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In Reply to: RE: Line amp/AVC/TVC thoughts posted by pix on February 23, 2017 at 23:18:09
Adjusted to -6db (the worse case) a 100K Ohms passive resistive ladder attenuator has an output impedance of 25k ohms.
The low pass filter created by 25k ohms driving into 400pF has a -3db point of 16kHz.
I know you said "the level (and HF-extend) where acceptable" but I disagree WRT the HF-extension. A -3db point of 16kHz will shift the phase all the way down to 1.6kHz
BTW don't forget to add the cable capacitance that will make the numbers even worse.
I think AVCs are great but I'm not sure your phono pre's output impedance of 2k ohms will properly drive one.
Ask Dave.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Tre' IIRC I saw a post you made somewhere about you using the 33 step TVCs offered on ebay, and stated the inductance. If so can you tell me what the inductance is?
I use the secondary as a autoformer.I've never tried them as transformers.
There's not enough inductance (for my use) in the primary. Besides, I can't think of a good reason to use them that way. (maybe if I were fighting ground loop hum?)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/24/17
I wonder if that would be enough for a phono stage with 1K output impeadance?
twystd
80Hy has a reactance at 20Hz of 10k ohms.
I'm driving mine with about 400 ohms.
It takes a good size coupling cap.
I'm running a 4.7uf in parallel with a 2.2uf for a total of 6.9uf.
6.9uf into 10k ohms has a -3db point of 2.3Hz, not that there's a whole lot of 23Hz on most of my records but I like to cover all the bases/basses :-)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
6.9uf into 10k ohms has a -3db point of 2.3Hz, not that there's a whole lot of 23Hz on most of my records but I like to cover all the bases/basses :-)
you really cannot extrapolate the impedance of a reactive load @ 20hz down to 2.3hz.
spice says your phase shift @ 20hz is -3.6º and -140º @ 5hz.... and if left undamped I'd watch out for tonearm resonance :-)
(in defense of the autoformers... I generally find that spice shows a much worse case than actually measures in circuit)
dave
"and if left undamped I'd watch out for tonearm resonance"
Would a larger cap help or hurt?
"if left undamped"
There is a 100k ohm resistor from selected tap to ground. Any help?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Changing the cap value moves and damps / undamps the resonance. Here are a few values (green is always the first value swept (3.5u))
Your 100K load helps damp it a bit but you really need a lower value to show a difference.
Now even though 5K looks nice and is still 10X the source Z to keep the tech heads happy, you now have the same phase shift increase that the guy in the original video complained about. (just at the other end of the spectrum)
dave
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Dave wrote:
"(in defense of the autoformers... I generally find that spice shows a much worse case than actually measures in circuit)"
Spice (psud, etc) are limited tools... that don't take into account, for example, the various core losses of an inductor.
Data retreived especially concerning resonances should be consumed with a grain of salt. They will almost always be overstated nor is the Q of the resultant complex impedances is not captured in these programs.
MSL
Builder of MagneQuest & Peerless transformers since 1989
Well. I'm usually play music at 1/10 of a 100k resistive ladder, but I got your point.
This is definitely what I am hearing.
By saying that I know my system is a bit overgained, and having a less gained (read Miller capacitance) input tube would solve. But this is a to big project right now.
But if it's all about the -3dB roll off point, how much headroom is needed for?
Btw, From what I have reading, Dave's AVC is a 20H loading, but can easily be restacked up to 10 times higher. This would make it an easier load with fairly low z out.
Well. I'm usually play music at 1/10 of a 100k resistive ladder, but I got your point.
This is definitely what I am hearing.
By saying that I know my system is a bit overgained, and having a less gained (read Miller capacitance) input tube would solve. But this is a to big project right now.
But if it's all about the -3dB roll off point, how much headroom is needed for?
Btw, From what I have reading, Dave's AVC is a 20H loading, but can easily be restacked up to 10 times higher. This would make it an easier load with fairly low z out.
"But if it's all about the -3dB roll off point, how much headroom is needed for?"
some say it doesn't matter but if you want 20kHz phase shift free, the -3db point of the filter would need to be at 200kHz.
"...Dave's AVC is a 20H loading, but can easily be restacked up to 10 times higher."
the reactance at 20Hz of 200Hy is 25k ohms.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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