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hello what can cause around 4vdc on Legato 3 se outputs have no problem trimming bal outputs powered from on Placid HD BP and no output buffers or inductors in place the thing plays but have serious hiss in the background 14,3vdc on both pos and neg and around 60mV shunt current best Leif Edited by user Sunday, March 4, 2012 9:30:25 AM(UTC)
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have tried replacing 4562 with op275 with no effect
best
Leif
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Has it always been like this or "has it happened" ?
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it´s built new and never set into operation
only had problems from day one
had initial problems with Placid HD BP, but think it´s stable now
maybe it killed some of the transistors
this is at least the 5th dac I build from TPA
also have problem getting stable lock on s/pdif
have ordered another legato to compare
best
Leif
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is the legato with or without buffers? Edited by user Sunday, March 4, 2012 7:38:21 PM(UTC)
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Did you install the jumpers that are required for omitting the buffers?
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hell o R you mean J5&6 ? thought they´re only if you´re omitting bal/se stage  Quote 1 manual: The Legato is a pretty straightforward build. C1-C4 can either filter to GND or the positive rail. I prefer GND. Please note J5 and J6 are only used if the buffers are omitted - otherwise do not populate. Quote 2 manual: Completely omitting only the SE buffer: Omit: IC2, IC3, and optionally L1 and L2. Then install jumpers at J5 and J6. best Leif PS sry for first missing the "o" in hello Edited by user Monday, March 5, 2012 6:25:03 AM(UTC)
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The two passages you quote seem pretty clear to me. If you omit the buffer but are still using BAL/SE you must have the jumpers (which basically replace the buffer with a wire) :)
"Please note J5 and J6 are only used if the buffers are omitted - otherwise do not populate."
If you omit the entire BAL/SE stage then of course none of those components are required at all. The buffer becomes a moot point. :)
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my fault
I did not see the lacking IC´s as a part of the se circuit
thxs anyway
as to the flickering lock through s/pdif pcb and sidecar I´ll try to put the minus s/pdif signal to gnd instead of minus
thxs again
I now have a good hunch about this ....
best
Leif
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That spdif signal should go to -IN and +IN, but it is ok to also join -In and GND for single ended SPDIF(Anything that is not differential like AES).
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it goes to - and + now, but I have problems getting lock on s/pdif ; 2 is worse than 1 ?
despite that they´re seemingly identical and positioned one above the other
I´ll try hooking up - to gnd
reg gnd:
the dac is built on a plywood bottom cover with aluminium back plate
incoming ac gnd goes to backplate
all connectors, both in and out , are "free-floating" with ref to backplate and ac gnd
is this ok?
thxs again
I´ll report back
best
Leif
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hello again
inserted the jumpers in j5 and 6
now I can adjust RCH to 0,0 mv but I cannot get below 330mv on LCH
the range is about 1,2 - 0,3vdc
LCH also plays noticibly lower in level
tried inserting op275 instead; no difference
any idea?
the s/pdif lock is stable on both 1 and 2 when jumper between minus and gnd on input
except when touching chassis on amp; obviously a grounding issue
best
Leif
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IS the balanced output still 0 offset? Make you you zero that first. The adjust SE, then go back and check balanced again.
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hello again Sir
latest development
can adjust both channels on both se and bal output for zero dc
BUT
when I lower BIII board onto standoff connectors both se and bal on LCH jumps up to minus 1,11-1.2 vdc depending on pot position
RCH stays stable
transistors gone?
best
Leif
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You need must adjust the offset while it is connected to the DAC. Also check that you supplies are still good to go.
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yes tried again
pluss 14,23 minus 14,24 shunt around 57ma on both
dac 5,2 shunt 50ma
minus 1,11-1,2 on both se and bal with piggybacked dac board on LCH
0,00 both on RCH
best
Leif
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looked at schematic:
could any or all faulty QN1-3 give this result?
best
Leif
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More likely a cold solder joint or something like that. The P-channel MOSFETs can be damaged by ESD, but this is pretty rare.
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I resoldered the whole board
best
Leif
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