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Sunday, July 24, 2011

The Octal Preamplifier Mk2: Circuit Update


Here is a minor update to the circuit of the MK2 version of the Octal preamp:

This is only a minor change. The high voltage for the phono section is better picked directly from the HV input from the PSU, rather than the linestage decoupling cap. The latter connection can result in some low frequency oscillations, depending on the DC resistance of the chokes in the filter chain. The scheme as pictured above is free from that. Of course the PSU needs to deliver an adequately smoothed supply voltage. I use a LCLC filtered supply with high inductance chokes. The last cap on the PSU output can be left in the PSU chassis, but can also be placed in the preamp section after the PSU input.

The optional bypass caps are left out in this version of the schematic. I don't use them. With the inductance values of the plate chokes as shown, they are not needed. Of course experimentation with this is encouraged!

Best regards


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