[Bill Baker]

Curious as to everyone's opinion and preferences in the battle between active and passive preamps when used with tube amplifiers.
 Your thoughts on sonic merits of both.
 Pro's and cons.

 Only looking for honest opinions. Not a battle.

[tom1356]

I think the issue needs to be divided into passive resistor based pre's and transformer based pre's. Almost all criticisms of passives are of the resistor based types.
I have a custom pre using TX-102 silver transformers. I use it with custom 300b monoblocks.
I will never remove this piece from my system, but I will surely try different amps, sources, and speakers.
I started selling high end audio in 1986. I've owned/heard countless high end pre's. Nothing compares to a system designed around a TVC passive.

[nathanm]

It seems to be a matter of gain stages and impedance whosits, all that stuff I still am struggling to make sense of.  In the case of my Cary tube amp which in its original incarnation was purposely designed with one less gain stage, using it with a passive pre was fine (Monolithic PA-1) but when played with an active preamp (NAD 3225PE integrated's pre-out) it really came alive.  Much more gain and consequently much better perceived bass and treble extention and punch which I normally associate with SS amps.  

The lack of this gain stage was purported to be a Good Thing, but geez you need the right amount of juice in there somewhere, right?  What difference does it make whether its in the amp or in your preamp?  Anyway, it's not too surprising then that Cary had a similar lightbulb go off and they redid the input section with higher gain tubes for the 'ol "R" version. (not cheap either, sheesh!)

With amps that have increased sensitivity\gain my passive pre works fine.  My assumption is that it's a matter of getting the input vs. output impedances complimenting each other but I'll be damned if I can wrap my head around the concept about what combination of numbers is most desirable.  

[Bill Baker]

With amps that have increased sensitivity\gain my passive pre works fine. My assumption is that it's a matter of getting the input vs. output impedances complimenting each other but I'll be damned if I can wrap my head around the concept about what combination of numbers is most desirable

  Well said. I have come across some amps that sound exceptional when being driven by a passive resistive pre and others that need the additional gain of a passive.
  I will admit that an active pre [can] add a bit of "magic" to certain amplifiers. System synergy is obviously key but I still like a passive pre built around a quality volume pot, suually stepped attenuators, with the right amplifiers which are usually those with an input impedance of at least 100k.

[Jon L]

Honestly, there can be no "conclusions."

This passive/TVC/active discussion is akin to dynamic/electrostat/planar discussion, and unlike the movie "Highlander," there CAN be more than ONE.  

For MY system and tastes, I prefer a simple, high-quality resistor-based passive.  I really like the EVS ultimate attenuators, which also allows you to bypass one set of extra interconnects as well, which is a big bonus IMO.  Just look at how much people are paying for top-flight interconnects these days.  The EVS works great in my system b/c my source has high-output tube gain+buffer stage and my tube amp has input impedance of 100K.  It also helps my customized speakers have 95dB sensitivity/flat 7 Ohms.  

In this context, my EVS was much more "straight-wire-with-gain" than even the Bent Audio silver TVC (Mk II) I had, which was admittedly "crippled" by having to use extra interconnects.  
I'd love to try the Mk III transformers, but when it comes down to it, transformers are made of many, many meters of wire.

I've also heard systems come alive with TVC in different contexts, and I generally prefer the presentation of passives to actives.

However, if the system has very good extension into low-bass and uses "difficult" speakers, my opinion is that you Need a superb SS active preamp if you want that last 5-10% of bone-crushing, crunching, defined, KILLER bass dynamics.  Most active preamps (SS or tube) out there do not meet the criteria for "superb" unfortunately.  

While many people would actually prefer a similarly priced tube pre over SS (and I am one of them), I still concede that much to a superb SS pre for those who enjoy a lot of music that lives in the lower bass.

In theory, I hate all of the above solutions and would like my digital source to output pure high-res digital signal without any analogue manipulation (including volume control) directly to pure digital amplifier.

[dado5]

This is a toughie.

In my limited experience, I would say I prefer active preamps overall, but this is a shaky lead for the powered boys.  

First, all (and I mean all) the active preamps I have used provide too much gain when combimed with my power amps. Often I am limited to the 9 o'clock position at best on the volume control. This means my gear's perfomance is being limited to the poor perfroming end of the potentiometer's operating range.  Honestly I don't know why pre amps with any more than 6-9 dB of gain are even marketed as most power amps have input sensitivities well below the 2 volts that a CD/DVD player puts out. Second, all the preamps I have tried increase the noise level, although this is audible only when no music is playing.  That's the bad news.  The good news is that active preamps seem to give more drive to the music.  Bass/midbass impact seems to improve in particular.

Passive pre amps are definelty cleaner sounding and far less noisy. But they tend to sap some of the impact and emotion from the music. Now I have only used a Creek OBH-10 and several home brew jobs using carbon and plastic pots. I have also used switched attenuators, but these were again built using pretty cheap parts (no DACTs or Goldpoints), so I will be the first to say that I have not heard a high end passive unit.

So I marginally prefer the extra jump factor that an active unit provides despite the faults that come along with it.

