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Schiit Freya S Preamplifier Review

JohnYang1997

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I was hoping for answers from those here more knowledgeable than I am. But I also studied this on my own to see if I could find the answer. What I read is that thermal noise doesn't depend on the voltage across or current through the resistor -- other than the indirect effect that higher voltage draws more current which heats it up, which increases thermal noise. This implies that thermal noise is inherently voltage/power. The dB ratio for thermal noise is given relative to an arbitrary 1 V reference. But that is arbitrary. Since it is the voltage & power of thermal noise that is constant depending on temperature but not on voltage, the actual dB ratio of noise depends on the voltage across the resistor.

For example, that 9400 Ohm resistor mentioned above has 1.74 uV of voltage noise at room temperature. If you pass a 1 V signal across it, that noise is 115 dB below the signal. But if you pass a 50 mV signal across it, that noise voltage doesn't change so it's only 89 dB below the signal.

The practical audio take-away is that there's no free lunch when attenuating signals to low levels -- thermal noise becomes a bigger factor for small signals.
Can you just stop? This is plainly wrong. Can you draw a suitable equivalent circuit with noise voltage sources and noiseless resistance and analysis the circuit? If you don't know how to do it then it's no help from me here. I remember some Texas instrument opamps have datasheet with how to calculate noise. You may wanna check those out.
If assuming the output stage doesn't have noise, and you turn the pot all the way down the noise is zero, period. If you still don't understand, I suggest learning KVL to analyze the circuit.
 

MRC01

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I don't claim to be an authority. I'm just trying to understand this. My EE textbook says thermal noise depends only on temperature and bandwidth; it does not depend on the voltage across the resistor. I will try the references you mentioned.
 

SIY

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I don't claim to be an authority. I'm just trying to understand this. My EE textbook says thermal noise depends only on temperature and bandwidth; it does not depend on the voltage across the resistor. I will try the references you mentioned.
That’s Johnson noise only. There’s also excess noise.
 

JohnYang1997

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I don't claim to be an authority. I'm just trying to understand this. My EE textbook says thermal noise depends only on temperature and bandwidth; it does not depend on the voltage across the resistor.
But that's not how you calculate SNR. The thermal noise ACROSS THE RESISTOR is only depending on the temperature, bandwidth and resistance but the noise at a voltage divider output is different.
 

MRC01

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If the the resistor's thermal noise doesn't depend on voltage, then small signals passed across the resistor will have lower SNR than big signals.
 

JohnYang1997

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If the the resistor's thermal noise doesn't depend on voltage, then small signals passed across the resistor will have lower SNR than big signals.
This is not technically wrong. But that's not how you think about this.
If you want a certain output, different input voltage will lead to different pot position. The noise is the parallel of two branches of the pot. Then you calculate the ratio.
 

MRC01

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OK so back to the voltage divider mentioned above: 9400 in series, 600 in parallel, -24 dB, 10 kOhm to the source. Thermal noise comes from the 600 ohm resistor, which is 0.44 uV. The actual SNR depends on the level of the music. If you're listening to a symphony with big dynamic range, at the crescendo where the signal input is 2 Vrms, the output is 24 dB quieter which is 126 mV and SNR is 133 dB. When the music reaches a quiet section 30 dB quieter, the signal input is 63 mV, the output is 24 dB quieter which is 3.98 mV and SNR drops to 103 dB.
 

MRC01

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Based on this I went back and updated the calculations in my earlier post.
The conclusion is that a passive attenuator should provide lower noise as you turn down the volume, compared to most of the active amps here, where SNR typically drops 1:1 with volume level.
PS: as I drew the circuit it became clear that the voltage across the parallel resistor is the voltage that the downstream device sees.
 
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solderdude

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After each attenuator there will be amplification with its own noise floor that is higher than that of the resistor.
That noise floor ultimately determines the S/N ratio.

Perhaps create a thread for it (or ask) as the discussion has little to do with the Freya (aside that it is both active and passive)
 
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regarding the concept, at last noise is noise thermal o whatever noise, and in the digital systems, it affect the measurement of SNR, isn't it? and I know here talk is over analog preamp.
 

solderdude

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That's what triggered this entire discussion - seeing measurements of Freya's passive mode having lower noise than active.
I understand that.
Noise from the amplifier itself is added + noise is amplified.
In passive mode you are simply looking at the noise floor of the AP input.
 

solderdude

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It has a remote control that can 'switch it on and off remotely' (the mute button) as well as on the front.
The on-off switch in the rear, when switched off will, disconnects the power and takes away the remote control functionality as there is no low power standby power supply in the Freya S.

I assume the Freya S doesn't do pass any signal anymore when the power is shut off.
 
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da Choge

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FYI, Amir has the new Freya S. I know the man is very busy but I really look forward to the results.
I am very interested in the re-testing of this "new" Freya S as well. Hopefully, if Amir has time to do it in the near future, he will post a notification in this thread? (even if creating another thread) so that we can all take note of it. This is a very affordable (compared to something similar from Tortuga) component with some simple, but very useful, features.
 
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amirm

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I have a general question. I have a denon x3300 avr preout to the schiit freya (tube) to the crown 1502 power amplifier. Now in this chain, I have the schiit freya's volume on to max, put the channel amps on the crowns to about the 2'oclick position and control the overall volume through the denon avr remote control. Does this sound about right in the way I have this setup? Also, for everyday listening (no where near pushing the max on the crown power) what is the thought on the sound quality of this? I am also using all balanced xlrs.

thanks.
 

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