Swapping Op-Amps... you have checked to see it's stable haven't you ? - Page 11 - diyAudio
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Old 21st March 2013, 11:16 AM   #101
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Thank you all - I can't find a schematic for the (older, or original not the new "Super") Eastern Electric Minimax DAC but I will continue to look. The LME9940 is a "common" (DIY, smart? effective? is another issue) swap (with the brown-dog adapters) for the stock I/V conversion stage and output. The stock opamps are I/V 2 x NE5532. After this opamps, there're 2 outputs stages, one based on a single tube (12AU7) and other SS with the opamp NE5534.

Scanning the net reveals many- many - comments about them (LME49990) running hot (in all kinds of audio circuits. It is *apparently* common. The specs allow for a range up to about 185F, but I didn't understand why my right and left channels seem to run at different temps. It could be my laser temp. reader is not consistently hitting the hot-spot on the U2 and U7 due to the angle at which I have to take a reading.

OPERATING RATINGS(1)
Temperature Range
TMIN ≤ TA ≤ TMAX –40C ≤ TA ≤ 85C
Supply Voltage Range 5V ≤ VS ≤ 18V

But, the LME4990 run so hot (occasional reading up to 140F on U1) that I might worry about putting the Minimax lid back on top and driving ambient up inside the unit.

Last edited by dmccoach; 21st March 2013 at 11:43 AM.
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Old 21st March 2013, 12:37 PM   #102
qusp is offline qusp  Australia
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actually the lme isnt exactly the best choice for IV, its a great chip for low noise VAS but not really fast enough for a ES9018 trans-impedance stage IMO. it seems its been chosen due to its (well earned) reputation as a nice low noise opamp for not much money, rather than being useful for the job at hand.

that being said, being on an adapter in a socket that isnt really very well decoupled (not close enough and only using audiophile approved leaded film caps, rather than something usefully small and low inductance...) the right side is very close to the clock, could mean you are picking up stray RF. is it the one on the right in this pic next to the clock thats hotter? I wouldnt worry too much about a tiny opamp heating up the chassis ambient too much, but if they really are that much different, then there may well be something undesirable going on
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Old 21st March 2013, 01:01 PM   #103
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Thanks would you recommend anything over the I/V stage stock NE5532?

Quote:
Originally Posted by qusp View Post
actually the lme isnt exactly the best choice for IV, its a great chip for low noise VAS but not really fast enough for a ES9018 trans-impedance stage IMO. it seems its been chosen due to its (well earned) reputation as a nice low noise opamp for not much money, rather than being useful for the job at hand.

that being said, being on an adapter in a socket that isnt really very well decoupled (not close enough and only using audiophile approved leaded film caps, rather than something usefully small and low inductance...) the right side is very close to the clock, could mean you are picking up stray RF. is it the one on the right in this pic next to the clock thats hotter? I wouldnt worry too much about a tiny opamp heating up the chassis ambient too much, but if they really are that much different, then there may well be something undesirable going on
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Old 12th April 2013, 01:37 PM   #104
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Quote:
Originally Posted by SY View Post
Get a scope. Seriously. Why in the world would someone try to do electronics without the basic tools? May as well try to get by without a soldering iron.
@Mooly : very interesting, thanks for posting.

How much is a used scope, and what sort of things should one look out for when buying one?

Thx
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Old 12th April 2013, 02:30 PM   #105
Mooly is offline Mooly  United Kingdom
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I would always recommend an analogue scope (using a CRT) and one that has (and everyone has their own ideas here) at least 20 to 30Mhz bandwidth with dual trace. Higher if you can get it but if not that will sufficer for most diy (and more) needs.

No idea on prices tbh although auction sites list dozens
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Old 12th April 2013, 03:19 PM   #106
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Default LME49990

I haven't detect temperature issues with the LME49990 used in I/V of my Minimax.

There may be more suitable opamps for the function, but it's the best direct drop option I've already tried. I have tried the opamps bellow that are ranked in my preference order:

- LME49990 (similar to 49720, but more smooth highs)
- LME49720
- NE5532
- OPA2604

For output, the model chosen is ADA4627-1BRZ.
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Old 13th April 2013, 07:44 PM   #107
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Nice thread and one I was looking for as I'd like to swap out the op amps on my MF V-VLPS. I don't own a scope and wondering if this might be an issue? I had "assumed" I could buy the op amps and simple replace them and possibly have a new sound for my amp-turntable??
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Old 15th April 2013, 05:46 PM   #108
Mooly is offline Mooly  United Kingdom
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Instability is always a possibility swapping opamps and is why you should always check with a scope. Without that check you just don't know for sure and as this thread mentions at the start, its not always audibly obvious that a device is oscillating.
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Old 24th April 2013, 02:48 AM   #109
mikej56 is offline mikej56  United Kingdom
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The best thread I've read in a long time - now where did I bury that scope?
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Old 24th April 2013, 03:23 AM   #110
jcx is offline jcx  United States
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new Rigol 'scopes and others have become ridiculously high value propositions in just the past year or so - with GHz sample rate the 50 MHz dual channel sufficiently oversamples for a "analog feel"

buying a new, working 'scope seems like a very practical way to go today
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