The best sounding audio integrated opamps

[AndrewT]

Originally Posted by Mooly

Just though of another thing (obvious really). The network of 1k and 68pf feeding an opamp. I haven't got that particular device to hand, but although it's a highly specified device, I'll bet it introduces more HF roll off than the RC network when used at modest gain or even as a buffer.

It seems I did not make my answer clear enough.
We are adding a 1k0 resistor and/or adding a 68pF/100pF cap to a system that already has an RF filter built into it.

The roll-off frequency of this parasitic RF filter is determined by the source resistance/impedance (Rs) of each filter.
The Pot has a source resistance for the downstream side of 8k//12k1 = 4k8
The parasitic capacitance (Cpara) after that could be 5pF or 10pF or 20pF.

The parasitic RF filter is effectively Rs & Cpara.

Before adding the extra components and assumed Cpara=10pF we have RC=0.05us. This will not be audible as an audio frequency filter.

Adding a 1000pF (=1nF) at the RCA input is ludicrous. Try 47pF at the RCA socket and come back with your audibility results.

Now add on 1k0, RC becomes 0.06us, still inaudible.
Now add 68pF alone, RC becomes 0.37us, some may argue this is just approaching audibility.
Now add 1k0 & 68pF, RC becomes 0.45us, even closer to audibility.

Replace the stacked film low inductance 68pF PP with a rolled foil medium inductance 100pF PS and the filter characteristics will be changed. This may be audible. In addition the RC becomes 0.58us. This slope is modified by the inherent inductance of the polystyrene.

I am not talking about swapping boutique /exotic parts for technically competent components. I am talking plain and simple engineering.

[AndrewT]

Originally Posted by Mooly

Just though of another thing (obvious really). The network of 1k and 68pf feeding an opamp. I haven't got that particular device to hand, but although it's a highly specified device, I'll bet it introduces more HF roll off than the RC network when used at modest gain or even as a buffer.

It seems I did not make my answer clear enough.
We are adding a 1k0 resistor and/or adding a 68pF/100pF cap to a system that already has an RF filter built into it.

The roll-off frequency of this parasitic RF filter is determined by the source resistance/impedance (Rs) of each filter.
The Pot has a source resistance for the downstream side of 8k//12k1 = 4k8
The parasitic capacitance (Cpara) after that could be 5pF or 10pF or 20pF.

The parasitic RF filter is effectively Rs & Cpara.

Before adding the extra components and assumed Cpara=10pF we have RC=0.05us. This will not be audible as an audio frequency filter.

Now add on 1k0, RC becomes 0.06us, still inaudible.
Now add 68pF alone, RC becomes 0.37us, some may argue this is just approaching audibility.
Now add 1k0 & 68pF, RC becomes 0.45us, even closer to audibility.

Replace the stacked film low inductance 68pF PP with a rolled foil medium inductance 100pF PS and the filter characteristics will be changed. This may be audible. In addition the RC becomes 0.58us. This slope is modified by the inherent inductance of the polystyrene.

I am not talking about swapping boutique /exotic parts for technically competent components. I am talking plain and simple engineering.

Adding 1000pF at the RCA socket is ludicrous. Try 47pF and come back and tell us the audibility result. The reason for such a low value at the input is that a 2pole filter is being created using the inductance of the preceding interconnects to create an LC RF filter.
The RF filter is not intended to alter and/or modify the audio signal.
It is there solely to attenuate the RF interference and thus allow the audio circuit to do it's job properly.

[abraxalito]

Originally Posted by alexg

I am almost done replacing all the PSU caps with Panasonic FC caps and have done a regulated power supply to replace the wallwart and these tweaks have improved the sound so much.

High gain circuits are especially sensitive to power supply hash, a lot of care is needed in the layout of phono preamps. You might want to check (by examination of the pcb) if the designer has used separate grounds for power and signal - one very common approach is to decouple the opamp supplies direct to the combined signal/power ground. My money would be on there being no distinction on that cheap unit

[HiFiNutNut]

Andrew,

I guess we have moved on from this. The roll-off is not an issue.

See below. Right from the CD output to the opamp input. I have not included parasitic inductance and resistance. But you know they won't affect the result of the roll-off as much. I have made the capacitance to the extreme.

Regards,
Bill

[Mooly]

Originally Posted by HiFiNutNut

After all those discussions, I did one easy experiment. I moved the RF shunt in front of the 20k pot. For convenience the caps are soldered at the input RCA jack. This should completely eliminate the possibility of adding additional parasitics to the opamp because of the RF shunt, or upsetting the bias of the opamps. I did some calculations and found 1nF before the 12k // 8k divider gives a -3dB point at 600kHz, at 16Mhz (gain bandwidth of the opa627 at unity gain) it is -46dB down. In theory, this is completely inaudible.

Hi Bill,
You are losing me on your descriptions of your experiments

You moved the RF shunt (68pf and 1k) to the front of the pot... that's fine, however the filter response is now determined more by what feeds the pot. I'm looking at your circuit in post 1707.
If you feed it from the wiper of the volume control, then the source resistance alters as the pot is turned, this in turn has a small effect on the reponse of the filter. In other words the response is different for every setting of the pot... but the effect is slight with the values used.
Feed the filter from the constant impedance of the CD player output and the response is "fixed".

[AndrewT]

Originally Posted by HiFiNutNut

I guess we have moved on from this. The roll-off is not an issue.

no, you haven't. You are still reporting anomalous results from your tests because you have not understood why an RF filter does it job and why it uses particular component/circuit values to achieve an inaudible attenuation of RF.

Post 1754.
Are R1, R2 and C1 inside the source CDP?

[Mooly]

Originally Posted by HiFiNutNut

I did some calculations and found 1nF before the 12k // 8k divider gives a -3dB point at 600kHz, at 16Mhz (gain bandwidth of the opa627 at unity gain) it is -46dB down. In theory, this is completely inaudible.

Hmmm... where's the 1nf come from ?
I'm looking at your circuit and you feed the 20 k pot from the CD player output which will be at a fairly low Z.

[Joachim Gerhard]

Hi Alexg !
You are on the right track. Pannasonic FC is a good replacement and the sound may get a bit smoother over time.
0.47 uF seems a bit small to me.
What is the input impedance of your next ( line ? ) stage ?
For 47 kOhm i whould recommend something around 2uF.
The RIAA curve in the V-LPS should be accurate. Those guys are not stupid but may as you say have used affordable components to keep the price down.

[HiFiNutNut]

Mooly, I guess we typed at the same time. Can you see post #1754?

[Mooly]

A$nd you have just posted a new circuit

1nf is too high to use on the output of an opamp (in practice).

Andrew keeps mentioning parasitic capacitance etc.
That's one reason I mentioned earlier that this was a real world measurement taking into account the 'scope probe capacitance. I nearly posted another picture of the result at HF using a 1 to 1 probe to show you.
When you are talking about small cap values (pf range) then circuit layout and stray capacitance starts to become more and more important... and you can even make small caps on the PCB using close running print, not just caps but inductors too, and this is often done in RF work.

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