Hmmm...
This is very different from my experience. I use a test signal from the Stereophile Test CD 2 to measure for output at 0 dB, and I get a bit over 2.1 volts differential with 187R up top and 820Rx2 on the bottom. My source is the Sonore USB interface feeding I2S to a B-IIIse (sync clocking) from a Linux based server or the Sonore µRendu (prototype).
I have made final output tweaks to my 3 Legatos and the good news is that they sound wonderful and their outputs now help compensate for different driver efficiencies. There is no audible noise and sound levels are adequate on quieter recordings. So, mission accomplished.
However, when I measure the output I’m still unsure what is going on. This is what I have observed from a 0dB sine wave produced via SoX on the BBB:
150/681 ohm - 1.2 v/differential channel
165/750 ohm - ~1.3 v/differential channel
180/820 ohm - 1.5 v/differential channel (~3v SE)
195/848 ohm - 1.6 v/differential channel
I was expecting 2 volts with the 195/848 ohm pair! Any thoughts about what is (apparently) limiting the output? What am I missing?B3 I2C registers 1-8 are at 0 and registers 20-23 are maxed. There is no amplitude difference between filtering the signal in ALSA and sending it to hw:0,0 in the BBB. I just wonder if the B3 is performing differently with output from BBB/Cronus than it would with a different I2S source.
One observation re the resistors - compared to the stock Vishays (very good indeed) the red PR9372s are at least equal. I think I’m hearing even more recording environment acoustics now compared to before the resistor swap.
Cheers,
Frank
This is very different from my experience. I use a test signal from the Stereophile Test CD 2 to measure for output at 0 dB, and I get a bit over 2.1 volts differential with 187R up top and 820Rx2 on the bottom. My source is the Sonore USB interface feeding I2S to a B-IIIse (sync clocking) from a Linux based server or the Sonore µRendu (prototype).
Comparing playback volumes between my old Legato 2 and current Legato3.1, the newer board is quieter. Both have 680/150 ohm resistors. On some tracks I could really use more output from the new boards. I ran a 440Hz sine wave from the BBB at -0dB and got the below trace (0.2 v/div). 1.2 volts max. For my current listening room I think I could use a full 2 volts at -0 dB. The Legato info doc suggests that 187 ohm lower resistors will produce that, but I'm doubtful if 150 ohms gives only 1.2v.
Comments or suggestions? TIA!
Frank
That does not look like a differential measurement on the scope.
Thanks Russ! I'm glad that there is nothing here to worry about because the output levels are just enough for quieter (usually older) recordings.
That scope trace used 2 probes, one on each differential output pin from Legato (0.2 volt/division). I probably used the wrong nomenclature... SE output from Legato (which I don't use) is about twice the numbers in the table.
I'm REALLY enjoying the system!
Frank
Legato and Lundahl - 8kHz cut off
I recently changed amp to Pass F5. The gain is quite low and got some hum when going SE so I put in Lundahls on the input.
When doing measurements I now see that with Legato3, SE output via buffer, I have a tweeter roll-off at 8kHz!
With my second dac with IVY3 I have no issue with this.
I thought the buffered SE output would be able to drive the Lundahls.
Any ideas or hints would be really appreciated
I recently changed amp to Pass F5. The gain is quite low and got some hum when going SE so I put in Lundahls on the input.
When doing measurements I now see that with Legato3, SE output via buffer, I have a tweeter roll-off at 8kHz!
With my second dac with IVY3 I have no issue with this.
I thought the buffered SE output would be able to drive the Lundahls.
Any ideas or hints would be really appreciated
Legato 2.5 Optimal Tuning Point
Hi all,
I've been running a Legato 2 board upgraded with Legato 3 parts == identical circuit except for VR1 / VR2 still trim the input offset rather than nulling in the L3 circuit. I use the balanced output only and stock 680/150R values. Placid BP with +-14.5V rails.
After reading all (!!) the pages in this and the tweaks thread I'm confused as to what the optimal operating point is that I should be aiming for.
e.g. Should I wind VR1 / VR2 to GND so the circuit is identical to a L3 and just use symmetrical rails or for best performance should I set the input bias to ~1.7V and/or the output bias to ?? by adjusting the -ve rail?
Can you please share the best operating point you've found for the 680/150R configuration?
@Russ what do you recommend?
many thanks,
Bruce
Hi all,
I've been running a Legato 2 board upgraded with Legato 3 parts == identical circuit except for VR1 / VR2 still trim the input offset rather than nulling in the L3 circuit. I use the balanced output only and stock 680/150R values. Placid BP with +-14.5V rails.
After reading all (!!) the pages in this and the tweaks thread I'm confused as to what the optimal operating point is that I should be aiming for.
e.g. Should I wind VR1 / VR2 to GND so the circuit is identical to a L3 and just use symmetrical rails or for best performance should I set the input bias to ~1.7V and/or the output bias to ?? by adjusting the -ve rail?
Can you please share the best operating point you've found for the 680/150R configuration?
@Russ what do you recommend?
many thanks,
Bruce
That has to be roll-off from the trafo. Legato SE direct into 32R test load and headphone load shows no roll off until well beyond 20khz.
Many thanks for the response, Russ.You are right on track. I designed for those values at +/-15V but 14.5V is fine
Just confirming to GND VR1/VR2 input bias like a L3 circuit is the best?
BTW it makes no sense to drive a trafo with an SE signal when you have a balanced signal to feed it.
Still if you wish to use the SE output to drive the trafo you could try and remove the output inductor and see if that is interfering with the trafo reactance. You shouldn't need it in that app anyway.
Still if you wish to use the SE output to drive the trafo you could try and remove the output inductor and see if that is interfering with the trafo reactance. You shouldn't need it in that app anyway.
Thanks for your reply Russ!
I did an impedance measurment on the input side of the Lundahl trafo.
What interesting is that exactly at 8kHz the phase goes positive (inductive load I assume).
I would like to use the balanced signal, but I think those output have less power to drive the trafo-load, or?
I will remove L1,L2 on the SE output of Legato3 and see if things improve.
I did an impedance measurment on the input side of the Lundahl trafo.
What interesting is that exactly at 8kHz the phase goes positive (inductive load I assume).
I would like to use the balanced signal, but I think those output have less power to drive the trafo-load, or?
I will remove L1,L2 on the SE output of Legato3 and see if things improve.
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I found the problem!
If I switch off oversamplig and reclocking (SW2, 5) in the Buffalo3 SE I get this phenomena.
Picture below is with oversampling on or off!
Thing is that I somehow liked the non-overampling, more instant and direct sound. No smearing if you understand what I mean. Perhaps more like a NOS 16-bit dac
But of course I want to keep the output above 8kHz
brgs
If I switch off oversamplig and reclocking (SW2, 5) in the Buffalo3 SE I get this phenomena.
Picture below is with oversampling on or off!
Thing is that I somehow liked the non-overampling, more instant and direct sound. No smearing if you understand what I mean. Perhaps more like a NOS 16-bit dac
But of course I want to keep the output above 8kHz
brgs
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