-
Feeling lucky? Check out the AK 24th Anniversary Giveaway thread and join in the fun!
Is a capacitor in the signal path if it has...
- Thread starter roger2
- Start date
Register to hide this ad
No..
ConradH
Lunatic Member
It certain can be. Consider an NPN transistor with an emitter resistor to ground. There's often a cap bypassing that resistor, which changes the AC gain of the circuit. The characteristics of that cap can affect the signal. Call it what you will, but to me that's part of the signal path. Parallel or series, it doesn't matter; if it affects the signal it's in the path. I don't consider bypass caps and power supply caps in the same way, though they can have an effect too. In the example above, you might have a bypass cap from collector to ground. IMO, still important, but is not in the signal path. Put another way, it's not intended to affect or set the response.
Chip Chester
Super Member
It would seem it's in the signal's path to ground.
As is the speaker.
Chip
As is the speaker.
Chip
Great question.
Obviously Conrad makes a good point on the emitter bypass caps. But what about power supply filter caps?
I've heard many very knowledgeable people state that power supply filter caps are in the signal path. They claim that all power supply current must return through them. I've often wondered what is meant by that. Obviously, DC current will not pass through a cap (for long), only AC current. The signal current could be returning through the filter caps, but does it? I don't see why it would. I could design a power supply that uses no filter caps at all, and I bet my audio gear would be just as happy. I don't understand their statement that all of the signal current returns through the filter caps. I'm hoping someone can explain it to me.
Terry
Obviously Conrad makes a good point on the emitter bypass caps. But what about power supply filter caps?
I've heard many very knowledgeable people state that power supply filter caps are in the signal path. They claim that all power supply current must return through them. I've often wondered what is meant by that. Obviously, DC current will not pass through a cap (for long), only AC current. The signal current could be returning through the filter caps, but does it? I don't see why it would. I could design a power supply that uses no filter caps at all, and I bet my audio gear would be just as happy. I don't understand their statement that all of the signal current returns through the filter caps. I'm hoping someone can explain it to me.
Terry
The discussion about whether or not a cap is in the signal path can get dicey, as we're seeing above.
However, unless you're getting music out of the wall outlet, a filter capacitor is not in the signal path.
However, unless you're getting music out of the wall outlet, a filter capacitor is not in the signal path.
ken kantor
Fowl Humor
Yes, without question. Just consider an active filter circuit. Any errors or distortions present in the signal caps going to ground will be manifest on the output.
There is no doubt about this.
-k
kyle
Super Member
Yes.
A power amplifier is a modulated power supply, particularly with single ended amps.
Whether SS or tube, you're just altering voltage or current in the supply to power the speakers.
A capacitor will change the impedance to ground at a center frequency and will affect the sound of the amp in some positions in the circuit more than others. 6dB per octave at the center freq. Teflon caps may make oil caps seem soft and undefined.
A phono section has more strict requirements than an amp for noise and sensitivity as an example. You need closely matched parts and tubes for phono but for a power amp 5 to 10% is fine.
I think that where a cap is in a circuit makes more difference than what kind it is but good caps in coupling positions is key.
A power amplifier is a modulated power supply, particularly with single ended amps.
Whether SS or tube, you're just altering voltage or current in the supply to power the speakers.
A capacitor will change the impedance to ground at a center frequency and will affect the sound of the amp in some positions in the circuit more than others. 6dB per octave at the center freq. Teflon caps may make oil caps seem soft and undefined.
A phono section has more strict requirements than an amp for noise and sensitivity as an example. You need closely matched parts and tubes for phono but for a power amp 5 to 10% is fine.
I think that where a cap is in a circuit makes more difference than what kind it is but good caps in coupling positions is key.
SoundOfSound
Super Member
The reason people say the power supply is in the signal path is because it supplies the voltage (Vcc) that is connected to the collector. If the power supply is dirty all that voltage ripple and noise will become part of the signal that is going to the next circuit. Say you have a string of 4 transistors in your amp. That noise on Vcc in the first stage is now going to be on the base of the next transistor, plus the power supply noise on Vcc. You can see how this will go downhill rather quick after a few stages.
The signal that is on the base is not actually passing through to the transistors collector.
That small signal is just controlling how much the transistor conducts and therefore the voltage drop present at the collector. These voltage swings at the collector are the new "amplified" signal. Any noise/ripple on Vcc will become part of this new signal.
That's why high end gear have power supplies built like a BSH. Their supplies are very clean and have lots of reserve current capability to keep Vcc stable.
http://www.learningaboutelectronics.com/Articles/What-is-vcc-of-a-transistor
Last edited:
nashvillebill
Super Member
But transistors exhibit power supply ripple rejection (PSRR), minor ripple isn't going to pass onto the base of the next transistor.
ConradH
Lunatic Member
Now it gets tougher. An amplifier (like an opamp) might have phenomenal PSRR at low frequencies, but far less so at high frequencies. Feed it a lot of ripple from the supply and maybe little happens. Feed it HF noise from the same supply and watch it show up at the output.
When you talk about signal path it has to be taken in context. If you're tracing signals with a scope or such, knowing the signal path in that context is obviously important, and it's usually straightforward. OTOH, if you're interested in sonic effects and how various parts affect the signal, the definition of "signal path" may be quite different.
The biggest thing I see is people without electronic background not recognizing that series and parallel components can have equal effects on the signal. The epitome of this is the shunt volume control, where somehow a pot to ground isn't considered as bad as a pot in series or used as a divider.
When you talk about signal path it has to be taken in context. If you're tracing signals with a scope or such, knowing the signal path in that context is obviously important, and it's usually straightforward. OTOH, if you're interested in sonic effects and how various parts affect the signal, the definition of "signal path" may be quite different.
The biggest thing I see is people without electronic background not recognizing that series and parallel components can have equal effects on the signal. The epitome of this is the shunt volume control, where somehow a pot to ground isn't considered as bad as a pot in series or used as a divider.
By that definition if the electric power generating plant is delivering noisy/dirty power and it's not being properly filtered, then the power plant is "in the signal path".
This^^^.
The main reason these discussions go circular is not because of analyzing a circuit to determine whether or not the component in question is in the signal path, it's because of dramatic differences of opinion regarding what the "signal path" means.
If there's no consensus on what the signal path is, then there can be no consensus on what's in or not in the signal path.
Capacitors in power supply are dependent on different properties than in low signal circuits. In power supply (filtering) important are low inductance and low ESR. In signal path linearity and leakage current are much more important. That is why different capacitor's types have their intended purpose described in datasheets.
SoundOfSound
Super Member
We can say that it affects the signal path. I'm sure we have all heard a pop of some appliance turning on or the noise of a drill or vacuum cleaner coming through our speakers at one time or another. We didn't hook up RCA cables to the vacuum cleaner. All that noise is coming through the AC wall outlet and making it impossible for the power supply to provide a stable Vcc voltage. This will have negative affects on the signal path. It is even easier to see these things degrade a video signal when watching a TV.
I guess the point that some of us are trying to make is that you cannot have a clean and stable audio/video signal without a clean and stable power supply. The power supply has a direct affect on the signal path, hopefully a good one.
I guess for my purposes, the cap would be in the signal path if non-linearities in the cap would be expected to show up as distortion in the output of the amplifier. I would not expect non-linearities in a cap used as power supply bypassing to cause distortion in the output. But a coupling cap or filter cap definitely would.
Terry
Terry
We're approaching a ratio of one new definition of signal path per post.
petehall347
the brandy coffee man
what is the signal path ? what path does it take in a properly working amplifier ?
From the input to the output, touching the supply lines and ground as little as possible
