Why do you need 4 LEDs as replacement for ZD1? ZD1 is used as level shifter and with 4 LEDs and with 6V output you are pushing opamps output bellow neg power supply???
You have pure luck, since OPA1641 is capable to swing that low, almost to the neg power supply rail
Your voltage reference: how do you calculate gain?
Another issue: watch for OPA1641 input range
Hi,
My head finally got it sorted.
I realised I had been following conflicting ideas. I saw an earlier post on this thread by Walt Jung that, to me, seemed to say that ZD1 should be double Vref, just as this design had. But elsewhere I had read that ZD1 and Vref should be the same...
So I changed zd1 to two red leds, and vref the same, and now the opamp's output sits mid rail (psu output is 7V, op amp output is 3.5V, both inputs 3.6V), and everything is behaving well.
So am I done ?
Thanks again,
Tom
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Equations here, should have linked it, sorry: An Unconditionally Stable Linear Voltage Regulator
That will work, but it is not necessary to have Vref and the ZD1 value the same.
Vref should be such that the opamp is working in its linear range. Mid-supply is good, but not necessary. It can be anything from say V+-2V and V- (or gnd) +2V.
The same goes for the opamp Vout. Mid supply is nice, but not a requirement. So, you don't have to be religious about it and you have some leeway to use whatever fancies you
jan
Success was short lived. :-(
I started using 16x tda1543 and the reg won't start up - vout is 3.8V. So I increased zd1 to 5.4V with a third led and this works.
However, VR1 won't set the voltage. Vout steadily declines to a minimum and VR1 only has a temporary effect.
I lowered R2 150R in the current mirrror and this boosts the min voltage but the behaviour is the same.
I have tried lf351, opa1641 and lme49710 - all the same behaviour but with different output voltage.
With 40R in the current mirror and lme49710, the voltage settles at 7.08V which is about what I want, but I'd like to be able to use VR1 to trim it. I'm also concerned that 40r might be allowing significant current to flow and might damage the transistors.
Opa690 seems unstable and the voltage steadily climbs.
Any ideas what is going wrong and how I fix it ?
I started using 16x tda1543 and the reg won't start up - vout is 3.8V. So I increased zd1 to 5.4V with a third led and this works.
However, VR1 won't set the voltage. Vout steadily declines to a minimum and VR1 only has a temporary effect.
I lowered R2 150R in the current mirrror and this boosts the min voltage but the behaviour is the same.
I have tried lf351, opa1641 and lme49710 - all the same behaviour but with different output voltage.
With 40R in the current mirror and lme49710, the voltage settles at 7.08V which is about what I want, but I'd like to be able to use VR1 to trim it. I'm also concerned that 40r might be allowing significant current to flow and might damage the transistors.
Opa690 seems unstable and the voltage steadily climbs.
Any ideas what is going wrong and how I fix it ?
Hello again,
I found the fault - a pcb trace burned through so vref wasn't reaching the op amp, so it was simply floating down to its min voltage.
So that is now fixed and everything returned to 'normal'.
I am relieved this is not my limited skills, but I am amazed - the current through that trace is tiny - how crap is this PCB ?
BTW I think the lme49710ha sounds a little better than opa1641.
Cheers,
I found the fault - a pcb trace burned through so vref wasn't reaching the op amp, so it was simply floating down to its min voltage.
So that is now fixed and everything returned to 'normal'.
I am relieved this is not my limited skills, but I am amazed - the current through that trace is tiny - how crap is this PCB ?
BTW I think the lme49710ha sounds a little better than opa1641.
Cheers,
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One thing with them regulators is they have no protection circuitry if things go wrong as you just seen. Or if something upstream shorts etc. It needs some current limiting on that pass transistor. Foldback limiting or something.
Remote Sensing for a Class A push pull O/P stage (headphone amp)
Hello,
I was wondering how you would connect up remote sensing for a split rail Class A output stage. It's a headphone amp in push pull I was interested in building.
Now I can see how it works driving a chip or load on a single supply rail. RS+ to positive pin and RS- to gnd pin or 0V pins. An output stage is not so easy at least to me.
You could just connect both RS pins from two regs (pos & neg one) to 0v rail but I am not sure that is optimal. Not sure being honest.
Any help appreciatted.
Hello,
I was wondering how you would connect up remote sensing for a split rail Class A output stage. It's a headphone amp in push pull I was interested in building.
Now I can see how it works driving a chip or load on a single supply rail. RS+ to positive pin and RS- to gnd pin or 0V pins. An output stage is not so easy at least to me.
You could just connect both RS pins from two regs (pos & neg one) to 0v rail but I am not sure that is optimal. Not sure being honest.
Any help appreciatted.
The "pre-regulator" described in the follow up article Improved Positive/Negative Regulators Audio Electronics 4/00 will limit the current. The pre-regulator will raise the output impedance a bit, however.
http://waltjung.org/PDFs/Improved_PN_Regs.pdf
http://waltjung.org/PDFs/Improved_PN_Regs.pdf
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Thanks - the vref is 7.25V so I guess that'd suit 12v or higher supplies, and I could use an lm329 on the op amp output for that, or drop the voltage from the vref with the datasheet's suggested potential divider. I shall give it a try... it should work slightly better than the standard 723.
Tom
Tom
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I'm not sure it would work without some additional hardware. The thing that differentiates the more advanced "super regs" is that the op amp + pass device actually regulates the output, and the op amp is fed directly from that output. I don't recall ever seeing a 723 circuit that does that.
I'm sure it could be done with added parts, however.
Walt Jung
Hi,
I'm quite happy to add a few parts. The vref is a buried zener and the noise is filtered with a low pass filter. Large caps (say over 5uF) in this filter cause the output to stay at vref, 7.25V, at start up, before dropping to the set output voltage. This would seem to lend itself to the super-regs start up conditions.
Anyway, I'll give it a try next week just out of curiosity.
I'm quite happy to add a few parts. The vref is a buried zener and the noise is filtered with a low pass filter. Large caps (say over 5uF) in this filter cause the output to stay at vref, 7.25V, at start up, before dropping to the set output voltage. This would seem to lend itself to the super-regs start up conditions.
Anyway, I'll give it a try next week just out of curiosity.
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