The Audio Research PH3 is a high-end tube phono preamplifier, produced in the 1990s by the historic American company Audio Research. Designed to amplify with the utmost fidelity the tiny signal coming from MM and MC cartridges (via step-up), it is appreciated by audiophiles for its neutral and dynamic sound. The stage is entirely based on ECC88/6922 tubes and follows the classic RIAA equalization, required to restore the frequency response of vinyl records to a flat line.
This unit was delivered to me in working condition, but the owner requested a preventive overhaul to ensure that performance remains at its best over time.
Component and tube check
Inside the PH3 there are few electrolytic capacitors, all of high quality: I checked them all instrumentally and they were found to be in perfect efficiency, with no leakage or out-of-spec ESR. The focus then moved to the three ECC88 tubes that form the heart of the circuit:
- V1 and V2: these are the gain tubes, which actually amplify the input signal.
- V3: works as a cathode-follower buffer, providing a low output impedance without adding gain.
Testing on the computerized curve tracer revealed that the two Sowtek tubes installed in V1 and V2 had unbalanced internal sections, while a third Sowtek tube was still in excellent health.
To ensure the best channel symmetry I therefore:
- Moved the remaining healthy Sowtek tube to position V3, where tube quality has less sonic impact.
- Installed in V1 and V2 a matched pair of Philips NOS ECC88, carefully selected and certified on my curve tracer.
Frequency response measurement
As a final step I checked the frequency response of both channels using a reverse RIAA tool. A RIAA phono preamp, as is well known, compensates the record’s original cutting curve: vinyl records are cut with reduced low frequencies and boosted highs, and the preamp applies the opposite correction to deliver an overall flat response.
The reverse RIAA generator, on the other hand, provides a signal already “pre-compensated,” so the output should ideally be perfectly flat. Any deviations highlight the residual equalization, that is, the particular “sonic fingerprint” the preamplifier may impart.
The graphs below illustrate this very characteristic fingerprint. Both channels showed overlapping curves, confirming that the circuit works in perfect symmetry and that the overhaul can be considered successfully completed.
This overhaul demonstrates that a quality component like the Audio Research PH3 can, even after many years, continue to deliver reference-level performance with a simple instrumental check and proper tube selection. The owner can therefore keep enjoying vinyl playback with the certainty of a perfectly efficient and well-balanced phono preamplifier.
If you own a PH3 or a similar model and would like a thorough overhaul or a professional repair, feel free to contact me. I can check the condition of the tubes, measure the RIAA response with dedicated instruments and restore your unit to its original top performance, preserving its value and musicality. Contact me for an estimate or for more information.