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Opamp Rolling, Does It Work?

solderdude

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Two 4556 can already give 200ma of output current.

For practical purposes it is a little less than that.
80mA (for symmetrical distortion free signal) per opamp = 160mA for 2, but this is peak current so 'merely' 115mArms.

To increase current capabilities perhaps adding 1 dual opamp (with VERY short wiring to the boards) and 2 extra resistors would be the easiest and best way to go about this.... in this particular O2 case.
 

JohnYang1997

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For practical purposes it is a little less than that.
80mA (for symmetrical distortion free signal) per opamp = 160mA for 2, but this is peak current so 'merely' 115mArms.

To increase current capabilities perhaps adding 1 dual opamp (with VERY short wiring to the boards) and 2 extra resistors would be the easiest and best way to go about this.... in this particular O2 case.
You need to look at the voltage vs current graph. It's easily 100ma for normal use cases. 100ma at 7V rms. Adding more is possible but very not elegant in o2 without redo the board.
 

JohnYang1997

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I think he means the feedback resistor is not necessarily in the way of connecting the outputs together.
But the output of each opamp is directly connected to the inverting input in the pcb layout in Objective 2. Unless redo the pcb, it will not work with o2. Or maybe he wasn't answering the guy who asked about o2?
 

syn08

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But the output of each opamp is directly connected to the inverting input in the pcb layout in Objective 2. Unless redo the pcb, it will not work with o2. Or maybe he wasn't answering the guy who asked about o2?

So, to summarize so far: you would need an op amp with low noise(?), stable at +1 gain and compatible with an existing PCB (no feedback resistor allowed, only PDIP or SOIC case).

Couldn't you specify these to start with? There are options (you already got a few, there are more, but then I'm sure you'll come up with more requirements). Do you have a point to make in perpetrating this charade?
 

JohnYang1997

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So, to summarize so far: you would need an op amp with low noise(?), stable at +1 gain and compatible with an existing PCB (no feedback resistor allowed, only PDIP or SOIC case).

Couldn't you specify these to start with? There are options (you already got a few, there are more, but then I'm sure you'll come up with more requirements). Do you have a point to make in perpetrating this charade?
It wasn't me.
Mr Silou asked
"Is there any good opamp that can deliver more current than the stock o2 opamps, is easy to implement and stable into low loads?" in last page.
I was basically answering his question.
 

ajawamnet

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Current feedback opamps are totally different beast than the typical voltage feedback like those used in audio

see this:
https://www.analog.com/media/en/training-seminars/tutorials/MT-034.pdf

A few years ago I had to design a circuit that used a CFB opamp - the THS3122 for a pulse amp that had to drive a 50' of crap wire at 2.5W into a 50 ohm load with a 20ns rise time @ 10Vpp.

They do not use the same topology and really shouldn't be used in place of a VFB type - from that app note:


• Unlike VFB op amps, CFB op amps do not have balanced inputs. Instead, the non-inverting
input is high impedance, and the inverting input is low impedance.
• The open-loop gain of CFB op amps is measured in units of Ω (transimpedance gain) rather
than V/V as for VFB op amps.
• For a fixed value feedback resistor R2, the closed-loop gain of a CFB can be varied by
changing R1, without significantly affecting the closed-loop bandwidth. This can be seen by
examining the simplified equation in Figure 3. The denominator determines the overall
frequency response; and if R2 is constant, then R1 of the numerator can be changed (thereby
changing the gain) without affecting the denominator—hence the bandwidth remains
relatively constant.


The CFB topology is primarily used where the ultimate in high speed and low distortion is
required. The fundamental concept is based on the fact that in bipolar transistor circuits currents
can be switched faster than voltages, all other things being equal.



