Power on. No smoke. But both speaker output terminals had +13VDC offset.
Checking all DC points found out -15V of the full differential OP amp is open. After power off and soldering, everything back to normal.
+SP has an DC offset -2.8mV, -SP -2.6mV, total DC offset at speaker terminals is -0.2mV.
Checking all DC points found out -15V of the full differential OP amp is open. After power off and soldering, everything back to normal.
+SP has an DC offset -2.8mV, -SP -2.6mV, total DC offset at speaker terminals is -0.2mV.
Attachments
Few tests about the amp DC offsets.
1. No ground in the middle of R18 and R8，C46 connnected，Speaker +/- terminals has 15mV offset to GND，but the SP +/- has only 0.2mV offset
2. No ground in the middle of R18 and R8，no C46, Pin 2 short to GND, Speaker +/- terminals has 0.3mV offset to GND, the SP +/- has 0.2mV offset
2. ground in the middle of R18 and R8，roughly same DC offset as 1. But for whole differential operation, 2 is used.
1. No ground in the middle of R18 and R8，C46 connnected，Speaker +/- terminals has 15mV offset to GND，but the SP +/- has only 0.2mV offset
2. No ground in the middle of R18 and R8，no C46, Pin 2 short to GND, Speaker +/- terminals has 0.3mV offset to GND, the SP +/- has 0.2mV offset
2. ground in the middle of R18 and R8，roughly same DC offset as 1. But for whole differential operation, 2 is used.
Attachments
Testing gears:
Power supply: 2xindustrial SMPS 24-29VDC/5A
Signal source: E70 DAC
ADC: E1DA Cosmos ADCiso
Input XLR for P<10W
Aux Input for P >10W
Load: 8 Ohm Resistor
Condition: Open on a work bench
Measurement of E70: XLR output, XLR cable open bare wire end with shield ,
E1DA Cosmos ADCiso: XLR input 10V, XLR cable with alligator clips on the output bare wire ends.
Power supply: 2xindustrial SMPS 24-29VDC/5A
Signal source: E70 DAC
ADC: E1DA Cosmos ADCiso
Input XLR for P<10W
Aux Input for P >10W
Load: 8 Ohm Resistor
Condition: Open on a work bench
Measurement of E70: XLR output, XLR cable open bare wire end with shield ,
E1DA Cosmos ADCiso: XLR input 10V, XLR cable with alligator clips on the output bare wire ends.
Attachments
Figure compares the THD+N with a straight XLR cable from E70 output to E1DA Cosmos ADCiso XLR input 10V. The number is better.
With the comparison, I'd like to show that:
Cutting a straight XLR cable into two and connecting it back by 3 alligator clips can introduce about 10db noise. THD stays almost same.
With the comparison, I'd like to show that:
Cutting a straight XLR cable into two and connecting it back by 3 alligator clips can introduce about 10db noise. THD stays almost same.
Attachments
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The noise performance of the Amp
FFT length1M 8 averge 20-20khz with REW, 8 ohm resistor load.
Fig 218: ADCiso XLR input short noise level
Fig 219: E70 XLR output noise level (direct connect to ADCiso)
Fig 220: Power off noise level of the amp with OPA1642 as the input buffer
Fig 221: Power on noise level of the amp with OPA1642 as the input buffer (input shorted)
Fig 220: Power off noise level of the amp with OPA2891 as the input buffer
Fig 221: Power on noise level of the amp with OPA2891 as the input buffer (input shorted)
From the comparison, it can be seen:
The measuring system has background noise of 0.1uV.
E70 has a noise level of 1uV
The amps have a noise level 5X higher tha E70. A hum of 60Hz(close 10uV) and its harmony can be seen
FFT length1M 8 averge 20-20khz with REW, 8 ohm resistor load.
Fig 218: ADCiso XLR input short noise level
Fig 219: E70 XLR output noise level (direct connect to ADCiso)
Fig 220: Power off noise level of the amp with OPA1642 as the input buffer
Fig 221: Power on noise level of the amp with OPA1642 as the input buffer (input shorted)
Fig 220: Power off noise level of the amp with OPA2891 as the input buffer
Fig 221: Power on noise level of the amp with OPA2891 as the input buffer (input shorted)
From the comparison, it can be seen:
The measuring system has background noise of 0.1uV.
E70 has a noise level of 1uV
The amps have a noise level 5X higher tha E70. A hum of 60Hz(close 10uV) and its harmony can be seen
Attachments
THD+N Performance
The performance exceeded my expectations.
The amplifier gain is set to 10x (20 dB). Clipping occurs at 38.5 Vrms into 8 ohms and at 32 Vrms into 4 ohms. During testing, the power supply (620 VA, 140,000 µF) voltage dropped from ±34.2 VDC at idle to ±28.7 VDC under a 240 W load into 4 ohms (one channel), and to ±30.9 VDC under a 180 W load into 8 ohms (one channel).
The performance exceeded my expectations.
The amplifier gain is set to 10x (20 dB). Clipping occurs at 38.5 Vrms into 8 ohms and at 32 Vrms into 4 ohms. During testing, the power supply (620 VA, 140,000 µF) voltage dropped from ±34.2 VDC at idle to ±28.7 VDC under a 240 W load into 4 ohms (one channel), and to ±30.9 VDC under a 180 W load into 8 ohms (one channel).
- Maximum output power into 8 ohms: 175 W with THD+N of 0.00023%
- Maximum output power into 4 ohms: 240 W with THD+N of 0.00042%
Attachments
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I tested both the OPA1642 (FET input) and OPA2891 (BJT input) op-amps as input buffers. While the overall THD+N is virtually identical for both, the harmonic distortion profiles differ noticeably.
At typical listening levels (1W–10W), the OPA1642 exhibits a dominance of even-order harmonic distortion, resulting in a warmer tonal character. The amplifier, as a result, also produces more even harmonics in this range.
At higher output levels (above 100W), the differences between the two op-amps become negligible, and both perform similarly in terms of harmonic structure.
At typical listening levels (1W–10W), the OPA1642 exhibits a dominance of even-order harmonic distortion, resulting in a warmer tonal character. The amplifier, as a result, also produces more even harmonics in this range.
At higher output levels (above 100W), the differences between the two op-amps become negligible, and both perform similarly in terms of harmonic structure.
