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 Post subject: OP27 vs OP37
PostPosted: February 3rd, 2014, 11:23 am 
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This is mainly for tomp, but I figure we could all benefit from the answer...

So, I was looking at the AD datasheets for the the OP27 & OP37. They're pin-to-pin compatible, so they could easily be substituted for each other in many circuits. tomp said/implied at the meeting last month at my place that the OP27 was a better choice for an MM RIAA stage, while he preferred to OP37 for an MM step-up. The OP37 datasheet recommends the OP37 over the OP27 for gains >10, with either being a good choice for gains between 5 and 10, OP37 not recommended for gains <5.

So, assuming a two-stage MM RIAA pre-amp, (is that correct for your design Tom?) I would assume the gain of each stage is >10, so why the preference for the OP27 in this application? :confusion-confused:

Roscoe

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 Post subject: Re: OP27 vs OP37
PostPosted: February 3rd, 2014, 6:35 pm 
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Roscoe:

I tried to answer this earlier but when I went to post the reply it crashed. So here I go again.

The RIAA stage is a single stage very similar to what is listed in the data sheet and the "Electrionic Design" 1980 article. Since the RIAA network is in the feedback loop and drives the amp gain to unity at 20KHz, you cannot use the OP37 as it is not stable for gains below 5. However, in the SUT replacement stage, the gain requirement of 10 is consistent at all frequencies. Therefore the higher gain bandwidth product of the OP37 gives lower distortion at the high end of the scale where the gain remains high.

You can use the OP27 for both stages but give up some minor amount of distortion at the highest frrequencies. Whether or not that would be audible is questionable but since both devicces have the same noise specs and price why not use the OP37 in the SUT stage? I'm including the gain vs frequency and output swing vs frequency curves for both. The OP27 curves are in this message and the OP37 curves in another because we can only atach three files to each message.


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OP27 gain vs freq.jpg
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OP27 output vs freq.jpg
OP27 output vs freq.jpg [ 62.08 KiB | Viewed 21211 times ]
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 Post subject: Re: OP27 vs OP37
PostPosted: February 3rd, 2014, 6:37 pm 
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Here are the OP37 curves.


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OP27 gain vs freq.jpg
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OP37 output vs freq.jpg
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 Post subject: Re: OP27 vs OP37
PostPosted: February 4th, 2014, 11:24 am 
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TI advertises the OPA227/OPA228 as improved replacements for the OP27/OP37 respectively. Any experience with the TI (Burr-Brown I imagine) parts?

Roscoe

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 Post subject: Re: OP27 vs OP37
PostPosted: February 4th, 2014, 11:54 am 
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Roscoe:

I looked at just a few specs of the TI devices and I don't see where they are an improvement in this application. The GBP of the OP37 is actually higher at 63 MHz than the 33 MHz of the OP228. The slew rate of the OP37 is 17 volts/us with the OP228 10 volts/us. The OP227 has a slightly faster settling time of 5ns, but none of those differences in either case will result in any audible difference. With the gains and frequencies required any of them will be indistinguishable. Again, the feedback components will be more important.

Tom


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 Post subject: Re: OP27 vs OP37
PostPosted: February 5th, 2014, 11:31 am 
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Interesting discussion! Do I see a shoot-out in the future?

(I still have to finish my tube phono amp, so don't expect me to participate soon. :oops: )

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 Post subject: Re: OP27 vs OP37
PostPosted: February 5th, 2014, 12:22 pm 
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If there would be some interest in a comparison we could build two phono stages with identical feedback components but different OP Amps and compare them. I have an ABX test box which will allow us to exactly set up the test, do the comparisons in a double blind manner to eliminate biases and then give us the results of our conclusions. Any takers?

If we are going to do this I would then like to write up the results in "AudioXpress". I would keep the identities of all the participants anonymous of course. As a side benefit, I would do the board layouts which would include servo control of offsets so that they could be direct coupled with no capacitors necessary. I would then make the boards available to anyone who wants them at my cost.


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 Post subject: Re: OP27 vs OP37
PostPosted: February 14th, 2014, 5:00 pm 
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tomp wrote:
Roscoe:
The RIAA stage is a single stage very similar to what is listed in the data sheet and the "Electrionic Design" 1980 article. Since the RIAA network is in the feedback loop and drives the amp gain to unity at 20KHz, you cannot use the OP37 as it is not stable for gains below 5.


Actually, I think it's well above 20kHz before it hits unity gain, but it would still create problems using an OP37... However, IF you include the controversial 3rd pole that Allen Wright was pushing ("Enhanced RIAA"), the gain never gets close enough to unity to be a problem...

Roscoe

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