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In Reply to: RE: Output impedance of a DAC to input impedance of Power amp?????? posted by Astrostar60 on August 10, 2014 at 12:36:39
If your dac is truly 10ohms output impedance with 2v out, you have no troubles what so ever using a 10kohm passive pre.
The least your dac will see at the worst position on the passive combined with the poweramps 47kohm is around 7kohm, which is nothing if it is 10ohms output impedance.
This should be the most transparent and dynamic you will ever hear your system.
Remember this quote from Nelson Pass
Nelson Pass,
"We’ve got lots of gain in our electronics. More gain than some of us need or want. At least 10 db more.
Think of it this way: If you are running your volume control down around 9 o’clock, you are actually throwing away signal level so that a subsequent gain stage can make it back up.
Routinely DIYers opt to make themselves a “passive preamp” - just an input selector and a volume control.
What could be better? Hardly any noise or distortion added by these simple passive parts. No feedback, no worrying about what type of capacitors – just musical perfection.
And yet there are guys out there who don’t care for the result. “It sucks the life out of the music”, is a commonly heard refrain (really - I’m being serious here!). Maybe they are reacting psychologically to the need to turn the volume control up compared to an active preamp."
Cheers George
Follow Ups:
To add to what Nelson said,
Maybe the output impedance of the passive preamp is too high and the highs are being sucked out of the music by the low pass filter created by the output impedance vs. the shunt capacitance of the interconnecting cable plus the Miller capacitance of the first stage of the amplifier?
At 10k ohms the (worse case) output impedance will be 2500 ohms at the -6db setting.
That should only cause HF roll off with very high shunt capacitance but not all passive preamps are that low of an impedance.
With a 100k passive resistive attenuator the worst case output impedance is 25k ohms and it would only take a little shunt capacitance to roll off the highs and shift the phase well down into the audio band.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi Tre', just to further add to what you also said for the interested readers.That is why a 10K external passive pre is the way to go, never 100kohm. As you said worst a 10kohm will be is 2.5kohm output impedance.
Good interconnects these days are always less than <100pf (picofard) per foot. I always say keep them under 1.5mts long when I suggest use with my Lightspeed Attenuator which mimics a 10k passive pot, but say you have 2mts at 100pf per foot. That's approx 600pf of capacitance per channel.This combined with the 2.5kohm ouput impedance of the 10k passive pot equates to a HF roll of -3db at 106khz!!!!!so there is far from any phase shift down into the audio band. Most amps can't reach this figure, let alone digital sources.
Cheers George
Edits: 08/11/14 08/11/14 08/11/14
While a -3db point of 106kHz will yield a (more or less) flat frequency response to about 53kHz it takes a decade for the phase shift to go away.That means there will be some phase shift all the way down to 10.3kHz
This is why all the tech books talk about bypassing cathodes to 2Hz if you want 20Hz flat and phase shift free (it's the same thing but on the high pass side of the equation) and if we want 20kHz flat and phase shift free, all of the low pass filters (and they are seemly everywhere in a system) need to have their -3db points no lower than 200kHz.
(It's impossible to reach that goal when an audio transformer is involved but that's no reason to introduce more phase shift when you can help it)
You can find good cables with far less than 100pF per foot.
I'm using 1 meter cables that measure 70pF. Not 70pF per foot but 70pF total! Measured on a Sencore LC53.
BTW Don't over look the input capacitance of the power amp, that would be added to the cable capacitance.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 08/12/14
Of course you can find cables less than 100pf per foot, and I used this as a maximum, that is why I said < less than 100pf.
As for frequency response no tube poweramp's output transformer I know of, can attain 100khz without masses of feedback around it, and then they sound rubbish.
Cheers George
"As for frequency response no tube poweramp's output transformer I know of, can attain 100khz......"
OK, but is that a good reason/excuse to add more phase shift in the audio band when it could otherwise be prevented?
Have a listen to 2.5k driving 600pF then drop that to 150pF. I would bet you will hear the difference even through an output transformer that can't make 50kHz flat.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hey Tre" I'm all for speed, my amps are -3db at 180khz. Why do you think I called my product Lightspeed Attenuator it goes from dc - ghz in speed.
The simple fact is all digital sources are limited to 20khz and DSD to 50khz. And vinyl is a joke trying to reproduce a 20khz sine let alone square wave.
When I give my specs for interconnects to be used with the Lightspeed Attenuator I say <100pf per foot and 1.5mt long, this with the Lightspeed's output impedance works out to be -3db at 141khz.
Cheers George
Thanks George, top answer to my question. I will post back here with my findings once I get the system hooked up. I agree, Passives could in theory be the answer to all our dreams of straight through music - source to power amp. I think (possibly) the ones slating passives have tried it with long interconnects and/or high output impedance of the source. Some DACs can be 500 ohms, so the 10 times rules would mean a minimum of 5k input at the power amp. And the power amp has to achieve full rated power at or below the rated output of the DAC. Mine is full rated power at .9v so my DACs 2v will be fine.
Bi-amping will bring the impedance down as well. I run a 10k impedance sub and the power amp at 47k, so with the 10k passive, the DAC would see about 8k +10k at the load, and climbing as I turn the volume down from 100% to 50% (for example). If the impedance matching in any system goes way out or the source has a weak output stage, frequency roll off or loos / lack of bass will result.
Another thing I found, digital volume is aweful, it sounds flat and lifeless. Analogue volume control seems much better.
Live long and listen!
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