Lower output impedance and oil caps questions for a phono stagepreamp.

[Elias]

Hi.

I have a Lafayette phono stage preamp made in japan with 2 12AX7. I
have 2 questions:

1) I modified the input impedance from 6K to 47K as suggested by the
manual and it already sounds better, but not great. The output
impedance is 100K (there is an 100K resistor from the PS before the
coupling capacitor). My preamp has a 47.5 Kohms input impedance. Can
I put a 47 Kohms resistor after the coupling capacitor between the
signal and ground to match the input impedance of my preamp? Any
better suggestion?

2) The phono stage preamp has King Oil Capacitors for all coupling
caps. Are they OK to keep or should I replaced them?

Thanks.
--
Elias

[Ian Bell[_2_]]

Elias wrote:
> Hi.
>
> I have a Lafayette phono stage preamp made in japan with 2 12AX7. I
> have 2 questions:
>
> 1) I modified the input impedance from 6K to 47K as suggested by the
> manual and it already sounds better, but not great. The output
> impedance is 100K (there is an 100K resistor from the PS before the
> coupling capacitor). My preamp has a 47.5 Kohms input impedance. Can
> I put a 47 Kohms resistor after the coupling capacitor between the
> signal and ground to match the input impedance of my preamp? Any
> better suggestion?
>
> 2) The phono stage preamp has King Oil Capacitors for all coupling
> caps. Are they OK to keep or should I replaced them?
>
> Thanks.
> --
> Elias


This does not make a lot of sense as it stands. Can you post a schematic?

Cheers

ian

[Paul D. Spiegel]

Elias > wrote:
> Hi.
>
> I have a Lafayette phono stage preamp made in japan with 2 12AX7. I
> have 2 questions:
>
> 1) I modified the input impedance from 6K to 47K as suggested by the
> manual and it already sounds better, but not great. The output
> impedance is 100K (there is an 100K resistor from the PS before the
> coupling capacitor). My preamp has a 47.5 Kohms input impedance. Can
> I put a 47 Kohms resistor after the coupling capacitor between the
> signal and ground to match the input impedance of my preamp? Any
> better suggestion?

While you can roughly set the input impedance with the value of the grid
resistor as described in the manual, output impedance is much more
complicated than value of the load resistor. There are a lot of
additional factors such as the value of the cathode resistor, presence and
value of a cathode bypass cap, etc.

Chances are, you are probably close to 50 Kohm as is, but it likely
doesn't sound too good driving a 47.5 Kohm (solid state?) load. Once
again there are lot's of interstage dependancies, but I'd guess that you
are getting a significant low frequency rolloff and rather harsh, bright
highs. The rule of thumb is often an input impedance of 10-12 times the
driving output impedance to get the smoothest and widest frequency
response.

If you can't increase the input impedance of the pre-amp to 500 Kohm or
greater (common for a tube pre-amp), then I would recommend adding a
cathode follower buffer with a 0.5 to 2.0 mF output coupling cap to
address the interface.

My 2 cents.

> 2) The phono stage preamp has King Oil Capacitors for all
coupling > caps. Are they OK to keep or should I replaced them?

No comment. I'm not familiar with these.