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Old 12th January 2013, 02:23 PM   #1581
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so now that's why at low volume ( 5K in series on my passive preamp) i have a small roll-off and not at mid-level with less than 2K ....

everyday is a learning day

thanks everybody!
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Last edited by juanitox; 12th January 2013 at 02:26 PM.
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Old 12th January 2013, 05:11 PM   #1582
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Quote:
Originally Posted by Blues View Post
Going thru the question and answer related to Nelson's answer above, I think he mistook your 100k for frequency(Hz) instead of the 100kohms that you meant so if you clear this up we might get a better answer.
No mistake. There are two resistors that people would be talking about.

The first is the high value (usually 10K or more) that biases the Gate with
a DC potential. This is also nominally the input impedance. This value likes
to be high for obvious reasons, but is limited by its interaction with the
Gate leakage current, which can be anything from less than 1 uA to maybe
100 uA. 100uA through 10K is 1 volt, and since the leakage current
varies with conditions and age, that combination might result in the bias
being a bit off at some point.

The Sony parts I have tested have low leakage, but I wouldn't push the
value of this resistor above 47K regardless, and there's no point considering
that the input capacitance renders that moot at high frequencies.

The second resistor is between the Gate and the signal source. I normally
put at least 47 ohms there to avoid parasitics, and assume that most
sources will have their own resistance. If the sum of the Gate resistor and
the source resistance is 600 ohms, then with SIT-1's we see a rolloff of
about 100 KHz. This sets a benchmark for you to think about.

If you are putting a potentiometer voltage divider on the input to use as a
volume control then you can figure the worst case will be 1/4 the value
of the pot, and a 10 Kohm pot will just about give you minimal audio
bandwidth at 25 KHz or so.

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Old 12th January 2013, 05:35 PM   #1583
generg is offline generg  Germany
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very clear, thank you Mr. Pass!
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Old 12th January 2013, 05:49 PM   #1584
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whoa!

Blue Car !

Pa made long post !!



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Old 12th January 2013, 05:58 PM   #1585
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I agree with Nelson if the 600-ohm source is directly connected at the 10kohm/47 ohm junction. But if we reference the L'Amp circuit we will find a bandpass filter created by the 10uF input cap, the 10kohm+5kohm(assuming P1 is midpoint)+2Kohm to ground, the 47-ohm gate resistor, and the 2SK82 gate cap at say 2500pF. The bandpass will be 0.94Hz thru 1.35MHz. Now this part might be bandwidth limited to 100kHz so we see it roll-off at that frequency anyway.

Now if we hang a 10k pot for volume control at the input I assume it will be quite benign to the L'Amp, unless your source has a very low output cap (<<10uF) creating another high pass filter with the pot.
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Old 12th January 2013, 06:14 PM   #1586
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too much problem with this Amp , i give up

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Old 12th January 2013, 06:49 PM   #1587
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Quote:
Originally Posted by Michael Rothacher View Post
The effective input capacitance and the output impedance of whatever drives it form a low-pass filter with cutoff = 2*Pi*R*C

So, driving the SIT with a 600 Ohm source gives:

6.28 * 600 * 2E-9 = 133 kHz

And, for example, hanging a linear 50k volume pot on the input, set at 50%, gives:

6.28 * (600+12500) * 2E-9 = 6 kHz (which is hi-fi for humpback whales)
Mike,
To clarify and complete the formula: (6.28 * 600 * 2E-9)^-1 =133kHz or f=1/(2*Pi*R*C)
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Old 12th January 2013, 07:04 PM   #1588
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Yes, I left out the 1/ while typing it out. Sorry boys. Thanks Blues.

Last edited by Michael Rothacher; 12th January 2013 at 07:26 PM.
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Old 13th January 2013, 02:34 AM   #1589
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Quote:
Originally Posted by Zen Mod View Post
Pa made long post !!
Coffee on slow brew.

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Old 13th January 2013, 09:30 AM   #1590
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Quote:
Originally Posted by juanitox View Post
too much problem with this Amp , i give up

Noo don't give up!
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