L'Amp: A simple SIT Amp

[Telstar]

Yes! Elma are my first choice. Used by goldpoint and not hard to find in Europe.
But those i found are with single pcb (per channel) with little spacing, for smd resistors. I'll look further (or get the pre-mounted one)

[permaneder]

Originally Posted by Telstar

Yes! Elma are my first choice. Used by goldpoint and not hard to find in Europe.
But those i found are with single pcb (per channel) with little spacing, for smd resistors. I'll look further (or get the pre-mounted one)

Yours are stepped attenuators that have always a chain of resistors in the signal path. Mine only one resistor at each step. A couple of years ago I purchased the last two from a British distributor. They seem to be hard to find meanwhile and if you get some, they are quite expensive.

But have a look at these high grade attenuators from Poland: High Quality Audio & Industrial Attenuators and Passive Preamplifiers

[Telstar]

Originally Posted by permaneder

Yours are stepped attenuators that have always a chain of resistors in the signal path. Mine only one resistor at each step.

Mine? I have got none yet.
You mean a shunt attenuator, i plan something like that.

But have a look at these high grade attenuators from Poland: High Quality Audio & Industrial Attenuators and Passive Preamplifiers

They had mixed quality issues, so I prefer to go with Elma.

[permaneder]

Originally Posted by Telstar

Mine? I have got none yet.



They had mixed quality issues, so I prefer to go with Elma.

What kind of quality issues?

[Telstar]

Search there is a thread in this forum.

[permaneder]

Originally Posted by Telstar

Search there is a thread in this forum.

I have read the comments at Khozmo 48 position stepped attenuators.

Sounds good, at least in my comprehension...

[juanitox]

Originally Posted by Michael Rothacher

Do you mean a 10k volume pot? The effective input capacitance of the 2SK82 under L'Amp operating conditions was a couple thousand pf as I recall.

can i repeat my "newbie" question ? what is the effect of this input capacitance and how to cure it on the Sit ?

[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)

[juanitox]

thanks michael i understand , so the 10K and 47K resistor loading the gate is not in the impedance calcul ? or perhaps for high pass with the coupling capacitor ? i understand but slowly

[Telstar]

From Michael's calculation I get that a 1k pot would be ideal to not have HF rolloff.
My source is very low impedance so I could use a 1k pot, the problem being it unobtainium
Therefore i'll build a stepped attenuator taking advantage of the 10k (which i erroneously called inpout resistance) in it.