DIY HiFi Supply ABS
Active Bias Supply Module












Active Bias Supply


Benefits: saturation of the output transformer is avoided, amplifier always operates optimally, better bass performance, more dynamics, danger of runaway tube current when amplifier is unattended is minimized, tube life is extended.
Also since most bias voltage supply filter networks tend to be very simple single diode or half wave, there is usually a great deal of ripple in the most sensitive signal path in the amplifier (the input grid of the output tube) An active bias system is an opportunity to design a bias supply with extremely low levels of ripple. The result is a sense of stillness and low-level detail not obtainable with standard bias voltage supplies.

Compact 4 x 9cm. easy to place in chassis.


Application notes (pdf)


The ABS eliminates the need for constant re-adjustment of bias to compensate for mains variations and valve aging. Regulation of cathode current of tubes from around 1mA to 250mA with a 10R cathode resistor. Current match between tubes depends on cathode resistor tolerance. With ± 1% resistorss typical mismatch << 0.1mA.
Perfect current balance produces better bass.

Current regulation is not influenced by music signal. Current set by standard 1/4W leaded resistor (exchangeable). Current can be adjusted from 0mA to 90mA per tube if a 10K Pot is connected instead of the resistor.

Reduces noise on bias supply by at least 40db (100 times). Lower noise improves resolution and imaging.

Turn on delay of 45 seconds, slow ramp up of current over a further 45 seconds.

Fault indication via LED's (Red) on PCB (these can be re-mounted on the amplifier chassis) Fault Indication via TTL compatible open collector output (can be used for additional protection circuitry or for our upcoming electronic choke PSU PCB). .

Can be fitted to any existing fixed bias equipped amplifier without changing the original circuit and with absolutely minimal modifications, except bias potentiometers (and possibly cathode resistor changes)

Multiple mounting holes for easy mounting. Small size due to use of existing supplies already present in your amplifier. No high voltage connections are required to be made.



 
Active Bias Supply Module.
for 2 tubes
€ 76.90
ex.VAT
€ 91.50
incl.VAT




Active Bias Supply Module.
for 4 tubes
€ 144.95
ex.VAT
€ 172.50
incl.VAT
pair







 

 

 

 

 

 

 

 

 

 

 

 

 

 

 





 


Experiences from customers:

Ella mods: Active Bias System
Posted by Goneill (A) on June 15, 2008 at 20:28:09

Originally I wasn't very interested in adding a digital circuit to my Ella but Brian convinced me that he had made sure no digital noise would affect the sound so I changed my mind.

The first job was to change a resistor on each ABS board. It comes with a 3K3 which sets the bias to 40mA but with KT88's I wanted 50mA. A table of values included in the instructions says that means I'd need a 4K3. The problem is that it would then be fixed, but if I ever changed back to EL34 (or 6550) I'd have to swap the resistor again. So I decided to use one of the existing bias pots in parallel with the resistor and wanted a range of 0mA to 60mA, which the table said would need a resistance range from 0R to 5K6. With a 22K pot in parallel that meant using a 7K5 resistor, because (1/22) + (1/7.5) = (1/5.6)

Then I had to make some space for the boards, which meant moving the bias bridge rectifier, removing the additional bias capacitors I'd added (which probably aren't needed now due to the extra filtering in the ABS) and rerouting the main A/C supply wires.

After that it was easy to mount the boards on L-shaped brackets off the tube sockets. I would have preferred to have points to solder the wires to on the boards instead of the screw fittings but I suppose if you use a fixed resistor instead of a parallel pot then you want the board as easy to remove as possible so you can get to the resistor to change it. And it would be nice if the LEDs on the boards could be moved so they were chassis mounted but I suppose that would make the manufacture of the boards much harder.

In use the bias goes to the value set and stays there regardless of the amp temperature and it does keep the pairs of tubes nicely balanced. Because I had already changed the bias diode to a schottky rectifier the improvement in sound wasn't as big as it would have been for a standard Ella, but there was still an improvement in details and bass. On a standard Ella the improvement would be huge.

