I am always happy to post with a morning cup of coffee.
Brian is likely busy putting in his much needed boiler.
If you are running balanced only. Then yes you have a lot of freedom to experiment. the 25V caps on the legato can be replaced or even omitted.
What I have found is that with the CFP there is a point of diminishing returns and that happens to be about 15V.
That said, if you want you can actually short the CFP resistor and omit the P channel devices and use a TO-220 N channel FET. If you do that you will have a classic "D1" type common gate stage. Then increasing the positive rail will help a lot.
You can always leave the negative rail relatively low. Its up to you. It's purpose in the cct is just to provide current for the uppper part of the circuit.
Here is how I am running my Legato (2.0.0 PCB with 3 FETs per side per half):
I/V resistor 105R(I don't need 2VRMs, 1.2 or so is fine).
Current source resistors are 475R (2 x in parallel)
Basically you can use the negative rail to tweak the amount of current and thus the bias point for the top half.
I aim to bias the output at about (VCC/2)-1V So in this case I was aiming for about 6.5V. This in my experience is a sweet spot.
Positive rail 15V.
Negative rail 13V.
It sounds delightful.
Now if you are going to experiment, be sure you understand the implications. Calculate the power dissipated and the supply requirements carefully. Your on your own here, but it is certainly manageable. I will help out if I can.
Cheers!
Russ
I have no idea what you are wanting to do.
You don't "have to" do anything.
What is your goal?
I need to know more specifically what sort of output level are you going to need?
Mine was pretty low with that setup, about 1VRMs, that way I listen with almost no attenuation at all.
Is that what you want? If so just do as indicated. But make sure you actually adjust the rails! The values I gave are just guidelines for you to tweak for yourself.
Mine was pretty low with that setup, about 1VRMs, that way I listen with almost no attenuation at all.
Is that what you want? If so just do as indicated. But make sure you actually adjust the rails! The values I gave are just guidelines for you to tweak for yourself.
Legato R1-4 @ 180R is more dynamic than the Opus and some redbook recordings got tiring. 150R is not bad, but I would prefer a more dynamic presentation. So I wanted to try about 170 R (maintaining the 2.2 ratio as well as possible) and am still looking for parts. I was thinking that 2X340R (.5W)/ 2X750R would be just about dialed-in...
The new Legato ver. 2 is up and running and I love hearing not only the instruments, but the space around them. Three thoughts:
1. Mission accomplished with matching dynamics among DACs - I ultimately decided on 2X330R PRP .5W for I/V resistors and 2X750R 1 W Holco for current source. At the moment I'm liking bias of about 6 volts ~= +15.25v, -13.5v.
2. I previously complained about listless performance in Legato 1.03 after changing I/V resistors but about a month after complaining, performance had clearly improved (though dynamics were still restrained). Maybe this extended 'burn in' is just a property of PRPs. But I think also the Legato needs considerable time to warm-up after powering-on.
3. Thumbs up on my current capacitors: Auricap 4.7uF for output and Relcap RTE polystyrene 7.5 nF for the stock 15 nF Wimas. No 'glassyness' or high frequency fatigue. [My tweeters are on L pads (which has avoided much angst in my digital past) and only now with BII/Legato running so nicely can I enjoy them without attenuation.]
Thanks Brian and Russ! Brian, you do a fantastic job with kit management - I could never gladly wrangle so many details!
...looking forward to Tridents when you get more ready...Frank / Mpls.
[IIRC] Leave out the buffer, triple up the BS250 FETs, and double up the current supply resistors. Rather than messing around finding BS250s that match, I simply ordered ver. 2 instead...
Frank
Just a quick note to confirm some of the discoveries others have reported. I finally have the dual mono Buffalo ll with Trident regulators, 2 Legato 2, full Placid power and three Ventus per side as my amp is single ended. While some things were better I felt I had lost some of the magic of the original Legato (without buffer). The high end seemed soft almost smeared as if there was a phase difference between the three fets. The bass was very full to the point of being bloated without the drive or extension of the IVY or the musicality and balance of the Legato 1. Everything stock, +-15 volt, biased to 1/2 AVCC at input and output, shunting 80 ma.
So taking the lead of others I replaced the .015 filter caps with some .0039 polystyrene and removed the bipolar output caps on the Legato. (if you are spending big $$$ on output caps the best cap is no cap and the Ventus may even be less expensive.) Re-trimmed the Ventus for 0V dc output and was stunned by the improvement. The polite over-damped impression of the stock unit was replaced with an open airy detail. Some of the most realistic cymbals ever. The bass lost its bloat gaining depth and drive for an overall more dynamic and better balanced presentation. I couldn't be happier with the result. Maybe I'll have to try some of those $$$ resistors next, but first I am just going to sit back and enjoy the music.
Thanks to all who have contributed to this process, and again thanks to Brian and Russ for the wealth of information that has allowed us to play with their great designs.
So taking the lead of others I replaced the .015 filter caps with some .0039 polystyrene and removed the bipolar output caps on the Legato. (if you are spending big $$$ on output caps the best cap is no cap and the Ventus may even be less expensive.) Re-trimmed the Ventus for 0V dc output and was stunned by the improvement. The polite over-damped impression of the stock unit was replaced with an open airy detail. Some of the most realistic cymbals ever. The bass lost its bloat gaining depth and drive for an overall more dynamic and better balanced presentation. I couldn't be happier with the result. Maybe I'll have to try some of those $$$ resistors next, but first I am just going to sit back and enjoy the music.
Thanks to all who have contributed to this process, and again thanks to Brian and Russ for the wealth of information that has allowed us to play with their great designs.
Glad you are enjoying your Legato Stephen, and thanks for passing on your results. I too found removing the DC blocking caps and replace the 15nF caps with small polystyrene versions significant upgrades. Still Using a LME49720HA for balanced to SE, but have transformers and a discrete opamp to try at some point.
It's a very unique sort of enjoyment, isn't it?!
It may be worth varying Vminus (between ~12.5 - 15?) to find the supply sweet spot (when all your gear is well warmed-up). It thought it was fairly obvious by ear - and then measure across the outputs of, say, R1 & R9/10 to see how close you are to 6-6.5v...Francolargo,
Because I am running the three Ventus off of the + - 15 Placid bipolar as well I prefer to keep the rails symmetrical. I have also replaced VR3 and VR4 on the Placid with fixed resistors.
Your point about measuring only after warmup is well taken. I now see =14.91 and -14.88 volts instead of the +- 15 at startup.
With stock r values the output is sitting at 7.38 volts. If as has been suggested things might be a little sweeter at (vcc/2) -1V or about 6.45 in my case couldn't this be obtained by leaving the voltage rails alone and adjusting the value of current source resister ? In fact if I don't change the current source r lowering the - V rail would move the bias in the wrong direction wouldn't it?
Because I am running the three Ventus off of the + - 15 Placid bipolar as well I prefer to keep the rails symmetrical. I have also replaced VR3 and VR4 on the Placid with fixed resistors.
Your point about measuring only after warmup is well taken. I now see =14.91 and -14.88 volts instead of the +- 15 at startup.
With stock r values the output is sitting at 7.38 volts. If as has been suggested things might be a little sweeter at (vcc/2) -1V or about 6.45 in my case couldn't this be obtained by leaving the voltage rails alone and adjusting the value of current source resister ? In fact if I don't change the current source r lowering the - V rail would move the bias in the wrong direction wouldn't it?
We're talking about coupling caps, the green bipolar Nichicons. Bypassing them won't harm anything, but will of course let any differential DC through (this can be trimmed by altering the value of one of the current source resistors). You are right, offset does drift around a little with temperature.
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