Electronics

Thomas PA - Performance

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Specifications

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• Circuit Type: push-pull, class AB, pure pentode
• Frequency Response: 15 Hz - 58 kHz +/- 0.4 dB
• Input Sensitivity: 16.6 dBV at 2 Vrms
• Input Impedance: 680 Kohms
• Output Power: 8 W per channel at cutoff (2.3 Vrms input)
• Tubes: 1 x 5U4GB, 1 x OA2, 1 x 12AX7, 1 x 12AU7, 4 x 6V6GT
• Power Consumption: 85 V-A at full output

Below you can see the Bode plots for the left and right channels with the amp running at 2.0Vrms at the inputs (about 85% full output).  The solid green line is the output response in dB and the dotted green line is the phaseObviously, a low pass filter would be a good idea to roll-off the response above 20kHz and eliminate the peak around 50kHz.  I haven't probed more closely to isolate the dip around 6.8kHz, but I suspect it has something to do with the feedback circuit.  I was suprised at the good low level response, given the physical size of the output transformers.  Again, this is probably due, at least in part, to the global negative feedback.

Left Channel (1dB/div)

Right Channel (1dB/div)