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Old 14th February 2010, 07:38 PM   #1
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Default Dumb op-amp question from complete newbie

Hi,
I am attempting the following mod project:

Photo 1 of 17, Rotel RQ970BX HFW -Andy Grove upgrade

My question is - can I use a pair of OPA627AP's or AD843JNZ's instead of the specified AD743JN without any other changes to the project instructions? They seem to be a lot easier to get hold of.

To be honest, I know nothing about electronics - but I can solder and I can follow instructions. I've got as far as mounting the existing op-amps into sockets without destoying anything

Thanks in advance for any help,
Tim.
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Old 14th February 2010, 08:14 PM   #2
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Check the pin-outs on your replacement set of OP-Amps...the pin-outs MUST be exact. Check for specifications also.
Are you replacing these Op-amps for a "cleaner" sound??
The general characteristics of Op-amps are:
Infinite input impedence
Infinite gain
Dead short output impedence
Infinite frequency capabilities
Obviously these characteristics are impossible..more like 10M Ohms input, 200K gain, 100 Ohm output, 50K Hz "F".
Although it doesn't quite hit the "ideal" characteristics it is close enough.
There is a fifth..but I have forgotten.
An Op-amp without negative feedback will crank the gain up so high as to slam an AC signal to the positive and negative PS rails effectively making a square wave generator.
Search Wiki-pedia for Op-amps...great things these little silicon beetles!

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Old 15th February 2010, 09:54 AM   #3
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Thanks, I appreciate your reply.

I did take a look at the datasheets before posting here, but I have to say that I'm a bit out of my depth, which is why I thought I'd seek some expert advice.

From the datasheets, the pins for the different chips are described as follows

AD744JN (existing chip)
pin1 NULL/DECOMPENSATION
pin2 -IN
pin3 +IN
pin4 V-
pin5 COMPENSATION
pin6 OUTPUT
pin7 V+
pin8 NULL/COMPENSATION

AD743JN (recommended in the mod project)
pin1 NULL
pin2 -IN
pin3 +IN
pin4 -Vs
pin5 NULL
pin6 OUT
pin7 +Vs
pin8 NO CONNECT

OPA627AP
pin1 OFFSET TRIM
pin2 -IN
pin3 +IN
pin4 -Vs
pin5 OFFSET TRIM
pin6 OUTPUT
pin7 +Vs
pin8 NO CONNECT

AD843JN
pin1 BALANCE
pin2 -IN
pin3 +IN
pin4 -Vs
pin5 NO CONNECT
pin6 OUTPUT
pin7 +Vs
pin8 BALANCE

I was unsure whether the differences in this table were significant, or even possibly just terminology differences (especially seeing as the AD743JN is the one being recommended to replace the AD744JN).

As you can see, I'm sruggling with the basics, so it worries me a little when you say to "also check for specifications"! I was hoping some kind soul would give me a yes or no as to whether these can be used as drop-in replacements.

Once again, thank you for your help.
- Tim

Last edited by indypants; 15th February 2010 at 09:59 AM.
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Old 15th February 2010, 11:58 AM   #4
godfrey is offline godfrey  South Africa
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Hi Tim

Not quite a simple yes/no but...

Stability
I don't know what the Rotel's circuit looks like, but both the OPA627 and AD843 are unity-gain stable so they should behave themselves nicely.

Note that the OPA637, for example, is not unity-gain stable and might decide to oscillate at high frequencies (depending on the circuit).

DC offset
All the opamps you mentioned are fet-input devices with very low input bias currents so DC offset shouldn't be a problem when swapping.

Bipolar input devices like the NE5532 could cause grief in that department though (depending on the circuit). btw: sorry to use the 5532 as an example, but I'm not too familiar with the fashionable new stuff.

Noise
Dunno if that's part of the reason you want to swap chips but here's the deal anyway:
AD743 is much better than AD744
OPA627 is not quite as good as AD743 but is still much better than AD744
AD843 is a bit worse than AD743

What you actually asked about
Pins 1, 5 and 8 are used differently in all these chips.
OTOH, they don't have to be used and probably aren't.

They're only for adjusting the DC offset at the output, so unless there's a little trimpot somewhere for that, they're probably not used.

If you're not sure whether they're connected to anything in the Rotel, you can cut the offending legs off the chips just to be sure (seriously!). These are pins 1 and 5 for the OPA627 or pins 1 and 8 for the AD843.

Hmm...
On second thoughts I suppose that can all be wrapped up into a short answer:

Yes - Either the OPA627 or AD843 should be fine, but cut off pins 1, 5 and 8 from either just to be safe. (and make very sure you're looking at the chip the right way up before you start cutting )

Cheers - Godfrey
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Old 15th February 2010, 12:35 PM   #5
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Hi Godfrey, thanks for taking the time to provide that very informative reply.... I think I even understood most of it too

This project was published in Hi-Fi World magazine in 1994. All the details, including the Rotel circuit diagrams, are available from the link in my first post - but they mean very little to me. Would I be asking too much for you to have a quick glance at that for me to see if there are any obvious pitfalls?

