Thomas – Performance
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Specifications
- Circuit Type: push-pull, class AB, pure pentode
- Frequency Response: 15 Hz – 58 kHz +/- 0.4 dB (see the Bode plots)
- 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.