The second reason for which separate potentiometers are nowadays so in fashion is that they make a balance control not necessary.
Balance can be necessary in a tube system for two reasons, either unmatched channel gain or unmatched sound diffusion in the environment.
Well, independently from the cause, suppose you need a balance control and take into account the situation stated above: think how easy it would be to move the two separate knobs so that the relative difference in the level of the two knobs remains the same; note that volume potentiometers are logarithmic, so that the relative position of the two knobs is very difficult to correlate to the desired level difference.
The only way to set the correct level is listening to the loudspeaker level, which could be really annoying.... and even misleading, if you do not perfectly know the music you are playing.
If you want to solve output level mismatching problems and live happily with separate potentiometer, you must use perfectly matched tubes and potentiometers build up of a precision, multi-position selector and a number of precision resistors.
Estimated cost for such a good potentiometer: a few hundred dollars... (and a very accurate tube selection is to be added...). There are low cost (around 60$) solutions, but they have a number of levels far lower than I think to be necessary.
I did not want to use high cost solutions and I did not want to use matched tubes but I needed an almost continuous control, even at low levels, and I wanted to have no problem in rearranging the output level. So I had to use two potentiometers, one for the volume and one for balance.
I decided to stick to double potentiometers: very low cost ones, by the way, as I have got to think that a simple carbon potentiometer sounds much better than expensive film ones. And I am not the one.
As low cost carbon potentiometers suffer from dust, then I closed it up completely in Teflon tape (now it seems the mummy of a potentiometer... but it works and has not yet started crackling: they especially dislike wood powder... and in a wooden case there always is a little (euphemism) wooden powder...
Anyway I did not place any balance control on the front panel. Actually, if it is necessary to alter a correct channel balance in order to let the user perceive the same apparent level from the two loudspeakers, this means that the environmental situation is so bad that a pre of this level would be widely excessive, or better, totally wasted... for example, in a certain position in my listening room there is an apparent difference of the two channel, instrumentally balanced, of about 2dB, but even if you make the apparent level perfectly balanced, the soundstage image in that position is not perfect.
Far better to force the user to solve his own environmental problem, than to give him an incomplete, far from correct solution...
Actually if you look at the MW Pre 01 cabinet you can see only two knobs, one for the volume and one for the input selector. But on the contrary I think that a balance control is absolutely necessary in tube amplification. An the balance control then?
The balance is only an internal set point. The line balance control has the same configuration as the SIMPREs one: it is a variable resistor placed in series to the volume control. But it's accessible only from inside the box: It is supposed to be used only to arrange for the same gain in both line stage channels, typically after tubes substitution. The balance control setting must be accomplished via instrumental check of the output level of each channel in front of the same line input signal.
Nothing obviously oblige you to choose the same solution: there would not be any problem in connecting the balance control to a knob on the panel, apart a further crowding up of the interior of the box, which is very far from empty...
Anyway, I think this to be a really correct approach to the problem. Actually, complete absence of any balance control would require a perfectly matched pair of tubes to be used: I admit that all couples of ECC88 I have tested have a very good matching (even though only a very rough matching had been granted by the reseller and only in a few cases) and that even between two ECC88 tubes of different type there is normally a reasonable matching.
But in this way I will be able to use any couple of tubes I could buy with a perfect output level matching, sparing money on supposed and far from safe tube matching procedures.
Here too nothing oblige you even to set up a balance control: I explained the reason of its presence, you can judge if it really necessary to you. Even with matched tubes, anyway, I prefer to have a perfect balance, and I know I can perceive even low differences.
One possible, very elegant solution could be to use separate potentiometers as dimmers at line inputs, as said before. These potentiometers could effectively substitute the balance control, but would work on a low level signal, while the balance control, placed at the output of line stage, works on a higher level signal, and attenuate it only slightly. Take also into account that separate instrumental setting of each potentiometer would be required at any change of line tubes.
Well, just to make things complete, obviously there is a matching problem even between the two halves of the phono stage tubes... and even here the solution is the same: in this case not a real balance control, but two separate potentiometers each one controlling the level of the signal sent to input selector.
Yes, I know, too many metal-to-carbon contacts points in signal path. If you prefer, you can get rid of the potentiometers and either buy perfectly matched pairs of tubes or solder a couple of fixed resistors instead of potentiometers and change them according to new tubes gain, whenever you change the tubes. I find both solutions far too annoying for lazy people like me....
