From: rbs@uk.ac.ed.aisb (Robert Scott)
Newsgroups: rec.audio
Subject: Passive preamps revisited
Date: 14 Aug 92 15:12:18 GMT
Reply-To: rbs@aiai.ed.ac.uk
Distribution: rec
Organization: Dept AI, Edinburgh University, Scotland


Several people, including myself, have asked about passive preamps,
types of, and their pros and cons.

Some people asked me to post any info I gained on the subject so
here we are. I warn you that it is quite a full explanation.

What's in an amp?
-----------------
A conventional hifi amplification systems consists of:

 - a preamp;
 - an equaliser;
 - a power amp.

The preamp amplifies small signals from the source inputs so that signals
at the output of the preamp are at the same level.
This is especially needed for very low source signals such as that
generated by a record player pickup.
Signals from cassette tape players, tuners, CD plares, DAT players
don't need preamplification but may benefit from buffering
to match their output impedence with that of later stages (such as the
power amp).
Somewhere in the preamp balance and volume controls are also added
and let's not forget the input source selector switch.

The equaliser is there to correct any nonlinearities in the characteristics
of the source. A common type of equalisation are the RIAA equaliser
used to correct the characteristics of record player pickups
(they have nonlinear characteristics to improve noise rejection).
The type of equalisation that most people will be familiar with is
the treble and bass left/cut tone controls on most amplifiers.
One can buy a fancy graphic equaliser to have more control over
the equalisation process which is useful for say tailoring
the characteristics of the amplifier to that of the listening room
(although most purists would argue that no equalisation is good
 equalisation).

Generally, the preamp and equalisation stages are integrated together
and sold under the name of "preamp".

The power amp takes the standardised level, equalised signal and 
amplifies it to the level needed for the speaker system.

One can buy amplifiers were the 3 stages are integrated together
(sold as an integrated amplifier) or as a separate preamp and
power amp (and even separate graphic equaliser).

Why a passive preamp?
---------------------
Since most signal sources (all except the record player) are already
at the line level (2V peak-to-peak) and have excellent output
characteristics it seems to follow that:

  - we don't need preamplification
  - we don't need equalisation

and if the source output is well balanced, as it is from most CD players
then:

  - we don't need balancing

Why get rid of these stages? The theory goes that he less components
there are in the path between source and speakers then the better the
sound so we might expect some sound improvement by dispensing with
these stages.

Now we have got rid of nearly all the functions of the preamp and 
equaliser except that:

  - we do need a volume control

So a passive preamp is basically going to be a volume control
made from passive components (components that do not use amplification).
A typical system using a passive preamp would be a CD player source
going into the passive preamp (volume control) which then goes directly
into the power amp.

Types of passive preamp
-----------------------

As far as I've found there are 2 basic types of passive preamp:

 - variable resistor type
 - switched attenuator type


Variable resistor
-----------------

The variable resistor type is the simpler and cheaper.
Here is a diagram.


       sig in  0------.
		      |
		      /
                      \<----O  sig out
from   		      /                   to power amp
source 		      |
	  gnd  0------.-----O  gnd

The variable resistor is a potential divider sending a proportion of
sig in to sig out.
The advantage of this system is that it is extremely simple.
The disadvantage is that it's output impedance varies depending
on the position of the control. This in effect causes some
unwanted equalisation to signal although this effect may be
small if the correct value for the resistor is chosen.
The other major drawback is that variables resistors are
inherently noisey devices although one can buy fancy low noise
versions for not much more expense.

Typically values depend on the impedance of the source and
power amp but I've seen between 22 k log and 100 k log pots used.

Switched attenuator
-------------------

A switched attenuator consists of a complex switching device
(in this case usually mechanical) and a resistor attenuator network.
There are many versions of these networks, like pi, lambda and T networks.
I'm not going to go into these networks topologies here;
best thing is to consult an electronics reference book.
As the switch is rotated to a new position, different parts of the
resistor network are brought into the signal path thereby 
giving different amounts of signal attenuation.

The advantage of this approach is that if one choose the right network
version then the input and output impedances of are constant 
(within the tolerances of the components).
The down side is that these networks are non trivial to wire up,
the resistors in the network must be matched,
and the switch has to be of a very high quality, plus it is easy
to get all sort of interference creeping into the system.
As a result good quality switched attenuators for audio use
can approach the cost of a conventional preamp.
Good ones should introduce less distortion than a premp, however.
Another downside is that the attenuation is in discrete steps
and not continuously variable.

The Verdict
-----------
I haven't built or heard any passive preamps so I can't
give any experimental insights.
My guess is though that the ranking from good to bad of different
preamp systems would go thus:

 - high quality switched attenuator
 - reasonable to high quality preamp
 - good variable resistor
 - bad preamp
 - bad variable resistor
 - bad switched attenuator

So unless you're going to shell out large wads of folding stuff for a
good switching box stick to your preamp (with the tone and balance controls
switched out).

This is all my humble biased and qualitative opinion.

Rob Scott
---------

--
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Newsgroups: rec.audio