I recently had the opportunity to overhaul and test the Audio Innovations Classic 25 tube amplifier, a piece of equipment that helped bring vacuum-tube amplification back to the forefront of the audio scene. Based on the integrated Series 700 line, the Classic Stereo 25 was sold as a kit, intended to make tube amplifier ownership more accessible. Because it was originally marketed as a kit, anyone buying a used unit should be cautious, as the assembler’s skills are unknown.

However, the unit I serviced featured flawless soldering and workmanship that looked factory-assembled. It’s unclear whether pre-assembled versions were sold, but the build quality of this example suggests remarkable attention to detail and precision.

Sound Quality

If you are looking for a sound that embodies the traditional essence of Audio Innovations tube amplifiers, the Stereo 25 comes closer than any other model I’ve tested. Characterized by a slight softness at the frequency extremes and a warm yet open midrange, it stands out for its engaging dynamics. This is largely due to its lower negative feedback rate compared with the 500, 700 and 800 models in the same line.

The Overhaul

The servicing of this Audio Innovations Classic 25 was straightforward and methodical. First, each tube was carefully tested using the uTracer curve tracer to ensure proper operation. Defective tubes were replaced, while those in good condition were retained.

Next, I checked all electrolytic capacitors on the board with an LCR bridge. Good capacitors were marked green, and those requiring replacement were marked red, allowing quick identification of components to replace, as shown in the photos.

After replacing the electrolytic capacitors marked as defective…

…I performed standard measurements to ensure the amplifier’s proper operation. This overhaul ensured that the Audio Innovations Classic 25 is ready to deliver optimal and reliable performance.

Measurements and Performance

Instrumental measurements of the Audio Innovations Classic 25 yielded interesting results. Power output measured about 22.5 watts RMS with the original Sovtek EL34 tubes. I am currently awaiting a matched quartet of new Tung-Sol EL34s to complete re-tubing, which may further enhance the amplifier’s performance.

Another noteworthy aspect is the damping factor, around 7.7. This is significantly lower than that of previous family models, indicating a lower overall feedback rate. The output transformers used are the same as in the 500/700/800 models, so a strong resonance between 20 and 90 kHz persists, as shown in the bandwidth graph. It is surprising that this issue has not been corrected despite the years since the first 500 model was produced.

Bandwidth at 1 watt showing transformer resonance

Square waves at 100 Hz, 1 kHz and 10 kHz

Finally, the series of graphs is completed with the one showing harmonic distortion…