Preamplifier struggle





February 2010

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.
















































































































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