Yes had a read a few times now and a very informative post.
Some nooby questions for you John so I am clear what you did, if you please.
A couple of fleas were delivered mid week and I have built one of them up to provide the housekeeping supplies to my ALWSR regs in Avcc duties on my Buffalo II. Even in basic setup, as per Rays supplied components, feeding the ALWSR opamp, ALWSR voltage reference via FET CCS and the ALWSR output stage CCS it was a substantial improvement over the ALWSR and its TPR reg feeding these loads for itself.
Just to be sure, you used the Flea to feed just the AD797 supply terminals of the ALW?
Fitting a single fet current source in place of the 7812 feeding the LED's was an improvement, though not as great as I expected, so this evening, if I can keep off the Vino, I'll be knocking up my own cascode current source for these ref LED's.
John
This was just a jfet with gate and source connected to deliver Idss?
This afternoon I changed the 7812 for a LM1086 regulator as on paper at least this has better line and load regulation. This was worth doing and may be something to consider in place of the 7812 on the final circuit. an extra 3 components though, two resistors and a Tantalum cap. The pin outs of the 78XX series and the LM317/LM1086's are different so doing a layout suitable for either will be a little tricky, but I think worthwhile.
Well if it improves why bother having an option, I am quite sure you have it plugged into a position that puts the spotlight on performance, on the other hand re-jigging for 1086 with a single 7812 option slot is no hardship.
The hardship would be writing a stuffing guide for the amount of options we now have.
Thoughts of others?
While I was at it I made some other changes to things that had disturbed me about the Flea for a while. More lily gliding perhaps, but don't we want to leave no stone unturned here?
Neither the 78XX series nor the LM317/LM1086's like to have a purely capacitive load. The 317/1086's want to see at least a series 2.7 ohm in the load and yet the standard Flea circuit only gives 0.5 ohms here. I put a further 2.2 ohms in series with R2 and R8 (R7 on Tony's design) to correct this. A nice little increase in dynamics and apparent volume resulted (normally a sign that a resonance has been removed IME). There must have been something untoward going on that this extra damping stopped.
I don't like putting two low ESR caps in parallel with each other without additional damping between them, yet this is how the two caps at the AD797 supply pins are connected. To sort this I added 0.5 ohms in series with the 4.7uF cap to damp any possible resonance between stray inductance in that cap and the 100nF ceramic across it. The result was a very slight increase in apparent dynamics, only discernible after extended listening to familiar music. Another way round this would have been to use a high ESR Electrolytic or Tantalum bead as the 4u7 cap, but at least 0.5 ohms of ESR is needed and the ESR of Electrolytics reduces with temperature.
Interesting and something I completely missed. Indeed my first Fleas were stuffed with 4u7 Tants as the 797 bypass, I changed to the films in later versions as Rays BOM specified Films and thought the change was due to improvement. The penny never dropped that I had contradicted what I learned in my TPR developments with Martins help. Mainly a TPR from Acoustica's 3 pin reg notes, boy that was a nice sounding regulator when feeding my SBTouch perhaps I should try it in other positions now my SBT is dead.
I might try the two BJT current source as well now, but I think further improvement is limited to improved layout after that. I will also be getting the scope out to sniff around for anything nasty.
John
I ordered some parts and have a little strip layout for Martins CCS of this thread and may get around to grafting them to the pair off Fleas on the Avcc terminals.
Please post a sketch of your CCS when your sorted.
In the midst of this process I realised that I was getting a better sound now than I really have ever expected possible from my PMC FB1 speakers. There's a depth and realism to the bottom end that I really hadn't expected of them, no matter what amp was driving them. It is quite spectacular at times. It was the switch to using the Flea for housekeeping duties on the ALWSR that really made that significant step up in quality. Two weeks ago I sold my Naim CDS3 after 9 years of ownership due to lack of use and I have no regrets or doubts. The modified Buffalo was already better in all departments IMHO and more so now I have a modified Flea in service.
Yep my CD5 got taken out of the system, it was tweaked a little but absolutely LoFi in comparison to the tricked out Buffaloes.
