L'Amp: A simple SIT Amp

[juanitox]

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!

 

[Nelson Pass]

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.

 

[generg]

very clear, thank you Mr. Pass!

 

[Zen Mod]

whoa!

Blue Car !

Pa made long post !!



Woah! A Blue Car - YouTube

 

[Blues]

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.

 

[juanitox]

too much problem with this Amp , i give up

 

[Blues]

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)

 

[Michael Rothacher]

Yes, I left out the 1/ while typing it out. Sorry boys. Thanks Blues.

 

[Nelson Pass]

Pa made long post !!

Coffee on slow brew.

 

[Telstar]

too much problem with this Amp , i give up

Noo don't give up!

 

[juanitox]

Noo don't give up!

no even i run mine without input cap and with a bypassed source resistor .. none of my ancient SET had sounded like this Sit amp..

put a tube buffer before and you 've got a Big Fat SET sounds , or just a low impedance passive device before ( LDR on my side) and you have a very refined and neutral sounds but lacking some extended bandwitch and low level detail

nothing is perfect in this world .... and that's why i love to play

 

[Zen Mod]

if your tube buffer is giving change to big fat SE sound , then that's no good buffer , in my book

any iteration of SLCF is way to go ; in short - cascode up for constant voltage mode , CCS down for symmetrical and benign THD spectra

it doesn't matter will you make it with toobz , sand , cork , whatever

 

[Zen Mod]

regarding some private correspondence with RK R , about grades ;

I hope someone is having exact grade tables ?

sorry if that's already posted

 

[Telstar]

For fun today with a friend i measured the resistance of my four K82 and two of them where nicely close with about 3,5 Ohm. Looking good

The hardest thing is to procure the ixys fets, if someone has spares, please sell me.

 

[Garuda77]

Hi all,

Amazing thread...I got inspired with all the Eastern promise of nirvana and bought 2 x the 2SK82's...but...I feel it will not have the power I crave..sooo rather than shelling out big bucks on heatsinks trafo's etc and being left wanting anyone here think I could use these in some sort of buffer/preamp in front of some quality T amp monoblock modules or similar? I only use CD as source at present (direct out from DAC via caps - soon to be sowters in balanced configuration) so could keep it really simple, was thinking maybe stepped attenuator then simple circuit with the SITS into class T somethings, maybe hifimediy mono blocks (still researching this part)

 

[OlavHaal]

Power ?????

Hi Garuda77!!

You said " - but...I feel it will not have the power I crave.. -"

And to that I can support some info. Today I have the same DirectOut Dac with Sowther`s - through the B1 passive preamp - then through my homemade monoblock`s SIT(2x8 watt ca) - then through my newly-made Alpair 12P(93 db) in PortedCabinett.
And when I today turn my volumepot up till around 50 % on the B1 it is just perfect - you can hear EVERYTHING - the little touch on the strings on the gitars - the way Rickie Lee Jones is wriggling her voice in The Weight on the album The Devil You Know. When earlier playing my chain on the Alp10.2 without the Sowther it culd be a little low on some album(earlier R.L.Jones and some) - but now with Sowthers and the Alp12P it is really filling the room with sound. especially Voices and acoustic instument is getting the full treatment.
So the SIT is PERFECT on efficient speakers(if you have Sowthers especially)!!

Best

Olav

 

[juanitox]

DCB1 and SITAMP

do you think it"s possible to drive directly the SIT with the DCB1 output without any capacitor or resistor just making the bias with the OFFSET of the DCB1 ?
( let say 11V+ and -16V negative rail)

 

[OlavHaal]

Questions...questions....

Hi Juanitox!

If you are asking me - I`m sorry to say that I don`t know. Let`s send the question further to the guy`s who are the experts: Michael or who else - Mr Pass?.

My SIT-version is selfbiasing and I have moved my 10microF Obbligato from the output of my B1 and to the input on the SIT.
And I have moved to building Dac`s for a while(even if I have a couple spare 2SK82 waiting for LumiaAria - if that is the chosen part!!!)
The sound is so full-bodied and elegant - as is - with the Alp12P!!

Best
Olav

 

[juanitox]

or ZM perhaps if he is in a good day

 

[OlavHaal]

Yepp-yepp