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)
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)
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?
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?
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.
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.
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?
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
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?
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?
**** 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 ****
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 ****
Kashmire said:
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
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:
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.
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.
Consider this quote from Doug Self
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.
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!
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.
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!
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
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
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?
Kashmire,
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
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
"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
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
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