Passive preamp ?

[seti]

They seem to be incredibly simple. When are they appropriate for use? What are the benefits and drawbacks?

[Mallette]

"Passive Preamp" is a contradiction in terms. I would assume we are talking about a volume control and source selector to be inserted in a line. It would be appropriate for interface between a source and an amp that had no control of volume or method to connect multiple sources.

Other opinions or experience?

Dave

[sfogg]

They can be used when you don't need gain from your sources. Matching between source and amp is important as the source has to drive a harder load with a passive pre-amp then a more traditional pre-amp. Amps with higher input sensitivities and higher input impedance help when running a passive pre-amp.

Benefits are very clean sound and no added noise. Drawbacks are the system matching requirements.

I ran passive pre-amps for a few years and really enjoyed them. Some nice units to watch for are the Mod Squad Line Drive, Adcom SLC-505, McCormack TLC-1 and Adcom GFP-750. The McCormack TLC-1 has straight passive outputs as well as a set of buffered outputs (no gain) which can help in the system matching. The Adcom GFP-750 can be switched from active to passive so if you wanted to run passive at lower levels but needed some gain for louder listening it can do that.

Shawn

[seti]

That is interesting. Thanks for the info.

[ZAKO]

Could you short,n those wires a bit more???

[Mallette]

Hey, that looks like a kit I could build...[8-|]

Dave

[Daddy Dee]

seti,

What's the unit in the pic? Is that in your listening room?

[PrestonTom]

Shawn hit the nail on the head (source Z, load Z, and gain).

Capacitance in the interconnect may also be a problem. The solution is low capacitance (or short) cables and this does not necessarily mean "fancy" cables. The implementations are typically a potentiometer that is conventionally wired (e.g. Adcom) or one with an extra shunt resistor (e.g. McCormack) or a switch with fixed resistors (e.g Placette, I think)), or a transformer based unit (potentially very expensive).

These systems are misunderstood by many regarding their proper implementation (they also can not be used with phono) and hence remain a well-kept secret. That's too bad since they can be of the great, inexpensive upgrades.

Good Luck,

-Tom

[Mallette]

>These systems are misunderstood by many regarding their proper implementation (they also can not be used with phono) ...

I know what you mean, but it might help to clarify that if you use a dedicated phono preamp like a bottlehead or whatever that outputs line level you can use it will a passive.

Dave

[seti]

Dee no that rooom is too clean to be mine : ) It is a Canary Audio preamp. Canary Audio is my favorite manufacturer. Most of their gear is 300B oriented. Also I have met someone in town that can give me soldering and diy tutoring. I was interested in passive preamps because of the simplicity but was wondering how they worked because there was so little to them.

http://cls.audiogon.com/cgi-bin/cls.pl?preatube&1223867322 $395 OBO

[greg928gts]

I have several Creek OBH-10's kicking around. I use one between my CD source and my SET amps (to Khorns). I like the purity of sound in this system. Balance control is affected by moving my chair from side to side. []

I use them in other systems to provide remote volume control, like when I'm using a vintage receiver or my Scott 299B. I cannot detect any difference in sound quality with or without the Creek in line.

Greg

[SET12]

They seem to be incredibly simple. When are they appropriate for use? What are the benefits and drawbacks?

Nice to see some thinking out of the box

I have not used a line stage in yrs.

Few line stages hold my interest.

After listening once! Its all that it took for me to set my FET line stage on the shelf.

So I run my Sony X77ES direct from its variable out directly into my SET mono's typically those controls are about 10K and being 5K at the mid point which is where I run at my loud playing volume.

My amps have a 500K input so no real issues along with 1/2 volt in for 10 watts out.

I recently had SWL here with his Peach II here! I held my breath because I knew if I liked what I heard I was going to have to design and build my self a line stage!

Well! Its a very nice line stage and pretty musical! But! It was out of its league vs CD direct into my amps. Dynamicly my player was just as dynamic and quite a bit tighter and faster in the low end. Direct was all so, so much more transparent that the Peach sounded veiled in comparison which doesn't surprise me due to all the wire and switch's and stuff the signal has to goe thru!

And BTW he later sold the Peach!

Also think about this most audio taper volume controls at mid seting are attenuating 80% of the incoming signal!

Yes great preamps can add life and body to the music! But unfortunately there are many that don't.

