6088 Based Battery Powered Full Feature Preamp Part 3


Posted on Feb 6, 2014

The main amplifying stages are triode connected 6088 stages. As indicated from the previous part, there are two amplifying stages. This provides a non-inverted input to output connection, as well as sufficient gain to incorporate tone control. The tone controls are switch selected so that `flat` bypasses all reactive elements; truely flat response


6088 Based Battery Powered Full Feature Preamp Part 3
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. The overall circuit topology is again shown here for completeness: Notice that the first stage is slightly starved. This provides a gain of about 9. This drives the tone control and balance circuits that introduce a loss of about 7. 2. The second stage is operated at slightly higher current, and has a lower plate resistor, so that the preamp "output" impedance is less than 10k. The second stage has a gain of about 8, so the overall "line" stage has a gain of about 10 (20dB). It will produce about 10VRMS without much distortion. The amplifier section is mounted on a separate board containing all 6 "line" side amplifying tubes. For consistency in any projected modifications to the circuit, I provided a DC ground path and isolated all inputs and outputs. That`s the extra r`s and c`s on the input and output sides of the amplifiers. You don`t always NEED to do this, but it does provide a guaranteed DC return no matter what you connect to it. (The exception is the input to the tape monitor, since the rest of the circuit guarantees the idling potential at this point. ) The "tape monitor" output is isolated. The uses a 6418 (an even smaller device) as a unity gain inverting buffer which has a low output impedance by virtue of the "feedback" localized to that stage. Interestingly, by comparing the sound of the phono run directly from the direct phono output to the phono output run from the tape monitor, you can get a good idea of...




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