7/1250A Measurements and Analysis
The rising output impedance with frequency is indication that Sherbourn may have placed a series inductor after the feedback stage and prior to the speaker terminals for added stability and noise reduction. This is not an uncommon practice in amplifier design.
Damping factor remained stable and uniform at full unclipped power into an 8 ohm and 4 ohm load. It dipped with frequency slightly into a 4 ohm load, but was close to our minimum recommendation of 50. For more information on this topic, read our article on Damping Factor: Effects on System Response.
Sherbourn 7/1250A Frequency Response @ 1 watt, 8 ohms
The Sherbourn 7/1250A exhibited reasonably flat frequency response, with a small amount of roll off at high frequency (about -0.45dB at 20kHz) when driving an 8 ohm resistor at one watt. The -3dB point was around 60kHz.
The output power of the 7/1250A was most surprising since it exceeded the manufacturers specification of 125 watts RMS by more than 25 watts! With two channels driven it was able to achieve 140wpc into 8 ohms but I noticed my line voltage began sagging from 120V to 118V. Into a 4 ohm load it delivered over 225 watts RMS and only slightly less into two channels. By employing two larger power supplies as opposed to multiple smaller units for each amplifier, allowed for the amp to exceed its power rating when only one or two channels were driven at full tilt into 4 ohms.
The power supply in the 7/1250A is certainly up to task for delivering its rated power and then some. For all you power nuts that love to see “All Channels Driven” tests conducted, by now you realize we don’t believe in that test condition thus you wont find it in this analysis. Besides, unless your wall outlet is capable of delivering in excess of 20 amps of current and you have a variac to hold the line voltage constant, you will never be able to achieve this test scenario, and you certainly are unlikely to simulate it listening to music or movies.
The frequency response at full power was at least as good as what I measured at one watt. Ideally all of the curves would line up with a variance of no more than +-0.5dB being tolerable. The Sherborn fell right within this window. Though the response looks slightly worse at 1 watt, I chalk it up to inaccuracies in my test setup and variance in the RMS meter when recording the data. Overall the linearity exhibited here is good especially considering I was driving the amp to more than its rated power without any signs of stress or distortion.
