Op amp supply Quiz - what sounds best ?

[KSTR]

Note just blindly paralleling capacitors can also cause problems. The capacitors' ESL and ESR can work together to again peak the noise at unexpected frequencies by causing resonances among the capacitors themselves. Ironically, this is often worse with low-ESR capacitors and low-impedance power pours (planes), since it provides higher-Q resonances among the capacitors.

This!

Most audio circuit engineers don't seem to use a network analyzer to check the supply impedance right at the component pins. The use of other methods like step exitation to check supply impedance is also not in wide use it seems. Once you do, you see the disaster that is happening there most of the time with any typical circuit that simply slaps 100nF at each opamp/component within a layout where the supply traces are long and inductive. Even worse is the staggered approach using smth. like 100nF in parallel with larger low ESR high capacitance film cap or one of those new ultra low-ESR polymer electrolytics. Anti-resonances peaking at 10's of ohms when the smaller C reacts with the ESL or phyical L!
I can only recommend to any designer: if you have money to spend on instrumentation don't aim for an audio analyzer (AP or what ever), rather buy a network analyser and learn how to use it.

The typical inverting I/V-stage draws high frequency pulse current from either supply, as much as the DAC chip can sink/source in any given moment which can be 10's of mA, way beyond the class-A range of the opamp's output stage, and that can severly exite those resonances more easily than in the class-A range because the former is half-wave rectified current with corresponding train of harmonics.

Obviously, local decoupling (in the true sense of the word, which means high series impedance and low parallel impedance) is an easy and effective way to avoid that and force the return current into the GND plane from where it can in turn return to the DAC chip's analog supply bypassing. The series impedance must be ohmic at the frequencies of interest and it doesn't need to be any higher than the value one would use for a parallel snubber, so 1...10Ohms works well. Higher, like 100Ohms doesn't hurt but requires larger parallel caps, otherwise the supply modulation can be larger than tolerable (depends on the PSRR of the opamp which will be good at DC/LF but poor at higher frequencies).

So to answer the OP's quiz: Replace the series resistor with a properly selected L//R, and use an equally well choosen cap. Eye-balling values is bound to fail, here. Get a network analyzer (and use its spectrum analyser features as well), invest in sniffer probes, etc. And sim things properly before building anything.

 

[solderdude]

Comparative measurements or simulations? Something to grasp.

There won't be measurements. The OP does not do audio measurements, he uses sighted subjective impressions.
If he could do such measurements his LDR volume control would already have been measured.
So whatever the OP found to be 'optimal' is, with a high probability, not of electrical origin.

What are the 400V thyristors for and how are they connected ?

Care to share the schematics instead of just mentioning what components are used ?

The funniest part (for me) is the usage of current sources which would only really act as a current source when there is a minimal voltage drop across them. This means each different opamp should be trimmed individually for current sources to actually operate as a current source.
It also means the power supply voltage will be considerably lower than when say a 100 Ohm resistor were used.
Usually opamps perform better on a higher voltage rails, not on a lower one.

A compound decoupling capacitor is enough (combined with inductor or resistor)

I pull the BS card.

 

[stereo coffee]

Comparative measurements or simulations? Something to grasp.

I am glad you qualified as something to grasp, as my measurement equipment is 2x multimeter and 2 x oscilloscope awaiting some new probes.

I have a PD8700 to compare it to, which has impressive layout with individual DACs for L and R so if we compare measurements of each
the PDS507 manufacturers data is not much help as the PDS 507 already has better specification.

But as its LR layout is so good the PD8700 should be the better sounding player, but is not the case when the 2x resistor are replaced in the PDS507


PDS507


Dynamic range: 99dB


Signal to Noise Ratio: 110dB


Total harmonic distortion: 0.002%


Line output: 2V



PD8700


Dynamic range: 97dB


Signal to Noise Ratio: 108dB


Channel separation: 102dB


Total harmonic distortion: 0.0022%

 

[stereo coffee]

There won't be measurements. The OP does not do measurements, he uses sighted subjective impressions.
If he could do such measurements his LDR volume control would already have been measured.
So whatever the OP found to be 'optimal' is, with a high probability, not of electrical origin.

What are the thyristors for and how are they connected ?

Care to share the schematics instead of just mentioning what components are used ?

