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Old 9th September 2013, 09:00 PM   #571
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R8 ONLY adjusts the bias on the front end in this configuration. If R7 was fixed you could use R8 to adjust the offset, as was done in many previous Aleph versions. Technically you can adjust bias with R8 when the Aleph Current Source is set to allow more current than the output device will pass with the Vgs set by R8 and R7.

Your laterals are a major change since they require a Vgs ~1.2V where verticals need ~4V. That's easier to deal with in the F5 since you aren't balancing a current source and the gain device.
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Old 10th September 2013, 04:56 AM   #572
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Quote:
Originally Posted by oliverniekamp View Post
Yes i was considering some really huge heatsinks, or some pc fan for airflow.
But the sinks i have got are way too big.

First i wanted to adjust the bias and this appears to be a bigger problem than expected.

Now i have R27 at 10K and the bias is at 406 mV, but i can´t adjust the bias with R8 anymore, it just doesn´t change.
When the R8 resistance gets to low the amp gets unstable and the offset starts moving up and down, or i have like 14 V offset.

With R8 at 2K i can adjust offset with R7 and everything is stable.
So it appears that i could leave R8 fixed at 2K and adjust the bias with R27.

I build Aleph 3, Aleph 30 and Aleph X before, but this never happened.
Of cause i never used lateral Fets before, only in F5 without any problem.
You're going in the right direction by lowering R27...very much lower than 68k as this sets the lower Vgs of the 2SK1530 compared to IRFP240. So if you find that 10k for R27 gets you your desired Output Stage bias current, measure the voltage from Gate to bottom of Source R of the Current Source mosfet this is the Vgs+VR17 say ~2.8V. Now match this voltage with the lower Output mosfet by adjusting R7. You do this by knowing the input stage current thru one of the jfets. If these jfets were matched at 8mA then it's 2.8/0.008 or R7=350 ohms. Set R8 such that it is double this current or 16mA for the input jfets.

Hope that helps...
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Old 10th September 2013, 01:29 PM   #573
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BTW, is 0.5 W for the 9.1 V Zener sufficient?
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Old 10th September 2013, 02:34 PM   #574
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Quote:
Originally Posted by ticktock View Post
BTW, is 0.5 W for the 9.1 V Zener sufficient?
Yes
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Old 10th September 2013, 02:47 PM   #575
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Thanks
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Old 17th September 2013, 05:23 PM   #576
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After drilling and tapping the heatsinks yesterday I wired up my Aleph J this morning and checked the voltages.The p/s is running at +- 22.5V and the offset was at 75mv on both channels which I was able to zero after it had warmed up.Voltage across the source resistors reads 0.55V on one channel and 0.56V on the other;is this a problem as the h/sinks are at approx 47degrees?Should I install the trimmer so that I can lower this?
The amp plays music and sounds pretty good but this is only through a pair of Tandy 4" drivers.
A big thanks to Jim for his excellent build guide
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Old 17th September 2013, 05:37 PM   #577
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everything is pretty OK

after some time of enjoyment , check AC gain of Aleph CCS
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Old 17th September 2013, 07:02 PM   #578
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If the temp is cool, you are good.

BUT, do check the temperature of the transistors themselves - pin 2 is the same as the face of the transistor, and verify that they are 65C or below.

Verify that the heat is being removed from the transistors. If it is, everything is fine.
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Old 17th September 2013, 07:22 PM   #579
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I say that you can go a bit higher for the case temp, like 70-75 maybe. It's just me though :-)
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Old 17th September 2013, 07:26 PM   #580
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Thanks guys;I'll hopefully get chance for a proper listen tomorrow and will check the temperatures.
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