Tube amp, output capacitor and headphones impedance

[00940]

On my old RKV, there are two big output caps. Electrolytic, 220uf, 350V. Fact is, they need to be changed, they made their time.

The question is : are such big caps useful for 300ohms headphones (I don't plan to use anything below) ?

I found this formula on headwize, giving the frequency corner (3db down) in function of the size of the output cap and the impedance of the cans:

C (uF) = 1,000,000 / [2pi (corner frequency) Rheadphones]

So :

47 = 1,000,000 / [2 x 3.1416 x corner frequency x 300]
=> 47 x 2 x 3.1416 x 300 = 1,000,000 / corner frequency
==> corner frequency = 1,000,000 / 88,593 = 11.30Hz

Which is fine.

The good thing now is that a 47uf, 400V polyprop audyn cap is only 10.25€ a piece. And a high quality electrolytic caps, with this voltage rating would cost arms and legs. And I'm not even speaking of 220uf/385V blackgate (around 130$ a piece).

Is everything here allright or is the output capacitor also dependant of the circuit as a whole ?

[fdegrove]

Hi,

Is everything here allright or is the output capacitor also dependant of the circuit as a whole ?

No, it's load dependent.

Cheers,

[TG]

No, it's load dependent.

Oh, really?
Actually it depends on both load and Rout of the circuit...

C (uF) = 1,000,000 / [2pi (corner frequency) (Rheadphones-Rout)]

[fdegrove]

Hi,

Actually it depends on both load and Rout of the circuit...

Sure but I considered that as a known factor not a variable.

Cheers,

[TG]

Sure but I considered that as a known factor not a variable.

Well it don't have to be a variable to cause troubles
With Rout=200 Ohm (not so uncommon with tube amps) the output cap has to be 3 times bigger to keep the same corner frequency...

Good luck

[00940]

how easy (difficult) is it to determine Rout ?

I don't know a lot about tube amps, it's just maintenance here

I should perhaps just get a pana FC of the same value and bypass it with a high quality polyprop

[fdegrove]

Hi,

An easy to read paper explaining basics can be found here:

AIKENAMPS

Cheers,

[00940]

hum, the design of the RKV is awfully complicated and i can't associate it with any of those schematics.

Is there a practical way to measure the Rout, i mean with a multimeter or such ?

http://www.audiovalve.de/rkv/rkvpatent.jpg here's a bad pic of the schematic; if you feel you could help, i can provide you the actual schematic but i've been asked not to make it public.

[TG]

Is there a practical way to measure the Rout, i mean with a multimeter or such ?

Yes, there is. First apply some signal to the input and measure output (say U0) unloaded. Then load output with resistor Rl(e.g. 0.5-1 kOhm) and measure output (U1) retaining the same input.

Rout = Rl*(U0/U1 - 1)

Good luck!

[fdegrove]

Hi,

The outputstage on the patent looks like some sort of SEPP circuit.

It should have very low Zout if the tubes are run in triode mode.
I can't tell from the patent drawing...

Cheers,