L'Amp: A simple SIT Amp
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 12th January 2013, 02:23 PM #1581 juanitox   diyAudio Member     Join Date: Dec 2006 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! __________________ French is not a crime. Last edited by juanitox; 12th January 2013 at 02:26 PM.
Nelson Pass
The one and only

Join Date: Mar 2001
Quote:
 Originally Posted by Blues 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.

 12th January 2013, 05:35 PM #1583 generg   diyAudio Member     Join Date: Jul 2009 Location: Near Frankfurt very clear, thank you Mr. Pass! __________________ and the First Wtt is......a really First Class PSS Idea
 12th January 2013, 05:49 PM #1584 Zen Mod   Official Court Jester diyAudio Member     Join Date: Jan 2003 Location: ancient Batsch , behind Iron Curtain whoa! Blue Car ! Pa made long post !! Woah! A Blue Car - YouTube __________________ my Papa is smarter than your Nelson ! clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa...© by Mighty ZM
 12th January 2013, 05:58 PM #1585 Blues   diyAudio Member     Join Date: Sep 2003 Location: The Milky Way 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.
 12th January 2013, 06:14 PM #1586 juanitox   diyAudio Member     Join Date: Dec 2006 too much problem with this Amp , i give up __________________ French is not a crime.
Blues
diyAudio Member

Join Date: Sep 2003
Location: The Milky Way
Quote:
 Originally Posted by Michael Rothacher 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)

 12th January 2013, 07:04 PM #1588 Michael Rothacher   AudioMaker.Tech diyAudio Member     Join Date: May 2001 Yes, I left out the 1/ while typing it out. Sorry boys. Thanks Blues. __________________ Visit AudioMaker.Tech Last edited by Michael Rothacher; 12th January 2013 at 07:26 PM.
Nelson Pass
The one and only

Join Date: Mar 2001
Quote:
 Originally Posted by Zen Mod Pa made long post !!
Coffee on slow brew.

Telstar
diyAudio Member

Join Date: Dec 2007
Location: Italy
Quote:
 Originally Posted by juanitox too much problem with this Amp , i give up
Noo don't give up!
__________________
"The total harmonic distortion is not a measure of the degree of distastefulness to the listener and it is recommended that its use should be discontinued." D. Masa, 1938

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