Ver. 0.1 - ©Phædrus Audio Ltd. 2011 All rights reserved.
Chapter 6 - Circuit descriptions
PHILTER
The Phædrus Audi o PHAB and the PHAME preampli fi ers and the PHILTER equali ser all share
roughly the same preampli fi er ci rcui t.
The ampli fi er block i s a tw o-stage desi gn i ncorporati ng an EF86 low -noi se pentode i n the fi rst
stage and a paralleled ECC88 hi gh mutual-conductance double-tri ode as the output stage. All the
landmark ampli fi er desi gns fr om Mullard's appli cati on laboratori es speci fi ed the EF86. In fact,
Mullard descri bed the EF86 as
a low noi se pentode i ntended for use as a RC coupled AF vol tage
ampli fi ers; parti cularly i n the early stages of hi gh gai n audi o ampli fi ers, mi crophone preampli fi ers
and magneti c tape r ecor ders.
The output s tage compri ses a dual tri ode valve i n w hi ch the tw o devi ces are wi red i n parallel. Thi s
paralleli ng of the tw o tri odes has the benefi ci al effects of doubli ng the mutual conductance and
halvi ng the anode resi stance. The latter characteri sti c is parti cularly valuable i n ensuri ng a low
i mpedance dri ve to the output trans former.
The output s tage di rectly feeds the output step-dow n transformer. The output i s balanced and
earth free. A modest degree of negati ve feedback i s taken from the anode of the output valve(s)
to the cathode ci rcui t of the i nput s tage to stabi li se gai n, extend frequency-res ponse and low er
di storti on. Thi s feedback resi stor i s selected to gi ve di ffer ent gai ns dependi ng on the par ti cular
product.
Performance
At recordi ng i ndustry li ne-up (0VV=+4dBu) the
magni tude of all di storti on components on a 1kH z si ne
w ave i nput are less then -55dB below fundam ental.
Thi s, and the A-w ei ghted noi se response relati ve to
maximum output, i s i llustrated below .
