Amplifier hiss with no input signal

[hifidon]

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.

I've noticed hiss while music is playing, but not very often. I've always just assumed it was part of the recording. But maybe not...

You didn't say how bit-depth relates to this discussion. Will you elaborate?

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.

You're right about this. If I power-off the DAC then the hiss is reduced significantly. There is still some small amount of hiss, but I have to put my ear close to the speaker in order to hear it. I would never notice it.

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.

I understand this now. Thank you for clarifying.

 

[hifidon]

You didn't say how bit-depth relates to this discussion. Will you elaborate?

Maybe I can answer my own question. Reducing volume digitally can reduce the effective bit-depth of the recording, if the digital processor does not itself operate at a sufficiently high bit-depth. Is this the gist of it?

 

[Pilover2000]

I don't know if anyone mentioned this earlier but if there is any way you can put a noise gate in the path it can quiet things down. I've got a processor right before my amps with a noise gate built in and using it makes a nice difference to the hiss level.

Grant.

 

[rkeman]

I have a beginner question. 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. It's not loud, normally I don't notice it. At night, when everything is quiet and the music stops playing, that's when it becomes obvious. Is this kind of hiss normal, generally speaking, for class D amps? Is there an amp specification that would indicate the level of hiss the amp will produce? Is it possible that the way I have things connected is causing the hiss, or making it worse? Thanks!

The Class D Audio SDS-250 has a variable gain control that can be used to optimize the noise floor and headroom of the system. At full gain (i.e. the volume control all the way over), the noise from the gain stage of the power amplifier is unnecessarily emphasized. Try reducing the gain on the power amplifier(s) and raising the gain on the preamplifier or other source component. The input section of the Class D Audio is very good quality and the amplifier has low self noise even with high sensitivity designs. My 6Pi cornerhorns generate about 95 dB/W/m and have no appreciable noise floor with a properly configured Class D Audio SDS-470C. The SDS series can be further modified for use with extremely sensitive loudspeakers (>103 db/W/m) if required.

 

[hifidon]

The Class D Audio SDS-250 has a variable gain control that can be used to optimize the noise floor and headroom of the system. At full gain (i.e. the volume control all the way over), the noise from the gain stage of the power amplifier is unnecessarily emphasized. Try reducing the gain on the power amplifier(s) and raising the gain on the preamplifier or other source component. The input section of the Class D Audio is very good quality and the amplifier has low self noise even with high sensitivity designs. My 6Pi cornerhorns generate about 95 dB/W/m and have no appreciable noise floor with a properly configured Class D Audio SDS-470C. The SDS series can be further modified for use with extremely sensitive loudspeakers (>103 db/W/m) if required.

As mentioned above, I've determined that most of the noise I've been hearing is from the DAC, not the amp. With the DAC powered off I can barely hear the amp and I would never have noticed it. But I have been thinking about using the volume control circuitry of the SDS for lowering the gain of the mids by 5 dB as is needed for the overall gain structure. It would be an easy hardware based solution. I just need to know what value resistor to use for a 5dB drop.

 

[mboxler]

I just need to know what value resistor to use for a 5dB drop.

Are you currently using pots or resistors on the SDS gain pins?

 

[abraxalito]

You didn't say how bit-depth relates to this discussion. Will you elaborate?

The bit depth of the recording sets a lower bound on the amount of noise you'll hear. But only when the music's playing out, it has no bearing on the noise when the DAC's quiescent (converting zeroes). A 16bit recording can at best do around 93dB SNR - for each extra bit this number increases by 6dB.

<edit> Just read your later post - if you use digital volume for attenuation then you're not necessarily reducing the bit-depth of the recording. It will depend on what happens to the low-order bits after multiplication of the signal by your fractional volume (a number less than 1). If they're retained and fed to a DAC with more than 16bits then you won't necessarily increase your noise floor, meaning no effective reduction in bit-depth. If say your DAC is 20bits (with true 20bit SNR) - with a 16bit recording then you can accommodate up to 24dB attenuation before losing bits.

 

[hifidon]

Are you currently using pots or resistors on the SDS gain pins?

They are connected by a jumper. As per Tom's instructions.

 

[mboxler]

They are connected by a jumper. As per Tom's instructions.

Oh...So you are currently at full gain, which I believe is 32dB(?).

Here are Tom's recommended fixed resistors.

For those who wish to change the pots for fixed resistors, here are resistor values to produce required gain. Gain is listed in dB: To change gain potentiometer to a fixed resistor, you would connect a fixed resistor between the yellow and black wires on the gain controls (of course, remove the gain pots). Leave the red wire disconnected.

Use high quality 1% resistors:

560 Ohm = 31 dB

1K = 30 dB

2.2K = 27 dB

5.2K = 23 dB

6.2K = 18 dB

7.5K = 16 dB

 

[RegaPedro]

I'm having the same issue of low level hiss with a ClassDaudio CDA-120 amp kit I just purchased. It is in an enclosure (including the toroid transfomer and power supply board) with a Pass Labs Korg B1 Nutube preamp board (kit I assembled). With the preamp off or disconnected the hiss is there. Even with the preamp on and with no input signal the hiss stays the same even if the volume is turned up on the preamp. I understand the ClassDaudio to be a low noise floor amp. I'm working with ClassDaudio to determine what the issue is.

 

[Mark Tillotson]

What's the output impedance of your preamp? It needs to be low to drive a power amplifier and get low noise.

Disconnecting a source from a power amp will produce significant hiss, you need to short the input of an amp to measure its own noise contribution.

I suspect trying to use a valve preamp is your problem - lots of Johnson noise from the high value resistors in that preamp and no output transformer to attentuate all that (voltage) noise.


Try shorting the input of the CDA-120 and seeing it that reduces the hiss - if so its nothing to do with the CDA-120

 

[Meocon]

Signal to Noise refers to active signal (i.e when the music is playing). The hiss noise is the background noise and it is a native design side effect, does not matter how expensive the amp.
The same 2 way speakers with fullrange (1)(using built in passive xover of the speaker) and bi amp (2) (two way passive with stand alone active xover) setup for example (my system) will be dead quiet if using class A or AB amp, swaped to any class D amp in both cases will exhibite this background hiss noise.
That to say, a resistor in series with the hf driver or any damping circuit will not help nor the gain structure because with the (1) setup, the amp just send fullrange freq to the speaker, and the built in xcover does the rest.
The gain structure calibration only needed in setup (2) because each amp drive each low-high driver directly with specified freq.