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Class D Switching Power Amplifiers and Power D/A conversion

Amplifier hiss with no input signal
Amplifier hiss with no input signal
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Old 6th December 2017, 07:43 AM   #11
abraxalito is online now abraxalito  United Kingdom
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Doing the 5dB attenuation in hardware rather than software will certainly improve the noise. However while I'm sure a ferrite transformer is well suited to the tweeter, I'm not so certain about the mid, owing to the lower frequencies. Ferrite is best suited to higher frequencies though it may indeed work OK at mid-range freqs, depending on where the lower cut-off is.

No, the two output power specs imply that he probably compromised between 4 and 8ohms. The only way I know of to get the damping correct in both cases is from using post-filter feedback. As far as I know only Hypex is doing that, though there may be others turning up now as some of TI's later designed chips can be operated in this fashion.

As to whether the noise is present at all levels, it depends whether you're using digital volume control for your system volume. If so, then yes.
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Old 6th December 2017, 04:18 PM   #12
hifidon is offline hifidon  United States
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Originally Posted by abraxalito View Post
Doing the 5dB attenuation in hardware rather than software will certainly improve the noise. However while I'm sure a ferrite transformer is well suited to the tweeter, I'm not so certain about the mid, owing to the lower frequencies. Ferrite is best suited to higher frequencies though it may indeed work OK at mid-range freqs, depending on where the lower cut-off is.
What about putting a resistor in series between the input signal and the amp? Would that be a good solution? If yes for the mids, would this approach also be good for the higher frequencies of a typical tweeter?

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Originally Posted by abraxalito View Post
No, the two output power specs imply that he probably compromised between 4 and 8ohms. The only way I know of to get the damping correct in both cases is from using post-filter feedback. As far as I know only Hypex is doing that, though there may be others turning up now as some of TI's later designed chips can be operated in this fashion.
Trying to get my head around damping. What is considered "correct" damping?

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Originally Posted by abraxalito View Post
ClassD amps normally have an LC output filter which relies on the speaker to provide damping. Insufficient damping (due to too high termination impedance) gives a rising HF response which will accentuate any hiss.
This confuses me. Perhaps I just need you to clarify what "termination impedance" is in reference to. I want to think it is the speaker impedance, but that wouldn't make sense given the definition of damping factor. I.e. higher speaker impedance equals higher damping factor.
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Old 6th December 2017, 11:18 PM   #13
abraxalito is online now abraxalito  United Kingdom
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Originally Posted by hifidon View Post
What about putting a resistor in series between the input signal and the amp?
You'd need two resistors to form a 'potential divider'. This would certainly work yes.

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Would that be a good solution? If yes for the mids, would this approach also be good for the higher frequencies of a typical tweeter?
In my experience use of a resistive attenuator is likely to reduce dynamics as you're driving your amp from opamps inside your MiniSharc and you're feeding a classD amp. Normally a classD has lowish input impedance and performs best when driven from a low source impedance. But its worth trying to see how you get on. Yes you can use this approach for the tweeter too, but you have no need to adjust its sensitivity right?

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Trying to get my head around damping. What is considered "correct" damping?
'Correct' damping is when you have a Butterworth alignment, i.e. a flat passband leading to a smooth (monotonic) roll-off with no peaking at the corner frequency. In such an arrangement the Q = 1/SQRT(2).

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This confuses me. Perhaps I just need you to clarify what "termination impedance" is in reference to. I want to think it is the speaker impedance, but that wouldn't make sense given the definition of damping factor. I.e. higher speaker impedance equals higher damping factor.
The damping I'm talking about has nothing at all to do with 'damping factor' which is an entirely misleading concept. 'Damping factor' is ostensibly about damping of the bass resonance of the speaker, here we're about damping the HF resonance of the amp's LC output filter. You can find a paper or two about LC filter design for classD amps on TI's website, this will fill in the details for you. App note sloa119b is as good a place to start as any : ignore section 1 and dive straight into 2.
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Old 7th December 2017, 05:12 AM   #14
hifidon is offline hifidon  United States
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Originally Posted by abraxalito View Post
Yes you can use this approach for the tweeter too, but you have no need to adjust its sensitivity right?
I was asking in general terms, for the future, and maybe better understanding.

