Use a Passive Preamp For Excellent Sound Quality

Unamplified Volume Control Gives Low Cost Audiophile Performance

Jun 27, 2009

Hi-Fi enthusiasts can save money and boost quality by omitting a traditional pre-amplifier, feeding the CD player or Ipod to their power amp via a passive volume control.

The traditional integrated stereo amplifier combines the controls with the audio power amplifier. In upmarket Hi-Fi systems the control unit is separated into the preamplifier, which selects the audio source, adjusts the volume and performs other functions like tone controls and compensating for the RIAA equalisation for a turntable.

The output from the preamplifier is at line level, ready to feed into a separate power amplifier, which increases the power level to drive the loudspeakers. The power amplifier typically has no operational controls apart from an on/off switch.

However, in a minimalist system with just one source like a CD player or an Ipod, many of the functions of the preamplifier are unnecessary. High end systems often dispense with tone controls, and a single source does no need a source selector. It becomes possible to replace the expensive preamplifier with a simple device containing a volume control. Even the most minimalist audiophile needs to control how loud the music is.

Because a volume control simply attenuates the signal going to the rest of the system, it is a passive circuit requiring no power supply or amplification. This is often called a passive preamplifier. Strictly speaking it cannot amplify, but the name has stuck because it replaces the preamplifier in an audio system, which is also placed between the audio sources and the power amplifier.

DIY Passive Preamp Or Commercial Product?

Because the passive preamp contains just a volume control and the connectors to splice it into the signal path, it is a particularly easy project for the DIY audio enthusiast. The parts cost is around $20. However, these are also available as commercial products such as the Placette Remote Volume Control.

The schematic is shown in the image below, and the whole project should take less than an afternoon to complete. It is worthwhile spending a little more money and getting a conductive plastic potentiometer as these tend to be better matched between channels than regular carbon track types.

Passive Preamplifier Cable Length Issues

Being passive, the volume control should have a very low distortion compared to a regular active preamplifier. The output resistance of a passive preamp is at most a quarter of the value of the potentiometer track, assuming it is being driven by a CD player which is a usually low impedance source. This works with the audio cable capacitance running to the power amplifier to make a low-pass filter like a tone control turning the treble down.

Keep Cable Length from Passive Preamp to Power Amplifier Short, Less Than 2m

Typical quality audio cable has a capacitance of about 150pF per metre, though some cheap cable can easily be twice that. Running ten metres of cable from the passive preamp to a power amplifier of typical 50pF input capacitance and 47kohm input will result in nearly 1dB loss at the highest audio frequencies.

This is not high fidelity. A chart of frequency response against cable length is shown below. The cable length from passive preamp to the power amplifier should be kept short, less than 2m. This is a shame, as the control position is not usually a good location for a large power amplifier.

This issue can be reduced by lowering the value of the potentiometer. However, this will increase the load on the source, and if the CD player assumes a standard domestic audio load impedance of 50k and uses isolating capacitors on the output, the low load resistance makes a high-pass filter, which will result in a loss of bass response.

Despite the issues, a passive preamp is well worth considering for a CD based high quality audio system. As part of an upgrade path, a home-brew passive preamp is a good way to defer the expensive preamplifier purchase and concentrate funds on a good CD source, power amplifier and speakers, adding a good active preamp as funds permit and perhaps more signal sources require. And while the high frequency loss is an issue in an audiophile system, a passive preamplifier is a good budget control unit for a minimalist student audio system based on an Ipod dock or CD player.

The copyright of the article Use a Passive Preamp For Excellent Sound Quality in HIFI Stereo is owned by Richard Mudhar. Permission to republish Use a Passive Preamp For Excellent Sound Quality in print or online must be granted by the author in writing.