Electrolytic Output Capacitor

[GRollins]

Originally posted by Kashmire


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

Take five colored lights. Shine them on a piece of paper. Reduce four of the five by random amounts, but not to zero. Call the fifth one an electrolytic capacitor. Call the other ones anything you want...resistors, gain devices, wire, power supply...anything. Assume that the goal is zero coloration. i.e. all lights completely out, but accept that this is an unattainable goal.
Given that you will never get the rest of the circuit to such a state of perfection that the electrolytic capacitor is the only thing holding the circuit back, it's pointless to beat yourself over the head with this.
No reasonable person argues that electrolytics are sonically transparent. The trick is to minimize the influence of the cap on the sound.
The best thing to do is to get rid of the cap entirely, but given the topology of the amps in question, that's not going to work. Yes, there are DC-coupled amps, but then you don't have a cap-coupled Zen any longer, you have something else.
The next best thing is to try to minimize the burden the cap places on the sound. Options include:
--Bypass the electrolytic with a film cap
--Drop the electrolytic entirely and use a film cap by itself; accept the loss of lower frequencies...perhaps even turning this into a benefit in the case of a bi-amp system
--Enclose the cap in the feedback loop
--Put two of the amps back to back and feed them a balanced signal--the speaker floats and no longer sees the DC. This comes close to changing the amplifier topology. You can argue this either way.
There are some who claim that film bypasses are bad for any of a dozen reasons. I'm not among them.
Those who want to argue based on the price of film caps are rather missing the point. Either it sounds better or it doesn't. Only after that question is settled can you reasonably try to look at a cost/benefit ratio. That will be up to the individual. But be careful to separate the benefit and cost questions.

Grey

[jacco vermeulen]

Olé

[Kashmire]

Thank you, G. I agree wholeheartedly. That's my my prior post concentrated on the necessity of the coupling capacitor in the Zen. I'm afraid I did myself a disservice, though. The Zen discussion is a distraction from what I'm really wanting to find out:

Originally posted by Kashmire

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

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

If I can rephrase it again,

Is a push-pull amplifier best served with:

1. a bipolar power supply (and symmetrical PSU filter capacitors), possibly a DC servo to maintain low DC offset, and potential hazard to your speakers? (i.e. if a signal-level bias transistor fails, the output MOSFETs can swing to full current, although there are ways to mitigate this, which increases complexity again.)

2. a monopolar power supply with a single filter capacitor and an output coupling capacitor?

It seems that you can trade an output capacitor for PSU complexity (bipolar vs. monopolar), robustness (hazard), and output DC offset (servo loop). You also trade the commutation between the two (biploar) filter capacitors as the waveform crosses the zero axis.

[Kashmire]

Consider this quote from Doug Self

The Doug Self website

Distortion Mechanism #8

... however, in AC-coupled designs the output capacitor may contribute significant distortion.

Doug Self maintains that an output capacitor belongs on his list of Eight Distortion Mechanisms of a Generic Power Amplifier.

But also consider this countering discussion by our Zen Master Nelson Pass.

Originally posted by Nelson Pass

It is my opinion that the role of capacitor is overstated as a source of distortion and sonic degradation.

I've never built a solid-state push-pull amplifier before. I have built tube SE, tube PP, and solid-state SE (Zen). I'm trying to understand why the solid-state guys use bipolar power supplies. Is there a good reason (i.e. is Doug Self correct)? If Nelson Pass is correct, why does he use bipolar supplies?

p.s. the famous audiophile quote, If one of these is right, the other must be wrong!

[GRollins]

You're taking the Zen amps out of context. Once you do so, you're going to have conceptual problems.
They are not intended as state of the art contenders. They are projects, rendered simply and foolproof-ly (look, Ma, I just made up a new word!). If they meet your needs...great. If not...great.
You're trying to read more into the circuits than is intended.
If you want to alter the circuits, then have at it. But you're spending a lot of time and effort criticizing a rock for not being a cloud. The rock doesn't care. Neither does the cloud. You're only going to frustrate yourself. Take a deep breath, back up, and reassess the Zen amps and their place in the universe.

