I/V using opamps

[jcx]

I thought I got the OPA627 sounding as good as an opamp could for an I/V stage, nulling out the dc offset from the PCM1704 to it seemed to do a lot of good for them, don't ask me why, all I can say is that it seems the OPA627 as an opamp I/V doesn't like to see dc offset from the dac chip comming into it

knowing op amp limitations I would rather point at the DAC - they often don't do a very good job on the datasheet of show limits, effects of offset V, termination Z


before widespread factory internal zener zap offset trimming some op amps benefited from selection for low offset, or those with trim pins could have input stage nonlinearity caused distortion minimized by the Vos trim


of course I have no idea what people are really talking about when attributing "frequency response" type "explainations" to op amp rolling - we can be fairly sure no "conventionally audible" frequency respose changes occur with decdent, "audio" op amps in DAC I/V circuits

but if some are so certain of these changes then why aren't they exploring really new op amps, composite op amp circuits instead of going down the descrete garden path, "nfb" rabbit holes

[georgehifi]

Hi JCX, no dc offset from the Dac's thanks to Abraxalito and the dac offset nulling circuit he posted, so the dac/s was zero'ed to within a tenth of a millivolt each time when each i/v opamp was listened to. And I listened to many not just these two mentioned.

PCM1704 Help needed with higher dc offset from one channel

Cheers George

[abraxalito]

I agree George, opamps for I/V just do not deliver transparency. I have changed my view based on experience, because I used to go for them a few years ago. Try a passive filter (steep one, based on inductors and capacitors) before discrete I/V and prepare to be surprised (Listening to passive I/V, 7th order elliptic filter now, can't go back).

[georgehifi]

Originally Posted by abraxalito

I agree George, opamps for I/V just do not deliver transparency. I have changed my view based on experience, because I used to go for them a few years ago. Try a passive filter (steep one, based on inductors and capacitors) before discrete I/V and prepare to be surprised (Listening to passive I/V, 7th order elliptic filter now, can't go back).

I would really like to do this Abraxalito, but please correct me if I'm wrong, the PCM1704's only have +-1.2mA output swing, this would die to nothing by the time it goes through a nice low 10ohms or so passive resistor i/v, then to bring it back up to 2v output with a decent low output impedance active stage maybe noise will become a big issue?

Cheers George

[abraxalito]

I've not played with the PCM1704 - why does the I/V resistor need to be as low as 10R? I know there are some issues with diodes conducting at the output which limits the compliance available. I have about 100mV signal on the output of my TDA1545s, noise doesn't seem to be an issue, but then I'm only feeding 16bits. The amp stage will dominate in noise terms for 10R source resistance so its worth exploring whether the resistor can't be bigger. AD603 works great for me, this has a built-in 100R resistor.

[georgehifi]

I've just read and been told that 10ohms passive I/V gets the best out of the PCM1704 I don't know who to believe to be honest anymore.
That AD603 looks the goods if 100ohms input is as good as 10ohms. 3v output at 2ohms with 50mA and I think most of the dac noise is taken out within the dac so maybe no low pass filter is required maybe?

Cheers George

[abraxalito]

If you delete the low pass filter it'll still sound better than pretty much anything else except TDA1541A. But with the LP filter in place, the sound goes up into another league altogether - completely gob-smacking (at least with my TDA1545s). This does rather depend on the amp stage after it, AD603 might not be as great performance as a discrete stage done well. Once you've heard the sound through the filter, I can't believe you'd ever want to go back...

[studiostevus]

Originally Posted by abraxalito

If you delete the low pass filter it'll still sound better than pretty much anything else except TDA1541A. But with the LP filter in place, the sound goes up into another league altogether - completely gob-smacking (at least with my TDA1545s). This does rather depend on the amp stage after it, AD603 might not be as great performance as a discrete stage done well. Once you've heard the sound through the filter, I can't believe you'd ever want to go back...

What's your take on filtering the output of a non-oversampling tda1541a with resistor as i/v?

I am currently not using any filtering (neither low pass nor otherwise to compensate the nos roll-off), and actually very happy as it is....but i read somewhere that bumping up the high frequencies make a huge positive difference ...

[abraxalito]

Definitely worth trying a filter, even with TDA1541A. Its definitely also worth bringing up the droopy top end by some means. You could do that by adding on two more TDA1541As (if you're feeling fairly rich) and feeding them with an HEF4517 delay line to create a 3 tap FIR filter. Thorsten also says undoing the droop is definitely worthwhile, I think he does it by some passive LC circuit, though not sure of the details.

[Trident]

Hi all,

This is my first post, so far and learned reading here.

I'm doing the mod in a CD player Technics SL-P999, is a great player.

This model mount a linear DAC converters: Four PCM56P (two per channel)

I replaced on I / V stage the original operational M5238 by OPA2604, and I'm not happy with the result ... sound, poor, muffled, dull, veiled.

I was thinking of replacing by AD825, because I need to be an double encapsulated SMD, and the power supply voltaje is a +7.9 and -8.1 (original design in factory)

I have skills and experience, it is worked in Panasonic/Technics/National for 15 years repairing audio, and I have instruments, parts, and Service Manual for these players

any suggestions?

greetings



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