A common error in using a current feedback op amp is to connect the inverting input directly to
the output in an attempt to build a unity gain voltage follower (buffer)
. This circuit will oscillate
because the equivalent feedback resistor value is zero
. The follower circuit can be stabilized by
simply connecting the inverting input to the output using the recommended feedback resistor
value.
Another difference between the VFB and CFB amplifiers is that the inverting input impedance of
the CFB amp is low (typically 50 Ω to 100 Ω), while the non-inverting input impedance is high
(typically several hundred kΩ). Therefore, the inputs of the CFB amp are not balanced, as is the
case with VFB amps.



One primary difference between the CFB and VFB amps is that the CFB amplifier does not have
a constant gain-bandwidth product. While there is a small change in bandwidth with gain, it is
much less than the 6 dB/octave we see with a VFB op amp. This is shown in Figure 4. As
previously mentioned, the bandwidth of a CFB op amp is proportional to the feedback resistor.
For every CFB op amp there is a recommended value of feedback resistor for maximum
bandwidth. If you increase the value of the resistor beyond this value, you reduce the bandwidth.
If you use a lower value of resistor below this value, the phase margin is reduced, and the
amplifier may become unstable.


In general, VFB amplifiers offer:
• Lower Noise
• Better dc Performance
• Feedback Component Freedom
while CFB amplifiers offer:
• Faster Slew Rates
• Lower Distortion
• Feedback Component Restrictions
 

NoNameNPC

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Maybe noob question, all opamp have noise, when sample amplified by opamp it also add noise/square root hz? Or I am wrong? Is this audible?
 

SIY

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Maybe noob question, all opamp have noise, when sample amplified by opamp it also add noise/square root hz? Or I am wrong? Is this audible?

There's a lot garbled in there, but the simple answer is that op-amp noise can be heard if you use a severely incorrect part choice or screw up the circuit.
 

trl

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Maybe noob question, all opamp have noise, when sample amplified by opamp it also add noise/square root hz? Or I am wrong? Is this audible?
If used in gain stage you will measure and even hear the diff. of noise between opamps.

I was able to spot the diff. in O2 (with NJM2068 I hear a lower noise than using NE5532, 6.5X gain) and in HPA-3B (with OPA1652 I hear a lower noise than using LME49860, in high gain) only when swapping opamps in gain-stage.
 

AnalogSteph

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That's funny, 'cause I've been arguing for years that the O2 gain stage is needlessly low-noise (generally swamped by typical sources), compromising on real-world distortion performance instead as the NJM2068 breaks a bit of a sweat at high levels. I've been suggesting upping feedback resistor values by a factor of 2 even in the regular version, especially if you need the 6.5X gain. (Things are a bit more relaxed in 2.5X, a lot more so in 1X.)

If you do the math, both NJM2068 and NE5532 have an equivalent input noise level of below -120 dBV (1 µV) in the stock 6.5X gain circuit (1k5/274R). That's -126 dBFS relative to a 2 Vrms DAC output. There should not be any meaningful difference left at a DAC dynamic range of about 116 dB or less (or 110 dB if you're stuck with onboard audio levels).

If noise down that far is even audible to begin with, your gain staging needs some work to say the least. If a noise level that low ends up at >=20 dB SPL, imagine where your peak signal output is - in all likelihood, 130+ dB SPL. Ouch.

Of course you can find a scenario in which the noise difference between the two types becomes plainly audible - input shorted, excessive gain, full volume, sensitive headphones / IEMs. But is that a realistic use case? Hell no. If you were to connect your regular source under the same conditions, you'd be getting a ton of noise and ear-splitting levels.

Opamp noise becomes critical when handling really low-level signals. We're talking phono or microphone inputs, where nominal signal levels may be mere millivolts, or headphone amplifier stages after the volume control when using sensitive loads. A line-level input just isn't as demanding by far. (That's pretty much the point of line level. It can be picked for comfortable signal handling, being unencumbered by the needs of any of these pesky transducers.)
 
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JohnYang1997

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That's funny, 'cause I've been arguing for years that the O2 gain stage is needlessly low-noise (generally swamped by typical sources), compromising on real-world distortion performance instead as the NJM2068 breaks a bit of a sweat at high levels. I've been suggesting upping feedback resistor values by a factor of 2 even in the regular version, especially if you need the 6.5X gain. (Things are a bit more relaxed in 2.5X, a lot more so in 1X.)