You have given me a lot to digest. Thank you.
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Old 7th December 2017, 05:08 PM   #15
hifidon is offline hifidon  United States
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Originally Posted by abraxalito View Post
You'd need two resistors to form a 'potential divider'. This would certainly work yes.
I hope you don't mind but I need to back up. I'm not seeing how, in terms of what the amp sees, doing volume control digitally is different than putting resistors between the signal and amp. When volume is lowered digitally, doesn't the DAC respond by reducing the voltage of its output? So it seems to me that the amp should see essentially the same signal in either case. What am I missing?
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Old 7th December 2017, 05:51 PM   #16
Pilover2000 is offline Pilover2000  Canada
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Originally Posted by hifidon View Post
I am using 3 ClassDAudio SDS-250 amp modules to drive a pair of 3-way active speakers. When there's no signal going into the amps, I can hear a faint hiss coming out of each of the speaker drivers.
You may want to read An Amplifier for all Mankind: Class D Audio SDS-440C Amplifier Review | Featured Reviews, Hi-Fi Systems Reviews | HighFidelityReview - Hi-Fi systems, DVD-Audio and SACD reviews .

In the manufacturers reply at the bottom of the page note " The amplifiers can easily be adjusted for super-high efficiency speakers, but in my opinion, if we make this adjustment to all amps, it might make the sound more like other class D amps on the market, meaning the sound would be a little flat compared to the exciting, lively sound and dynamics our amplifiers are capable of. "

In other words you can likely tame the hiss, but at a price.

Grant.
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Old 8th December 2017, 01:34 AM   #17
abraxalito is online now abraxalito  United Kingdom
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Originally Posted by hifidon View Post
I hope you don't mind but I need to back up.
Quite the opposite of minding, I'm encouraged that you're concerned to get clarity.

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I'm not seeing how, in terms of what the amp sees, doing volume control digitally is different than putting resistors between the signal and amp. When volume is lowered digitally, doesn't the DAC respond by reducing the voltage of its output? So it seems to me that the amp should see essentially the same signal in either case. What am I missing?
I'll take the opportunity to back up too, as I've made an assumption about your system which might not be valid. I've assumed that you want your system's noise to be primarily determined by the noise on the recording and that you may wish to play higher resolution music than RBCD. If you only want to play RBCD (16bits depth recordings) and you don't at all care about noise when no music is playing then you probably have no need to be concerned about gain structure.

Since your opening post was about noise with nothing playing it seemed to be a reasonable assumption to make. You're correct about what happens when the volume is lowered digitally - the DAC is then given smaller (amplitude) numbers to convert back to analog. However the noise of the DAC when no music is playing comes from the DAC's output circuits and not from the digits fed to it, since with no music playing the DAC's converting a value of zero constantly. It cannot generate a true zero in response to this input, there will always be some noise and its this noise which is most likely responsible for any hiss you hear under quiescent conditions. To put it another way, your DAC's ultimate signal-to-noise ratio is the bottleneck in the system.

When you attenuate digitally your maximum signal is reduced (by 5dB in your case) and so the signal-to-noise ratio (SNR) decreases by the same amount, 5dB because the DAC's output noise is constant. With a resistive divider at the DAC's output, provided the resistor values are low enough not to contribute significant noise of their own, the DAC's residual noise will also go down by 5dB, along with the signal. So the SNR remains the same.
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Old 8th December 2017, 01:58 AM   #18
abraxalito is online now abraxalito  United Kingdom
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Quote:
Originally Posted by Pilover2000 View Post
In other words you can likely tame the hiss, but at a price.
For the price of a pair of toroidal output transformers to step down the voltage you can likely have your dynamics cake and eat it.
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Old 8th December 2017, 11:18 AM   #19
Pilover2000 is offline Pilover2000  Canada
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Is it possible that the way I have things connected is causing the hiss, or making it worse? Thanks!
Here is another thread in case you didn't find it in your search. It's been asked and discussed before. It's not the way you have things connected I suspect.

Class-D and The Elephant in the Room = Hiss

Grant.
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Old 9th December 2017, 06:00 AM   #20
hifidon is offline hifidon  United States
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Originally Posted by Pilover2000 View Post
Here is another thread in case you didn't find it in your search. It's been asked and discussed before. It's not the way you have things connected I suspect.

Class-D and The Elephant in the Room = Hiss

Grant.
My takeaway from that thread is that Class D amps are not inherently hissy simply because they are Class D, and that hiss can be caused by a lot of things.
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