# Cathode Follower output cap

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#### alexg

In most cathode follower schematic on the net, the output capacitor is usually around 2.2uf.

Can these be lowered? Is the computation of output capacitor for cathode follower different the other topology?

Thanks.

#### SY

The size of the output cap is determined by the load and by the required LF rolloff by the usual formula (f3 = 1/(2piRloadC). For a load resistance (e.g., power amp input impedance in the case of a CF output of a preamp) of 100k, a 2.2u cap gives a LF -3dB point of 1/(2pi*2.2E-06*100,000) = 0.72 Hz.

#### TheGimp

The output capacitance is dependent on both the driving resistance (impedance) and load resistance.

If you lower it, you effect the low end transfer characteristics (that is, you raise the low end cut off, -3db point, etc).

You need to specify the load before you can calculate the output capacitance.

(SY beat me to it with a more detailed answer.)

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#### DualTriode

Hello,
Remember there is more going on at that -3db roll off point than lower output. There is also phase angle shift happening. Minus 3db is a JND just noticeable difference. The phase angle is 45 degrees off as well.
Take a look at Douglas Self’s’ books, he documents an increase in distortion near roll off. Capacitor caused distortion. Good reasons to select capacitors for roll off well below audible range frequencies.
DT
All just for fun!

#### alexg

@Sy, thanks. I thought CF follows a different formula.

I asked because I need to filter the low frequency on my preamp, I can see my woofer moving quite slowly when playing LPs, I was thinking if I use lower output cap, I can get rid of these low frequency going to my speakers.

#### SemperFi

That's what's called a 'rumble filter' and was common in the old days when vinyl was the norm. I have the same 'problem' in my system, but not worried about it. (I don't use a rumble filter).

#### Brit01

I asked because I need to filter the low frequency on my preamp, I can see my woofer moving quite slowly when playing LPs, I was thinking if I use lower output cap, I can get rid of these low frequency going to my speakers.

I had this issue with my SS power amp.

I bought 2 devices which fit on the RCA inputs of the amp which filter out frequencies below a specified level. I chose I think 20 or 30Hz. can't remember the company right now.

Nice pieces, didn't effect the sound at all and solved the woofer wobbling instantly. You won't hear bass frequencies below this level anyway or your speaker won't be able to produce them.

#### SY

I asked because I need to filter the low frequency on my preamp, I can see my woofer moving quite slowly when playing LPs, I was thinking if I use lower output cap, I can get rid of these low frequency going to my speakers.

Yes, you can, but you don't want to overdo it. I wouldn't set the f3 much higher than 2 Hz for the reasons DT mentioned. You may want to dig a bit deeper to see what the LF noise is and try to eliminate it at the source. Possible causes include power supply (poor regulation), heater supply (ditto), heater-to-cathode leakage, and the tube itself.

#### DF96

Valve amps with feedback often have an LF response peak around a few Hz. This is caused by the accumulation of LF phase shifts bringing the feedback loop near to instability. This is in the same region as record warps, arm resonance (with a poor cartridge-arm match) and mains voltage variations. Two options: stop the subsonic stuff from getting in by using a smaller capacitor, and accept the resulting phase shift; reduce the feedback by increasing the feedback resistor, which will increase the amp sensitivity but reduce the LF peak.

First thing to do is confirm the source of the subsonic: is it coming from the cartridge or the phono preamp? Do you get cone flap when you are all set up for LP, but the stylus is not actually touching the record?

#### kevinkr

Paid Member
Hello,
<snip>
Take a look at Douglas Self’s’ books, he documents an increase in distortion near roll off. Capacitor caused distortion. Good reasons to select capacitors for roll off well below audible range frequencies.
DT
All just for fun!

Practically speaking this is mainly applicable in electrolytic capacitors which have a variety of distortion generating mechanisms (voltage coefficient, etc) that good film caps do not.

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#### mashaffer

Sounds like a mismatch between the cartridge compliance and tone arm mass. Some google searching will give some tips on choosing an appropriate cartridge I bet. My memory of the details is insufficient to provide any detailed help.

#### Tubelab_com

If you can see the woofer cone moving there are likely two different distortion generating mechanisms at work that haven't been mentioned yet.

First is woofer linearity. As the cone moves some of its x-max is used up. depending on how the speaker is designed the linearity and efficiency of the speaker is reduced as the cone moves away from its resting location. This is more an issue with high efficiency speakers.

Second is OPT saturation. The OPT doesn't pass the 1 to 5 Hz energy very well, so more is hitting the transformer than you see in the speaker. It is using up the magnetic capability of the OPT. In extreme cases you can hear the sound change as the speaker cone nears its extreme excursions. This is more of an issue with P-P transformers.

This used to make my speakers go crazy when I had a 400 watt Carver ? Phase Linear SS system and a few warped records. Some could not be played loud for fear of losing a speaker cone.

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