohm/2=31W average class A.
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Old 13th August 2019, 08:21 PM   #5697
BigE is offline BigE  Canada
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Average is 1/2 peak, so the amp will operate in Class A until it is asked to produce over 62 watts.
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Old 16th August 2019, 02:23 AM   #5698
rjm121012 is offline rjm121012
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Each side is biased at 1.4 amps per pair. That would allow each pair to operate class A between 0 and 2.8 amps (1.4 amp +/-1.4 amp). RMS amps = .707 Peak amps. .707 x 1.4 amps peak = 1 amp rms. Even if I double this for push pull still is not correct.
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Old 16th August 2019, 02:36 AM   #5699
rjm121012 is offline rjm121012
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I think I got it.
2.7 A peak times .707 = 2 A rms
I squared x r = 4 Amp rms x 8 ohm = 32 W Class A
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Old 16th August 2019, 03:11 PM   #5700
BigE is offline BigE  Canada
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Quote:
Originally Posted by rjm121012 View Post
I think I got it.
2.7 A peak times .707 = 2 A rms
I squared x r = 4 Amp rms x 8 ohm = 32 W Class A
Peak current is 2.8. Average is 1/2 Peak.

Peak Voltage that the amp can deliver is rail voltage -- likely a bit less due to sagging.

This does NOT mean that the current of 2.8 amps happens at clipping!

Since V = IR, the voltage when the amp leaves class A when driving an 8 ohm speaker is 8*2.8 = 22.4 V. It is half that when driving 4 ohms, since current doubles into 4 ohms.

When providing more than 22.4 Volts to an 8 ohm speaker, the amp is operating in Class B.
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