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Old 15th April 2008, 07:33 PM   #1
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Default OPA627 Buffer Based Preamp

Hi,

I am trying to get to the bottom of a small problem that I am hoping some of the smart bods on here will be able to assist with.

I am building an OPA627 buffer to go in my pre-amp which will drive my ChipAmp.com (LM3886) power amp. I have collected together the parts based on the information on this website:

http://www.platenspeler.com/diy/prea...chooser_3.html

What I am stuggling with is whether I can use the polypropylene capacitors I have for decoupling the op-amp as I couldn't find any ceramics as suggested (though I have now found the 0.1uF, but not the 4.7uF)? Are the capacitors I have too big and mounted too far away from the chip to work effectively?

I have attached a picture of the layout to give you an idea.
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Old 15th April 2008, 08:24 PM   #2
dfdye is offline dfdye  United States
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Holy cow! Those caps are HUGE!!!!

To decouple the power supply at the chip power inputs, you don't need anything even CLOSE to that voltage rating (and I have no idea what the rating is, but it is overkill!). What you have shouldn't cause a problem, but they are overkill. You seem to have them pretty close to the power supply pins, so you should be OK, but the next time you are in an electronics store or making an order, just buy a bag of 100V 0.1uF ceramic caps! You WILL use them.

I am not really sure what you are talking about with the 4.7uF caps, but you should be fine using an electrolytic or tantalum for that size instead of a ceramic or film. The ceramic or film would be physically quite large at that voltage/capacitance rating. If it is in parallel with the ceramic/film decoupling cap, then you will definitely be fine with tantalum or electrolytic.

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Old 15th April 2008, 09:41 PM   #3
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Default Re: OPA627 Buffer Based Preamp

Quote:
Originally posted by ianpengelly
What I am stuggling with is whether I can use the polypropylene capacitors I have for decoupling....
Yes, if you don't have anything smaller but plain 0.1uF/63 V polyester would be just fine.
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Old 15th April 2008, 09:47 PM   #4
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I get good results with small 0.47uF Panasonic film caps for bypassing, and even that's probably more capacity than needed. The real trick is keeping them close to the op-amp. I'd isolate the outputs with 100 ohms, not 10 as per the original design. I also have to wonder about those resistors- are they carbon films? Seems an odd choice to use with such an expensive op-amp.
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Old 16th April 2008, 01:01 AM   #5
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I always use 10uf to 47uf electrolytic for bypassing.

I believe under 1uf value was too small to analog opamp.

Zang
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Old 16th April 2008, 03:46 AM   #6
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Somewhere upstream there should be more capacitance, but the 627 is a 16 MHz op-amp, and needs *local* bypassing appropriate to its speed. No electrolytic, even an Oscon, is really suitable. I'm not saying it won't work, or will break out into gross oscillation, but you really need a small ceramic or film, close to the pins, to get all the performance the part is capable of. The National parts like LME49710 are even faster (55 MHz) and the same caution applies. The part doesn't care that it's in an audio circuit- it needs low impedance supplies at the frequencies where it has gain.
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Old 16th April 2008, 04:16 AM   #7
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Default Re: Re: OPA627 Buffer Based Preamp

Quote:
Originally posted by peranders

Yes, if you don't have anything smaller but plain 0.1uF/63 V polyester would be just fine.
Not that I have a preference, but why do you recommend polyester over ceramics? I'll use either without thinking just based on what I have lying around, but if I am ordering parts, I have always defaulted to ceramics. Any insights?
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Old 16th April 2008, 09:46 AM   #8
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Default Re: Re: Re: OPA627 Buffer Based Preamp

Many thanks for all the input so far!

Quote:
Originally posted by dfdye
Not that I have a preference, but why do you recommend polyester over ceramics? I'll use either without thinking just based on what I have lying around, but if I am ordering parts, I have always defaulted to ceramics. Any insights?
Quite, are there any addvantages other than they tend to be much smaller?

I am pleased the layout will work with the caps I have, at least they aren't going to be wasted.

What should I do with the op-amp trim connections? Should these be grounded or left floating if I am not going to use them?

Conrad, when you say you should isolate the outputs with 100 ohms instead of 10, are you refering to the signal outputs from the op-amp, and if so why?
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Old 16th April 2008, 02:18 PM   #9
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I've looked at lots of caps on a vector impedance meter, and for serious RF you're better off with physically tiny ceramics. Still, the film caps are good to very high frequencies- my guess is they're limited more by physical dimensions, than by material properties. As soon as you have 1/4 - 1/2 inch of lead length, you lose the properties of either one. Films seem to give very good results with typical audio op-amps.

I prefer to isolate cable capacitance with 100 ohms (ok, 50-200) for stability reasons. High performance, high bandwidth op-amps aren't that stable into capacitive loads, especially at low gains, and I don't detect any sonic penalty. Try various values and see what you hear. Advice from fools on the 'net is always risky
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Old 16th April 2008, 02:35 PM   #10
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That seems a reasonable explination regarding the decoupling capacitors, I may consider adding some 0.1uF ceramic or film capacitors across the chips pins as well to be on the safe side.

fools on the net? Surely they are no different to the fools in the pub?
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