Grey

[Kashmire]

Originally posted by Kashmire

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

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

Originally posted by Kashmire

I'm trying to understand why the solid-state guys use bipolar power supplies.

[Kashmire]

The Doug Self Site

The distortion generated by an AC-coupled amplifier's output capacitor is more serious, as it is not confined to low frequencies. A 6800uF output capacitor driving 40 W into an 8-Ohm load gives mid-band third-harmonic distortion at .0025%, as shown in Fig 32. This is five times more than a Blameless amplifier generates mid-band. Also, the LF THD rise is much steeper than in the small-signal case.

According to measurements performed by Doug Self, using an output capacitor on a power amplifier increases his amplifier's distortion by a factor of five.

I don't fully subscribe to all of Self's pontifications, although, I have never built one of his "blameless" amplifiers, either. I suppose I should build one and listen before I make any more specific commentaries on his theories.

Nevertheless, the fact remains that Self measured that a capacitor imparted fives times the distortion than the active devices. This doesn't make any sense to me, because according to the illustrations above (previous posts), the PSU capacitors are in the output signal ground-loop path in push-pull amplifiers (such as the Self blameless amplifier).

So here's a starter list:

1. Self measured distortion of a small signal capacitor (into a 680 Ohm load), with and without a DC bias. The signal got worse with DC bias applied. In his power amplifier measurements (8 Ohm load), no DC bias was used, as his push-pull output rests at zero volts.
2. Since we know the PSU capacitors are part of the output signal ground-path return loop, shouldn't the PSU capacitors impart similar sonic contributions?
3. The PSU capacitors are held a large DC bias (full PSU voltage), all of the time.
4. The blameless amplifier has a lot of feedback, so everything was operating within the feedback loop, with the exception of the output capacitor in his tests.

Since he was measuring something outside of the feedback look, I contend his measurements of 5x distortion due to an output capacitor was an "apples to oranges" comparison.

The question I am posing may be altering by tiny bits, but remains the same: "why use a bipolar PSU with Class-A push-pull amplifiers, when a monopolar PSU can be used with an output capacitor?" From an engineering standpoint, I can't see why the Pass F4 in Class-A mode would be significantly different in either configuration.

Anyone care to comment?

[bananatuan]

Dear all
Now I'm doing a Alpeh P but I only have 4 capacitor GE 5uF 400V for input cap. I wonder can 5uF use in input cap instead of 10uF as original design ?
Thank for your advises / Banana

[Pan]

Kashmire,

Therefore, in a true class-A amplifier, the filter caps really aren't part of the signal path.

The filter caps are very much in the signal path in a single ended Pass style amp as well as in a typical complementary class A amp.

You need to go bridged to get dynamic influence of caps out of the equation.


Or you can use Andrea Ciuffoli's capacitance multiplier solution as used in the power follower of his. Still you have that cap left for DC blocking on the output though.


/Peter

[KSTR]

"Therefore, in a true class-A amplifier, the filter caps really aren't part of the signal path"

A class-A topology is only one of a set of pre-requisites for that. "Out ouf the signal path" translates to "don't see any signal current" in the end, and that's is the only thing that counts.

Thus:

- the output stage must draw constant current from the supply, regardless of signal and load (class-A is not necessarily equivalent to constant current, it just says that no output device ever gets turned off)

- if split supplies are used, this must be valid for both of the supply halves. With split supplies a bridge output topology is needed to fulfill that requirement, otherwise only the sum of currents is constant (while the indiviual currents are copies of the output current). Peter wrote that already, I like to emphasize a certain point again: With a constant current bridge, the center tap is freed from load currents, and that allow the use of a single supply (you must still provide a "GND" reference point, of course).

With a single supply and output cap, there also is bridge topology, albeit one of the halves is passive and not actively controlled with a signal. Redraw the schem and you see it... without constant current draw there is no benefit, the supply cap sees a full-wave rectified copy of the output current, whereas in the split supply each cap sees different half-wave rectified currents. Depending on the location where you set the split point (for AC), any combination of these variants are possible.

The rectifying action is mainy what causes the problems, forcing the supply to handle signal current harmonics up into the hundredths of kHz.

- Klaus