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Old 27th August 2006, 01:36 AM   #1
legarem is offline legarem  Canada
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Default Best op amp for I/V conversion

Hi

Last month, I bought a MHDT labs parasidea non oversampling dac which I find really great for the price. Originally, there's a OPA2604 for the TDA1545 I/V conversion. This IC for me is only mediocre so I tried OPA627 to replace the OPA2604. After many days of listening, I really didn''t like the OPA627 sound in this dac. Surprisingly, the sound was harsh in the mid -high frequencys with this op amp.. There's a tube follower after the I/V conversion. I also tried an AD826 with good results. Some peoples talked to me about LM6181 for doing I/V conversion. I never tried this op amp. Any recommendations about what should be the best op amp to try for I/V?

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Legarem
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Old 27th August 2006, 08:59 AM   #2
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Default Lm6181

Hello Legarem
I'm not sure that the current feedback op amp LM6181 is a good solution:this is not an audio op amp.

Moreover National Semiconductor advises an other product:LMH6682.

Normally the high speed op amp(designed for video applications,ADSL modems,line driver ) have a disagreeable sound.
Why not tests whith a discrete circuit?
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Old 27th August 2006, 09:45 AM   #3
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Default Re: Lm6181

Quote:
Originally posted by parsecaudio
Normally the high speed op amp(designed for video applications,ADSL modems,line driver ) have a disagreeable sound.
Why not tests with a discrete circuit?
I can agree to a certain level regarding your remark about discrete circuits.

But think again when saying that high speed opamps have a disagreeable sound. Did you hear AD844 and AD815 in audio applications ?

Legarem, did you decouple the OPA627 as recommended ( including a film cap from the + to - pins ) ?
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Old 27th August 2006, 10:23 AM   #4
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Default High speed op amp sound

Hi jean paul
I don't know these Analog Devices op amp,but only the National semiconductor's H.S. amp like LM6171, LM7171,LM6165,LM6103:it does not appeal to their sound to me.
I prefer discrete circuit because they are more easy adaptable to the requirements(sounds or others) that we have need.
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Old 27th August 2006, 11:56 AM   #5
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I agree that a discrete circuit should normally be better than an op amp. First, I have to find a good discrete circuit. Second, in the Paradisea, there's not to much place so it will not be easy to implement one.

About decoupling OPA627, there's a 0.1 mfd from V+ to ground and V- to ground near the op amp.

I am also tempted to try an I/V with a resistor but in the past when I tried this topology in another dac (AD1862) I found that it lacked dynamic.

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Old 27th August 2006, 12:06 PM   #6
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Change them for 10 uF 50V Panasonic FC electrolytics very close to the supply pins.

Add one 0.1 uF MKT filmcap ( 5 mm pitch like Wima MKS 2 ) from + to - directly ( not to ground ) on the chip or under the pcb. You'll like the result.

I believe it was CarlosFM who said values of up to 220 nF can be used. Never tried that, I mostly use 10 nF or 100 nF.
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Old 27th August 2006, 12:30 PM   #7
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Default Supply coupling

Hi,
what you say is exact jean paul.
I think that to add 1- 2.2 nF silvered mica capacitor,above all in the digital supply,is a little improvement in the power supply.
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Old 27th August 2006, 12:35 PM   #8
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I don't have Fc Panasonic on hand but I have tantalums which I heard great things about them for decoupling. Some peoples seems to love them other seems to hate them.

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Old 27th August 2006, 12:52 PM   #9
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Default Tantalum cap

Tantalum caps is one of a better solutions for supply coupling.They can go well also a low impedance 105 degrees cap designed for SMPS.
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Old 27th August 2006, 04:30 PM   #10
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It is the questionable quality of tantalum caps in the past ( they often blew when used directly on supply lines ) and the mining/war scandal that keeps me from using them.

Besides that they sound worse than conventional electrolytic caps. At least they did the last time I tried them.

When you send me a PM with your adress etc. I can send you some 1 uF/25V caps with sealed ends I recently obtained. You are the guinee pig then I will appreciate your report on them though.
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