[BrianM]

Can anyone here point me to a good discussion on the "debate" about passive vs. active preamps? (Assuming no one wants to restart that debate in this forum?)

Here's a link where somebody takes up the cause for passives in wake of nefarious calumniation:

http://www.rothwellaudioproducts.co.uk/html/passive_preamp_tech.html

[avahifi]

A quick search herein found what I said before, I will repeat myself again.

A passive preamplifier actually is not a preamplifier at all, inasmuch as it does not amplify.  Well - there are some that use some kind of step up transformers, but that is another can of worms.

Essentially a passive preamp is a volume control in a box with some switching functions.

The advantage musically is that it eliminates all active preamp circuit functions, and if the active preamp circuits are not of very high quality they deserve to be eliminated.

The disadvantage musically is that is eliminates all active preamp circuit functions.  Those functions should be designed to provide a easy pure resistive load for the sources to drive, eliminate out of band garbage, drive the sh--- out of all downstream loads no matter how goofy they are, and not screw up in and of themselves.  Lots of designers drop the ball here.

A passive preamp essentially connects your sources directly to the power amp, through the likely significant amount of distributed capacitance of the cables between the source and the passive preamp, its internal load, the distributed capacitance of the cables between the passive preamp and the power amp, and the internal distributed capacitance of the amplifier's input circuits.  Whew!!!  If you wonder what a capacitive load does to the music signal, see all the test results done over the years showing this, they cause a leading edge spike and ringing on the signal, the bigger the load the worse the results.  In addition now that load makes demands on the drive current capability of the source that it likely does not have.

So with a passive preamp you have the distortion of the additional capacitive load on the source, and from taxing its drive current capability.  With an active preamp, you have the distortion of the preamp's active circuits themselves.

Which is worse?  It depends on the quality of the preamp active circuits, the load driving capacity of the source, and the amount of distributed capacitance of the cables.  Your results may vary.   

In general we would suggest short, low capacitance well shielded interconnect cables, sources with excellent current drive capability (a spec not usually talked about) an amplifier with a pure resistive input impedance, and a very very low distortion active preamp line stage providing a resistive input load and high output drive current.

Hope this has not confused you too much.

Frank Van Alstine

[JLM]

Brian, 

My, what big words you use!


Frank,

Me very low, your talk very high.  (Yes you went over at least my head on some of that.)  But I love your style a ton.   

How about transformer based volume controls?  What's your opinion oh, wise one?

[WEEZ]

Frank, you are correct, for sure. But, there is only one thing you left out; the fact that most systems don't require the excessive gain that most preamps have.

I'll bet that if you made a 'preamp' from your buffered line driver (i.e., all the features of a preamp but without any gain) you could sell 'em. Watcha' think? 

WEEZ

[TomW16]

Hi BrianM,

I have been using a passive preamp to very good effect between an AVA DAC and an AVA 350 FetValve Amp.  As Frank mentioned, it is not as straight forward as using an active preamp but if your souce components are up to driving the amp and you keep your low capacitance interconnects short, you can get away from another active stage.

Tom

[marvda1]

how about offering variable gain as ird does on it's purist preamp as the best of both worlds?

[bundee1]

How short is very short with passives and interconnect lengths?

[dB Cooper]

Frank also didn't mention something he brought up in his old Audio Basics newsletter: A passive has a high output impedance and tends to interact with the load. Am I quoting you correctly, FVA? (Ever since Stereophile, all audio bigshots must apparently be referred to by their initials    )

[Steve Eddy]

A passive preamp essentially connects your sources directly to the power amp, through the likely significant amount of distributed capacitance of the cables between the source and the passive preamp, its internal load, the distributed capacitance of the cables between the passive preamp and the power amp, and the internal distributed capacitance of the amplifier's input circuits.  Whew!!! If you wonder what a capacitive load does to the music signal, see all the test results done over the years showing this, they cause a leading edge spike and ringing on the signal, the bigger the load the worse the results.

The more pertinent question is, what is the frequency of that resonance? If the resonance is up say around 100kHz, then it's not going to be affecting the music signal.

In addition now that load makes demands on the drive current capability of the source that it likely does not have.

Not necessarily. Yeah, if you've got the volume control turned all the way up, then the source will effectively see the combined capacitive load of the two cables, but in a more typical situation, one isn't listening with the volume control turned all the way up.

Let's say your passive is made using a 10k pot. At half its resistive rotation, which for a log taper will be somewhere past twelve o'clock, you've got 5k of resistance in series between the source and the second cable. So even if the second cable were so capacitive as to be a dead short, the source will just see 5k ohms in parallel with the capacitance of the first cable.

se

[Steve Eddy]

Frank also didn't mention something he brought up in his old Audio Basics newsletter: A passive has a high output impedance and tends to interact with the load.

Well, the output impedance combines with the cable's capacitance to form a low pass filter. The higher the output impedance, for a given cable capacitance, the lower the cutoff frequency of that low pass filter.

se

[avahifi]

It may be a low pass filter, but it remains a capacitive load and that can be death to the driving component.  In general it will slow down the feedback loop, make feedback arrive late, changing negative feedback to positive, and generate an underdamped resonant peak somewhere high out of the audio band.

While you cannot hear this, your amplifier and tweeters can.  The out of band resonant peak can cause either or both your amp and or speaker voice coil to saturate and while in saturation or cutoff at any frequency, all audio information is erased!

Worse case,  Threshold amplifier without an output inductor to protect against capacitive loads, and Polk's infamous Cobra Cables, a multi-wire braided speaker wire that was very capacitive.  That combination caused the amplifier to go into full bore oscillation and melt.  The resonant frequency was way way above what you could hear, but you could smell it! 

Avoid capactive loads as much as possible and your system will like you better.

Frank Van Alstine

[Wayner]

It sounds like the term "passive" is a misnomer. Perhaps a better term would be "reactive".

W