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Legato 3 - Look ma! No caps!

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#142
MartinC700 said:
Russ,

Is that a Zobel network on the output of the buffer? - If so presumably that is to eliminate any possibility of instability when driving a capacitive load?

Regards

Martin
Click to expand...

No not exactly. 🙂 A zobel is a parallel (to the load) R in series with a C. This is simply an impedance shelf for very high frequencies (hundreds of KHz).

The purpose of the inductor is indeed to insure stability even into the most demanding load. It is as Brian said, optional, but I would not ever remove it. It ensures that at audio frequencies there is next to zero impedance while protecting the cct from difficult loads.

Cheers!
Russ
 
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#143
I'm gonna connect 2 x Ventus to the Legato 3.1. Both have a pot to get DC +/- 0V, I Believe VR1 & VR2 on the Legato 3.1 are to get rid of DC ? Iwant to remove them and connect the pin's 1,2,3 with each other, so I have only the Ventus for adjusting DC out.
Can I do this ?

- I'm going to try to change (after tuning DC to 0V) the offset pot on the Ventus for 2 fixed resistors, tried it before on my ONO and it lowers the DC drift. Not sure if is needed here, it's already a first class product 🙂

I'm very glad that I was able to order a Legato3.1, waiting another few weeks would have give me cold turkey symptoms :geezer::yell:
 
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#149
Bassivus said:
Sorry if I'm asking something that might be obvious to others but I still don't get it. Does Legato 3 cancel common mode DC in balanced?
Click to expand...

Well it depends on how your using it. 🙂 If you going dual mono then the SE outputs become balanced outputs and there will be no common mode DC there at all.

If you are using it stereo there will be common mode DC (but no differential DC) only at the balanced outputs which for most balanced amplifiers will not be a problem in the least.

Here are some strategies you can take if you like.

- For balanced output only connect GND on one end, the DAC or the Amp or don not connect GND at all. Most balanced amplifiers really don't need GND connected at all. Make sure with your DMM that you do not have any onnectivity between GND on the DAC and GND on the amp.

- For the very cautious Use AC coupling caps between the DAC and the XLR jack. This is foolproof. 🙂

Most people will not even need to worry about the common mode DC, but everyone should do their homework as to the requirements of their particular setup. That is why this is "DIY use only." You may need to do a little research on your particular amplifier.

So I guess the bottom line is, if you have any doubts get a couple good 4.7-10uf film caps and put them on the balanced outputs. 🙂
 
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#150
Supertex missing in BOM

Russ White said:
Correct. The same transistor will work for Legato 1/2 but you need to install them backward on those boards as the pin-out is reversed.

That part has a number of advantages, first is that he Vth is very consistent and low! This gives better headroom and THD. Second the low Ron. It is a great workhorse for the CFP. Especially in parallel. 😎

Cheers!
Russ
Click to expand...

Please note that the Supertex has not been included in the BOM
 
#154
ichiban said:
Uggh! Will I be able to mod my newly purchased v2 into this configuration?
Click to expand...

I'm looking to convert my unbuilt V2. I hate to waste the board especially since V3 boards are already sold out. After looking at the V3 schematic, here's what I have so far:

(12) Supertex TP0604 (replacing BS250KL). Install them backwards as the pin-outs are reversed. (mouser $0.87)
(2) LME49600 added in SE output stage (digikey $10.50)
(8) 2.2nF (replacing 1nF and 15nF caps)
(2) 100pF cap added across IC1.1/1.2 pins 6 & 7
(2) 10uH inductor added in SE output stage (anyone know a mouser part#?)
(2) 10R trimpot added across R9/R11 and R14/R15 with wiper to (–)15V
Jumper positions VR1, VR2, C5-C8

The trickiest part will be the LME49600.

Any comments or errors spotted?
 
#155
VR3 and VR4? 1K trimpots

Too bad you can't get your hands on a v3.1 pcb. I'm really enjoying my v2 right now, very dynamic and detailed with almost no listening fatigue. Added the Supertex TP0604's, Panasonic ECHU smd caps (2.2n,3.3n), removed C5-C8 and replaced with links, 180r Shinkoh's for the I/V resistors. Rail voltages are +15V and -13.5V (Salas shunt regulators). Anxious to try out some 3-leg shunts on the Buffalo.
 
#158
I'm not sure what the effect is feeding the bal/se converter with asymmetrical rails like this, probably not ideal. Balanced out topology is still identical for v2 and v3.1. So v3.1 rail tweaks should still apply. You can always leave out J1-J4 and give each section it's own optimized supply. But, maybe not necessary? I think it sounds very good....
 
#159
glt said:
Would this optimized voltage setting also apply to L 3.1? Seems the LME's would like +/- 15v
Click to expand...

No, L3.1 is a different circuit from L2 with input always centered at GND. That type of voltage skew is only valuable with the input centered at AVCC/2 (as it can be on L2).

My research showed that there was really no benefit because of the symmetrical nature of the circuit in centering at AVCC/2. So that feature is out and DC nulling is in. 😀

For L3.1 I recommend 12-15V rails. Symmetrical is just fine. Skewing won't hurt, but does not really have the same effect in this cct. 🙂

Use 12V if you want to keep the temps down, 15V for theoretical (simulated) max performance. There is actually almost no penalty for running at 12V BTW.
 
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