Which opamp to use?

[Carnesd]

Hi everyone, I'm currently looking for my first electrical project and I think I've found what I wanna start with.
Its a guitar line splitter so that I can record two amps at the same time with a DI as well.
Here is the schematic
http://www.geofex.com/FX_images/splitter.gif

I have a couple of questions on the schematic.

1. I see no labeling on the opamp so I'm guessing I have to figure out which one to use but i'm not sure what I need to be looking at to figure this out. Or are there just some generic opamps that would be used here?

2. Instead of using the input being plugged in to turn on the circuit I was thinking of just using a switch so I could remove eveything in the top box and use a switch on the 9v correct?

3. Maybe a dumb question but the 100nf cap in the middle of the circuit would be polarized? or just a fixed cap? The nomenclature I found on the internet is contradicting as some say this is a fixed cap and some say polarized.

4. Also, on the transformers, the two dots on either side mean that I will leave the middle pins alone?


Thanks for reading, as you can see I am still in the noob phase but I am trying by best to take in as much knowledge as possible.

[BobEllis]

Any FET input op amp will do - TL071, OPA132, OPA627, AD797, etc. Low current devices will be preferred for battery life, but I am not familiar with those parts. The last two will need special attention to layout.

You can also use a dual op amp and double the circuit - perhaps one side drives the D/I and the other the transformers. The pin connections will change, obviously, Another way to use a dual is just ground the inputs of the unused section.

As the circuit is drawn, the loads on the transformers may affect the DI output. Ideally you'd have separate op amps for each output.

Mouser only has the TL071 and OPA627 in stock, but plenty of opa2134 if you want to try something perhaps a bit better sounding than TL71 and don't want to spend $28 on an IC.

Yes, you could use a switch instead of the turn on circuit, but risk forgetting to turn it off and killing the batteries. You can also use a +/-15V supply powered by a mains transformer.

100nf is non polarized. You may find different dielectrics sound different. Polypropylene is a good choice. Anything over 50V is fine.

The dots denote the "positive" end of the winding. This means paying attention to the connection will ensure that the output is in phase with the input. Yes, leave the middle pin alone.

[TheSeekerr]

Remember that you can also get 2134 as a single (OPA134), or as it's very-slightly-superior brother (OPA132). OPA227 (2227 for the dual) is another good choice for an easy-to-use opamp.

[BobEllis]

I suggested the OPA2134 since the schematic suggests that the OP will be shopping at Mouser and they are out of stock on OPA134 and OPA132

[TheSeekerr]

Ah. The vagaries of Mouser escape me - they don't have an Australian presence, so I shop with Farnell, who don't tend to suffer stock shortages on such common parts.

[Carnesd]

Originally Posted by BobEllis

Any FET input op amp will do - TL071, OPA132, OPA627, AD797, etc. Low current devices will be preferred for battery life, but I am not familiar with those parts. The last two will need special attention to layout.

You can also use a dual op amp and double the circuit - perhaps one side drives the D/I and the other the transformers. The pin connections will change, obviously, Another way to use a dual is just ground the inputs of the unused section.

As the circuit is drawn, the loads on the transformers may affect the DI output. Ideally you'd have separate op amps for each output.

Mouser only has the TL071 and OPA627 in stock, but plenty of opa2134 if you want to try something perhaps a bit better sounding than TL71 and don't want to spend $28 on an IC.

Yes, you could use a switch instead of the turn on circuit, but risk forgetting to turn it off and killing the batteries. You can also use a +/-15V supply powered by a mains transformer.

100nf is non polarized. You may find different dielectrics sound different. Polypropylene is a good choice. Anything over 50V is fine.

The dots denote the "positive" end of the winding. This means paying attention to the connection will ensure that the output is in phase with the input. Yes, leave the middle pin alone.

Thanks for the reply!
I think I will just get a 2134,

If i wanted to use the second side of the 2134 to drive the di,
I would split the inputs to each opa and have the output of the second opa go to the 1k resistor for the DI?
Like this?

http://img.photobucket.com/albums/v1...sd/dualopa.jpg

Or would I have to change resistor values because of the load change?

[5th element]

Originally Posted by BobEllis

Any FET input op amp will do - AD797

The AD797 has BJT inputs.

[BobEllis]

Originally Posted by 5th element

The AD797 has BJT inputs.

Oops! Thanks for the catch.

Carnesd - you'll need separate feedback resistors and cap for each op amp - duplicate the 11K, 10K and 2.2uf. You are correct in your use of output resistors.

[indianajo]

If you change the input resistors to 1megohm instead of 2, you can use BJT input 33078 dual dip op amps which are $.35 ea this week at Newark (US affiliate of Farnell). I'm doing it on my Herald disco mixer, droped hiss about 20 db versus 4558 op amps originally installed. Different continents and generations have different shorthands, I think your 2M2 resistors at the input jacks were 2 megohm. I don't know what the second 2 was for, you surely don't need 2 watt resistors unless you have more than 300 volts of static buildup at the inputs. (2 watt resistors are big enough to take 500 volts without arcing over, usually).

[BobEllis]

2M2 = 2,200,000 ohms

I'd be careful using a BJT input op amp - even substituting 1M resistors at the input you will get a fair amount of DC offset (.125V at typical input current) due to the difference between the inverting and non-inverting resistances. Not hard to deal with by adding a blocking cap to the DI output, but necessary. You also don't want to go much lower than 500K input impedance with passive guitars.