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Old 14th November 2014, 10:25 AM   #31
chrapladm is offline chrapladm  Australia
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Yes I am not so worried about the subwoofer. I know I would have to have a MONSTER of an amp for that one. And having plenty of headroom for dynamics n a movie and Crest factor in music has always had me wanting much more than is probably needed.

BUT when building these amps I dont want to build something that is never going to happen because of parts costs and complexity. While it would be fun to build a XA1000, lol, its never going to happen. I know it wont because of other reasons but that was just my point.

BUT back to your point I would be about 2.5m - 4m away when watching movies. BUT I dont listen at reference level so 30watts may do it. Or I just double that to make sure I do have enough. And I will have a pair of 18's that will have to have a amplifier channel each going to them or mono. Those re about 2.5meters away. And I will be adding another single 21" down the road. BUT I dont want to talk about speakers just getting power to them is all.

F5Turbo sounded like an option but it seems like all designs can be ramped up for whatever output.
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Old 14th November 2014, 10:41 AM   #32
AndrewT is offline AndrewT  Scotland
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The standard F5 is 25W.
The F5t can be just about any power you choose.
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Old 14th November 2014, 11:08 AM   #33
chrapladm is offline chrapladm  Australia
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Thats what I was reading. At what point or rail voltage or other does the design have to change and incorporate a lot more things because of the amount of wattage being produced?

Or is the design simple enough that you just have to change value of components to reach a persons wants?
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Old 14th November 2014, 11:56 AM   #34
AndrewT is offline AndrewT  Scotland
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For ClassAB amplifiers the output device total Pmax is approximately 4times to 6times the maximum output power. i.e. for 80W maximum output, use one pair of 150W to 250W devices

For ClassA the device temperature is set by the device dissipation and this determines the maximum ClassA output power.
For the F5 and F5t the maximum device dissipation is approximately 30W for long term reliability.
That 30W sets the limit of the product of ClassA bias current and supply rail voltage.
examples:
1A bias and 30Vcc = 30W device dissipation
1.5A & 20Vcc = 30W
2A & 15Vcc = 30W
0.75A & 40Vcc = 30W
0.667A & 45Vcc = 30W
0.5A & 60Vcc = 30W

You need to set a target and then CHOOSE what combination achieves your target.
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Last edited by AndrewT; 14th November 2014 at 12:02 PM.
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Old 19th November 2014, 05:04 AM   #35
BigE is offline BigE  Canada
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Quote:
Originally Posted by chrapladm View Post
Thats what I was reading. At what point or rail voltage or other does the design have to change and incorporate a lot more things because of the amount of wattage being produced?

Or is the design simple enough that you just have to change value of components to reach a persons wants?
Alot more things are dependent on the size of the toroid, and on the size of the capacitor bank. As far a rail voltage goes. you either cascode or you don't. You must cascode if the rail voltage is higher than Vds of the 2sk170/2sj74.

Your wattage will depend on rail voltage.
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Old 19th November 2014, 06:03 AM   #36
chrapladm is offline chrapladm  Australia
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Yes thanks for that. Havent chatted with 6L6 more on questions I had I now know what I will need for my wants. For me the F5v3 would be best for me. BUT I just need to find the space for it. I will slowly collect parts for one of these builds. And hopefully when its time to solder I will have the room.
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Old 8th December 2014, 01:20 AM   #37
craigtone is offline craigtone  United States
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Halfway there! We had some technical difficulties for this build. I didn't see the note about pre-setting the pots until after they were soldered to the boards and then I could not measure them since they were in-circuit.

We powered it up with a Variac to 50V and dialed out both channel offsets to 0V and the bias to 0.35V drop. I then raised it to 70 and one channel behaved fine but the other my bias went to the negative rail (-21V). My fear is that R8 was at one extreme end of its range ... would this fry the JFETs?

I ended up removing the R8 pot from both channels and replacing with a 1K and the good channel still functioned fine but I still get -21V offset on the bad channel and adjusting R7 does nothing. I can adjust bias but not offset. I also noticed the JFETs are VERY hot.

I removed the V+ and V- from the bad channel, set the offset and bias on the good channel and listened and it sounds good albeit veiled. Of course it needs to break-in, but so far so good.

Any ideas on the bad channel? Did I toast the JFETs?
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Old 8th December 2014, 01:23 AM   #38
6L6 is offline 6L6  United States
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If you have a rail on offset, it's most likely a mosfet shorted to chassis or a bad solder joint somewhere. Check everything.

The last few people who reported a similar problem had a bad solder joint.
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Old 8th December 2014, 01:50 AM   #39
craigtone is offline craigtone  United States
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Thanks 6L6 ... I think I found it! Q2 appears to be backwards on the bad channel.

Stay tuned ...
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Old 8th February 2015, 05:04 PM   #40
Nosian is offline Nosian
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Default input stage?

i have seen some very simple input stage from NP. only putting together with a volume control for impedance matching. can't find it now. any help please?
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