Power amp without decoupling capacitor, what does this mean?

[Freighter]

Hi all,

I currently have a Marantz SR7011 AVR and I'm looking for a power amp to connect to it (pre-out). I think the NC252MP would be a good choice and I found a seller, however on their website they post this warning:

"This amplifier does not have a decoupling capacitor, so it lets the DC component of the signal through. Make sure your source has no DC component present at the output. The DC protection of the module switches on at 12VDC which is very permissive."

AUDIOPHONICS MPA-S250NC RCA Class D Stereo Amplifier Ncore 2x250W 4 Ohm - Audiophonics

The Audiophonics MPA-S252NC XLR is a stereo amplifier based on an amplification module Hypex Ncore NC252MP. This solution is ideal for anyone who wants to have a large power reserve (2x150W) without sacrificing sound reproduction quality.

www.audiophonics.fr

I don't quite understand what this means. I was hoping someone could explain what this means exactly, and what the implications would be for me for using it together with my AVR?

Thanks!

 

[sq225917]

It means you need to measure the output of you pre with a voltmeter in DC mode to see how much dc it outputs.

Less than 1mv would be ideal, any more than that and I'd want to measure out the back of the connected power amp before connecting speakers.

20-50mv DC output from a power amp is considered acceptable. My own are sub 1mv, but YMMV.

 

[Speedskater]

Pre-amps often have decoupling capacitors in their outputs.
Power amps often have decoupling capacitors in their inputs.
This is redundant, only one is needed.

 

[Suffolkhifinut]

Hi all,

I currently have a Marantz SR7011 AVR and I'm looking for a power amp to connect to it (pre-out). I think the NC252MP would be a good choice and I found a seller, however on their website they post this warning:

"This amplifier does not have a decoupling capacitor, so it lets the DC component of the signal through. Make sure your source has no DC component present at the output. The DC protection of the module switches on at 12VDC which is very permissive."

AUDIOPHONICS MPA-S250NC RCA Class D Stereo Amplifier Ncore 2x250W 4 Ohm - Audiophonics

The Audiophonics MPA-S252NC XLR is a stereo amplifier based on an amplification module Hypex Ncore NC252MP. This solution is ideal for anyone who wants to have a large power reserve (2x150W) without sacrificing sound reproduction quality.

www.audiophonics.fr

I don't quite understand what this means. I was hoping someone could explain what this means exactly, and what the implications would be for me for using it together with my AVR?

Thanks!

Coupling capacitors are used to let the AC signal through and block the DC power supply. Each stage in an amplifier must be DC biased and the bias levels may vary from stage to stage.

 

[DVDdoug]

DC (direct current) is zero Hz. If you connect a 1.5V battery (DC) to a speaker, and if you can see the cone, you'll see it pop-out or pop-in depending on the polarity. You'll also hear a click or pop when the cone moves but once it gets into position it doesn't make any sound while the DC current flows. It makes another click or pop when you disconnect it.

Audio isn't supposed to have DC but some gear has a DC offset and the DC can exist along with the AC audio signal. It wastes power and it can cause or contribute to distortion because the positive or negative is closer to clipping. And it can push your woofer toward it's mechanical limits.

A series capacitor blocks DC. It makes a high-pass filter so normally the capacitor value is chosen to pass bass while blocking DC and subsonic frequencies. Speaker crossovers have capacitors which means you can't DC into a midrange or tweeter.

The proper capacitor doesn't hurt the audio and it can sometimes prevent problems.

The Audacity Website has a bit of information about DC offset in digital files (a bad thing). In a digital file it's not actually current, it's just an offset in the digital values where the waveform is offset and silence is not at a value of zero. But if it goes through a DAC, then it becomes direct current.

Some audio circuits, especially older discrete designs without ICs or op-amps, require DC bias to work, so capacitors are required on the input & output to filter it out. Most battery operated circuits only have positive voltage as a power supply so they are internally DC biased.

 

[Suffolkhifinut]

Pre-amps often have decoupling capacitors in their outputs.
Power amps often have decoupling capacitors in their inputs.
This is redundant, only one is needed.

The use of decoupling capacitors is entirely different and are quite often used (as an example) to keep the DC voltage dependant on emitter current. As an example a capacitor could and often is connected across any resistor connecting a power rail to the emitter in a bipolar transistor.

 

[bo_knows]

Wow! Thank you for bringing this up.
I have had a very bad experience with this (DC going into the woofers) in the past.
I believe my Sony preamp malfunctioned and DC was sent to my Plinius amp which I believe doesn't have decoupling capacitors on the output stages and it RUINED my Dynaudio woofers twice! It was a nightmare! I had to send the boxes to the Dynaudio center in Chicago to be replaced (the first time around). The second time it happened, I purchase the woofers from Dynaudio and replaced them myself. A very expensive lesson. Needless to say, I fixed the Sony preamp and sold the Plinius amp. The amps that took place after Plinius departed, were Parasound JC-1 monoblocks. The following features made that purchase justified much easier:

• Direct-coupled signal path, no capacitors or inductors in the audio path
• DC servo & relay protection circuits

 

[Freighter]

DC (direct current) is zero Hz. If you connect a 1.5V battery (DC) to a speaker, and if you can see the cone, you'll see it pop-out or pop-in depending on the polarity. You'll also hear a click or pop when the cone moves but once it gets into position it doesn't make any sound while the DC current flows. It makes another click or pop when you disconnect it.

Audio isn't supposed to have DC but some gear has a DC offset and the DC can exist along with the AC audio signal. It wastes power and it can cause or contribute to distortion because the positive or negative is closer to clipping. And it can push your woofer toward it's mechanical limits.

