ruffrecords
Well-known member
ruffrecords said:The -134dBu EIN is certainly impressive. How did you measure it?
Cheers
Ian
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ruffrecords said:The -134dBu EIN is certainly impressive. How did you measure it?
Cheers
Ian
A-weighted... so it's probably about -125/126dBu unweightedruffrecords said:The -134dBu EIN is certainly impressive. How did you measure it?
abbey road d enfer said:A-weighted... so it's probably about -125/126dBu unweighted
There is a major mistake in your routing. You made the ground entry on the input side. As a result, the current spikes that circulate in the ground trace create a noise voltage that is directly superimposed to the input signal.ricothetroll said:Any opinion on the routing ?
abbey road d enfer said:There is a major mistake in your routing. You made the ground entry on the input side. As a result, the current spikes that circulate in the ground trace create a noise voltage that is directly superimposed to the input signal.
What source impedance have you used for your measurements? that is very important; the "optimum" Zobel is for a source impedance of 150-200 ohms.ricothetroll said:Am I not understanding the concept of optimal load correctly ?
Your results are consistent with the basic simulation results. Really 9k is loading too much the secondary; it is not normal nor acceptable to clamp the signal by 4dB in order to optimize the transient response. Typically a good Zobel should not clamp by more than 1dB. That would suggest about 50kohms for the resistor.ricothetroll said:I used 180R in series with the AD2 generator output, which has a low output impedance (opamp driven) :
I did some measurements to compare both loads with different source impedances, as I would like the preamp to behave well with different mics. The load without the 3n3 capacitor is still more linear, even with 1k source impedance (see attached charts)
Microphones are typically designed for 5-10x nominal Z, so designing mic pres for 2kohms is a safe bet. Note that some users appreciate the possibility to explore higher values, since some mics seem to "open up". I tend to agree with that. Some mics will react more than others to seeing a high impedance. The effects are well understood; they can be easily EQ'd. The electrical damping in microphones is so low that microphone loading has no electromechanical effects, only the electrical interaction betewwen the mic impedance and the preamp impedance.ricothetroll said:Experimenting with different sources shows that it still doesn't stand high source impedance well.
BTW, what's the maximum microphone impedance one should expect ? I thought 1k was already high but I could find references to things as high as 50k....
I would tend to somewhat disagree with the formulation. The Zobel network is an image impedance of the reflected impedance at the secondary, i.e. the capacitive effect compensates the inductive nature of the xfmr. The result should a purely resistive impedance. It will certainly reduce HF, compared to the original resonant (peaky) response. But the resulting response should be maximally flat.ricothetroll said:The purpose of the RC load is to give high frequency attenuation, as this is considered to results in a better-looking square wave response, in particular when the source impedance is low.
"Exaggerated" is sure bad, but there should be a nice compromise. I'd really like to know the values of nominal and leakage inductance as well as parallel capacitance. A graph of the response without any load would help understanding.Personally I have become skeptical to exaggerated RC loading.
That is not the experience I had with other mic xfmrs; proper Zobel optimization flattens the resonant peak, but the asymptote after cut-off is pretty much the same.Even if you are outside the 20-20k frequency range, in my experience RC loading makes the sound more dull.
ricothetroll said:Hi,
Here is the graph without load, input drectly connected to the generator and output directly connected to the scope. Still done with AD2.
I tried to measure the primary inductance but couldn't obtain consistent results with the resonant circuit technique described here :
http://sound.whsites.net/articles/audio-xfmrs.htm
For example (input signal is 10mV peak) :
- Series cap = 10nF-> peak at 900Hz-> 3.13H
- Series cap = 100nF -> peak at 160Hz -> 9.65H
- Series cap = 1uF -> peak at 30Hz -> 28.14 H
How should I interpret those results ?
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