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The idea behind this unit is to remove the sub-energy from the main amplifiers and loudspeakers.  For a given voltage level, the lower the frequency the greater the magnetic flux in the signal transformers.  So rather than implementing  classic bi-amplification, I felt that it would be more effective sonically to remove the sub frequencies entirely from the main amplifiers and let the sub-woofer plate amplifier handle them.  Similarly, for a given sound pressure level, the lower the frequency the greater the loudspeaker cone displacement.  Consequently the sub frequencies cause doppler distortion, therefore there should be a real benefit in removing the sub energy from the main loudspeakers.  I was astonished when I first tried this unit.  There were clearly audible improvements in resolution and clarity but the biggest gains came with improvements to the sound stage, especially the tricky aspect of sound stage depth.

The topology uses input transformers (Lundahl 1540) connected 1:1.  Each secondary feeds a 2A3 grid.  The 2A3s plates are choke fed with a potential divider across each choke to B+,  the tap on each potential divider driving a 2nd order CL high pass filter, the final passband gain being around 6dB.  The corner frequency of the high pass filters is around 50Hz.  The series element of the potential divider appears in series with the plate resistance.  Thus increase of Rp becomes a fraction of a larger resistance thereby reducing the effect of tube aging on the filter corner frequency.  The shunt element further helps to swamp out the effects of tube aging since it appears in parallel with the plate and series resistance, thereby restoring the source impedance from the 2A3 output circuit to around 1kohm.  The math looks like:

No PD, Source Z = Rp  = 800.  If we say that Rp increases by 50ohms, we have the  % change increase in source resistance:

100 x (850 - 800) / 800 : % increase in source resistance equals 6.25%

The potential divider series resistor is 1.2k and the shunt resistor is 2.2k, so for a new tube where Rp = 800, the source resistance is equal to:

(800+1200) x 2200 / (800+1200+2200) = 1045ohms

If Rp increases by 50 ohms we have:

(850+1200) x 2200 / (850+1200+2200) = 1061ohms:

% increase in source resistance 100 x (1061 - 1045) / 1045 = 1.5%

This is a significant improvement at a cost of 3.8dB gain.  I can accept this loss since I do not need the full gain the 2A3 can provide.

The filters terminate in a 6k resistor for an aggregate output impedance around 860ohms.  I wound my own chokes on junkbox ferrite cored bobbins.  As with air cored coils, there is a critical trade between inductance and DCR.  Initially, I wound up with 0.8H.  Later, I added some external iron magnetic circuit elements¹ which increased the inductance to 2.1H.  To minimize the crossover region loss, I arranged that the filter characteristic is close to Butterworth (flattest frequency response).  I thought about going with a Bessel response (flattest phase response) but decided against it, having experienced some crossover region loss while experimenting with the concept. 

The sub channels are buffered by separate emitter followers that have no filter, relying on the low pass filter on the sub woofer plate amplifier².  A nice benefit of using the 1540 transformers is that the units can accept  both balanced and unbalanced inputs.  (I use the XLR outputs from my Meridian 508-20 CD player also my DV563a player has XLR outputs.)  Volume control is accomplished using a 12k series resistance in the 'live' leg of the transformer secondary winding feeding a variable shunt (Alps potentiometer).  I used this technique previously on various projects and have found it to be superbly transparent.  Using this technique enabled me to incorporate the remote volume control using a kit available from Mikkel Simonsen mcs@post5.tele.dk  He sells kits including all parts for the receiver/driver (it can do power up and mute as well as volume), a nice instruction booklet and the motorised pot for a very reasonable price.  I am using a 'One For All' universal remote transmitter (UCR-4640B00) programmed with Phillips code, 0054 (TV, VCR).

I used 2A3s because I wanted to use a filamentary triode and needed a very low anode resistance.  300Bs would also be suitable.  The 2A3s are microphonic, but I can live with that.  I will get some nice fat silicone O rings one of these days!

The high pass filter chokes are home made, wound on ferrite cores for an inductance of 0.8H.  I installed them into a copper box covered in self-adhesive mu-metal.

¹There is a pocket of around 3/16" between each end of the cores and the walls of the box.  I put a 1/16 build of EI lams (robbed from a wall wart transformer)
in each pocket and wedged them hard against the ends of the ferrites using thin rubber. 

Click to see the line stage schematic:

In line with what has become my standard practice for the heating of filamentary tubes, I use filament feed chokes and a current regulated supply.  In this case, being a line stage and especially given that the volume control is on the input, filament supply noise suppression is critical.  In a departure from my normal practice, to further reject line and rectification noise, I have incorporated voltage pre-regulators to the current regulated filament supplies

Click to see power supply schematic:

Not so clear picture showing the receiver board (which can be powered from 6.3V ac) and Alps pot.  I checked the HF radiated from the unit, it appears to be minimal so I did not bother with shielding the unit.

Picture showing the receiver located in the unit base plate, (just to the right of the control bracket and below the board).

²Sub Woofer Channel Summation:  Originally, I had the two sub channels connected to the two line inputs on the sub-woofer.  While fooling around with the line amps off, I noticed a kind of 'cracking' distortion coming from the sub, akin to slew limiting on transients.  I tried mixing the two sub outputs from the line stage using equal value resistors and feeding the result to one input on the sub-woofer, no better.  Thinking (well perhaps not your actual thinking, more like muddling) about this, I came up with the notion that the two channels may contain fairly similar sub-bass signal content but at very different levels and also out-of-phase.  So, I wondered if mixing the two signals in parallel was causing some form of destructive interference.  What I wanted to do was to actually SUM the signals, not mix (muddle) them in parallel.  What was needed was to feed each sub signal to separate transformers so as to isolate them from one another and then sum the signals by connecting the transformer secondaries in series.  Since the sub-woofer has a plate amplifier and the sub paths in the line stage have less than unity gain, I tried using of all things, a (then redundant) pair of Lundahl LL9206 cartridge transformers, connected 2.5:1.  The line stage sub outputs go to each "primary" while the secondaries are connected in series.  Eureka! (Well I guess you may not.)  No more cracking.