FYI Schiit is sending me another Freya S for testing....
So that was in active mode? I wonder how passive performs at these lower volume levels. Given the other measurements, probably even better.That's correct. I adjusted the volume until I got the desired output, summed THD+N for each channel as expressed in SINAD and plotted it.
It was. In passive mode you are just seeing the performance of the source then.So that was in active mode? I wonder how passive performs at these lower volume levels.
With passive attenuators the signal will get smaller.
The smaller the signal gets (distortion does not increase) the smaller the distance to the always present noise floor of the equipment after the volume control.
Basically when you attenuate 20db the S/N ratio also drops 20dB as the noise floor is constant.
In Amirs tests the noise floor of the AP.
FYI Schiit is sending me another Freya S for testing....
I am so far behind in testing electronics that I don't want to ask for more stuff.Ask if You can measure their tube Freya. Would be interesting to see it measured with different tubes too.
Imagine the full scale signal as a musical waveform with a tiny bit of ripple on it. The ripple is the noise at 120 dB smaller than the signal. A conventional active amp has a fixed gain ratio, so much of that noise ripple comes from the gain-bandwidth product, so imagine it as a fixed voltage level. As you turn down the volume, you shrink the signal while this noise remains constant, so the SNR shrinks more or less 1:1 with volume as you turn it down.
Now imagine that same full scale signal with the ripple. Instead of attenuating the signal independently of the fixed noise level, you pass the entire signal to a downstream voltage divider. The noise ripple is part of the signal, so it all gets attenuated together, which means the SNR stays the same -- except for whatever noise the metal film resistors of the voltage divider add. So if that noise is smaller than the noise that was in the signal to begin with, the SNR won't drop as much.
So the reason you may get better SNR at lower volume settings with the passive setup is because:
(1) The thermal noise of metal film resistor is probably less than the noise of the active amplification circuit. This seems reasonable, since the noise of most active amps is worse than 120 dB, and the noise of a 5k metal film resistor at room temp is about 120 dB, and that is the worst case -6 dB volume position. All the other volume positions of the passive will have lower resistances with lower noise.
(2) Thermal noise is just the theoretical minimum noise. All resistors have somewhat more noise than this minimum. Metal film resistors are among the best, quietest, closest to this minimum. Most analog pots of equivalent resistance are higher.
Anyway, that's my reasoning. Measurement will tell if it's right or wrong.
You are quite welcome.Kudos to Schiit's finance manager T.M.Noble
Edit: I updated the numbers below based on information from further discussion here.The contributions in passive mode to worsening SINAD at lower levels , should be a fascinating composite of 1. Thermal noise, 2. Resistance material noise and 3. What very few ever discuss - Contact noise involving degradation of passing audio signals with contacts, worsening with use https://en.wikipedia.org/wiki/Contact_protection ,related then to the bathtub curve.
1. Thermal noise - every resistor has this. Thermal noise at a 10k attenuator's worst case -6 dB is about -120 dB.
2. Resistance material noise (above thermal noise) - potentiometers have more of this than metal film.
3. Contacts - lifespan of the relays shouldn't be an issue. Typical EMRs have a life of about 1M actuations, which is 27 years switching 100 times per day. If it uses SSRs the lifespan is effectively infinite. But Amir said he hears a mechanical click as he rotates the knob, so it is probably using EMRs not SSRs.
PS: I went back and studied the spreadsheet I made when I built my passive attenuator years ago. If we consider how much SNR drops when you turn down the volume from full scale, assume that SNR is 120 dB at full scale, and the active drops 1:1 as you turn it down. Also assume the passive noise level depends on the resistor in series, not its output impedance (this will be less favorable to the passive since the lower volume levels use bigger resistors in series, smaller in parallel). Then let's look at a few points along the curve as we turn down the volume:
Active vs. passive should be equal at -6 dB. That's because active will drop about 6 dB and passive will have -120 dB of noise which adds to the -120 dB in the signal being attenuated, which doubles it, which is 6 dB worse. So they're both -114 dB.
At -12 dB the passive should be 4 dB quieter. The active will be 120 dB - 12 = 108 dB. In a 10k ladder at -12 dB, the resistors are 7500 and 2500 so the signal passes through a 7500 ohm resistor which has noise of -116 dB, plus the -120 dB in the attenuated signal makes -112 dB.
At -18 dB the passive should be 9 dB quieter. The active should be 120 - 18 = 102 dB. The passive resistors are around 8700 and 1250 so the signal passes through a 8750 resistor which has noise of -115 dB, plus the -120 dB in the attenuated signal makes -111 dB.
At -24 dB the passive should be 15 dB quieter. The active should be 120 - 24 = 96 dB. The passive resistors are around 9400 and 630 so the signal passes through a 9400 resistor which has noise of -115 dB, plus the -120 dB in the attenuated signal makes the same -111 dB.
Overall, the passive's biggest resistor worst case is 10 kOhm which has noise at -115 dB. So it is quieter than the active at every volume setting below -6 dB. And the more you turn it down, the bigger its noise advantage becomes.
If the SNR of the downstream amp is worse than the preamp, then sure, the preamp isn't the limiting factor. But the purpose of the preamp is to attenuate the signal without altering it. In my example, in both cases the SNR starts at 120 dB and worsens as you turn down the volume. The passive doesn't magically improve it, it just has less degradation.You are talking about the signal behind the device (be it passive or active) that really does not matter because after a volume control (being passive or active) there will always be amplification which always has its own fixed noise floor. ...
Only when the gain of the last stages is controlled that noise floor would vary with the gain but in then would always end up in a noise floor.
The real question is (depending on the efficiency of the transducer, noise level in listening conditions and ear sensitivity (which drops as we age) whether or not the noise level reaches audible levels or not.
Both in passive and active pre-amp conditions the bottom noise depends on the noise level of electronics behind the volume control. That can only increase when the source (before the attenuator) has a worse signal to noise ratio. It can not become better.