Reply to: RE: Ella mods: Active Bias System posted by Goneill on June 15, 2008 at 20:28:09


I guess with all those surface mount active devices one could easily suspect one amongst them must be a digital Devil -- but the ABS is all analog, all the time.
It's encouraging to read that despite the modifications you have already made over the stock bias supply, the ABS still gave you some improvement in sound. since the ABS eliminates DC current through the transformer core, the improved bass response to noticed is to be expected. Probably the bias voltage we apply to the grid is one of the most overlooked factors in our quest to squeeze ultimate performance from our amplifiers.
thanks for the write-up, and your well drawn out diagrams.
Brian

 

Posted by bcherry (M) on June 17, 2008 at 17:36:57
Reply to: RE: Ella mods: Active Bias System posted by Goneill on June 15, 2008 at 20:28:09


yesterday we received our first new-style Ella with the ABS installed. The ABS locks down the bias and it's nice to see whether cold/hot/in between, all tube pairs are 100% balanced. since it only takes one or 2 milliamps of imbalance to seriously impact on the performance of the transformer (push pull Transformers have no air gap and cannot tolerate DC current), we now enjoy the full potential of the amplifier at all times.
a little more:
The fixed resistor on the ABS is intentional to make installation as simple and easy as possible, just as the screw terminals are, as well as the very concept.

The concept:

Take an existing fixed bias amp, select the correct resistor for it, fit with minimal changes and forget about bias. Very result oriented. No mucking about.

For those that dislike screw terminals and absolutely want to solder, the screw terminals are easily unsoldered. However these connections have absolutely minimal impact on sonics (unlike most connectors in amplifiers and tons less than tube sockets) and most people can strip a wire and place it in a screw terminal.

Equally, it is possible to desolder the LED’s and replace each by external LED’s on long wires, but with already 8 or 9 wires to each PCB, adding another 8 would have been overkill for most DIY’ers, so simplicity meant putting the LED’s on the PCB. Anyway, you only have to worry if they stay red and in future the matching PSU board will take care to shut the amp down and light a “sumpting wrong” light in case of red-light, so, no worries.

If using a Pot & Resistor combo to adjust the Bias it is recommended to take the two holes on the PCB holding the resistor to attach the wires in the usual fashion and wire the resistor directly to the adjustable resistor terminals.

In addition it may be worthwhile considering a series resistor to apply a lower limit, as adjusting the Bias to 0mA usually has little value.

Eg. in Goneill’s case: use an adjustment range of 30 – 60mA, using a 2K4 (2.4k) resistor in series with the combination of the 22k adjustable resistor paralleled with a fixed 3K9 (3.9k), which allows an extremely precise adjustment of the current over the sensible usable range, with the lowest resistance 2.4KOhm (I = 29mA) to 5.7KOhm (I = 60mA).

Again, for simplicity all this was not put on the PCB, as there are so many tubes and possible ranges that it is hard to come up with a single setup that fits all and is flexible. The worst case could have been two adjustable resistors on the PCB (to set lower and upper limits or current without external R) added to the (optional) external pot.

For the more advanced DIY’er the needed changes are not difficult, for others the ability to have a simple resistor take of all might be godsend.

Finally, yes, it is all analog. One can make a computer in a digital way or an analog way.

Before Russia gave in and copied IBM’s digital computers (in the 1960, before IC’s were invented) they used to do cutting edge analog and hybrid (as in analog for speed, digital add on for final precision) computers, that had impressive performance.

The ABS uses basically analog computing circuits based on this sadly forgotten technique, using the “always just right” paradigm compared to the digital principle of “always around 1/2LSB wrong”… ;-)

regards
Engineering Resource Team
DIYHFS





ABS tames wild Convergent Audio Technology JL-2



The story is simple. The amplifier had blown several tubes with subsequent damage and in the process of a thorough overhaul and repair the ABS was fitted. The 1R resistors in CAT cathode circuits were changed for 10 ohm and the Meter was re-adjusted to read correctly. This done, the usual ABS behavior cuts in, the bias is ramped up slowly and stays rock-steady afterwards, with no mismatch between halves.