A series capacitor blocks DC. It makes a high-pass filter so normally the capacitor value is chosen to pass bass while blocking DC and subsonic frequencies. Speaker crossovers have capacitors which means you can't DC into a midrange or tweeter.

The proper capacitor doesn't hurt the audio and it can sometimes prevent problems.

The Audacity Website has a bit of information about DC offset in digital files (a bad thing). In a digital file it's not actually current, it's just an offset in the digital values where the waveform is offset and silence is not at a value of zero. But if it goes through a DAC, then it becomes direct current.

Some audio circuits, especially older discrete designs without ICs or op-amps, require DC bias to work, so capacitors are required on the input & output to filter it out. Most battery operated circuits only have positive voltage as a power supply so they are internally DC biased.

Wow, thank you, that is very thorough. I only partially understand, so I’m a little reluctant to buy this amp now. Since the website mentions the lack of decoupling capacitor explicitly, does that mean that it’s a feature that amps normally have? For instance if I buy a more consumer oriented amp like a NAD or a Denon or something like that, would it typically have a decoupling capacitor?

If I do choose this amp as far as I understand

sq225917’s advice is what I would need to do (check the DC output of my AVR on the pre-out)?​

 

[bo_knows]

DC (direct current) is zero Hz. If you connect a 1.5V battery (DC) to a speaker, and if you can see the cone, you'll see it pop-out or pop-in depending on the polarity. You'll also hear a click or pop when the cone moves but once it gets into position it doesn't make any sound while the DC current flows. It makes another click or pop when you disconnect it.

Audio isn't supposed to have DC but some gear has a DC offset and the DC can exist along with the AC audio signal. It wastes power and it can cause or contribute to distortion because the positive or negative is closer to clipping. And it can push your woofer toward it's mechanical limits.

A series capacitor blocks DC. It makes a high-pass filter so normally the capacitor value is chosen to pass bass while blocking DC and subsonic frequencies. Speaker crossovers have capacitors which means you can't DC into a midrange or tweeter.

The proper capacitor doesn't hurt the audio and it can sometimes prevent problems.

The Audacity Website has a bit of information about DC offset in digital files (a bad thing). In a digital file it's not actually current, it's just an offset in the digital values where the waveform is offset and silence is not at a value of zero. But if it goes through a DAC, then it becomes direct current.

Some audio circuits, especially older discrete designs without ICs or op-amps, require DC bias to work, so capacitors are required on the input & output to filter it out. Most battery operated circuits only have positive voltage as a power supply so they are internally DC biased.

"Audio isn't supposed to have DC but some gear has a DC offset and the DC can exist along with the AC audio signal. It wastes power and it can cause or contribute to distortion because the positive or negative is closer to clipping. And it can push your woofer toward it's mechanical limits."

Oh, boy did my Dynaudio woofers got pushed out (bulged) beyond their mechanical limits with Plinius (class A) amp when DC was present! Ugly sight.

 

[captainbeefheart]

They don't know what a decoupling capacitor is because what they describe is a coupling capacitor not a decoupling capacitor. The latter would shunt signal to ground which isn't what you want. The former, and correct term of "coupling" basically couples stages of amplifiers together that have DC present. The coupling capacitor is sized to allow the AC signal to pass through while blocking the DC element.

If your power amp has an input capacitor than it's not a problem that your preamp may contain come DC, I still think it's silly to willingly send DC off to equipment and the preamp should have an output coupling capacitor sized appropriately for a load down to 10k ohms. I'd say minimum of 1uF but higher would be better, if you can add a 3.3uF film output cap it would be ideal as long as the input of your power amp isn't lower than 10k impedance.

 

[Cars-N-Cans]

Wow, thank you, that is very thorough. I only partially understand, so I’m a little reluctant to buy this amp now. Since the website mentions the lack of decoupling capacitor explicitly, does that mean that it’s a feature that amps normally have? For instance if I buy a more consumer oriented amp like a NAD or a Denon or something like that, would it typically have a decoupling capacitor?

What it really means is that the amplifier can pass (and amplify) DC just as though it is audio. Normally in linear amplifiers there is usually compensation in the feedback loop to roll the gain off to unity at DC so it does not amplify DC offsets in the source. The other common thing is to have a servo circuit to remove the bias so the amp can still play down to low frequencies, but wont pass DC to the devices its powering. Pretty much any device is capable of having a DC offset and many do, and this will be amplified by the amplifier (say, 20x) and then appear at the output. This means a 25mV offset from a DAC or other device its connected to becomes 0.5V at the output, which will displace the woofers quite a bit, and cook an IEM if its a headphone amp. The insidious part is that the DC offset is not directly audible. The DC offset protection circuit is just there in case the amplifier ends up saturating at one extreme. This could mean something like 40-50V of DC at its output, which would be catastrophic for anything hooked to it. As such, the DC protection will open the output relay if there is excessive offset detected. Probably wont be fast enough to prevent damage, but at least there wont be woofer cones dangling out the front of the speakers with flames coming off the voice coils.

This amp is not alone in this characteristic. There are a lot of amps out there that will happily treat DC no differently than audio. If you have a multimeter you can measure it at the speaker terminals with the source connected, no speakers, and nothing playing. There will be some small amount of DC present naturally since its hard to get things to be perfect, but it shouldn't be more than something like 25-50 mV. More than that and it will produce a distinct audible tick at start-up. Even more, say around a few tenths of a volt, and there will be a loud thump at turn on and turn off, and the speakers may sound distorted. Still more will produce loud bangs or sharp cracks from the woofers going to their extremes of travel, and they will be damaged. If you don't have a multimeter, a quick check is to watch the woofer cones when the amp is powering up. Once the internal relay(s) close, its acceptable to have a faint tick in the speakers, but the woofers should not visibly move.