Topic: Passive Preamp Discussion

[wraith]

*** please merge with other threads if there is a similar one.  Also, I'm not sure if I should start this thread here, or under the Amplifiers section ***

I would like to start a thread on passive preamps because its application for 2-channel audio use intrigues me.

I'm not an electronics engineer; I just try to understand as much as I can.  Please correct me if there are misconceptions in my thoughts.

Potentially, because of their minimalist design, they are the most un-coloured means of volume control as compared to line-stage preamps that amplify the signal.  Other good points are that they have no need for a power supply, and they can be constructed relatively inexpensively.  These qualities attract me to the idea of using a passive pre.

However, the disadvantage is that because passive pre's are basically a network of resistors, it is resistive by nature.  This will cause impedance mismatches, which may lead to frequency response anomalies, and lack of current to drive the power amps.  This worries me.

For those who have used Passive Pre's, kinldy share your experiences.

Here are the my thoughts/questions.

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1.  Resistive Passive Pre's vs. Transformer Volume Controlled (TVC) Passive Pre's

- from my research, TVC's are suppossedly superior because there is no current loss.  Unlike resistive implementations wherein attenuated signal is transformed into heat and loses some current, TVC's simply step-down the voltage using a transformer, but current is preserved.  So is it true that TVC's are superior?  Do TVC's have any audible or electrical disadvantages compared to resistive designs?

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2.  The all-important sound

- I have read that "dynamics are reduced".  I'm not exactly sure if this just refers to overall volume, or if also refers to the speed, articulation, weight, and tautness of the sound.  I can live with volume reduction, but having a potentially muddy and thin sound worries me.  Ironically, others reports contradict this by saying that bass tautness and articulation actually improved, and that some warmth is added as well.  Your thoughts and experiences please...

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3.  Use with SS or tube power amps 

- I have read that because tube amps have a higher input impedance, they will match passive pre's better because they are a relatively easier load to drive.  So does this mean than using passive pre's with SS amps are not advisable?  How does the input impedance of the power amp affect the sound?

Thanks.

[ATJr.]

==========
1.  Resistive Passive Pre's vs. Transformer Volume Controlled (TVC) Passive Pre's

- from my research, TVC's are suppossedly superior because there is no current loss.  Unlike resistive implementations wherein attenuated signal is transformed into heat and loses some current, TVC's simply step-down the voltage using a transformer, but current is preserved.  So is it true that TVC's are superior?  Do TVC's have any audible or electrical disadvantages compared to resistive designs?

transformers are not as linear as resistors, they have hysterisis and magnetizing current losses.

==========
2.  The all-important sound

- I have read that "dynamics are reduced".  I'm not exactly sure if this just refers to overall volume, or if also refers to the speed, articulation, weight, and tautness of the sound.  I can live with volume reduction, but having a potentially muddy and thin sound worries me.  Ironically, others reports contradict this by saying that bass tautness and articulation actually improved, and that some warmth is added as well.  Your thoughts and experiences please...

the volume control in front of the amp is indeed a pre-amp...
 

3.  Use with SS or tube power amps 

- I have read that because tube amps have a higher input impedance, they will match passive pre's better because they are a relatively easier load to drive.  So does this mean than using passive pre's with SS amps are not advisable?  How does the input impedance of the power amp affect the sound?

yes, it is true that tube amps have higher input impedances compare to ss amp, this is no reason to say that passive preamps can not find use in it, in fact my leach amp has a 50k volume control in front of it, that makes it a preamp.



passive preamps have been with us for as long as ohm's law was invented...
it is just semantics, nothing more than voltage or current deviders..

have no fear of using them...

rule of thumb, know the output impedance of the preceding stage, the "load" presented by your preamp should at least be 10x in order to get maximum voltage swing from the preceding stage...

[RU9]

Potentially, because of their minimalist design, they are the most un-coloured means of volume control as compared to line-stage preamps that amplify the signal.  Other good points are that they have no need for a power supply, and they can be constructed relatively inexpensively.  These qualities attract me to the idea of using a passive pre.

Yes, I agree. I have a "Killer" Passive linestage by Rene and a Tube Passive Pre By JojoD.

==========
1.  Resistive Passive Pre's vs. Transformer Volume Controlled (TVC) Passive Pre's

  Do TVC's have any audible or electrical disadvantages compared to resistive designs?

==========

JojoD assured me he did not observe any impedance mismatch problem with the Tube passive.

Also, factor in the cost.  TVCs are very expensive, have not heard a TVC yet.

2.  The all-important sound

- I have read that "dynamics are reduced".  I'm not exactly sure if this just refers to overall volume, or if also refers to the speed, articulation, weight, and tautness of the sound.  I can live with volume reduction, but having a potentially muddy and thin sound worries me.  Ironically, others reports contradict this by saying that bass tautness and articulation actually improved, and that some warmth is added as well.  Your thoughts and experiences please...
==========

Depends on how the source material is mastered. I have problems with CDs mastered "softly".
You want to squeeze out more but even at full volume, you have reached the limit.

Have some  success using a computer as a source. I can use wavegain to increase the volume.

I can assure you that the sound is not muddy and thin.  I would characterize the sound as "raw" or neutral.  I can still feel the "punch"  while listening to hip-hop and rock.

I suggest that you borrow/listen to an active pre and a passive pre before buying.  There is no harm in owing both.

3.  Use with SS or tube power amps 

- I have read that because tube amps have a higher input impedance, they will match passive pre's better because they are a relatively easier load to drive.  So does this mean than using passive pre's with SS amps are not advisable?  How does the input impedance of the power amp affect the sound?

I have an EL84 amp and a Rotel RB03 amp.

For the Rotel, no problems with the Killer and Tube passive.

For the EL84, no problems with Killer, For the tube pre, good for jazz and vocals but not suitable for rock and hiphop.
 

[wraith]

Thanks for the feedback guys. 

My current setup consists of a cd player > tubed preamp > ss power amp (connected to the fixed input).  To simulate a passive pre, i bypassed my preamp, then connected my cd player directly to the varibale input of my power amp (controlled by what i think is a resistorized attenuator at the back).

I hear no loss in dynamics and bass weight.  In fact, this setup has a lot more drive to give since its very loud at only 1/4 the way up of the attenuator.  However, what I observed missing is the larger-than-life staging/presentation that is usualy attributed to tubes. 

I hope to hear some feedback from those who have used TVCs.

By the way, this is the passive pre that I'm eyeing... by Zenn Audio:


Very minimalist and zen-like to say the least