[CBMC]

I am in the process of building a meta42. I have a pair of ER-4S headphones and bb opa637ap opamps. My question involves the values of the r3, r4, r5, and r6 resistors. I will be using this amp with a discman most of the time. Anyway, what would be optimal resistor values? So far this is the best I could figure, let me know if this would work or if there are better values to use:

r3: 2.55k

r4: 11.5k

r5: 1k

r6: 200k

Any help is appreciated.

 

[tangent]

I would make R6 1M or 470K and R5 3.3K.

I wouldn't bother using E96 values for R4 and R3. Use common E24 values instead like 10K and 2.2K to get the same overall gain level.

 

[CBMC]

I know I am an idiot, but what are e96 and e24 values. I am new to this whole amp building thing.

 

[CBMC]

I know I am an idiot, but what are e96 and e24 values. I am new to this whole amp building thing.

 

[tangent]

Download a copy of the Mouser catalog and look in the resistor section. You'll find copies of the E24 and E96 tables of common resistor values in there. 5% resistors use the E24 table, and 1% resistors use the E96 table which is a rough superset of the E24 table.

The E24 series use two significant digits, and they correspond to 4-color resistor stripe codes. The E96 series uses three significant digits, and 5 stripes on resistors that use stripe codes. For instance, you have "47" in the E24 table, giving values like 47 ohms, 4.7K ohms, 470 ohms, etc. The closest value in the E96 table is "475", for 47.5 ohms, 4.75K ohms, and 475 ohms. A few 1% resistor lines combine both series, so you might also be able to find 470K in that series.

EDIT: The reason I bring this up is because the E24 values are much more commonly used. You'll almost certainly need to get extra resistors, so having extra 2.2K resistors will be more helpful in future projects than having extra 2.55K resistors.

 

[CBMC]

So a gain of around 5 would work well for these headphones and this opamp? Or would I want a higher gain?

 

[tangent]

Sorry, I didn't check that. I'd recommend a gain of 7 or more in this setup. 10 wouldn't be out of line.

 

[CBMC]

Thanks, I thought 5 might be kind of low. Is there anyway that you could give me a ballpark figure of what the output independence at the phono jack would be with a gain of 7 and the opa637 opamp?

 

[CBMC]

A gain of 7 for the outer loop, so a gain of around 200 for the outerloop? Would that be about right? I figure I would need about 2.5-3 volts of input voltage for the er-4s's. I really appreciate all your help and will try not to ask so many questions in the future, I just want to make sure my resistor loop values are correct for my configuration. Anyway, thanks.

 

[CBMC]

You said a gain of 7 for the outer loop. Would I just do about 40x that for the inner loop(gain of 280)?

 

[tangent]

Quote:

Is there anyway that you could give me a ballpark figure of what the output independence at the phono jack would be with a gain of 7 and the opa637 opamp?

It requires calculations I don't know how to make. Why is it important for you to know this?

Quote:

I figure I would need about 2.5-3 volts of input voltage for the er-4s's.

Unlikely, unless you're talking about peak-to-peak voltage. 1Vrms is more likely for those phones.

Quote:

Would I just do about 40x that for the inner loop(gain of 280)?

There is no necessary relationship between the outer loop gain and the inner loop gain. You set them separately.

 

[CBMC]

The reason that I ask is because I am building a seperate headphone eq for the er-4s's for the 2.5 khz and 7.5khz notches in their frequency response and the values of the caps and so on depend on the output independence of the amp. I know those headphones are safe up to 3Vrms.(putting out somewhere around 122 db's, which might not be safe on the ears for long periods of time).

By any chance could you give me a suggestion on what the inner loop gain should be, if I am using a outer loop gain of 8? I know that I am probably getting annoying with the questions, I just want to get it right, so I don't have to do a lot of resistor value guesswork.

One more newbie question here involving jumpering across positions on the pcb. What do you need to do this? What do you occupy the posistions on the board with? Do you use 0 ohm resistors?

Again, thanks for all of your help. I really do appreciate it.

 

[tangent]

Quote:

I am building a seperate headphone eq

The output impedance without adding R8 or R9 is as close to 0 as makes no difference. You can add additional output resistance to equalize the headphones without worrying about what the amp will add.

Quote:

around 122 db's, which might not be safe on the ears for long periods of time

That's right at the threshold of pain. I.e. you immediately feel pain from hearing such a loud sound. It's not safe for even one second.

90dB is about as loud as you should go for any length of time. Some say even this is too much.

Quote:

By any chance could you give me a suggestion on what the inner loop gain should be, if I am using a outer loop gain of 8?

I told you, the outer loop gain has no bearing on the answer. And, the rules for determining inner loop gain are given in the docs.

Quote:

One more newbie question here involving jumpering across positions on the pcb. What do you need to do this? What do you occupy the posistions on the board with? Do you use 0 ohm resistors?

You can get 0 ohm resistors, but most people use resistor lead clippings. You'll have more clippings than you can ever use early on in the project. Keep some extras if you are going to keep playing with electronics, as they are also useful when breadboarding experiments.

 

[CBMC]

I think I got it now. How do these values look

r3: 1.5k

r4: 10.5k

r5:10k

r6:1m


I wanted to use a 200x gain for the inner loop but couldn't do that unless I used a 2m resistor in r6's spot. If you see something glaring you in the face that is wrong, please tell me. I know the opa637 is a fairly fast chip which requires higher gain, which I tried to take into effect.