741 opamp Replacements, upgrades

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shuchoco

Well-known member
Joined
Mar 20, 2009
Messages
89
Hi,

I just bought this weird and kindof cool little one-off console made by a company called Decibelab (mostly homebrew i think -- though i understand the guy who built this was an engineer at Aengus).  I'm planning on doing a recap,  and decided i should investigate other component upgrades.

The console was built in 1976,  which I understand was the heyday of the 741 opamp -- this thing has lots of 'em. 

I've attached the schematic, and a photo of the populated module that points out where the 741s are (not sure if thisd is useful).  On the actual module there are 741s where the schematic shows a 536 opamp.  I'm not knowledgeable enough to understand how or why the 536 is replaced with the 741. 

The EQ has the 741 in each point of the array,  and the mic pre has the 741 where the 536 shoulc be (as mentioned).  Is there a better sounding opamp that i can substitute for the 741?

I realize the topic of 741 replacements has been covered a bit on here, so sorry in advance if this is a redundant post. 

I am a serious noob with this stuff, so feel free to castrate me.  ;D

Any help or advice or flaming would be most appreciated.


2isyr7l.jpg

1538xuv.jpg




 
With adding a couple of stabilising capacitors, NE5534 could probably be used. See it's data sheet for proper stabilisation. I'm not sure you can get them in those packages, so you may need to solder extension-pins to them to get them to fit.

That output driver stage freaks me out.... I hope it's just an error in the schematic.

Jakob E.
 
thanks jakob.

i'll look into the NE5534.

also, are you familiar with this 536 opamp that is in the schematic?  why might it have been replaced in the actual module?  does it even matter?

what about that output stage?  i'm in over my head here!
 
shuchoco said:
thanks jakob.

i'll look into the NE5534.

also, are you familiar with this 536 opamp that is in the schematic?  why might it have been replaced in the actual module?  does it even matter?

what about that output stage?  i'm in over my head here!
It is very likely the 536 would tend to oscillate; it was a common plague with "fast" opamps in the 70's.
Proper fix was to install a decoupling resistor and a Zobel, but many designers facing oscillations took the easy way of replacing with sluggish opamps.
The 5534 is a nice improvement over the 536,, but you also have to find a substitute for the 741's in the EQ section (I can imagine it would be quite noisy). The 5534 can be used there also, but I'm afraid the current draw would be excessive. TL071's, although not being very low noise, are yet better than the 741, and the current draw is good. There are a few better single opamps, but the price is deterrent.
 
if price was no issue what would you suggest to replace these babies?  how much are fancy opamps?

thanks for the input!
 
LME49710HA metal-can from National. Read the datasheet for decoupling requirements.
 
The  LME49710HA is certainly fine, if cost truly is no object.

Pretty much anything you can buy and plug into the 741 hole, that works, will be a sonic improvement.

I'd start with a broad upgrade to something like the 5534, and then after you get used to that new improved sound, selectively look at upgrading some of the more difficult circuit positions, like summing amps and such.

JR
 
for using the 5534 should be connected its 5 and 8 pins with a 22 pF cap to avoid oscillations since the 741s work at unity gain configuration
 
the internet seems to think the opa637 is great for audio.   is that a potential drop-in substitute here?

i get the sense that the community consensus that the 637 is appreciably "better" sounding than the 5534,  though significant;y more expensive. any thoughts on this in relation to the circuit here? 

i'm thinking, to save cost, maybe i want to use the "nicest" most expensive opamps in key places (if that means anything) and maybe a lesser expensive one (like the 5534) in the EQ array.   is this strategy realistic/possible/stupid with what i have here?

 
opa637 are not indicated for unity gain like run the 741 in your eq, opa627 is indicated , I council you moreover the LME49710 (National) that are more cheap than opa627 .
Pier Paolo
 
opa627 is the opa637 unity gain stable version, sound better than NE5534 . 5534 is used by in neve, amek , ssl and more gears , and sound good but with a vintage sound. If you want a good vintage sound you should use the 5534 but if you want a modern sound council you the opa627 (very expensive) and the cheap LME49710 (but sound better than 5534). For use 5534 in your gear you should put between 5 and 8 pin a 22pF cap, if you want the opa637 you should use its unity gain stable opa627.
Pier Paolo.







 
the direc replacement for 5534 is the opa627.  If there is a cap on the pin 5 and 8 of the 5534, this cap must be removed before to replace the 5534 with opa627 or another op amp.
Opa627 is a very exspensive op amp so I council you to replace 5534 with  LME49710 tha's a very good amp but cheap.


Pier Paolo.
 
"49710HA"-s aren't really cheap by any stretch of imagination, are they? But they are _definitely_ more cost-effective than 627's. TBH, these "HA"-s are the "better" "single"-LM4562.

