Diamond Audio Four Channel Car Amplifier

CosmeticsBreathtaking. That's how I'd have to describe the appearance of the Diamond Audio D7054 amplifier. All of the connections on this amplifier are gold-plated, starting with the power connectors: These ultra heavy duty connectors can accommodate either a 4-gauge ring lug, a 4-gauge bare wire connection, or both! Nice idea, as an external capacitor could be added directly to this connection. The speaker connections are top quality as well; the set screws can accommodate #10-gauge speaker wire. The RCA connectors have a nice external machined appearance, but are quite ordinary. The overall external appearance is absolutely gorgeous: the best I've seen. The frosted aluminum finish, combined with chrome plating, and a translucent Plexi cover is easy on the eyes. Even the mounting feet are beautifully machined aluminum. All the crossover controls and configuration switches are accessible from a recessed area on top of the amplifier. Normally the 3" height would be too high for many mounting locations, but this D7054 is so stunning, one would never hide it.

The amplifier is cooled by a variable speed fan, which pulls air in through the endplates and over the internal heatsink, and exhausts it through the end plates as well. While the external aluminum case appears massive, it is not actually thermally connected and does not aid in cooling the amplifier. The amplifier is quite large for 400 watts, so it never thermally cycled during our tests.

Power vs Distortion @14.4V1khz
Frequency vs Distortion
@14.45V50W4Ohm
Crossover Response
Frequency Response

LayoutInternal layout is nice as well. The power supply is at one end, far away from the sensitive analog input circuitry. OP amps are low-noise and moderately fast slewing. Top quality film caps are used to couple the power stages to the input circuitry. On the downside, 85* C supply filter capacitors are used instead of 105* C caps required in an automotive environment. This amplifier uses four 2SA1303 output devices per channel. These are excellent audio bipolar transistors. Using four per channel 100 watt channel is more than we would normally see, but you'll never see one fail.

Power SupplyThe power supply is isolated and fully regulated. The supply has an extra rail to help the output stage pull closer to their rails. The supply fell out of regulation when the DC input fell below 9.8 volts, while delivering 400 watts into two ohms. This is well below operating range and should not create any problems.

CrossoversThis amplifier has three internal crossovers, front, rear, and auxiliary. Each crossover is 18 dB / Octave Butterworth, and can be configured for high pass, low pass or band pass. Frequencies can be chosen from 50 Hz to 5 kHz. Exceptional!

Protection The D7054 never got hot enough to thermally cycle, and it recovered from our short circuits tests without damage. Loads lower than two ohms cause the power supply to reduce voltage, so that the output devices are not overdriven.

PerformanceThe amplifier was ruler-flat from 10 Hz to 20k, as expected. The D7054 produced 50 watts x 4 into four ohms, and a healthy 97 watts x 4 into two ohms at 1 kHz. The power supply is fully regulated, even down to DC input voltages as low as 9.8 volts. So it should be an easy matter for Diamond to adjust the rail voltage up to exceed the published specification.

Two problem areas were separation and THD. Diamond rates separation at 80dB. With the amplifier unloaded, this spec was easily met. But, the separation fell to 50dB at 1kHz with four 4-ohm loads connected. This indicates an internal layout problem. I'd like to take this time to clarify what this really means in terms of sound quality. The properties of an amplifier such as separation and damping are often misunderstood and as a result are often given more importance than they deserve. What does 50dB of separation really mean? Let's say that you have 100 watts of music coming out of the right channel. With 50 dB separation you would have just 1/1000th of a watt leaking into the left. Take an audiophile's $1000 phono cartridge: 50 dB would be quite respectable. The same phono cartridge's separation would fall to only 25-30 dB at 20kHz! So, don't let this spec throw you off. For an amplifier, peak output current capability, slew rate, and ability to drive reactive loads are far more important parameters.

Another area where the amplifier fell short is THD. The D7054 was close to Diamond's spec with only one channel loaded, but rose to .09% @ 1kHz with all four channels loaded into four ohms. This indicates the same internal grounding problem discussed earlier.

Damping Factor measured greater than 300. Anything over 50 is gravy. I'd like to take this time to dispel another myth, that super high damping factors make an audible improvement. A damping factor of 50 means that the amplifier's actual output impedance divided into the load impedance is 50. In this example the amplifier's internal impedance is .08 ohms. A typical four ohm speaker typically has 3.2 ohms of internal DC resistance. So, the actual damping factor could not be better than 4/3.2 = 1.25! That is why emphasis on ultra high damping means nothing sonically.

The amplifier passed our reactive load test, working flawlessly into all of our reactive loads. Efficiency at full power into 2 ohms was 54%. At 1/3 power the efficiency fell to 36%, which is typical for class A/B amplifiers.

Listening TestWe tested the Diamond with Morel Duets and the Altec Lansing Voice of the Theater speakers. These old Voice of the Theater speakers are extremely efficient and help to reveal distortion at low power. The amplifier was completely neutral; we could not detect any coloration of the music. The amplifier was silent when turned on, but made a very slight tick when turned off.

ConclusionThis is a great sounding amp; it looks fantastic, and has very versatile crossovers. The cosmetic and build quality of this amp is quite substantial. A bit pricey, but beautiful nonetheless.

Diamond Audio Amplifier Specifications