I would rather dump an extra couple of grand into a great CD player than a preamp in Peach's price range.

I also use DVD direct for outstanding sound as well!

I added a C4S current source to my driver stage of my power amp and a LED for bias the combination is very dynamic and very low distortion as well.

So its just 2 gain stages for my amp and the signal input is right next to the driver stage.

And I'm just loving it.

SET12

[mike stehr]

or a transformer based unit (potentially very expensive).

This would be a DIY approach I would like to try sometime whenever I have money for it. It's not too costly.......

http://www.intactaudio.com/atten.html

I've had pretty good results running a older Pioneer CDP with the digital volume control direct into a tube amp, and using the remote to turn it up or down. It doesn't completely attenuate the signal, but I like the no noise factor of the direct approach. I love the idea, but the ol' Pioneer isn't the greatest sounding beast.....

I bought three of the radio shack ALP's 100K stereo pots the other day, and one actually measured around 95K on both sides....How lucky is that? These pots actually work pretty good. I'll throw it my little wood box so I won't have to be commited to one CD player.

[rroobbcc]

In my home office system (the system with the Forte IIs) I use a PS Audio 5.6 preamplifier. It has a "bypass" feature which eliminates the balance control from the signal path, and a "straightwire" feature which bypasses the active output, essentially turning it into a passive source selector and volume control. I use it exclusively in this mode and am VERY happy with it.

Rob

[seti]

This may be something I want to try to satisfy my curiosity. I found some info on a passive preamp that looks interesting. It uses sowter attenuator transformers and shalco stepped switch. http://homepages.tcp.co.uk/~sowter/attens.htm

Does this look like a good start on a simple design?

[SET12]

Sure,

They should sound better than most Pots!

SET12

[seti]

Sure,

They should sound better than most Pots!

SET12

Thanks for all the info. I'm glad I asked.

[Erik Mandaville]

I often see the reference to the contradiction in terms issue in reference to passive preamps. I sort of just thought of that term as a description for a device (which happens to be passive) that is before the amplifier in the signal chain.

Radio Shack has the parts needed to build a simple in-line volume and switching box to get an idea for what they sound like. Having also been among those who have built and used them in the past, the references to source output impedance and load input impedance can't be over emphasized. That is the reason behind transformer-based passives: They provide a comparatively high impedance on the input, yet extremely low output impedance figures on the output to the amps; which in turn in my experience seem to work better with high input Zs. Several months ago I built a small, dual mono control (no source switch) with my Dyna ST70, which has a suitably high input impedance of around .5 megohms. A long, long time ago, someone once suggested that the 50kohm pots (linear taper controls) I was using were totally inadequate for this application, and that I should be using a value of at least 1 meg. So, I tried it, and the results were terrible. They had the situation in reverse, since a pot of that value offered extremely high impedance on the output side, and high output impedances have trouble driving capacitive loads, such as is sometimes created by long runs of high pfd/foot IC.

I agree it's possible to get very good clarity with no additional noise, however, it helps to have a source, such as the generally higher outputs of a CD player (most of which are more than capable of driving many amps into clipping). It's sort of ironic, really, that more often than not active line stages are used to attenuate the signal rather than amplify it. What they do provide is impedance buffering between components, which means that an amplifier will see a lower source impedance from most active line stages than they would with most passive units, except transformer coupled examples. I also once experimented with a sort of passive-active vacuum tube buffer configured for unity gain but very good impedance matching. The problem with that is the need for a power supply, which of course can be a problem in some cases.

With conventional carbon comp pots of 10K, 50K, or 100K ohms, it's possible to install a very small high-pass filter between the input and wiper of the control. This would be in the range of maybe 47pfd, or so (I have to go back and find the actual value I used). It seems to somewhat offset the HF rolloff problems that can be encountered at reduced volume rotations, where the cap is essentially out of the circuit at higher ones. Tube instrument amps have something like this called a 'Bright Switch.'

Fixed resistors can offer better tracking if you're using a single stereo volume control, but it won't provide the impedance matching benefits of a transformer-based preamp. Those can be more expensive than many active line stages.

Have fun, you might end up with something you really like! Again, Radio Shack has the RCA jacks, volume controls, source switch (though they are not 'make-before-break' switches the last I looked), hook-up wire, and small chassis to build one of these in a couple of hours.

Erik