[GainHead]

Earwax, 100ohm should do the trick. It did on my previous prototype.

 

[The Speaker Guy]

On an IC datasheet, many times you see typical numbers posted prominantly. Device to device variation can be significant.

You may have an op amp whose Vos and IOS cancel each other with your component values. In the next circuit, they add to each other.

Also, I agree you could have an oscillation. Your amp is driving a sub 100 ohm load; reducing the feedback impedance will not cause noticeable distortion. You will get less thermal noise in the reistors too.

Yes, as you mentioned, your cap values will increase.

 

[GainHead]

The only values I am coming up with that are lower and still keep R2 at 50k is R3 and R4 = 100k so a gain of 2. Any thoughts from the audience?

Tangent, you have had success with the values posted on your website. Perhaps you would be willing to post a photo of your layout?

 

[tangent]

Quote:

Perhaps you would be willing to post a photo of your layout?

It wasn't anything special. Just a perfboard based amp, reasonably tight layout. The feedback cap and the bypass caps were far more important to stability than the layout.

 

[peranders]

Tangent, I would say when you are using >100 V/us devices the pcb layout is starting to get more important and >1000 V/us I would say it is the difference between failure and success.... together of course with careful decoupling.

 

[morsel]

How about reducing resistors by 10x? Do you really need that input cap?

 

[amb]

Quote:

Originally Posted by morsel How about reducing resistors by 10x? Do you really need that input cap?

The input cap is a almost necessary on opamps with BJT inputs, because it blocks the input DC offset current from being varied with the volume control pot setting (if equipped), as well as the output impedance of the signal source. Without the cap, the output DC offset would be rather unpredictable.

As for reducing resistor values by 10x, it means having to do that for the feedback resistors as well as the resistor that goes from the opamp + input to ground. This reduces the amp's input impedance by 10x and may cause difficulties with some signal sources.

 

[morsel]

I feel compelled to point out that plenty of LM6171 circuits, including those by Jan Meier and the National pdf use resistors in the 1-10k range.

 

[GainHead]

I found some text by Jan Meier over at Headwize stating that he was successful with the LM6171 using R4 = 300k, R3 = 56k and R2 = 47.5k. I modified my board layout and will try my same values (R2 = 100k, R3 = 127k and R4 = 464k), as well as Jan's and report back.

 

[GainHead]

I just found this post here http://www4.head-fi.org/forums/showt...3&highlight=r2 where Jamont describes that taking out C1 caused his DC offset problem. This is another difference between my new layout and the old - I took out C1. I did not think this was relevant to my question above though. I am going to test this out tonight and see if my problem is solved.

Assuming it does solve the problem, can anyone explain to me why or how? Thanks.

 

[amb]

As I already mentioned. C1 (the input capacitor) is almost mandatory for BJT inputs because it blocks the volume pot (or the input source) from mucking with the DC input offset. The overall DC offset at the output is influenced by a balance of the values of several resistors as described in tangent's "cranky opamps" article. If you don't have C1 then whatever drives the input will completely throw that balance out the window.

 

[GainHead]

I threw in C1 and it helps but does not eliminate the offset which is now 0.265-0.267 V on each side. I am getting these measurements with no source connected and no headphones connected. This opamp is f#@* pissing me off!

Edit: Actually the offset is now 0.066V on each side.

 

[amb]

Quote:

Originally Posted by GainHead This opamp is f#@* pissing me off!

LOL. I guess tangent didn't call this a "cranky" opamp without a reason.

 

[ppl]

Quote:

Originally Posted by GainHead I just found this post here http://www4.head-fi.org/forums/showt...3&highlight=r2 where Jamont describes that taking out C1 caused his DC offset problem. This is another difference between my new layout and the old - I took out C1. I did not think this was relevant to my question above though. I am going to test this out tonight and see if my problem is solved. Assuming it does solve the problem, can anyone explain to me why or how? Thanks.

i belive this was just answered By amb
"As I already mentioned. C1 (the input capacitor) is almost mandatory for BJT inputs because it blocks the volume pot (or the input source) from mucking with the DC input offset. The overall DC offset at the output is influenced by a balance of the values of several resistors as described in tangent's "cranky opamps" article. If you don't have C1 then whatever drives the input will completely throw that balance out the window."

 

[ppl]

Quote:

Originally Posted by GainHead I just found this post here http://www4.head-fi.org/forums/showt...3&highlight=r2 where Jamont describes that taking out C1 caused his DC offset problem. This is another difference between my new layout and the old - I took out C1. I did not think this was relevant to my question above though. I am going to test this out tonight and see if my problem is solved. Assuming it does solve the problem, can anyone explain to me why or how? Thanks.

Quote:

Originally Posted by amb i belive this was just answered By amb As I already mentioned. C1 (the input capacitor) is almost mandatory for BJT inputs because it blocks the volume pot (or the input source) from mucking with the DC input offset. The overall DC offset at the output is influenced by a balance of the values of several resistors as described in tangent's "cranky opamps" article. If you don't have C1 then whatever drives the input will completely throw that balance out the window. .

I think amb has answered this question