Passive Pre-amps

Hi,

Given all the hype over the passive pre-amps, I was wondering if any forum members would be kind enough to explain how they work?

I don't understand how they can amplify signal without a power supply and transistors.

Also, there must be some compromises/negatives to using passive pre-amps - what are they?

Thanks,
DT

 

They simply attenuate the signal from source to power amp by use of a volume control, so no need to amplify.
A pre doesnt amplify, only attenuates the signal.

Some say the negative is an un-exciting sound.

 

oh dear, I clearly need to research my hobby more thoroughly!

I thought preamps amplified a source signal, increasing the amount of amplification the higher the vol pot is turned?

So they only "attenuate" the signal then? What does that mean? If they don't amplify the signal, why can't I plug a source straight into a power amp?

Sorry, these are really basic questions, but I've never studied amplifiers/electronics before (obviously!).

DT

 

If you plugged a source direct to power amp, be prepared to buy new speakers

The signal from source is high, so a pre attanuates it, low volume means the highest attenuation, high volume the least.

 

So attenuation reduces the strength of the source signal? Come to think of it - that's what those rothwell in-line attenuators do - help reduce the volume of "loud" sources! Doh!

So "preamp" is a misnomer then? Shouldn't we be calling it an "attenuator" instead?

Are the technical benefits solely due to a shorter/clearer signal path and the removal of (non-linear) volume potentiometers?

DT

 

Hi Dynamic Turtle,

A passive preamp maybe not the best term for the device. But they can offer very good sound quality. They are basically a volume control that uses a potentiometer (variable resistor) or a stepped attenuator (various resistors). They don't need a power supply to work (unless it has remote control).

They offer less in the signal path to affect the signal. There will also be no power supply noise and line amp circuit noise or distortion.They can offer very good detail, resolution and transparency to the sound.

The downside is that they are affected by the load of the interconnect cable, the input impedance of the power amp and the value of the passive. You are at the mercy of the quality and design of the volume control.

They work well with high input impedances on the power amp, low capacitance cable and high sensitivity on the power amp. Also a input device with good drive capabilities is good to use.

Some people like them, so people don't.

The two links go into more detail.

http://www.dact.com/html/passive_preamp.html

http://www.goldpt.com/

Another type of passive is the transformer based preamp. This uses multi tapped transformers to give the attenuation. But these offer cable driving ability so they don't have the potential wealness of normal passive preamps.

SCIDB

 

Very interesting & informative, thanks SCIDB ;-)

From the characteristics you've described, standard passives need to be matched very carefully with the rest of your kit (cable capacitance, power amp impedence etc.) in order to work at their best?

Sounds like the sort of thing you have to build your system around, rather than just slot-in?

DT

 

Hi Turtle,
It is better to term them as a 'Passive-Pre' rather than a 'Passive-Preamp', as that is a more accurate term for a device which just gives source selection and volume control by attenuation.
As a general rule, to use a Passive all your sources need to have highish output (500mV+ ?), and your power amplifier to need a similar or smaller voltage for full output.

But, the transformer based Passives that Dean referred to are capable of giving some 'amplification'. They do this by having tappings on the transformer that offer a couple of step-up ratios as well as many more step-down ratios.

 

I enjoyed the sound my Lumley passive pre made (or rather allowed through!) with the Lumley valve power amps - very transparent and detailed. I spent nearly 10x the Lumley's cost to get better!

BTW, it's still for sale, and it's lovely in chromed stainless...

 

I believe the term "pre-amp" dates from before the days of CD players and their high outputs when sources (such as TTs) actually did require a bit of "pre" amplification.

Active pre-amps do actually provide some amplification with the volume knob turned up high. The unity gain (ie, no amplification or attenuation) position is often around the 12 o'clock mark on active pres. On passives it's obviously at the max position (no attenuation).

Michael.

 

AudioSynthesis are well regarded for their passive pre-'s, and were so keen on removing things from the signal path that they built digital attenuation into their Dax Decade, this obviating the need for a pre, and that extra interconnect.

 

wadia, msb, mf, krell and others also offer a source with volume type of arrangement. certainly if your system is as simple as a single source then it can make for devastatingly good performance as you are able to put the money you would have spent on a pre and cable into the source or power amp (or speakers). it's always seemed a very elegant way of doing things to me but unfortunately if you want an analogue source or recording then you can be stuffed (though some do offer analogue solutions).
cheers

julian

 

to follow-up the analogue source problem, the way I was directed to go was to buy a 'switchable' integrated amp - ie one that had the (AV) power amp mode for use with digital sources, via the Dax, but could revert to pre/power for an FM tuner, say. The Audiolab 8000S was such a box. I have now acquired an ADX for the analogue sources, which in my case are a tuner and a cassette deck.

 

Passive pre's are something that l have been interested in for a few years but only recently did l bite the bullet. Some of you probably read my recent thread on my Bent NOH. Its a transformer based pre and l believe thats the reason its so good. l am now convinced that resistor based attenuators are far inferior, especially if using the very cheap version fitted to a CD player for example.

My bent NOH is an absolute revelation, l will put my head on the chopping block and say l doubt there is any pre amp at any price that could better it.

l also believe that the ultimate no holds barred pre amp doesnt exist unless it uses transformers and not resistors for attenuation.

 

la toilette - was meaning an ordinary source that does not have its own volume control, i should make myself clear

 

I would tend to agree with you...as would many others who either own or have heard a NOH. When you consider that mine (3 RCA inputs, 1 XLR input, RCA/XLR switchable outputs, remote volume) cost around US$ 1500 ready-built that makes them simply stunning value. All the more pity that John Chapman has stopped making them for the time being

Still - there are people who have never heard a NOH with preconceived ideas about transformers being fundamentally "bad" who laugh at the idea of a transformer volume control

Michael.

 

Ha ha, sorry penance, I really didn't mean to sound obtuse, just remembered something that someone had recommended to me ages ago. Of course, as a passive pre is pretty much just a volume control (+/- input selector), there probably isn't much of a difference between using a variable output source direct to an amp and using source-to-pre-to-amp anyway...... Ho hum.