Maybe I'm looking at it wrong, but it looks like the precision binned version of the OPA1652, but maybe not (it's hair lower noise and wider GBW). They look a lot alike, nonetheless.
But, CMOS, the horror the horror!
Scott, I was trying to head off your 'cheap shot' relative to Richard Marsh. Now it is out, so let's visit history: We are all proud of our accomplishments and 'breakthroughs' and Richard was certainly one of the FIRST to consider servos added to an audio system to REMOVE the coupling caps. No, he was not the only one, Jim Bongiorno of GAS put one in his Thaedra preamp about the same time as Richard mentioned it in TAS in the middle-late 70's. But Richard has a right to his claim. Jim's GAS servo was way compromised and is not practical today. I 'blew' that one and stayed with cap coupling till about 1979. I did make my first 'common mode servo with a Harris 911 controlling the DC offset of a balanced bridged power amp design in 1969, however.
I am amazed of your good memories, though I didn't exactly use those words as above.
I have not used servos for ages. I use matched devices and good thermal coupling whenever I can. The last circuit I built with servo was Scott's phono pre, and only because I wanted to be true to his circuit. And because of the high gain at DC.
To illustrate my point, here is a circuit of a headphone amp we built, with an optional servo. It is ZGF and has a gain of 10. The circuit is already superceded by later versions, so it is here only for the servo discussion. As you can see, with a first-order 1Hz servo, the servo injects ~0.1% of a 100Hz signal back into the output. And that is already after an attenuation of 10 between servo IC out and injection point.
You can of course argue that even a lousy opamp has a THD of << -60dB at 100Hz. And that will then be a -120dB contribution to the output distortion. Who cares !!!
All correct.
But a TL071 costs 0.5€, a OPA134 2.50€, an ADA4627 7€. What is 7€ in the total costs of a "hi-end" amp ? I know I don't have to make commercially viable amps. But we are here at a DIY forum, right ? Isn't that the whole point about DIY ?
Oh, and of course the SL HPA I built for myself has no servo. It is stable to < +/-5mV after 5 minutes. The output bias is ~150mA.
Cheers, Patrick
7 euros for the ADA 4627 isn’t nessecary.
the OPA 189 has 40 times lower offset, and 7 times lower low frequency noise where it counts in a servo and is 30% of the cost
Offset 3 uv. Vs 120 uv
Noise 1khz 5.2 nv. Vs 6.1 nv
LF noise p to p. 0.1 uv Vs 0.7 uv
Digikey single piece $2.69 Vs $8.69
the OPA 189 has 40 times lower offset, and 7 times lower low frequency noise where it counts in a servo and is 30% of the cost
Offset 3 uv. Vs 120 uv
Noise 1khz 5.2 nv. Vs 6.1 nv
LF noise p to p. 0.1 uv Vs 0.7 uv
Digikey single piece $2.69 Vs $8.69
Hi Patrick,
I have found that if you can get by without a servo you are much further ahead. I get my low DC offsets the same way you do. It's more work, but well worthwhile.
In preamp or source circuitry, I'll still use a capacitor to make sure there is no offset. For low signal levels it is easier to find high quality capacitors, DC servos can cause more damage to the signal than they are meant to improve. If the error signal is injected into the diff pair, you are causing higher distortion than the circuit is capable of.
-Chris
I have found that if you can get by without a servo you are much further ahead. I get my low DC offsets the same way you do. It's more work, but well worthwhile.
In preamp or source circuitry, I'll still use a capacitor to make sure there is no offset. For low signal levels it is easier to find high quality capacitors, DC servos can cause more damage to the signal than they are meant to improve. If the error signal is injected into the diff pair, you are causing higher distortion than the circuit is capable of.
-Chris
I beg to differ, this is an unfair comparison. OPA189 is a chopper op amp and, as such, autocorrelates the 1/f noise (since the 1/f noise appears as a slow varying offset voltage).
If OPA189 is good for servo, then any chopper op amp (with virtually zero offset and offset drift) would be good as well. Quite a few to pick from the last 75 years. I know some tube amplifiers with relay chopper were developed, such a servo in a tube amplifier would be something to brag about.
OPA192. Not a chopper, and much improved DC specs compared to even the OPA140.
And OPA2156 is definitely not a binned version of the OPA1652. It is a significantly more complicated architecture, hence the rail-to-rail input, higher slew rate, and much lower distortion, even at much higher output currents. It also burns twice the power supply current of the OPA1652, so it's not perfect for everything.
OPA192, OPA2156, OPA1652...all CMOS!
And OPA2156 is definitely not a binned version of the OPA1652. It is a significantly more complicated architecture, hence the rail-to-rail input, higher slew rate, and much lower distortion, even at much higher output currents. It also burns twice the power supply current of the OPA1652, so it's not perfect for everything.
OPA192, OPA2156, OPA1652...all CMOS!
> If you don't want a chopper, OPA140 is less than half the price of ADA4627 .....
I had this on the BoM of the Super Linear and on an AD844 portable :
http://xen-audio.com/documents/xen-zgf/XEN ZGF Portable Description V1.pdf
On the Super Linear Current Drive, I had to go for OPA604 as I was using +/-24V rails ;
http://xen-audio.com/documents/cdha/Current Drive for Headphones Public 181006.pdf
Will look at OPA912 in the future. Thanks for the tip.
Patrick
I had this on the BoM of the Super Linear and on an AD844 portable :
http://xen-audio.com/documents/xen-zgf/XEN ZGF Portable Description V1.pdf
On the Super Linear Current Drive, I had to go for OPA604 as I was using +/-24V rails ;
http://xen-audio.com/documents/cdha/Current Drive for Headphones Public 181006.pdf
Will look at OPA912 in the future. Thanks for the tip.
Patrick
Only when I need to cope with 24V rails. No other reasons.
And not for audio amplification. Only for servos and other stuff.
I used to use the OPA2604 for DC detection in a headphone protection circuit.
XEN SHPP -- Simple Headphone Protection
Now my only choice is ADA4522-2.
But it is good to have a 2nd choice if possible.
Patrick
And not for audio amplification. Only for servos and other stuff.
I used to use the OPA2604 for DC detection in a headphone protection circuit.
XEN SHPP -- Simple Headphone Protection
Now my only choice is ADA4522-2.
But it is good to have a 2nd choice if possible.
Patrick
Scott beat me to it, and he's exactly right, an MCM (multi-chip module) consisting of two single op amp dies to make a dual would be pretty tough to bond out, and I'd almost guarantee that it would require a very suboptimal layout for the die. On the other hand, this approach is sometimes used in quad channel amplifiers (two dual op amp dies).
I'm a little removed from the OPA2604 status since changing roles last year, but I can walk across the office next week and ask what the latest is. Consider that the OPA2604 was released to the market in 1991. Which means the masks for it were likely cut in 1990 or 1989 and the semiconductor process on which it is built is even older. Fixing problems in a product that has been manufactured for almost 30 years is not as simple as you'd think!
If you need high voltage supplies, why not just go crazy and use the OPA462 or the ADHV4702? Some very cool HV op amps hit the market at the end of last year! Microchip also came out with the MCP6V51 which is a chopper that can operate on up to 45V supplies. It's not quite as high voltage as ADA4522, or at the ADA4522 / OPA189 level of precision, but definitely an interesting new entry into the market.
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