Phono stage tests

FREQUENCY RESPONSE

WHAT IT TELLS US

This is a measure of RIAA equalisation accuracy, shown in a graph here. LPs are cut with reduced bass and strengthened treble, to prevent groove excursions becoming too great, and suppress the hiss and crackle of vinyl surface noise. When played back bass must be boosted, and treble cut by exactly the same amount they were changed in the cutting process, as set by the RIAA curve. Phono stage frequency response is a measure of this, telling us how accurate equalisation is and what the resulting tonal balance of the phono stage will be. Rising treble means a bright sound balance and this common; it boosts apparent detail. Modern phono stages rarely, if ever, have falling treble and a warm sound.

The RIAA characteristic of a Rega Elicit amplifier, showing warp filter roll off at left.

Below 20Hz, in the sub-bass region, if the RIAA and derivative standards are followed gain remains constant down to 0Hz or d.c. Many phono stages simply follow this as it conveniently requires no additional filtering. However, this strongly amplifies disc warps, causing cone flap in reflex loudspeakers, something best prevented. The IEC issued an amended curve with additional 7950uS time constant but when used in replay equalisation this audibly lightens bass and is rarely used in phono stages. Many manufacturers use a compromise, and we measure gain at 5Hz to check for warp suppression, or lack of it.

HOW WE MEASURE IT

We inject white noise in the phono input, through an accurate inverse RIAA network. Output is then displayed on our Rohde & Schwarz UPL spectrum analyser, as shown here, providing a picture that is published in our Measured Performance section of the magazine review.

The Rega Elicit, measured using a pre-equalisation network to show frequency response.

In some cases where low frequency signal-to-noise is an issue, we inject noise unequalised and use post-equalisation, applied within the UPL spectrum analyser. This gives a smoother looking curve.

The quoted frequency response is usually wider than the 20Hz-20kHz spectrum shown and is measured using a manually swept sine wave generator.