Halo A 21 Amplifier Measurements & Conclusion
This test shows demonstrates the frequency response linearity of the power amplifier within the audible range at 1 watt and full power. As you can see, the A 51 maintains + 0.2 / -0.36 dB from 20 Hz to 20 kHz which is excellent. Notice that the linearity remains even during maximum unclipped power output into 8 ohm and 4 ohm loads indicting that this amp doesn't suffer from slew induced distortion typical of lesser designs.
Editor's Note: Parasound specs this amp at +/- 3dB from 5Hz to 100kHz at 1 watt.
I was able to measure about 250 watts into an 8 ohm load and about 400 watts into a 4 ohm load across the entire audio bandwidth. Please note that during the 4 ohm load test, my wall voltage sagged from 116VAC to 112VAC. Without having a VARIAC to hold the line voltage constant, it is difficult to accurately measure all channels driven with such a powerful amplifier. However, given the fact that the A 52 is endowed with a 2.2 kVA transformer and 164,000uF power supply, I think it's safe to assume this amp will deliver its rated power into all channels at 8 ohms. It will deliver marginally close to its rated power into 4 ohms. In order to do a true 400 watts/ch into 4 ohms continuously, would require a power transformer about 65% larger than the one in the A 52. Of course this all assumes you hold the line voltage constant and have at least a 20A dedicated line to power it. Since a continuous all channels driven scenario is both unrealistic, impractical, and unlikely this should be of little concern to most users.
We ran 1 kHz, 5 kHz and 10 kHz square wave tests at 1 watt into 8 ohms and found no excessive overshoot or ringing or slew induced distortion. The Parasound A 51 amplifier is more than capable of driving any speaker load in even the most demanding home theater systems. Only folks dealing with sound reinforcement applications may thirst for more power, but usually at the expense of higher noise and distortion. The linearity of this amplifier is excellent at any power level it is operating in, and its freedom from audible noise and distortion makes it a true winner!
Conclusion
The Parasound Halo A 21 and A 51 amplifiers are top rate amplifiers at a great value. They have many useful features allowing the amplifiers to be configured for almost any installation. Parasound seems to have attended to every aspect of their design including the excellent build quality internally and externally matched with great looks. They had the ability to control my speakers to the point where the soundstage was impeccable especially via balanced connections and although I did not have another $5,000, $10,000 or $20,000 dollar amp to compare the A 21 and A 51 with. However, I would not hesitate to put them up against other amplifiers in their comparable price range.
Parasound wisely decided not to combine their amplifiers into 7-channels perhaps due to size and weight constraints or probably due to their desire to optimize performance without compromise. If you happen to be searching for a 7 channel amplifier then we suggest combining the Parasound Halo A 51 with the accompanying A 21 amplifier as you will not be disappointed in your investment. Likewise, if your only requirement is for two channel stereo, then the Halo A 21 amplifier will satiate your stereo digital and analog appetite.
The Score Card
The scoring below is based on each piece of equipment doing the duty it is designed for. The numbers are weighed heavily with respect to the individual cost of each unit, thus giving a rating roughly equal to:
Performance × Price Factor/Value = Rating
Audioholics.com note: The ratings indicated below are based on subjective listening and objective testing of the product in question. The rating scale is based on performance/value ratio. If you notice better performing products in future reviews that have lower numbers in certain areas, be aware that the value factor is most likely the culprit. Other Audioholics reviewers may rate products solely based on performance, and each reviewer has his/her own system for ratings.
Audioholics Rating Scale
— Excellent
- — Very Good
- — Good
- — Fair
- — Poor
| Metric | Rating |
|---|---|
| Frequency Response Linearity | |
| Measured Power (8-ohms) | |
| Measured Power (4-ohms) | |
| Multi-channel Audio Performance | |
| Two-channel Audio Performance | |
| Build Quality | |
| Fit and Finish | |
| Ergonomics & Usability | |
| Features | |
| Performance | |
| Value | |
