[kyrill]

Hi Hugh
I am just curious

you mention the SS stage of the GK-1 has a very low Z-out, 45 ohms or so
but i am curious to the Z-in of the following  tube section

Kyrill

[AKSA]

Aha, Kyrill, secret men's business!!

The Zin of the tube input section, a self-biased cathode follower, calculates out at 4.3 megohms.

Zout of the SS section is in fact 32 ohms - it's a match made in heaven for the pot.

Cheers,

Hugh

[kyrill]

the Z in of the tube section is in mega ohms? 10⁶?
but then, if so, it can accommodate any pot any value?

[AKSA]

Yes, Kyrill, that's true,

BUT, the open loop gain and hence the feedback factor is determined by the value of this pot.  Make it too high and the fb factor increases, taking the stage outside the Bode/Nyquist stability criteria.  Stick to 10-25K, nothing higher or lower, otherwise you need to alter the value of the tiny silver mica at C3, changing sonics, possibly for the worse.  Everything balances beautifully as it is;  this is a sweet spot in the circuit!

Cheers,

Hugh

[kyrill]

Sir
is there an easy way to heighten the Z-in of the preamp from its original 47k?
I mean to 100K even 500K?

[AKSA]

Kyrill,

Don't call me Sir, or I'll court martial you!!   

You can increase the Zin to 100K, but this will increase noise somewhat and reduce offset control (no big deal since it's only a pot we are driving).

You can't increase it to 1M unless you use a jfet input, and this would require a redesign, as transconductance is MUCH less than bipolar transistors, greatly reducing feedback factor.

Ain't no free lunch, hombre........

Chars,

Hugh

[kyrill]

ok Hugh
how can i do this?

1M is not a goal for me
but 100K or 200K as a try how it sounds with different sources with higish Zout
i would like to try out

Greetz
Kyrill

[AKSA]

Hi Kyrill,

Hmm, change the following for a Zin of 100K:

1.  R1 47K to 100K
2.  R11 68K to 150K
3.  R9 47K to 100K
4.  R10 100K to 220K

This will deliver more noise, I believe, but the gain will be the same at (150 + 100)/100 = 2.5, little different to the original 2.44.

Offset will be preserved at the top of the volume pot;  that is, less than 5mV.

You may find a little more bass, and slightly less top end resolution.

Met Vriendelijke Groet,

Hugh

[kyrill]

ah it is worth trying