On a final note, I have gone DIY the whole way recently and I will be building my next amp with an input sensitivty close to 2 volts and utilizing a built-in quality stepped attenuator with an optimized range.  Theoretically, this should be the ideal solution.....We'll see in a month or so.

Thanks,
Rob[/list]

[Marbles]

Put me down for another VERY satisfied Silver TX102 (Bent) owner.

[Bill Baker]

First, all (and I mean all) the active preamps I have used provide too much gain when combimed with my power amps. Often I am limited to the 9 o'clock position at best on the volume control

 Hi Dado,
  This is true. Many of the active units have far too much gain. This can easily be overcome with a voltage devider allowing your preamp and amplifier to have a much happier marrige.

[JLM]

Synergy is a factor, yes.

Simplier IMO is better.

As mentioned above, wouldn't a high output source (as least largely)take care of the lack of dynamics of a passive versus an active pre-amp?

So if this is indeed the case I'd vote for passive.


I don't understand the advantage of listening to resistors versus transformers in passive designs.  Could someone please explain that for the class?

[JoshK]

Simple easy answer is resistive passives shunt the extra volume to ground, so some say they loose detail in doing so as shunting isn't always linear as you shunt more and more volume.  Transformers trade voltage for current and don't shunt.  They also are much less inclined to have bass issues (dynamics).  For further explanation read bentaudio's website.

[zybar]

When properly matched I would vote for the passive.

I use a Bent NOH.

Before the Bent, I tried the following active preamps:

Sonic Frontiers SFL-2
Audio Note M2
Manley Shrimp, Control Master
Musical Fidelity A3.2CR
Electrocompaniet EC 4.7

plus

Placette RVC passive preamp (both single ended and balanced - balanced was significantly better)

George

[mb]

My experience with passive attenuators v. active (buffered) attenuators:

1) My AKSA 55N was used with the 41-step TKD stepped attenuator. After several months of testing I eventually opted to put in a battery powered buffer after the TKD pot. The drive and 'body' was not there without the buffer (attenuator value was as recommended by Hugh Dean, the designer of AKSA, and the input impedance is a comfortable 47Kohms).

2) I use a Creek 4340 for my bedroom system. It has a remote (passive) volume control and the option to add in an active linestage. I put together a regulated power supply + buffer to plug into where the linestage would be. It sounded like the little Creek had gained 2-3x the power. No noticeable downside to this upgrade.

3) A TDA7294A chipamp project I built also had a 47k input impedance and sounded rather bland and gutless. Once again I had to add a buffer to bring the music and life into the amp.

I actually like the clean, neutral sound of a good passive attenuator; it's just that in my cases above, in spite of the relatively 'easy' input impedance, it seems that buffering of an good active stage helps.

[bundee1]

I just got my Luminous Audio Axiom which I think is one of those volume shunt type pres. Cable and amp matching is critical as I had to fill out a small questionaire to get the pre matched to my system. You have to be very careful because of all the negative effects a mismatched system can have. 2 things I noticed are:
1) this thing is incredibly neutral. So neutral in fact that I could easily tell the difference between 3 different IC's. These werent subtle differences either.

2) The neutrality makes cable selection very important. When I switched from some cheap Acoustic Research cables to Synergistic Research Alpha Sterlings the bass came back tight and punchy and details came out. Now I think I have to get another pair for my amp to pre because vocals seem a little thin up top.

Dynamics are very good. Im really surprised.

[Russtafarian]

MB wrote:

1) My AKSA 55N was used with the 41-step TKD stepped attenuator. After several months of testing I eventually opted to put in a battery powered buffer after the TKD pot. The drive and 'body' was not there without the buffer

What are you using for a buffer chip?

[gonefishin]

and add another "one more" for the Tx102 based NOH preamp.

[Doc Jr 8156]

Another Bent Audio TX102 MK III satisfied user.  I tried active preamps like Morrison ELAD, Audio Research, Dynaco PAS, and Mapletree Tube Pre.  The NOH is both the a new beginning and an end to the search.  Godspeed.

[zybar]

Hmmm...

Anybody notice the trend here?

Too bad John isn't making his preamp right now.

George

[nathanm]

What numbers constitute "high" vs. "low" impedance?  Where is the line drawn?

[mb]

MB wrote:

1) My AKSA 55N was used with the 41-step TKD stepped attenuator. After several months of testing I eventually opted to put in a battery powered buffer after the TKD pot. The drive and 'body' was not there without the buffer

What are you using for a buffer chip?

I'm using the HA5002. Has a fair dose of output dc offset when the input load is ~10-20k. Other suspects for buffer would be the EL200x and BUFxx series.  Also do note that these very high speed buffers tend to oscillate unless power and ground are clean.

[mb]

What numbers constitute "high" vs. "low" impedance?  Where is the line drawn?

I won't even attempt to draw the line. However as an example:

- passive attenuator (depending on pot value, volume setting, source output impedance): several K ohms??
- after an opamp: 100-200 ohms?
- after an active buffer: 10 ohms or less?

I'm sure it's not just impedance. Distortion, harmonic structure, etc, etc all come in, and the simplicity / neutrality of two resistors or two coils as volume control has its advantages too.