Legato 3 - Look ma! No caps!

[regiregi22]

Originally Posted by BrianDonegan

Yes, it is fully optional depending on what you are driving.

In which cases is it advisable to use it?

[Russ White]

Originally Posted by MartinC700

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

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

[acidbrain]

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

[Russ White]

Even if you going to use Ventus I would leave the pots. The cct will perform better when properly trimmed.

[glt]

Originally Posted by Russ White

Ok folks, I almost have the manual done, but here is the schematic for you to look at in the mean time.

Cheers!
Russ

Since my little DVM can also measure transistor gain, which transistors to match?

It seems from the diagram that QP1, QP2 and QP3 and their cousins would benefit from matching.

Thanks...

[acidbrain]

Originally Posted by Russ White

Even if you going to use Ventus I would leave the pots. The cct will perform better when properly trimmed.

Thanks Russ, then I will trim both Legato and Ventus.

[Russ White]

Originally Posted by glt

Since my little DVM can also measure transistor gain, which transistors to match?

It seems from the diagram that QP1, QP2 and QP3 and their cousins would benefit from matching.

Thanks...

Transconductance is so high in the cct that its honestly not critical.

But if you were going to match the only Qs that would produce any results at all would be QN2/3 and QN5/6.

[Bassivus]

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?

[Russ White]

Originally Posted by Bassivus

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?

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.

[pirastro]

Originally Posted by Russ White

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

Please note that the Supertex has not been included in the BOM