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Old 21st May 2008, 01:24 AM   #821
fab is offline fab  Canada
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Default Re: Re: PROFET Question

Quote:
Originally posted by Nelson Pass


That appears to be consistent with "Chuck" Hansen's review of the
Profet in AX magazine, in which he measured a 6.8 ohm output
impedance. That doesn't mean that the amp sounds bad, but it
tends to limit its application.
So 6.8 ohms output impedance is far away than a R2R design thus a lot of power "lost" for a common source output stage...

Also I could not get THD (1KHz) lower than 0.6% at 16Vac with 23Vdc power supply (at higher level it clipped but with a gradual rounded shape instead of flat) ... THD at low level was about 0.2%....is it consistent with Profet amp spec?

Unless using a hig hvoltage supply, I guess such a power amp design (Profet or F5) is not for Laterals... I have a few that I wanted to use...
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Old 21st May 2008, 02:29 AM   #822
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Offhand, the numbers look consistent with Hansen's measurements
of the Profet.

Perhaps you are being harsh in judging your amplifier. Those
numbers do not necessarily mean that it sounds bad, nor does it
mean that you are done tweaking this until it sounds better yet.
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Old 21st May 2008, 10:47 AM   #823
zinsula is offline zinsula  Switzerland
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Default Re: Re: Re: PROFET Question

Quote:
Originally posted by fab
Unless using a hig hvoltage supply, I guess such a power amp design (Profet or F5) is not for Laterals... I have a few that I wanted to use...
Don't give up so soon.
1. How many laterals do you have? Parallel them. You will not need source resistors even if you parallel them, laterals have positive tempco already at very low currents (if I recall correctly, these have zero tempco at around 100mA.)

2. Q9 and Q10 are not really needed at this low supply voltage. Btw, with the biasing resis-tor values shown in your scematic, they allow for only 2.5-3V over the JFets. This is low for high input signal levels.

3. I am puzzled about the values for P1 and P2. The laterals need around 1.5 - 2V G-S for 0.6A bias. That means only 2mA through the JFets??

I think that your main problem is the low transconductance of the laterals and the low Gate Source voltage needed to "open" them. Both result in a much lower OLG than the F5.

I suggest you a workaround according to this schematic:
Current sources with parallel resistors instead drain resistors P1/P2. This allows to increase OLG despite low voltage drop.
I have used your cascode transistors for that

Use the F5 feedback arrangement, it allows for lower degeneration despite same source R.

The resistor values are starting values. No sim values! You have to tweak them, maybe massively.

Success, Tino
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Old 21st May 2008, 04:56 PM   #824
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I have some questions before starting the building :

1) does F5 need a preamp?

2) may I decrease R11 and R12 slightly to 0,5 ohm?

3) If I use Fostex drivers as FE206 , are applyable the same tecniques described in " current source amplification " when my needs comes down to equalization? For Both F5 and F3 ?

edit: 4) Are ok 2sj109 here ?

thanks
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Old 21st May 2008, 06:20 PM   #825
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1) does F5 need a preamp?

No, it has enough gain depending on the sources

2) may I decrease R11 and R12 slightly to 0,5 ohm?

You mean increase? Yes, no problem.

3) If I use Fostex drivers as FE206 , are applyable the same tecniques described in " current source amplification " when my needs comes down to equalization? For Both F5 and F3 ?

The F3 and F5 are not current sources - the equalization
technique for current source amps will not work.

4) Are ok 2sj109 here ?

Yes.
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Old 21st May 2008, 07:15 PM   #826
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thanks ; yes I meant increase R11 and R12 .

Thats ok , so I am nearly ready , just need to find the thermistor ...
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Old 21st May 2008, 07:32 PM   #827
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Yes, I was wondering which thermistor are guys buying. I was looking for one whith a tab and hole, but can't find any on that package.
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Old 21st May 2008, 07:52 PM   #828
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Default thermistor

I'm going to try some Cantherm thermistors from digiKey, part # 317-1360-nd. My plan is to drill a small hole in the heatsink next to the mosfet and stick the head of the thermistor in it, insulate the leads and run them back to whatever pcb I decide to make for this amp.

George
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Old 21st May 2008, 08:19 PM   #829
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That's a good solution, hole in the heatsink and silicone grease
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Old 21st May 2008, 08:26 PM   #830
Zen Mod is offline Zen Mod  Serbia
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any sort of clamp is good enough , too .
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