Freq Band

Bal output circuit not worky (pic)
« on: August 05, 2006, 05:01:06 AM »
OK, let me have it....
Why won't this work?
Quad opamp. Matched resistors.
No signal is getting through. There is a r 50k at the input, output to a line stage mixer. It just gets real hot.
=FB=

http://www.analog.com/UploadedFiles/Data_Sheets/151309759OP471_a.pdf
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Boswell

Bal output circuit not worky (pic)
« Reply #1 on: August 05, 2006, 07:32:49 AM »
There's a mistake in the OP471 data sheet. Pin 13 is the inverting input and pin 12 is the non-inverting input. The pin numbers are correct in the diagram but the + and - symbols are interchanged. Check that you have wired it up according to the pin numbers.

Freq Band

Bal output circuit not worky (pic)
« Reply #2 on: August 05, 2006, 08:31:18 AM »
I am 98% sure I went by the pin #s, because that's how I laid out the circuitboard, by looking at the ic diagram on page one of the pdf.
Clever of you to find that though!

I'll check for that 2% error margin I left myself. Boards are at home. I made two of these , same results each time.

So you think it should work ?

=FB=
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Moby

Bal output circuit not worky (pic)
« Reply #3 on: August 05, 2006, 11:00:53 AM »
Looking ok to me except op-amp pinning :cool:
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Freq Band

Bal output circuit not worky (pic)
« Reply #4 on: August 05, 2006, 01:29:52 PM »
Here is a similar circuit (correct pinning) but also the input is a bit different ?
Why is that?
(DRV134, 135 pdip and soic, still available)
=FB=
http://focus.ti.com/lit/ds/symlink/drv135.pdf
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Bal output circuit not worky (pic)
« Reply #5 on: August 05, 2006, 01:50:02 PM »
Yup, the circuit looks OK, except for signal pinning as already pointed out.
Is the +/- supply pinning OK? Can you trust the data sheet..... there is already one error :?!

You have queried the +/-18V text; I don't know this op-amp, but +/-18V is not unheard of in some consoles; bear in mind that some 5532's are spec'ed to operate as high as +/-22V!! (Be very careful with replacement ICs if the supply is anything over +/-18V).


However........ a couple of things that could be a source of trouble:

1. There are no frequency-limiting caps accross the op-amps. Suggest 22pF in parallell with R4, R5, R8, R9. This is standard-practice when designing with this type of cross-coupled driver. Your op-amps may be going into oscillation and/or driving the outputs hard-to-rail. Have you 'scoped the circuit?

2. There are no output coupling caps. This is not really relevant to your present problem, but accidentally shoving phantom up an op-amp's output  has been done more than once, and usually with unhappy results, except for the bank-balances of freelance techies :grin: . For good LF response, use a 220uF with the +ve out. Insert between the outer end of R11 and the x-coupling feedback line. Do the same for the other output.

3. The 10k balance setting pot (R13) is way too high in value, and setting the null point will be a life-time's work..... unless you have a 40-turn device!! Drop it to about 2k2. Even 5k is a hassle to set.


With reference to your latest question about the input configuration being different between your postings, the original post has the input buffer providing a low-Z non-inverting drive to the outer-legs of the balanced output, with the inner legs being grounded. (For what it's worth, these x-coupled outputs like a low-Z drive).
Your most recent posting has the input stage expecting an external low-Z drive to the inner legs of the balancing stage, the buffer provides a low-Z drive to the outer legs. Interesting ideato drive all input legs with low-Z.


 

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