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Old 17th February 2012, 04:28 AM   #1
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Default F2J technical question

Dear Mr Pass,

I have some technical queries on the F2J. I have a SEAS FA22 in an open baffle which I intend to run with this amp. The SEAs FA22 is 8 ohms, so I figured, the bias current that you are using at 2.4A is probably on the high side. I suppose 1.2A is probably enough for me, just to let it run a bit cooler in the tropics. Is there any major impact of reducing the bias current this much for 8 ohms, I can understand why you would need to use this much for 4 ohms, to reach a 20Vp-p, but for 8 ohms, we would already hit by using a 1.2A.

If so, than My guess for the change in your circuit to accomodate the lower bias current. If I understand your circuit, I should be changing R16 + R17 to 1ohm each. And R7-R12 to 2 ohms each?

Would that be correct?

Thanks in advance

Oon
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Old 22nd February 2012, 11:30 PM   #2
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Anybody else out there want to comment on this?

Oon

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Old 22nd February 2012, 11:44 PM   #3
eyoung is offline eyoung  United States
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I think you want to see the amp leaving Class A sooner than later . ck tour output to see if crossover distortion happens at your listening levels...

see Papas article Leaving class A...


El
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Old 22nd February 2012, 11:50 PM   #4
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Hi El,

F2j is single ended (instead of push pull) and hence it won't leave class A. It will simply just clip.

Oon

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Old 23rd February 2012, 10:56 PM   #5
tp147 is offline tp147  Germany
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Hi,

I'm not inside the schematics of the F2, but I plan playing arround with bias with my F3

At F3 increasing R5, R6, R7 from 0R4 to 0R6 will decrease the current from 1.7 ampere to 1.1 ampere. increasing R5,6,7 also R2, R3, R4 have to be increased to provide Ugs of Q1 > 1V.

Power consumption will be 100 W instead of 160 W and I will lose output power from 15 W to arround 8 Watt - I guess.

cheers
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Old 24th February 2012, 11:05 AM   #6
ianc13 is offline ianc13  United Kingdom
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Hi Oon,

If I remember correctly the F1J runs at ~1/2 the current of the F2J. The F1J is 2 F2Js running opposite each other with the output blocking caps removed.

You could look at the resistor values used in the F1J for R16, 17, 18, 19 and 20 as a starting point. It looks like the equivalent of R16, R18 and R20 are different.

Ian
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Old 27th February 2012, 10:30 AM   #7
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Hi Ian,

Thanks I looked it up. As it turns out current was more like 2 amps. But in any case Nelson pass mentions how he worked out they values in the f1 service manual.it is just vbe of the STD transistor 0.6V divided by the resistance. So 0.5 ohm should be able too give the current I need.

Thanks, looking up the F1 service manual was a great idea.

Oon

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Old 27th February 2012, 06:14 PM   #8
Tea-Bag is offline Tea-Bag  United States
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Oon, not sure about this.
The combo of resistors on the R100 source are .66 in series. If one wants to go higher, then one would shoot for increasing the R of one or several of the 6 1R's on the schematic. Of course just biasing lower will burn less heat.
But it sounds best to me running hot as hell.

Last edited by Tea-Bag; 27th February 2012 at 06:23 PM.
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Old 27th February 2012, 11:41 PM   #9
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Hi teabag,

Can't wait to get the matched pair from you and start this project. I plan to increase the resistor at the source to 2 ohms each.

I would be interested to hear your listening impression at different bias current. I remember Nelson mentioning somewhere that it sounds better running hot. Running it at 2.4 instead of 1.2 would put it at different area of the Vgs vs Id curve. How does the sound improve. Warmer, richer, clearer? What was the impedance and the sensitivity of the speaker you were using for the evaluation. Many thanks in advance.

Oon

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Old 29th February 2012, 07:42 AM   #10
ianc13 is offline ianc13  United Kingdom
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Hi Oon,

You should be able to test the 2 currents relatively easily and figure out which is best for you.

You could run it at ~1.25A until it breaks in, get a feel for the sound and then piggy back another set of resistors on top of the others to get up to ~2.5A.

If you like it there then leave it, if not then cut the additional resistors away to get back to 1.25. That would also allow you to figure out how hot the amp will run at both currents too.

You could even pick a mid point using that method, always having the ~1.25A to fall back to.

If you stand the power resistors a little proud from the PCB then there is plenty of room to get another resistor vertical on top.

Personally I am very, very happy running at ~2.5A, but then it doesn't get to 35C or 100% humidity here in the UK very often!

Ian
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