No problem if not - you have been a great help already, and from what you've said, I think I'm confident enough now to try my luck with one of the alternatives if I'm unable to get hold of the AD743JN. When I soldered the sockets in place I did notice that some pins were unused, so I will check to see if this tallys up with what you said.

All the best,
Tim
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Old 15th February 2010, 01:30 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
I see two choices,
1.) start on something much simpler and gradually build up your knowledge.
Maybe a passive pre-amp to develop assembly skills and recognising components and values. Then add on a +6dB gain stage to make an active pre-amp. Learn about dB and compensation and decoupling and grounding etc.


2.) insert a socket in place of the 744. and plug in your 743
Listen to how it sounds. Make the modifications exactly as AG has described.
Build up your knowledge until eventually finding why the 743 performs better than 744 in this implementation.
Do NOT modify Andy Groves' design until YOU are able to make informed decisions on what each change may do and how to ameliorate the problems.

3.) soldering in the wrong opamp alternative is not an option with your present level of knowledge.
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Old 15th February 2010, 02:24 PM   #7
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Hi Andrew, thanks for your advice, but I should probably have explained that this is a "once-in-a-blue-moon" project for me - I'm not really a hobbyist, but I do enjoy building stuff. Clearly I don't have the skills to modify this design myself, but having said that, the article does hint at trying different op-amps, so I was here for some advice on the alternatives - especially as the 743 is proving difficult to find at a reasonable price. I definitely take your point about listening to them first. As I said, I've already put in some sockets for this.

Just here taking advantage of the wealth of of expertise on these forums

Cheers,
Tim.

Last edited by indypants; 15th February 2010 at 02:29 PM.
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Old 15th February 2010, 02:28 PM   #8
AndrewT is offline AndrewT  Scotland
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don't go cutting off pins.
bend the pin 1, and/or 5 and/or 8 to just miss the socket. You may need some or all of those pins to get each opamp sounding it's best. They are unlikely to sound the same if you simply swap between them.
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Old 15th February 2010, 07:56 PM   #9
tomchr is offline tomchr  United States
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Default Op-amp nulling - potential pitfall

Check the data sheet for the op-amps involved. Specifically, look at the typical application schematic - or any other application schematic that shows how to hook up a potentiometer to the NULL/BALANCE/OFFSET adjust inputs. You need to follow this exact hook-up when you use the op-amp. In most cases, you hook the ends of the potentiometer to the two balance inputs and the wiper to the negative supply. But on some op-amps the balance pot goes to the positive supply - or is wired completely differently. If you mess this up, you'll fry the op-amp. Not a huge deal, but it's pretty annoying to flush $5 down the toilet that way.

~Tom
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Old 15th February 2010, 09:24 PM   #10
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by tomchr View Post
Check the data sheet for the op-amps involved. Specifically, look at the typical application schematic - or any other application schematic that shows how to hook up a potentiometer to the NULL/BALANCE/OFFSET adjust inputs. You need to follow this exact hook-up when you use the op-amp. In most cases, you hook the ends of the potentiometer to the two balance inputs and the wiper to the negative supply. But on some op-amps the balance pot goes to the positive supply - or is wired completely differently. If you mess this up, you'll fry the op-amp. Not a huge deal, but it's pretty annoying to flush $5 down the toilet that way.

~Tom
Thank you!

Actually, I did some homework before posting above and the suggested replacement chips actually are different to the original in terms of the balance connecting to positive instead of negative.

Hence my suggestion to chop the legs just in case.
As you say - "pretty annoying to flush $5 down the toilet"

@Andrew:
Bending the pins out the way works too. Actually that's probably how I would do it, but for different reasons. The cash-strapped miser that I am, I would be thinking: "what if this doesn't work - I might want to use the chip for something else later, and then I might need those pins"

@Tim:
Quote:
Originally Posted by godfrey View Post
...I'm not too familiar with the fashionable new stuff.
Quote:
Originally Posted by indypants View Post
This project was published in Hi-Fi World magazine in 1994...
D'oh - OK, so I'm that old - thanks for the reminder!

I did follow your link - couldn't find an article or explanation for the swap, just a photo gallery. Anyway, I did find the circuit for the Rotel which shows that:
a) The chip-swap should be fine
b) The Rotel also uses NE5534, which makes me feel a bit better

Well done on putting in a socket, btw. That'll help no end with quick swaps for evaluation etc. When the dust has settled, it might be worth soldering in the chip of choice directly though (without socket) to avoid problems down the line with bad connections etc.
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