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Ultra-LD Mk.3 Amplifier Tweaks & Performance By NICHOLAS VINEN Finally, we present the specifications for the new Ultra-LD Mk.3 Amplifier along with a couple of minor tweaks to the design to maximise its performance. T HESE FIGURES and graphs show the performance of the complete Ultra-LD Mk.3 amplifier. The test signal source was set to 2V RMS and connected via the RCA inputs of channel 1 on the rear panel of the unit. The performance was measured at the speaker terminals on the rear of the unit, with a resistive load connected via 1m of twin lead. The volume control was set to deliver 100W into 8Ω and 200W into 4Ω with the 2V RMS input signal. The overall performance of this amplifier is much better than the vast majority of commercial amplifiers, even expensive models sold as “high fidelity”. Distortion figures for commercial units are often quite vague; those that do provide graphs typically show quite a dramatic rise in distortion above 1kHz. As you can see from the graphs published here, our Ultra-LD Mk.3 retains the low high-frequency distortion characteristics of the individual modules featured in the July 2011 issue.. The signal-to-noise ratio is also very good. The left/right channel performance differs, despite the fact that the amplifier modules are identical. This is because one module is closer to the power transformer than the other; we purposefully arranged it this way because otherwise, the transformer would be close to the sensitive input circuitry of the right-hand module and that would be worse. Specifications Continuous power, both channels driven (THD+N < 0.1%): 100W into 8Ω; 135W into 4Ω Music power, both channels driven: ~150W into 8Ω; ~200W into 4Ω Total harmonic distortion plus noise: <0.0025%, 1kHz, 20Hz-22kHz bandwidth, 90W (both channels driven) Signal-to-noise ratio: -109dB (left channel), -115dB (right channel) with respect to 90W into 8Ω Frequency response: +0,-0.3dB (8Ω), +0,-1.3dB (4Ω) 20Hz-20kHz Channel separation: approximately 50dB, 4Ω and 8Ω, both channels 96  Silicon Chip The performance of the right channel is almost as good as that of the module by itself, with very low distortion up to 20kHz – see the red and mauve traces in Fig.6. This graph was produced using a wide analyser bandwidth of 20Hz-80kHz, so that it includes the first and second harmonics of the higher frequency test signals. Despite this, distortion is down around 0.001% at 1kHz and below 0.004% at 20kHz. The left channel results are slightly worse for the reasons explained earlier, but still very good. The higher distortion for the left channel with both channels driven is due to the increased magnetic field around the transformer as it delivers nearly twice the current. Channel separation is virtually flat with frequency and insensitive to load impedance at -50dB. Further refinements We made a couple of additional refinements to the amplifier design in order to achieve this level of performance, not described in the previous articles. Both changes reduce the amount of ripple from the power supply that couples into the signal earth. First, we changed the 10Ω 0.25W siliconchip.com.au 0.1 THD+N vs Frequency, 90W, 20Hz-80kHz Bandwidth, 8 04/10/12 14:32:38 +1 04/10/12 14:59:31 0 0.05 Left channel, both driven Left channel, one driven Right channel, both driven Right channel, one driven 0.02 -1 8 4 -2 0.01 Relative Amplitude (dBr) Total Harmonic Distortion + Noise (%) Frequency Response, 10W, 4 & 8both channels identical) 0.005 0.002 0.001 -3 -4 -5 -6 -7 0.0005 -8 0.0002 -9 0.0001 20 50 100 200 500 1k 2k 5k 10k -10 10 20k 20 50 100 200 500 Frequency (Hz) Fig.6: distortion versus frequency into an 8Ω load at 90W per channel. The right channel has lower distortion than the left channel due to its proximity to the mains transformer and the hum/buzz coupling that results. Measurements with a 400Hz high-pass filter show the performance of the two channels is virtually identical if hum is ignored. 0.1 THD+N vs Power, 1kHz, 20Hz-22kHz Bandwidth, 8 04/10/12 14:43:57 0.1 10k THD+N vs Power, 1kHz, 20Hz-22kHz Bandwidth, 4 0.02 Total Harmonic Distortion + Noise (%) Total Harmonic Distortion + Noise (%) 5k 20k 50k 100k 04/10/12 14:49:53 0.05 Left channel, both driven Left channel, one driven Right channel, both driven Right channel, one driven 0.01 0.005 0.002 0.001 0.0005 Left channel, both driven Left channel, one driven Right channel, both driven Right channel, one driven 0.01 0.005 0.002 0.001 0.0005 0.0002 0.0002 0.0001 0.5 2k Fig.7: frequency response of the complete amplifier which is virtually flat from 20Hz to 20kHz. Very little bass roll-off is evident. The high-frequency roll-off is due to the output RLC filter, which is necessary to isolate the amplifier from the speakers and cabling, ensuring stability. As a result, the 4Ω high-frequency roll-off is significantly higher than for 8Ω. 0.05 0.02 1k Frequency (Hz) 1 2 5 10 20 50 100 200 300 0.0001 0.5 1 Fig.8: distortion versus power for 8Ω loads. Again, the right channel is noticeably lower in distortion than the left channel. Note that the power supply limits the available continuous power when driving both channels to around 100W while around 135W can be delivered if a single channel is driven. Music power is about 150W even if both channels are driven. resistor on each amplifier module to 47Ω. This resistor is located to the right of the RCA input socket and connects the signal ground to the power supply ground. If you have already built the modules, it’s simply a matter of clipping off these resistors, removing the lead stubs, clearing the holes with a solder sucker and soldering the new resistors in place. The second change is in the amplifier power supply wiring. While it’s siliconchip.com.au 2 5 10 20 50 100 200 300 Power (W) Power (W) Fig.9: distortion versus power for 4Ω loads. As is typical for power amplifiers, the distortion is somewhat higher when driving 4Ω loads than 8Ω loads, partly due to the increased noise that results from the lower load impedance. The power delivered is higher than for 8Ω, with around 135W per channel available when both are driven and about 200W with a single channel driven. convenient to wire up each amplifier module to its own terminal on the power supply board, this results in a relatively high ground resistance between the two modules. Performance is improved if both are wired to the same supply terminal, with a longer cable running from the left-hand module to the supply terminal on the right side. This requires one 3-wire supply lead to be longer than previously speci- fied, around 150mm. This is then run around the capacitors at the bottom of the power supply module to reach the power connector for the left channel amplifier. Keep it as short as possible and use heavy-duty wire as lower resistance means lower distortion. Twist the leads together before plugging it into the connector on the module. With these changes, your amplifier SC will give the best performance. May 2012  97