(The word of the www is such that these like the "A-class" pull to the positive rail better than to the negative, if you decide to perform an all-out pimping.. -similar to the 5534s in this regard)

there's user's "benchmarks" all over the internet.. diyaudio etc..
http://www.head-fi.org/forums/f6/lme49710-vs-opa627s-294317/

I'd say: if you decide to go with either of expensive "metal-cans", you should (!!!) upgrade the other parts of your circuit as well - recapping, cleaning, etc. AND do the proper DC decoupling procedure - as prescribed in datasheet.

(I know the "sound" of 741, it mushes and glues the signal together, which may be a Good Thing in some guitar circuit - in fact it is - but when you put it in some other chain, the opamp-swapping can become an eye-opener.)

 
tv said:
"49710HA"-s aren't really cheap by any stretch of imagination, are they? But they are _definitely_ more cost-effective than 627's. TBH, these "HA"-s are the "better" "single"-LM4562.

(The word of the www is such that these like the "A-class" pull to the positive rail better than to the negative, if you decide to perform an all-out pimping.. -similar to the 5534s in this regard)

there's user's "benchmarks" all over the internet.. diyaudio etc..
http://www.head-fi.org/forums/f6/lme49710-vs-opa627s-294317/

I'd say: if you decide to go with either of expensive "metal-cans", you should (!!!) upgrade the other parts of your circuit as well - recapping, cleaning, etc. AND do the proper DC decoupling procedure - as prescribed in datasheet.

(I know the "sound" of 741, it mushes and glues the signal together, which may be a Good Thing in some guitar circuit - in fact it is - but when you put it in some other chain, the opamp-swapping can become an eye-opener.)



LME49710HA metal can is circa 9 USD

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LME49710HA-ND

opa627 is circa 18 USD

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=OPA627AP-ND

there is a circa 9 USD of differences between them .........

Pier Paolo


 
ppa, you should compare apples to apples (IMHO). SHADY tactics here !!!!

missing info:
------------------------------------

OPA627AM TO99 - 39,37EUR @farnell
http://de.farnell.com/texas-instruments/opa627am/op-amp-precision-fet-to99-8-627/dp/1459637

- 34.88USD @digikey
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=OPA627AM-ND

PDIP8 - 18.38USD @digikey
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=OPA627AP-ND


LME49710NA PDIP8 - 2,65USD @digikey
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LME49710NA-ND

- TO99 9,15USD @digikey
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LME49710HA-ND

------------------------------------

For refitting a board, the LME49710 seems _THE_ posh part to me (cca 7'85USD in reasonable quantities for the metal-can - OUCH already). The only drawback against the OPA627 would be the inevitable DC offset generated as a byproduct of LME inputs' base current draw.. and IIRC some guys @GeekSlutz calculated the optimal input impedance for '4562 circuits to be @1,6KOhm according to Jensen's formula ..or so.

...only putting things in perspective, not my business whatsoever.
ymmv
 
The only drawback against the OPA627 would be the inevitable DC offset generated as a byproduct of LME inputs' base current draw.. and IIRC some guys @GeekSlutz calculated the optimal input impedance for '4562 circuits to be @1,6KOhm according to Jensen's formula ..or so.

...only putting things in perspective, not my business whatsoever.
ymmv

- the offset with LME49710 is circa 0,7mV as typical value in the preamp mic section of this circuit.........and the offset voltage of LME49710 is near to zero (0,05mV) in the eq section.....in pratics the voltage offset is very very low with LME49710.
 
- the offset with opa627 vs. LME is lower in the preamp section but bigger or equal than LME49710 in the eq section.

I would precise that the original op amps have input bias currents bigger than LME (more and more), so considering LME offset is academic.


regarding the preamp mic section's noise we should compare opa627 and LME noise performance them have in this circuit, so even if LME49710 has an optimal impedance value around 1,6K input impedance, opa627 has a bigger noise voltage than LME49710, but the real problem are the input transformer's noise figure and the 1k resistors that produces much noise so the "real" differences in noise level using opa627 and LME49710 are so low that them are a little academic, but the differences in costs between the two op amps are not so academic..... ;)

Pier Paolo



 


 
the important thing is eliminate the resistor across pin 5 and 8 in the 536 op amps before changing this op amp with opa627 or LME49710 to avoid problems... 
Moreover for decupling the op amps i council to use LME49710 only to avoid many problems. LME49710 generally has not many problems with a poor decupling but I can not escluding oscillations in this PCB. To avoid oscillations should be made a decupling using caps between op amps' supply voltage pins and ground. All caps and their pins must be near the op amp that should be decupled.

 
there are some errors in the schematic  ???
- the 62K resistors near the 741s must be connected to ground and the non inverting input of these op amps.

- the 4.7k resistor near first 741 must be connected to the negative terminal of the cap near the bjts

- the 4.7k resistors must be connected to the bjts' bases. 

Pier Paolo
 
- regarding 741 op amps decupling should be used 10nF (not more)caps between 741's supply pins and its nearest ground point that's the ground point connected to the 62k resistor connected to its + input pin.

- regarding 536 in the preamp mic section should be used 10nF (not more)caps between 536's supply pins and its nearest ground point that's the ground point connected to the resistor 220k near the same 536.
 
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