Legato Tweakers Thread...
 

I figured I would start a thread for those interested in tuning/tweaking the parts selection on the Legato to their preferences. No disrespect to Russ and Brian's work here, as their parts selection is very good, but of course one of the pleasures of kit DIY is being able to select different parts to suit individual tastes. Here is what I am thinking about so far:
Output coupling caps: 4.7 uF, Zen polypropylene (going into 11 kohms per phase minimum) leads from these become output wiring.
Power decoupling caps: maybe upgrade to Elna Silmic II
Signal path resistors: perhaps Vishay Z-Foils ($ though...), or PRP ( my fav affordable resistors)
What about the analog filter stage caps? Any ideas for improvments here?
Would love to hear other's thoughts on tweaks.

OK, some...
 

folks have their Legatos now, lets hear some news. Here is what I am up to:
Balanced only, going to run from a single Placid BP. After having a look at the schematic I decided to eliminate as many steel leaded parts as I could, and do some other parts changes:
R5-8 will be "naked" Z-foils, the other .25 Watt resistors will be PRP. Caps, C9,10,13-15 will be Elna Silmic II (yes, one must get creative to fit them). C19-22 will be Amtrans. After searching for output coupling caps that will fit neatly in my chassis design, and (hopefully) be quite transparent, I have selected Mundorf Zn, 2.7 UF 100 V film/foil caps. The Zns are not cheap, but they cost nothing compared to a good interconnect. I am jumping the board cap mounts, and will wire the Zns to the terminal outputs and then directly to the XLRs.
I still have not decided if I am going to use the buffer stage or not. Russ advises not to with my partnering gear, and I know from a numbers perspective he is right, but I also like the idea of super low output impedance. Anyone have any feedback on whether or not the buffer stage is a good idea?

wonder how this would go with +/-9vdc, i'm running battery power in a portable buff II and my battery pack is about 23v, with the LDOs im using i'm down to a stable 8.8vdc per phase. at the top of charge its 23v p2p, but at the bottom its 18.3 (6 x 3v (per cell) is drained, but I cut off at 3.1v)

yeah the greenies will be the first to go, all small films will be evox/rifa and i'll probably run some VCAP CUTF as bypass caps in parallel to nichicon KZ or silmic II. at the moment i'm using a OPA1632 based opamp stage (not IVY) but would love to have a portable discrete IV, i'll use balanced only, but will need the buffer as i'm driving low impedance headphones directly. not that fussed about the AC coupling after getting so used to DC coupled sound. Maybe I could use a servo instead.

is there any benefit to using a pricier grade of BC550 for higher HFE in this circuit?

qusp,

It will work just fine with 9V rails. There are a couple changes you would want to make:

Use a BC560 instead of BS250. Change R5-R8 to 392R. Done! :)


HFE is not that critical in the application because of the CFP. Still it won't hurt anything at all to use higher HFE. Give it whirl. just keep in mind, there is a filter cap there that will swamp most of the effect of the higher HFE. It is much better to have lower noise than high HFE in this application.

I fully expected some would use external caps. Thats really the only reasonable way to use film caps that large. Otherwise you make a PCB that is taken up completely with a few caps, not a great solution.

Cheers!
Russ

thanks Russ, OK well, be expecting an order for PCBs from me shortly then. i've also been playing with burson discrete opamps (duals running 'balanced') but prefer a fully differential stage, the balanced opamp IV is really nice and due to the nature of good opamps with well matched components, DC coupling isnt such an issue.

nice to know I can leave the high HFE rated parts, because I can buy 100 of the one in the BOM and match them well, for less than 14 of the higher rated parts would cost, then I would have some left for other projects. I think I already have some 560 and 392R here, so thats handy.

the CuTF are only 0.1uf, so arent huge, i'll see how they go anyway, I may just end up bypassing the silmics with SMD PPS films under the board.

qusp,

If you wanna get tricky, i might throw in the idea of using an assymetrical power supply for your battery powered legato. The negative rail doesn't really do much other than set the biasing from what i can see.

If you went with a 4 cell + 2 cell setup (+12V / -6V) you should be able to get much better performance than a 9/9. You'd just need to change the biasing resistors to suit the lower voltage across them.

It wouldn't be that great if you still wanted to use the ballsie, or the 9/9 for something else though.

Maybe there's a good reason not to do this, but i can't see one off the top of my head.

Any opinions on Black Gate non-polar type N? I have never used them. The 16V 100uF version should fit nicely on the board.

If you are going to bother changing the output caps, go for some polypropylene films. As far as bipolar electrolytics go, the Nichicon Muse are very very good. Since they haven;t made Blackgates in years, you would likely end up with counterfeits anyway.

what's the minimum value film cap that would be advisable to use instead of the 100uF Muse?

That depends on your load. What is it? Are you using the on-board BAL/SE?