john.luckins
pfm Member
I just thought I would share my ideas and findings on this great little circuit. In its standard form the B4 has replaced the Starfish in my system. I think as I age I’m tiring of the Naim preamp circuit sound and moving from the snappy and up front presentation to something a little more “pipe and slippers”. I’m very happy with the B4, fed by a CDS3 and into NAP135 clones. To me it is a clear upgrade from my old NAC 52 as well but better than a passive pre.
Now I’ve read a great deal about how good the B4 PSU rejection is and concur with the figures given. However I can easily hear the changes made when I go from a Toroid to an R core transformer driving my B4, so I know there must be interaction between the supply and signal here that matters and isn’t insignificant to the sound (for me).
Very specifically I have found that I much prefer the B4 when hooked up to a large (432VA) R Core Transformer compared to a very nice 500VA screened Canterbury Windings Toroid. I struggled to admit this as I had bought the Toroid specifically to use with my final B4. It is really nicely built by Terry but the R core is somehow better sounding, particularly in the bass.
Separating supplies
I have now got a completely separate supply for the constant current sources D2 and D4 that drive the LED’s and the cascode/bootstrap. I got an improvement by using a Cap multiplier (PFM gyrator) off a separate transformer and CLC filter rectifier.
Cascoding the constant current diodes
After a bit of googling I found a useful paper by Walter Jung on Constant Current sources – here
http://waltjung.org/PDFs/Sources_101_P1.pdf
Figure 9A shows how a constant current diode can be given improved rejection at low frequencies by being cascoded by just one jfet. I did this on D2 and D4 using 2N5486 devices and could hear an improvement to the sound. Two devices per channel can give the B4 a 10 to 20dB improvement in PSU rejection in the audio range. I’d recommend you give it a try, its any easy and reversible change and mines in for keeps. I got my 2N5486’s from ebay.
http://s869.photobucket.com/albums/ab257/John_Luckins/?action=view¤t=CCSCascode.jpg
I’m considering cascoding the other two CC diodes (D1 and D3) to see if the effect is also audible here, but I will need to add extra LED’s into the bootstrap/cascode to give enough voltage for it to work. This makes me a bit wary as the extra LEDs will slightly degrade things elsewhere – but hopefully not by too much.
Input/output caps
After a lot of experimentation I’m using a 100kohm input resistor and 0.47uf input capacitor. It is a Russian Teflon. Big, but only about £6 and a really obvious improvement over polystyrene and polypropelenes. On the output I managed to track down a couple of 3.3uF Teflon/Oil caps. All these caps have excellent clarity and balance. The difference they make is not marginal.
Shunt regulators
To finish off I’m now building separate Shunt regulator supplies to replace the LM317/337 regs and to feed D2 and D4. That means 8 separate regulators on a separate board. I’m using the latest version of the Salas Shunt regulator circuit form DIY audio forum. One extreme option would be to feed each of these regulators from separate transformers or windings. I’m considering rewinding some old R core transformers myself to do just this at low cost.
Thereafter a good case, a remote controlled Lightspeed Attenuator, careful layout and maybe screening of the transformers will see me to the end of my search for the best preamp I can put together.
Of course I'm interested in what other people have done to their B4, chances are I've missed a trick or three. Don't be shy.
John
Now I’ve read a great deal about how good the B4 PSU rejection is and concur with the figures given. However I can easily hear the changes made when I go from a Toroid to an R core transformer driving my B4, so I know there must be interaction between the supply and signal here that matters and isn’t insignificant to the sound (for me).
Very specifically I have found that I much prefer the B4 when hooked up to a large (432VA) R Core Transformer compared to a very nice 500VA screened Canterbury Windings Toroid. I struggled to admit this as I had bought the Toroid specifically to use with my final B4. It is really nicely built by Terry but the R core is somehow better sounding, particularly in the bass.
Separating supplies
I have now got a completely separate supply for the constant current sources D2 and D4 that drive the LED’s and the cascode/bootstrap. I got an improvement by using a Cap multiplier (PFM gyrator) off a separate transformer and CLC filter rectifier.
Cascoding the constant current diodes
After a bit of googling I found a useful paper by Walter Jung on Constant Current sources – here
http://waltjung.org/PDFs/Sources_101_P1.pdf
Figure 9A shows how a constant current diode can be given improved rejection at low frequencies by being cascoded by just one jfet. I did this on D2 and D4 using 2N5486 devices and could hear an improvement to the sound. Two devices per channel can give the B4 a 10 to 20dB improvement in PSU rejection in the audio range. I’d recommend you give it a try, its any easy and reversible change and mines in for keeps. I got my 2N5486’s from ebay.
http://s869.photobucket.com/albums/ab257/John_Luckins/?action=view¤t=CCSCascode.jpg
I’m considering cascoding the other two CC diodes (D1 and D3) to see if the effect is also audible here, but I will need to add extra LED’s into the bootstrap/cascode to give enough voltage for it to work. This makes me a bit wary as the extra LEDs will slightly degrade things elsewhere – but hopefully not by too much.
Input/output caps
After a lot of experimentation I’m using a 100kohm input resistor and 0.47uf input capacitor. It is a Russian Teflon. Big, but only about £6 and a really obvious improvement over polystyrene and polypropelenes. On the output I managed to track down a couple of 3.3uF Teflon/Oil caps. All these caps have excellent clarity and balance. The difference they make is not marginal.
Shunt regulators
To finish off I’m now building separate Shunt regulator supplies to replace the LM317/337 regs and to feed D2 and D4. That means 8 separate regulators on a separate board. I’m using the latest version of the Salas Shunt regulator circuit form DIY audio forum. One extreme option would be to feed each of these regulators from separate transformers or windings. I’m considering rewinding some old R core transformers myself to do just this at low cost.
Thereafter a good case, a remote controlled Lightspeed Attenuator, careful layout and maybe screening of the transformers will see me to the end of my search for the best preamp I can put together.
Of course I'm interested in what other people have done to their B4, chances are I've missed a trick or three. Don't be shy.
John