If you do the math, both NJM2068 and NE5532 have an equivalent input noise level of below -120 dBV (1 µV) in the stock 6.5X gain circuit (1k5/274R). That's -126 dBFS relative to a 2 Vrms DAC output. There should not be any meaningful difference left at a DAC dynamic range of about 116 dB or less (or 110 dB if you're stuck with onboard audio levels).

If noise down that far is even audible to begin with, your gain staging needs some work to say the least.

Of course you can find a scenario in which the noise difference between the two types becomes plainly audible - input shorted, excessive gain, full volume, sensitive headphones / IEMs. But is that a realistic use case? Hell no. If you were to connect your regular source under the same conditions, you'd be getting a ton of noise and ear-bleeding levels.

Opamp noise becomes critical when handling really low-level signals. We're talking phono or microphone inputs, where nominal signal levels may be mere millivolts. A line-level input just isn't as demanding by far.
hmmm. That not exactly correct is it?
The noise that matters is the output noise not the input referred noise. Also how input referred noise has anything to do with gain?
However tho, the SNR is preserved because the volume control is after the gain stage. There's potential clipping issue but you get lower noise with higher gain(i would argue it's meaningless on the other hand). The pot is the actual reason that the noise of gain stage is not audible.
In o2, the output stage is relatively noise free especially for the time but 2uV is not exactly low noise, some iems produce audible noise even with 0.8uV noise. -120dBV is not so clean when sensitivity is over 145dB/V.
 

AnalogSteph

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The noise that matters is the output noise not the input referred noise. Also how input referred noise has anything to do with gain?
Always depends on what you are looking at. On the input side, you'll have some given input signal and noise levels to compare to, so it's most useful to look at input-referred performance.

Gain does not figure in directly but indirectly, through the choice of feedback network resistors that will (at least in part) determine surrounding impedances.
However tho, the SNR is preserved because the volume control is after the gain stage. There's potential clipping issue but you get lower noise with higher gain(i would argue it's meaningless on the other hand). The pot is the actual reason that the noise of gain stage is not audible.
Pretty much. In practice, it is dwarfed by source noise and both get attenuated into oblivion.
In o2, the output stage is relatively noise free especially for the time but 2uV is not exactly low noise, some iems produce audible noise even with 0.8uV noise. -120dBV is not so clean when sensitivity is over 145dB/V.
To be fair, at least some of these grass growing detectors are meant to be used with a 10 ohm output impedance, making things several dB less critical. Noise requirements for IEMs seem to be somewhat relaxed in general due to all the blood circulating racket generated just by having something plugged into your ear... I don't think you need a great deal less than 20 dB SPL or so. Feel free to point me towards observations indicating the contrary though.
 

trl

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@AnalogSteph, feel free to try a pair of sensitive IEMs on O2 for yourself, with few opamps in gain-stage. What I stated below is what a fact of I've heard, because I personally heard the difference in background noise (tested with pot. to the max. and to 1/2). Also, LME49720 and LME49860 are having identical specs (besides the max. voltage rails) but inside the HPA-3B the former is having a way bigger noise than the first; unless the former is a fake (I doubt that), then probably some opamps are outside TI's specs (better or worse, depends). Same happened to me with the DC-output on many direct-coupled headamps: an opamps gives me -1.5mV of DC (@25C) and another one gives me 35mV (@25C), although I'm using identical opamps and from the same lot/batch (OPA1602/1612/1652 etc.). With AD opamps things are better (AD797/8599/8672), opamps sharing similar DC values on the output.

However, this is my personal experience, so please forget about the datasheet and do some test by yourself, maybe your findings will differ. Thanks!
 

maverickronin

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Where on earth are you guys finding low enough background noise to hear electrical noise from the O2?

My O2 is dead silent with 119dB/mW Shure SE530's.
 
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