Op-Amp substitution?

[ldervish]

First post here, so please be kind to me....

I am wanting to put a bass guitar preamp together from a schematic, and have some electronics experience, have successfully repaired a number of ss and tube amps. Just so you know where I'm starting from.

My question is whether I can substitute 2 8-pin opamps (LT1115, LT1028, LT1128 and LT1351 are available) for the TL074 the schematic calls for? One section of the TL074 is unused in the circuit and is shown in the schematic margin with output connected to -IN, with +IN grounded.

If so, are any of the LT chips more preferable than others?

Thanks, I appreciate your time.

 

[Oliphant]

I haven't tried any LT opamps before but you will get a much better response from others on this website if you post the schematic. When you do build it, use sockets, that way you can change opamps and test the result. Good luck

 

[JMFahey]

If a bass preamp, you won't notice using a "better" Op Amp.
Won't hurt, of course, but that's something else.

 

[dchisholm]

First post here, so please be kind to me....

I shall be on my best behavior and refrain from using phrases such as "cranial-rectal inversion".

. . . My question is whether I can substitute 2 8-pin opamps (LT1115, LT1028, LT1128 and LT1351 are available) for the TL074 the schematic calls for? . . .

How do you define "substitute"?

If you mean, "Can I simply plug a couple of 8-pin duals into the socket (or PCB pattern) where the TL074 goes?", the answer is a definite "NO!". At the very least, the pair of 8-pin duals requires 16 (total) pins, while the TL074 is a 14-pin package. More significantly, no matter how you rotate or shift it, the placement of the power pins prevents the 8-pin devices from being a direct plug-in substitute for even two of the opamps within the TL074 package, much less all four. If there was a compelling reason to use a pair of 8-pin duals as a direct plug-in replacement for a TL074 (or other quad opamp with the customary pinout) you would probably design a small printed wiring board to hold the duals and make the correct connections to pins that mate with the 14-pin socket.

If you mean, "Can I functionally replace the opamps in a TL074 with other opamps, such as those found in 8-pin dual opamp packages?", the answer is "Probably.". Of course you will have to change the connections so they go to the proper pins on the 8-pin packages. The dual opamp you choose would need to have characteristics compatible with the original circuit design using the TL074. Going mostly from memory, the four IC's you mention exceed the performance of the TL074 in every significant parameter except input bias and offset current. Consequently they should be acceptable UNLESS your circuit requires DC accuracy and uses high impedances in the gain-setting networks. (On second thought - you may need to add a compensation capacitor if you use the LT1115. I don't think that chip is unity-gain stable.)

But like "JMFahey" pointed out, it's unlikely that substituting opamps in your application will make a noticeable improvement. The TL07x series are far from state-of-the-art, but they have a reputation for decent performance in common audio applications. And, they are cheap and readily available. Rather than simply substituting opamps in the hopes of making some kind of "improvement" I suggest that you put some effort into characterizing the performance of your existing circuit, then decide what parameters you want to change, then look for an opamp (or other component(s)) that will move it in the desired direction.

Dale

 

[jcx]

guitar pickups can be very high Z at audio, even fairly high DC R

fet input op amps are a better choice - LT1028, 1115 are heavy bias BJT input that really are only superior for <<1 kOhm source

LT1022 is an OK LT jfet - they just don't make too many models of fet input op amps

some CMOS input op amps have some of the specs but haven't been used much in board level audio designs despite their use inside virtually all Vout audio DAC

if you are doing your own PCB, can deal with the different packaging, pinouts then dual op amps in smt give you the widest choice of performance today

Search username jcx, "pickups": diyAudio - Search Results

 

[JMFahey]

Post the schematic you'll build, to see whether any gain stage is heavily stressed or not.

 

[ldervish]

Thanks for the replies. DC, I've heard that term used before and it could have been directed at me....

This is a new perfboard build, not improving an existing one, just to see if I could do it and have it sound decent. The LT chips were furnished by LT for another project so they are on hand. To DC's question they would replace the TL074 functionally, at least that's what I was thinking, to save me a trip to the parts store. For the few $ involved it may be worth it to just skip the sub and go with the original IC.

Here is a link to the schematic.
http://home.centurytel.net/flanneldrawers/B50R_schematic.pdf

 

[ldervish]

Note, I am interested in the preamp portion only, up to and including the Line Out. R19 and beyond are X'd

 

[JMFahey]

Nice preamp.
If you don't want to use original TL074 and feel comfortable splitting the design into 2 DIP8, go for it.
I'd use 2 x TL072 and sleep like a baby.
You can use that spare Op Amp and build a balanced line out, to go straight to a mixer, how's that?

 

[ldervish]

Interesting idea, can you sketch it out or point me in a direction to find it? The only other schematic I have with a DI uses a transformer not an op amp.

 

[dchisholm]

Here is a link to the schematic. . . .

Like "jcx" said, the first stage really should be a FET-input opamp (like LT1022 or something from the TL07x family). The 470K resistor (R2) on the non-inverting input pretty much tells you that's what the original circuit designer had in mind. The next two sections could be any opamp intended for audio applications.

If you're working on perfboard then modifying the design to use 8-pin duals (or even 8-pin single opamps) is a good idea. There are a lot of parts there, and it will be difficult to make all the connections in the small area around a 14-pin package. By using two or three 8-pin IC's instead of the one 14-pin, you give yourself a little more "elbow room" to work with.

Dale

 

[JMFahey]

Try this:

An externally hosted image should be here but it was not working when we last tested it.

 

[ldervish]

Much appreciation to all, and special thanks to JMF for his time and trouble. I am digesting the points each of you has made and studied some threads mentioned by JCX.

First I am going to try a build as per schematic, since that was used successfully for a while in Ampeg's product line. Then I can try adjustments with a baseline to work from. Time to go shopping.