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Old 16th January 2002, 07:11 PM   #11
Geoff is offline Geoff  United Kingdom
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HB

Please do not confuse the numbers. You refer in your last post to the NE5532 and then make reference to Rod's diagram (which uses the NE5534) and 22pF capacitors.

The NE5534 is a single op-amp which is stable for gains equal to or greater than three. For gains below this, additional external compensation is required to ensure stability. This takes the form of a capacitor connected between pins 5 and 8 with a typical value of 22pF.

The NE5532 is a dual op-amp that is unity gain stable and does not require external compensation. It is basically two NE5534s in one case, each with additional internal compensation to provide the unity gain stability.

The TL071 (single), TL072 (dual), TL081 (single) or TL082 (dual) could be used in your application and have the benefit of being cheap. However, the NE5532 and NE5534 are better op-amps for audio applications. I would suggest that you use the NE5532 since it is cheaper than two NE5534s and also saves the cost of the compensation capacitors. You will also see if you visit Doug Self's site that it's measured performance, so far as %THD is concerned, is good even when compared with some op-amps costing many times more.

Geoff
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Old 16th January 2002, 07:19 PM   #12
paulb is offline paulb  Canada
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Quote:
Originally posted by hugobross
I just wonder if someone can hear in this case the difference in quality between those relative expensive opamps and the cheap ones such as the NE5532?
I wonder that too. My guess is, in most cases, probably not. You can always use sockets (not cheap ones!) and try it out.
My understanding is that the NE5532 is a workhorse in the audio and recording industry, so it's a good reference point anyway.
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Old 16th January 2002, 07:44 PM   #13
Geoff is offline Geoff  United Kingdom
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AudioFreak

Many manufacturers indicate in the general description or features section of the datasheet whether the op-amp is unity gain stable or not. If it is not, they normally quote the gain below which additional external compensation is required and give suggested capacitor values for different gains somewhere later in the datasheet.

National Semiconductor do not make things so easy in that they do not make reference to unity gain stability. You need to look for statements such as "External components are used to frequency compensate the amplifier." or "The LMxxx is a general purpose operational amplifier with external frequency compensation." or "The amplifier is internally compensated and stable for gains of 10 or greater."

In the absence of any of these statements it is reasonable to assume that the op-amp is unity gain stable. As a cross check, have a look at the pin-out diagram. If there is a need for external compensation under certain conditions of operation you will find a pin labelled 'comp'. The schematics in the 'typical applications' will also show if a capacitor is required, as will the the graphs of 'frequency response for various closed-loop gains' which will normally quote the capacitance value at the various gain levels plotted. In addition, sometimes there is a graph showing the compensation capacitor value at different gains etc.

If you check the Nat Semi datasheet for the LM411 against these criteria, you will find (by default) that it is unity gain stable, despite there being no specific statement to this effect.

Geoff
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Old 17th January 2002, 12:44 AM   #14
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Thanks alot Geoff.
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Old 24th January 2002, 09:45 AM   #15
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Hi HB
Sorry about the time delay, been away for a while (got to earn soe money now & again). Trying to get it to a size I can attach, will do it soon.

Dieterd
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Old 24th January 2002, 12:38 PM   #16
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HB
Here is the schematic

Dieterd
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Old 24th January 2002, 12:44 PM   #17
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thanks!!,

looks nice and simple, that's the way we like it....

HB.
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Old 6th February 2002, 11:04 AM   #18
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Hi, I've finished my exams with success, so I'm back on the road...

I was thinking of something I've publiced here. I'm quite sure it will work, but I've still a doubt: will there be a bigger influence of distortion at the input because of the high impedance of the opamp (is an extra parallel resistor needed here?)

Best regards,

HB.
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Old 6th February 2002, 02:09 PM   #19
paulb is offline paulb  Canada
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Congratulations on finishing your exams with success!
A couple of suggestions: You should have an input resistor to ground to keep the first op-amp's input at zero if there's nothing connected. You should probably have an input capacitor unless you can guarantee that your source will always have no DC offset. And don't forget to bypass the opamp's power supply pins.
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Old 6th February 2002, 04:13 PM   #20
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Thanks PaulB!!!

Indeed, you're right, I will place a 10k (or 4k7) input resistor to avoid/reduce distortion. There's no extra capacitor needed because the source (a quality mixer) is DC-protected.
After building this design, I will build a own designed amp that I will publish soon here; I've had bad experiences with other designs on the net so I just decided to do it myself. At school everything runs fine by now, especially on designing amp's (thanks to this forum).

HB.
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