The funniest part (for me) is the usage of current sources which would only really act as a current source when there is a minimal voltage drop across them. This means each different opamp should be trimmed individually for current sources to actually operate as a current source.
It also means the power supply voltage will be considerably lower than when say a 100 Ohm resistor were used.
Usually opamps perform better on a higher voltage rails, not on a lower one.

A compound decoupling capacitor is enough (combined with inductor or resistor)

I pull the BS card.

As usual a nasty unnecessary reply. Your lesson to learn is to from today on-wards, metaphorically always tackle the football ( the subject ) not the player ( those discussing) in any discussion.

 

[Wombat]

Those figures are within normal variation due to component and measurement tolerances. Your judgement seems to be based on sighted subjective listening.

 

[stereo coffee]

Those figures are within normal variation due to component and measurement tolerances. Your judgement seems to be based on sighted subjective listening.

Back to the competitive spirit of the topic, can you suggest 4 components ( or even less ) to improve upon Pioneer's use of 2x resistor ? Or is 2x resistor the best that can be done ( I hope not )

 

[solderdude]

As usual a nasty unnecessary reply. Your lesson to learn is to from today on-wards, metaphorically always tackle the football ( the subject ) not the player ( those discussing) in any discussion.

Your lesson of today is to answer the questions asked regarding the usage of the thyristors, how they are used (what is done with the gate) and if the minimum 2.5V voltage drop for the current source is not problematic.
How is the current source adjusted and what voltage is across the opamp.
Why would a current source be 'better' and result in audible changes ?
Why have you not even used a simple external soundcard to do at least basic measurements and show differences ?

 

[solderdude]

can you suggest 4 components ( or even less ) to improve upon Pioneer's use of 2x resistor ?

A: Does it need improvement ?
B: What would improve ?
C: Why ignore the already mentioned 'solutions' ? (i.e. usage of inductors, regulators that are way better than LM317/337, other decoupling caps)
D: why only address 1 opamp ?
E: What will be the 'gain' of changes ?
F: Will all changes lead to improvement ?
G: Why not use another opamp to get (measurable and verifyable improvements) ?

 

[BDWoody]

As usual a nasty unnecessary reply. Your lesson to learn is to from today on-wards, metaphorically always tackle the football ( the subject ) not the player ( those discussing) in any discussion.

Wow...
Yeah...definitely a troll crossing. Normal humans aren't this condescending or obnoxious. You've got a few responding, so it seems to be working.

 

[stereo coffee]

This!

Most audio circuit engineers don't seem to use a network analyzer to check the supply impedance right at the component pins. The use of other methods like step exitation to check supply impedance is also not in wide use it seems. Once you do, you see the disaster that is happening there most of the time with any typical circuit that simply slaps 100nF at each opamp/component within a layout where the supply traces are long and inductive. Even worse is the staggered approach using smth. like 100nF in parallel with larger low ESR high capacitance film cap or one of those new ultra low-ESR polymer electrolytics. Anti-resonances peaking at 10's of ohms when the smaller C reacts with the ESL or phyical L!
I can only recommend to any designer: if you have money to spend on instrumentation don't aim for an audio analyzer (AP or what ever), rather buy a network analyser and learn how to use it.

The typical inverting I/V-stage draws high frequency pulse current from either supply, as much as the DAC chip can sink/source in any given moment which can be 10's of mA, way beyond the class-A range of the opamp's output stage, and that can severly exite those resonances more easily than in the class-A range because the former is half-wave rectified current with corresponding train of harmonics.

Obviously, local decoupling (in the true sense of the word, which means high series impedance and low parallel impedance) is an easy and effective way to avoid that and force the return current into the GND plane from where it can in turn return to the DAC chip's analog supply bypassing. The series impedance must be ohmic at the frequencies of interest and it doesn't need to be any higher than the value one would use for a parallel snubber, so 1...10Ohms works well. Higher, like 100Ohms doesn't hurt but requires larger parallel caps, otherwise the supply modulation can be larger than tolerable (depends on the PSRR of the opamp which will be good at DC/LF but poor at higher frequencies).

So to answer the OP's quiz: Replace the series resistor with a properly selected L//R, and use an equally well choosen cap. Eye-balling values is bound to fail, here. Get a network analyzer (and use its spectrum analyser features as well), invest in sniffer probes, etc. And sim things properly before building anything.

