Increase size of cathode bypassed cap?

[lordvader]

Hey all,

Quick question - I have a phono preamp, with a c3g on the output stage.
Currently, it has a 470uf cap for the bypass cap, but was wondering if I can replace it with a 1000uf cap?

 

[zintolo]

Yes you can. 470 uF is there to make a full bypass down close to DC, and you’ll go simply twice closer to DC. Nothing you can ear.

 

[6A3sUMMER]

Just be sure the phono preamp cuts off two undesired low frequencies:

1. Tone Arm and Cartridge resonance (about 10Hz).
A result of the cartridge mass plus the effective arm mass at the cartridge, and the cartridge compliance.

2. Record warp frequency (about 3 Hz).

When it comes to extreme low frequency response, too much of a "good thing" can become a "bad thing".

Think about how your amplifier's output transformer laminations respond, and how your woofers respond to signals at 10Hz and 3 Hz.

 

[zintolo]

Thanks 6A3sUMMER,

I was not aware of those limits in this specific application.

 

[6A3sUMMER]

I do not have a schematic, but any change of a single pole frequency response generally changes the response of the whole circuit.

As I say, one small change often requires 3 more changes.

 

[azazello]

Nominal of katode cap depends to nominals of plate resistor and interstage cap.
There is diagram for depends. It is small, I tried to photo, but picture is bad. If You tell me nom. of plate res. and interstage cap, I will answer you.

 

[lordvader]

Sorry for the delay - this is the schematic I'm using. This version has 1000u in the bypass position (C5), but the built version has 470u.

 

[ggetzoff]

There are some excellent online cathode bias calculators. You can simulate what you will get both frequency wise and gain.

Just google it.

Cheers,

Greg

 

[lordvader]

You know, I probably used one to get to 470u!