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Placid HD BP v2.1: negative voltage fixed at -2.8V -- FIXED
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Yeah have to have a bad component, there is nothing wrong with the cct. It has been tested thoroughly.
Also your simulation is not really accurate - you need a better approximation of the actual opamp among other things.
My guess is you have a bad transistor or a short some place. It is hard to debug from afar. :)
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Keep in mind that the two transistors (actually three because the one is a darlington) form a signal device - a CFP. The voltage drop (effective VBE) of that CFP is important to the operation of the circuit. Edited by user Tuesday, September 9, 2014 6:07:35 PM(UTC)
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(info) I've just replaced QN3 by BC549 transistor (just smaller Vcbo and Vceo) and also "slowing" C8B capacitor and nothing has changed - switch is still needed to fix start-up from -2.8V.
Any ideas? Which other transistor could have cause that? Or where could be that short? Thanks.
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I know about inaccuracy of simulation ... I'm not able to set-up LTSpice to simulate well with OPA209 model. It is either painfully slow or it "loops" within stepping source or damped pseudo-transient analysis. Also I'm using just model of LT1009 because LT1034 is not available. BTW: This circuit is my first time with circuit simulator and circuits in general... it's interesting, however I see that I miss much of theory behind :D Edited by user Tuesday, September 9, 2014 9:19:40 PM(UTC)
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Simulation speed-up success: I've switched LTSpice solver to "alternate" and now it simulates OPA209 too... :-) EDIT: Rser of voltage source higher than 0.6 seems to be important too... Edited by user Tuesday, September 9, 2014 11:25:29 PM(UTC)
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I've tried to model recovery situation from my faulty state using initial conditions matching my state and ideal opamp. But these darlingtons do not get properly computed DC operating points... Edited by user Wednesday, September 10, 2014 2:23:58 PM(UTC)
| Reason: .asy to work with original ltspice File Attachment(s): placid-fullneg-recovery-sim2.zip (43kb) downloaded 3 time(s).miero attached the following image(s): placid-fullneg-recovery-sim2.png (47kb) downloaded 10 time(s).You cannot view/download attachments. Try to login or register.
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I'm out of ideas... there is just the last one... exchange opamps in positive/negative sides ... but I do not have proper tools, so it would be very likely damaged :-|
Any other?
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I don't like your Vref. It should say 2.5v (negative).
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Yes, that's strange.... In the simulation I've found that LT1009 consumes 135uA to generate voltage drop from 2.78V to 1.77V. When generating reference 2.5V it consumes about 186uA. Nevertheless datasheet of LT1034-2.5 specifies range of operating current 30uA (max.min) to 20mA (abs.max). By computation of currents at R6B and R13B there is clear that voltage reference will not work because there is not enough current consumed. R6B ... vdrop= 1.005 V, res=1992 Ohm => current= 504uA R13B ... vdrop= 0.232 V, res=467 Ohm => current= 497uA So for voltage reference there might be just 7uA available. Therefore one of C8B of IC2 (or something else?) consumes that 497uA current. I've already replaced C8B by another one and nothing has changed. Datasheet for original capacitor EEU-FC1H470H specifies leakage current(max.) is 23.5uA. The new capacitor rated to 35V has max. leakage even smaller - 16.5uA. Bias current of OPA209 should be in nA range, but OPA209 datasheet specifies minimum +/-2.25V so in this case it does not have enough power to operating properly - just 2.75V instead of 4.5V; so that current ~0.5mA current might be used to powering it up via Vin+. Edited by user Wednesday, September 10, 2014 11:51:34 PM(UTC)
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Resistance between pin "1" of R13B (desoldered to check this) and GND or OUT- is at least 1MOhm.
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Maybe... R13B could be increased to reduce that leaking current into IC2. Then VREF2 might create 2.5V reference. Also C8B can be smalker then?
Russ, what do you think? Can you suggest some R & C values to try?
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You could try that - but if that were the issue the circuit wouldn't work for anybody. :) Yet it certainly does. :)
You could short that resistor and see if it makes any difference. It is a simple RC so you just won't get the benefit of the filter.
I am starting to suspect a dead opamp on your board.
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Maybe... but dead a-like - Because "the switch" zombifies it on :) ... ok, I'll try shorting that. Thanks.
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Measurement of the zombie....
R13 shorted ..... Vref -1.56V ; Vout -2.78V
R13 = 4.7kOhm ..... Vref -2.32V ; Vout -2.76V
no R13, ICin+ on GND .... Vref -2.48V ; Vout -2.78V
no R13, ICin+ on ICVin- ... Vref -2.48V ; Vout -9.3V (100 Ohm load) or -10.4V unloaded
The last case is very strange... I'd expected that Vout- would be equal to Vin- but it is not.
Resistance ICin+ to GND is ove 5GOhm with R13 removed, so no short circuit on that side.
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Negative part of Placid 2.1 fixed by changing value of one passive element... ;-)
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Decreased R8B. While powering up the QN4 stays closed longer and opamp gets its minimum operational voltage 4.5V faster. This also forces opamp to work more, because lower R8B means also greater opamp output current. But it should be around 10mA, so I hope it does not influence performance too much. R8B: - default 470R => does not work (2.8V) - 230R => still does not work (2.8V) - 220R => works most of the time (it started with 2.8V once) - 100R => works So now I'm using 100R instead of designed 470R. EDIT: all other components are from TPA kit Edited by user Wednesday, September 17, 2014 7:36:09 AM(UTC)
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Russ/Brian ... is the opamp IC2 in my Placid 2.1 faulty? Should I seek replacement? Thanks.
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Even with that hack my broken Placid 2.1 seems usable with Legato... miero attached the following image(s): b3+placid21+legato+spdif.png (27kb) downloaded 12 time(s).You cannot view/download attachments. Try to login or register.
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Placid HD BP v2.1: negative voltage fixed at -2.8V -- FIXED
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