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Old 20th October 2006, 05:15 AM   #121
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John,

Looking to the last schema I think we could find the bias in amps just seeing the voltage drop on R6.

Voltage drop/0.5 (R6) = Amps

Guess your circuit would need at least 2 Amps per device so a total of 4 amps or more. Shure enough R6 should be big
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Old 20th October 2006, 05:36 AM   #122
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One other thought, I guess R3 is used for biasing the output transistors and not necessary for feedback.

You also have a full loop Feedback to the gate of the input so a local FB to the output seems redundant.

If this is the case I would try biasing the output directly from the rail. Also R3 seems very low in value. Most of the feedback is going to the power FET and very little to the input.
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Old 20th October 2006, 05:45 AM   #123
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And yet another thought,

I see you are using quite a big C2, try using a very small one and very good quality, guess a 0.1 uf would be much better in sound specialy if you are using the amp over 400 hz.
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Old 20th October 2006, 05:53 AM   #124
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Default Wow, thanks for the input!

I hadn't thought of relocating R3 to the + rail. But you're right, feedback is already being applied to the input. I'll have to try this out. 10K is what Nelson used for R3, so I kept the value.

If you have other input, I'm all ears, errr, I guess I'm all eyes!

Thanks a million,

John
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Old 20th October 2006, 05:56 AM   #125
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I was wondering about C2. I had thought of using 10 uF, but 0.1uF would probably offer cleaner highs? What's your favorite brand of cap in the 0.1uF range?

Again, thanks!

John
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Old 20th October 2006, 06:27 AM   #126
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use the smallest one you can get away with for C2, of course this will affect the roll off of the amp (hi pass filter).

Size matters, but try different brands and sizes to see how this affects the sound, at least on other zen's this make a diference on sound and overall signature, generaly speaking a small coupling cap will show better highs.

Coming back to Nelson's comment I think he suggested a resistor at the output of the buffer, before C6, have you tested this?

On the R3 comment, I think if you are not looking for FB it should be in the order of 100 or 200K but you will also need to change the pot to a higher value to compensate and be able to set the bias you need. This change will also affect the gain of the amplifier.

May be, if you decide to test a higher value for R3 you would also like to lower the FB resistor to the input.
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Old 20th October 2006, 01:57 PM   #127
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Regarding the R3 bias network, I got the impression that you get less distortion with lower resistance values. What's your take on this?

I can't recall off hand which Zen article I read this in, but I do remember reading it just the other day...

John


btw, I'm still designing the buffer and have not yet given it a home. Soon...
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Old 22nd October 2006, 01:36 AM   #128
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Default swapped IRFP240 for IRFP044N

Another good addition to this amplifier. I was able to lower the gain on my preamp by 35 percent or so. I can't tell a great deal of difference in the sound; the IRFP044 transistors are perhaps a bit creamer. But then again, we all know how our minds play tricks on us...

Almost all adjustments remained the same. I only had to lower P1 a fraction to get the DC offset down from 85mA to 14mA. I decided that was good enough for now. The slightest amount of torque on that shaft can send the offset sailing!

I'll let those in my home with the "golden ears" offer the final analysis.

John
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Old 22nd October 2006, 01:52 AM   #129
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carpenter-get some sleep.....now you measure DC offset in mA......
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Old 22nd October 2006, 02:21 AM   #130
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Default hahahahahaha...........

Gosh, I'm such a dope sometimes.....
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