Quick 4558 question.

[GREEN FUZ]

If I understand correctly a 4558 can be substituted by a TL072. Why then would a circuit ( In this case, the Tremulus Lune) call for an example of both on the same layout? I guess there must be a reason or it would just specify 2 072`s.

The reason I ask is that the 4558 is proving difficult to source locally and I was wondering if another dual op-amp would work just as well.

[johngreene]

If I understand correctly a 4558 can be substituted by a TL072. Why then would a circuit ( In this case, the Tremulus Lune) call for an example of both on the same layout? I guess there must be a reason or it would just specify 2 072`s.

The reason I ask is that the 4558 is proving difficult to source locally and I was wondering if another dual op-amp would work just as well.

I think the TL072 is used in the audio path because it is low noise and has a high input impedance. The opamp in the LFO doesn't really matter and the 4558 runs about 50% of the cost of the TL072. So for a company making a lot of these, it makes a difference. For a DIY'er making 1 or 2, it doesn't matter.

--john

[Mark Hammer]

Excellent advice and summary.  Spot on.

Many circuits will call for different dual-opamp chips within the same overall circuit.  Sometimes, these may make little difference in the overall sound or functioning of the circuit, and have the simple function of perhaps reducing current draw should one have the luxury of not having to worry about other aspects/considerations.  In other cases, they mave a substantial bearing on whether the circuit is noisy in some manner.  For instance, you will often see the TL022 or LM358 in LFO circuits.  These appear to be used in those subcircuits because of their reduced current draw.  That, in turn, reduces the likelihood of audible ticking from the LFO.  In still other cases, a bifet op-amp may be recommended for the input stage due to its high input impedance, but no subsequent stages require that property. 

In short, there are a bunch of reasons why a given circuit may employ different varieties of otherwise pin-for-pin-compatible dual op-amps.  If you are stuck short, there is every reason to expect that a circuit will "work" with any dual op-amp chip inserted into that slot.  It just may not work quite as well with respect to one or more parameters.  No reason why you can't use other duals as a temporary measure just to get the circuit up and running, and stick another chip into the socket once your next parts purchase arrives.

[GREEN FUZ]

Thanks for the quick response and considered replies to my question gentlemen. Looks like socketing is the way to go.

[JDoyle]

In short, there are a bunch of reasons why a given circuit may employ different varieties of otherwise pin-for-pin-compatible dual op-amps.  If you are stuck short, there is every reason to expect that a circuit will "work" with any dual op-amp chip inserted into that slot.  It just may not work quite as well with respect to one or more parameters.  No reason why you can't use other duals as a temporary measure just to get the circuit up and running, and stick another chip into the socket once your next parts purchase arrives.

I would add the addendum that before you swap out the called for op amp for one that you DO have, take the time to check the specs of the original part. Download the datasheet for it and look for one thing:

See if the original called-for op amp can swing to either power rail (V+ or V-), or if it can get to within less than a diode drop (0.6V) of either. If it can, find the original chip, as that ability is probably why the chip was chosen.

In actuallity, this normally only applies to control signal side chains, as some LFO/FWR circuits need to be able to be able to swing to a rail in order to properly and fully control whatever it is they are supposed to control.

In the audio path, unless the absolute maximum amount of headroom is required, swinging to the rail doesn't have any practical advantage to getting a circuit to function. So, if you KNOW that the op amp is only used in the audio path, you can use what you have on hand; but if you know that the chip functions in the control path, and the original can swing to either rail or damn close, save yourself the possible frustration of having a functioning circuit unable to function because you fed it the wrong chip; in that case, find the original chip.

Regards,

Jay Doyle