This is the best reply so far,

I would instead try everything though NOT to force return current into the ground plane to intermodulate with the analog supplies bypassing, as the analog supplies bypassing has its own purpose. Star earthing though may circumvent.

 

[stereo coffee]

Here is how I have TL431 and MCR100-6 configured presently.

 

[solderdude]

O.K. so it looks like the power supply current is only flowing through the thyristor gate ?
Very similar to just using a diode which has a rather low AC resistance instead of the desired high impedance of the voltage source.
Basically bypassing the TL431.
In that case the voltage drop across the TL431 is probably around 0.8V and not the minimum required 2.5V (for the TL431 to actually do something)

What is the measured voltage across the TL431 ?

 

[KSTR]

This is the best reply so far,

I would instead try everything though NOT to force return current into the ground plane to intermodulate with the analog supplies bypassing, as the analog supplies bypassing has its own purpose. Star earthing though may circumvent.

Ideally in theory, we could split the return current path so the DAC's GND is bypassed to the OpAmp's +5V and vice versa (assuming both are running single supply), just like one would bypass a bridge amplifier, the reference GND potential would be free of any load current. In practice it doesn't work as you still need the local 5V to GND bypasses and for a bunch of other reasons. That's why proper ground plane(s) architecture is the best approach. You still got to know where and how all the currents flow exactly and make sure any higher currents (with higher edge rates, that is) don't share paths with sensitive reference potentials.

 

[stereo coffee]

O.K. so it looks like the power supply current is only flowing through the thyristor gate ?
In that case the voltage drop across the TL431 is probably around 0.8V and not the minimu required 2.5V (for the TL431 to actuallu do something)

What is the measured voltage across the TL431 ?

On the positive rail 0.963 and negative 0.974. Yes I am not using the TL431 conventionally ( I rarely do ) . There is higher voltage on each thyristor anode than each output.

 

[solderdude]

So it looks like the TL431 can be left out. It does not seem to do anything.
For reverse currents from the opamp the diode of the thyristor is much lower impedance than a 100 Ohm most likely.
A diode in conduction has a lower dynamic resistance than the calculated static one (rule of thumbs about 1/40th so around 3 Ohm).
It looks like the circuit is lower impedant than the 100 Ohm ?

What happens with the voltage drop when the TL431 is removed ?

 

[SIY]

Well the thread is done, answer provided. I dont know if the OP was pulling legs or just not knowledgeable of the subject.

Chris is famously ignorant of basic electronics. It's painful watching you guys indulge him.

 

[stereo coffee]

Chris is famously ignorant of basic electronics. It's painful watching you guys indulge him.

Yet again attacking the player ( those discussing ) and not the football ( subject ) , If you have a circuit to improve the use of 100 ohm resistors,
lets see your effort in that direction please.

 

[BDWoody]

Yet again attacking the player ( those discussing ) and not the football ( subject ) , If you have a circuit to improve the use of 100 ohm resistors,
lets see your effort in that direction please.

I'm still trying to figure out what audible issue is being addressed.
Can you explain how this will improve what comes out of my speakers?

 

[SIY]

I'm still trying to figure out what audible issue is being addressed.

None. Not worth engaging. Do not feed.

 

[stereo coffee]

I'm still trying to figure out what audible issue is being addressed.
Can you explain how this will improve what comes out of my speakers?

An op amps Power Supply Rejection ratio ( PSRR ) and its Common Mode Rejection Ratio ( CMRR ) are discussed here
https://www.arrow.com/en/research-and-events/articles/why-cmrr-and-psrr-matter-when-choosing-op-amps

The discussion aims to examine the use of components immediately beyond the op amps voltage pins ( to see if there is anything better out there than as exampled 2x 100 ohm resistors in a Pioneer PDS507 ) to assist how current is drawn from the comparative ( to the op amp specifications ) raw supply.

Boldly we are attempting a discussion to encourage the provision of the promise that op amps provide with their magnificent specifications - but strangely rarely deliver in audio circuits. We have focus on what might improve them at their voltage pins - at little additional cost ( hopefully )

Suggesting 4 components or less for both voltage rails, its a challenge I agree.