By the way, at the end I did non mount anything at all... not even the potentiometers: the balance was just good enough without them.
BEFORE DESCRIBING YOU THE STRUCTURE OF THE CABINET, I WANT TO MAKE IT CLEAR THAT THE STRUCTURE IS DEFINITELY UNCONVENTIONAL, FROM MORE THAN ONE POINT OF VIEW CAN BE DIFFICULT TO MANAGE AND CAN CAUSE BIG PROBLEMS AND DAMAGES IF NOT CAREFULLY HANDLED.
IT IS MADE OF AT LEAST THREE LOOSELY CONNECTED PARTS, EACH ONE WITH SUCH A WEIGHT AS TO BE REALLY DANGEROUS FOR BOTH PEOPLE, ESPECIALLY CHILDREN, AND THEIR PROPERTIES, IF UNPROPERLY HANDLED.
IF YOU FOLLOW THE GIVEN INSTRUCTION YOU MUST BE ABSOLUTELY SURE THAT NEITHER YOU NOR ANYONE ELSE WILL NEVER LIFT THE BOX WITHOUT HANDLING IT FROM THE BOTTOM, AS THE CONNECTION OF SIDE WALLS TO THE BOTTOM IS NOT ABLE TO SUSTAIN IT.
I REPEAT, THIS CAN MAKE THE OBJECT EXTREMELY DANGEROUS IF HANDLED BY PEOPLE WHO DO NOT KNOW ITS PECULIAR FEATURES: IF ONE OF THE 10KG SLAB GETS LOOSE FROM THE BOTTOM OR FALLS DOWN FROM THE TOP IT CAN BECOME EXTREMELY DANGEROUS.
TAKE ALSO INTO ACCOUNT THAT THE WEIGHT OF THE SLABS IS NOT SO HIGH AS TO MAKE IMPOSSIBLE FOR A CHILD TO MOVE IT, BUT IT IS TOO HIGH FOR A CHILD TO BE ABLE TO HANDLE IT CORRECTLY.
SO THE CABINET CAN REALLY BECOME A SERIOUS, AND EVEN MORTAL, DANGER IF A CHILD CAN HAVE UNCONTROLLED ACCESS TO IT. MOREOVER IF THE CABINET IS NOT PROPERLY CLOSED, THERE ALSO IS A VERY SERIOUS RISK OF ELECTROCUTION.
IN ANY CASE BUILDING AND HANDLING THE CABINET IS COMPLETELY AND TOTALLY LEFT AT YOUR OWN RESPONSIBILITY AND AT YOUR OWN RISK.
In order to achieve best circuit insulation from environment, a very special cabinet has been designed. The cabinet has really high-mass, vibration-proof top and bottom, and the side panels, even though less insensitive, are almost totally detached from inner circuitry.
The dimensions of the box, not small at all (43cm wide, 30cm deep and 13cm high), are just another problem to face: the structure must be rigid, and larger the dimension, more difficult to achieve a good rigidity.
To solve the problem, top and bottom are two black granite slabs two centimetres thick and about 8kg each; the sides are two-cm thick wood, in two layers, each layer made up of a few separate long blocks glued together.
Note that the pre is about 20kg in weight: it is really very difficult to move in or out of a typical hi-fi rack.
The side panels have been accurately set up and tested for best isolation. I set up a very simple and ultra low cost isolation test system to detect the resonance critical frequencies of the box and reduce them as far as possible.
I used a CD player with a test disk as a white noise and sweep generator, amplified the signal through my hi-fi amp and loudspeaker in order to "stimulate" the box walls, detected... "frequency response" and resonance frequencies of the box with a ultra low cost microphone placed inside the box and plotted them with a very simple PC spectrum analyser software using a preinstalled sound card... Less expensive than this...
Obviously no absolute measure could take place with such a low cost measurement system, but just comparing energy distribution reaching the microphone in different situations I found out a few things I wouldn't believe.
A few of the problems that has been detected are absolutely general, so I think it useful to report them to you (well, quite a few are absolutely obvious... at least after you have detected or read about them...)
© Copyright 1998 Giorgio Pozzoli for TNT-Audio, http://www.tnt-audio.com
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