heaveno,
I am looking for a good design of a line level preamplifier, gain 15..20 dB. I have built the QSXM2 phono preamplifier and I am very pleased with it's sound. I like symmetrical designs, I like discrete designs. So I was thinking of using the first stage of the QSXM2 as a line level preamplifier.
Is this a good idea? What should be considered if doing this? Any changes to the schematics you wold recommend?
I guess the preamplifier should be able to work with higher level signals, so I was thinking of running it at a higher supply voltage. Is this ok? I guess the servo cannot be powered from, say, +- 30V, so it should have a separate supply. Would it still be working?
I will be glad to read any advise about the idea!
Thanks in advance!
Best Regards,
Angel Sinigersky
Sure you can use the first stage as a line stage and you can also increase the supply voltage but pay attention to how much power each part can dissipate. Opamps have a max voltage, look in the datasheet for this. The transistors max voltage is when the output is max positive/negative mind that. I leave it to you when it comes to adjust things. Try to keep all currents at the present values except for the output stage. Reduce them so the power dissipation will be within the limits. Mind also the feedback network and the power in those resistors.
Per-Anders, thanks for the answer! I am intending to put some mid power driver-transistors at the output, something like BD140/139, or japan transistors similar to these "classics".
Is there a danger for the servo opamp, if I power it at +-15V and the stage is powerd with +-30V? Is there a possibility to avoid the servo? I have built a symmetrical power amplifier from the book of R. Slone, the OptiMOS, and it has inherently a very low offset (1-2 mV without any adjustment, without pairing any transistors). I can't quite understand why, but you write the QSXM2 needs the servos...
Using DC is a matter of taste. Sure you can have a fixed (but trimmable) DC voltage. There is no problem to use lower voltage to the DC servo as long as you don't overload it at low frequencies.
The offset will be a couple of mV times the gain and in this high level application I'll guess offset voltage is not a problem.
A possible advantage to have BC5xx is that the Hfe is > 450 and the speed 300 MHz. BD139/140 have approx. 100 as Hfe.
Thank you once again for the valuable comment! I think I will stick to the BC5xx transistors, and will leave the supply voltage not very high, say +- 24V. I hope the stage will be able to handle 5-10V of output amplitude (this is actually much more than necessary)...
+-24 V is possible and your power amp will surely clip at 1-2 V so there is no need for extreme voltage levels in this respect.