DISPRE - discrete preamplifier without global negative feedback

After 2 years of evaluation of opamp based preamplifiers I decided to design completely discrete circuit, very fast, with high frequency range, and powerful fast output stage. The excellent stability was the design goal as well. I decided not to use global negative feedback and DC stabilizing aids.

The schematics  is as follows:

(click to enlarge)

Gain of both channels is matched by R8, R9. Volume pot is at the input. Q1 and Q2 create emitter coupled input differential pair, its gain is set by R8, R9. Further gain is performed by Q3. Overall voltage gain is 12 dB. Ouput stage is based on diamond buffer with  Q4, Q5, Q6, Q7, Q8, Q9. Its voltage gain is 1x, it has very low output impedance, quiescent current of the output buffer is 20mA.

Component values

Here you can see schematics with component values.

Parameters

Frequency range (-3 dB)                 7 Hz - 3 MHz
Rise time of step response               130 ns  approx
Output voltage swing                       24Vp-p
Output impedance                           < 3 Ohm

noise background / related to 1Vrms     (influenced by measuring card)

THD spectrum of 6kHz    (loop-back method, some residuals affected by measuring card)

Construction

PCB can be seen here:

(click to enlarge)

Sound

Open, dynamic, with very good detailed midrange.

PCBs available on request.

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Pavel Macura 17.10. 2005