Tascam 48 Op Amp replacement

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I'd like to replace U001-U004 (NJM2043 & NJM4559-D) Op Amps with LME49860NA Op Amps. These are the higher voltage tolerance LME49720s... According to my schematic for the Tascam 48, there is close to 20v on some of the rails...

I've done this successfully with my Tascam M-312B stereo buss, but that was more straightforward with .1uf ceramic decoupling caps on pins 4 and 8 to ground. Worked great.

My question is here with the Tascam 48 amp card, on U003, Pin 4 goes to ground... Do I need to do anything with that pin? Obviously, its not used for power, so putting a .1uf ceramic to ground is pointless, correct? Not sure. Need some damnnnnn help. Thanks!

Ecs787s said:

My question is here with the Tascam 48 amp card, on U003, Pin 4 goes to ground...

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Are you sure? I see that's what the schemo shows, but that could well be a mistake, because it shouldn't really work as shown. There are indeed cases where an opamp is powered from a single rail and the -V pin connected to ground, but in that case, the non-inverting inputs are biased at half-rail, which I don't see here; there is no good reason for doing it.
So measure the actual voltage on pin 4, I guess you'll see the negative rail voltage.

Yup, looks like a schematic mistake.

JR

I'll take the amp card out and see if there is continuity between pin 4 and ground and get back to you... This is interesting. Thanks for your replies!

You guys are absolutely correct in that it is a schematic error. There is continuity on pin for of both U003 and U004 as well as the connection at R125. Thanks for helping me out. Now its time to do some modding.

I'm thinking of recapping C006, 007, 031, 032, 050 & 051 with higher values. What if any would you all think would be the other best caps to replace... Is it worth recapping all the bipolar signal caps or should I just bypass them with something like .01 or .1??? Will I hear that big of difference? Thank you!

Ecs787s said:

I'm thinking of recapping C006, 007, 031, 032, 050 & 051 with higher values.

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they are very hard to identify on your pic. I tried downloading butPostimage just doesn't work for me!
There are endless controversial discussions about recapping. Consider that designers usually increase the calculated values by a factor 2 to 10, knowing that capacitance decreases with time.
It was not uncommon to find 'lytic capacitors manufactured in the 60's that were 100% dry in the 80's.
Not so today; elcaps have a much longer life expectancy. So I would just consider replacing the old ones with new ones of same value; they will be more durable and probably have a lower ESR (which may or may not prove beneficial). In fact you can find very affordable low ESR 105°C types. Increasing caps may create problems, just as by passing 'lytics with film. The issue is that these problems may appear in a very sneaky way, as distortion or granularity. 

What if any would you all think would be the other best caps to replace... Is it worth recapping all the bipolar signal caps or should I just bypass them with something like .01 or .1??? Will I hear that big of difference?

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That's one of the most common audiophool tweak. There is no evidence that it improves the operation of a well-designed circuit.
In most cases, it is harmless, but in some instances, the added stray capacitance can cause some issues.

Thanks for the good info! I'm surprised you can't click on the picture of the schematic for it to come up....

I think I'll just upgrade signal electros with the same values but better ones and upgrade the powersupply decoupling ones with higher values... and leave it at that.

Thanks again.

Ecs787s said:

Thanks for the good info! I'm surprised you can't click on the picture of the schematic for it to come up....

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check the link; it just goes to postimage.org

If you just click on the image, it should open the larger image in a new window... Let me know if that doesn't work because it works fine on my end. Sorry for the inconvenience.... Thank you.

Ecs787s said:

I think I'll just upgrade signal electros with the same values but better ones

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Look for something with a very long life at 105 degrees C.

and upgrade the powersupply decoupling ones with higher values... and leave it at that.

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Why would that be necessary?

Not sure, Andy... I've read that these "economy" manufacturers would use the lowest capacity necessary and by adding in some extra capacitance can't hurt... Would this be a waste of money? I know its not really that much more expensive to bump them up double...

Btw, I believe the new LME49720/49860 op amps draw some more current than what is currently in there... I'd have to check.

Thank you.

Ecs787s said:

Not sure, Andy... I've read that these "economy" manufacturers would use the lowest capacity necessary and by adding in some extra capacitance can't hurt... Would this be a waste of money? I know its not really that much more expensive to bump them up double...

