Electrolytic Output Capacitor

[Kashmire]

I've often wondered if the electrolytic output capacitor of the Zen amplifiers will ultimately limit the amplifier - i.e. the Zen 9 is better than the preceding versions, how long until new innovations no longer help, because the design is held back by the output coupling capacitor?

I've searched the forums a few times over the past months looking for this topic, but haven't found a serous discussion (other than occasional ramblings about bypassing with film types).

How much sonic contribution does the output capacitor make? It is my opinion that it is low, and there are much more important things to address. However, in a new amplifier design, should efforts be made to remove the output capacitor, or accept it as a mechanism that allows for other improvments?

(the tube guys fight over this regularly, especially with the parafeed configuration)

[Kashmire]

Wouldn't you know it? More searching and I found what I was looking for:

http://www.tubecad.com/2004/blog0014.htm

Comments welcome. Especially this question about push-pull amplifiers:

Op-amp servo loop to control DC offset or output coupling capacitor?

[h_a]

If you follow Nelson Pass and Erno Borbely, then you don't need to care too much about an output cap.

If you don't, feel free to investigate the topic - preferably with nice lab equipment - and please come back and tell us!

Cheers, Hannes

PS: oh and don't forget, the same applies to the tons of caps in your crossover. True there are mainly PP caps used there, but things like non-polar elcos do exist and are used.

[analog_sa]

Originally posted by h_a


If you don't, feel free to investigate the topic - preferably with nice lab equipment - and please come back and tell us!

And what could the "nice" lab equipment possibly show? That capacitors have leakage?

I used to be terrified by large coupling electrolytes. After listening to a Pass preamp for a while i am now merely frightened.

[ctong]

I am also interested in the subject. I have read at different places that one should bypass a electrolytic capacitor with a small film capacitor for high frequencies when bypassing a power supply but not when coupling a signal. Can't think of an explanation for the latter claim. Is there any merit to it?

[h_a]

And what could the "nice" lab equipment possibly show?

THD+N, Intermodulation distortion...just as Douglas Self has shown that bad caps can cause distortion in certain places. And he proofed it with measurements.

@ctong: the idea is to lower the impedance at higher frequencies.

By the way: guys, don't start to argue with me, please. I just wanted to state my thoughts on this and I'm really not keen on flame wars about merits and the pure evil of elcos. With argueing I mean subjective flaming, in case somebody has some links to articles with technical background - you're welcome!

We all know that everybody has a different opinion on this, really.

Cheers, Hannes

[ctong]

Originally posted by h_a




@ctong: the idea is to lower the impedance at higher frequencies.



Cheers, Hannes

Hannes,

What I mean is that is there any reason to believe that bypassing a coupling electrolytic capacitor with a smaller film one is a bad idea.

[jcx]

Capacitor Distortion

Good John Curl Article on Capacitors

http://www.diyaudio.com/forums/showt...6875#post96875

[Kashmire]

No flames here.

Nelson Pass wrote an article about bypassing large electrolytic capacitors.

http://www.hometheaterhifi.com/volume_4_2/nelpass.html

Regarding Doug Self, you can read about his theory of where distortion comes from here.

I agree with Broskie that PSU caps are commonly in the output signal path. Below is his example of the signal path for a push-pull amplifier with a bipolar power supply (+/- voltages):



Notice during the up or down cycle of the output waveform, different parts of the power supply (and filter capacitors) are being stressed.

Below is Broskie's other example. The amplifier on the left is the push-pull with bipolar power supply with the standard + and - filter capacitors. The amplifier on the right is push-pull with monopolar power supply, using a singular filter capacitor and an output coupling capacitor.



Now, since the monopolar PSU design uses the same capacitor for both up and down cycles of the waveform, instead of commutating between PSU rails (from plus to minus), shouldn't this be considered a benefit?

[Kashmire]

**** please don't let this post-script divert from the original intention of this thread, which was using an output capacitor on push-pull amplifiers ****

Here's the hidden part of the Broskie analysis: in the Zen, the PSU filter caps really don't see much of the AC waveform. The amplifier acts mostly as a constant-current load, so the PSU is rarely being modulated by an AC signal. Therefore, in a true class-A amplifier, the filter caps really aren't part of the signal path.

But an output capacitor is. But the output capacitor is also needed to function, so it plays the role as a bulk storage capacitor. How is this? Picture the Zen amplifier. During positive waveforms, the active MOSFET pinches off current, forcing the CCS to re-route its current through the load. During negative waveforms, the active MOSFET goes low impedance. All of the CCS current shoots right through it. So how does the load see a negative signal? The output capacitor discharges into the active MOSFET, adding its current the CCS's current. Therefore, the Zen is truly a special case!


**** please don't let this post-script divert from the original intention of this thread, which was using an output capacitor on push-pull amplifiers ****