Preamp ? Who needs it anyway ?
What is a preamp - a relict of an old era, where the preamp was
equipped with phono stage and tone controls. Today, we purists dont use
tone controls and phono is usually separate, so the pre is just a box
with volume control.
If we have mono amps - at least the pre allows to adjust both channels
at the same time - equally.
But does it serve any other purpose ?
Well, despite the fact that apparently the pre is unnecessary, somehow
- the sound via PRE is better than without it. It does something thats
important.
Pre adds space definition, power, depth, heigtht, slamm, full
timbre - it is important but I dont know why.
That little \something - for some folks is PRICELESS. It is of course
dependent on the source characteristics and amp characteristics.
Anyhow, my best friend needed a pre and he bought all the parts I
prescribed so it was time for me to start building.
The design criteria:
Tubed circuit
balanced topology
remote control
passive mode and active mode at the same time
single ended inputs
Input selection via remote
good looks
BEST SOUND
The list looks easy and innocent BUT IT IS NOT !!!
W imported from Hong Kong the box with the remote controlled stepped
attenuator with 99 steps and best possible Japanese resistors and
relays - the top variant of A-03 from www.DYICLUB.BIZ
I designed an overkill scheme:
Totally dual mono all the way
2 x 100 VA transformers (1000 % overspecified)
Rectification by dual diode tubes 5u4C (500 % overspecified)
4 x tubes ... you guessed right - 6H6Pi (6N6P)
Gold plated ceramic sockets for all 6 tubes
The filters are: C-L-C-L-C with two chokes 40 mH / 2000 Ohm / 20 mA per
channel
first C is a paper in oil 4 uF
Second cap is a Rubycon 200 uF / 400 V
Third cap - is an overkill MKP 100 uF / 400 V. This cap feeds directly
the tubes so I wanted it to be fast and strong - hence NO ELECTROLYTE
but MKP foil. It is 1000 times faster to discharge than any electrolyte.
Total supply under full load is 210 V DC
Since it is balanced design - there are FOUR identical amplification
stages - each is an SRPP - actively loaded triodes. Tues are after the
volume and input selector.
Unfortunately, despite having ultra clean power to anodes, despite
clean heating supply, despite screened signal wires - this unit is
humming. the hum comes from the volume control quadruple relay PCB. It
is unshielded and has long and complex signal handling and it picks up
50 Hz hum.
No matter what I tried - it hums.
What is the solution ?
Solution A is to put the volume section in copper sheet sub chassis.
(very very difficult, because there is so many wiresgoing out, needing
dedicated holes in copper shield. )
Solution B is to change tube config and break it into two stages.
INPUT stage is a voltage amplifier - anode follower triode. Then goes
the volume and after - an OUTPUT STAGE - a cathode follower buffer to
drive the amps.
In this mode - the signal entering the volume ladder would be 22 x
stronger than the source signal so the hum would be proportionally
reduced 22 times.
The load to the volume section would be in megaohms because the load
presented is just second tube grid.
That should not hum but I havent tried it yet.
I will post the results later.
At the moment the PRE sounds already spectacular but the hum is
really disturbing.
The text will appear lTER in March, this is just a teaser to raise
your appetite.