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"Lowest capacitance necessary" is a design rule based on how much ripple you can tolerate. (There's a spreadsheet floating around that calculates capacitance for a given ripple.)

I suppose the right thing to do is to measure the ripple at the smoothing caps, and then to measure the ripple at the output of the regulators, and see if that ripple is "tolerable." Don't forget that the op-amps themselves have some amount of PSRR. You can overspecify if it makes you feel good, and you can fit the cap in the alloted space, but the factory value is most likely adequate.

Was TASCAM really an "economy" manufacturer?

Was Tascam, an economy manufacturer? Not sure! That's why I'm here for help, Sir. I know the 40 series was a step up from the 30 series... So maybe not...

Thanks for your help.

Ok, so I just checked the quiescent current of the "stock" op amps and they run between 3-6mA while the new lme49720s run around 10mA... So based on this, would it be a good idea to bump up the capacitance on those power supply rails?

Andy Peters said:

Was TASCAM really an "economy" manufacturer?

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TASCAM was what I would consider a semi-pro equipment maker. Perhaps not the cheapest value product, but not premium high end recording gear.

JR

Tascam is PRO equipment maker. I have lot of experience with tapes (42B,  16ch, 44, 48) and these was better machines than Revox PR99 etc. in all segments. And so on.

http://tascam.com/applications/pro_audio/

moamps said:

Tascam is PRO equipment maker. I have lot of experience with tapes (42B,  16ch, 44, 48) and these was better machines than Revox PR99 etc. in all segments. And so on.

http://tascam.com/applications/pro_audio/

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Revox was the consumer division of Studer, so the PR99, derived from the B77, was a semi pro machine.
Tascam, although they now call themselves "Pro", are also a semi-pro manufacturer.
In fact, they defined almost singlehandedly the "semi-pro" category, that didn't exist before.
Indeed, their tape machines were very nice, but I would not consider them superior to the PR99.

moamps said:

Tascam is PRO equipment maker. I have lot of experience with tapes (42B,  16ch, 44, 48) and these was better machines than Revox PR99 etc. in all segments. And so on.

http://tascam.com/applications/pro_audio/

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Sorry i don't mean to sound like a gear snob...  researching a TASCAM product list it looks like they did make one 2" 24T tape machine and one 1/2" half track mastering machine which are pro tape formats, and lots of small format tape machines they called pro. Putting rack ears and XLR jacks on a cassette deck may help pros use it, but it's still a cassette deck.  I never saw their 2" machine in the wild.

The Revox was consumer gear.

Hell I worked at Peavey for 15 years, I designed lots of MI gear.  ;D ;D

JR

I didn't said that Tascams 42, 44, 48 and Revox PR99  are professional machines. Although, Studer said here http://wegavision.pytalhost.com/Revox/PR99IIp/revox03.jpg that PR99 is intended for professional use. I just did comparison according my experience of using and servicing between the models what can be seen on web very often (Tascam/Teac vs.  Studer/Revox).
When I say that Tascam is PRO, I have in my mind machines as ATR60, CD601, CD701, DA60, DA88 we used here in professional (OB) recording and broadcasting. Of course, YMMV.
@ ECS787s
You can replace C006, 007, 031, 032, 050 & 051 with higher values and higher max. voltage (25V instead of 16V). If new opamps will  draw more current, consider lowering the values of resistors R205, 206, 124,125, 83, 84.
Also replacing bipolar capacitors placed between stages is a good idea.
P.S.
There is a nice low noise discrete opamp (fet input) used as tape head preamp. 
   

There are so many meanings to "professional" that it really becomes a matter of semantics.
In the audio biz, the difference between pro and semi-pro (prosumer) is  caricatured by the +4 vs. -10 paradigm.
In a sociological view, pro is anything that allows a person or company to produce work for which he (they) are paid.
And there are individuals, who have been trained in a certain way (brainwashed?) and will draw an arbitrary line; as an example, in the 80's, truck drivers in the showbiz industry used to draw a line between Volvo (undoubtedly pro) and Renault Industries (not pro)! Unfortunately, in the absence of an accepted definition, snobbery rules!