Reference Theater Series IPS-1 Measurements & Analysis

I did some quick spot-checking on the IPS-1 amplifier gain structure to ensure it could be properly driven with a wide assortment of preamps or receivers. My personal criteria for amplifier gain structure is that it should be able to hit full power when driven with 2Vrms. The IPS-1 fell well within this criteria.

Amplifier Power & Efficiency

Due to the inherent multi rail switching scheme of the IPS-1, my efficiency measurements are much lower than actual for this amplifier design since I use continuous sine wave testing. My measurements indicated how efficient the amp is when driven continuously at full power (0.1% THD + N), hence when the amplifier is operating in linear A/B mode. During normal music listening, expect to see around 65-67% efficiency which is about the best you can get from a linear amplifier design.

As you can see, I measured the IPS-1 to deliver about 145wpc into 8-ohms and 188 watts into 4-ohms at 0.1% THD + N using my APC S-15 and SBATT to ensure the line stayed at 120V +- 2V. Keep in mind most review publications test at clipping and don’t do continuous power measurements so our power numbers are usually a lot more conservative than what you typically find from other reviewers.

For more info, see: The All Channels Driven (ACD) Test

The Emotiva website specs this amp as a 150 / 200 wpc (8/4 ohm; respectively) amplifier but upon further inspection, I have come to realize their power ratings are based on a 1% THD + N rating which explains why I measured less.

I asked Emotiva to furnish their power measurements at a constant 0.1% THD + N fidelity firewall to see if their ratings jived with my findings.

Emotiva Power Measurements

Figure 1. Emotiva Power Measurements vs 0.1% THD + N

As you can see, the true continuous power rating of this amplifier (Audioholics style at least) is 140wpc into 8-ohms and 175wpc into 4-ohms at 0.1% THD + N. Their data is very consistent with my own, except I actually measured a tad more power into a 4-ohm load.

Frequency Response

Figure 2. IPS-1 Frequency Response

The frequency response was smooth and extended to a -3dB point of 125kHz.

FFT Distortion Analysis

Figure 3. FFT Distortion Analysis at Full Power

At near full rated power (140wpc) into 8-ohms, the IPS-1 exhibited impressively low distortion (30.364+60.194)dBv = 90.6dBv or 100*alog^-1(-99.2/20) = 0.003% This figure is even lower than what I measured on the MPS-1 (.008%) under similar load conditions. I suspect the removal of the soft clipping circuit has paid dividends in this regard (lower noise floor, lower distortion).

As many amps approach their full power rating, their power supplies tend to run out of gas and show large harmonic nasties of the fundamental test tone. This was not the case with the IPS-1 and perhaps one of many indications as to why this amp maintains excellent composure at high power levels and sounds more powerful than it really is.

Output Impedance and Damping Factor

Figure 4. Amplifier Output Impedance vs Frequency

Amplifier output impedance when the IPS-1 was driving an 8-ohm load at 1 watt was below 100 mohm for almost the entire audio bandwidth with a gradual rise above 20kHz. We like to see amplifier maintain 150 mohm output impedance or less so that it will sound consistently good for a larger variety of speaker and cable loads with rather complex impedance profiles. This is a very good measurement and one of the primary design attributes as to why I felt the transient bass response of this amplifier was excellent, rivaling my more powerful digital subwoofer amplifiers I am currently using to power the subs in my reference speakers.

Figure 5. Amplifier Damping Factor vs Frequency

Again the IPS-1 did NOT disappoint. An amplifier with a low output impedance will also maintain excellent damping characteristics since these are related metrics. The IPS-1 maintained a damping factor of around 100 for 8 ohm loads and ½ that as expected for 4-ohm loads. This fell right within our benchmark criteria.

Signal to Noise Ratio (SNR) Measurements

The IPS-1 maintained very good SNR performance (about 2dB better than the MPS-1). SNR @ 1 watt: 127mVin / 1 watt out was 75.7dB unweighted or 83dB using 40kHz SPCL filter in my AP. Normally, I’d like to see an amplifier measure below 80dB at 1 watt unweighted, but I never found the IPS-1’s noise floor to stand out during critical music listening, or just idling by.