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 Post subject: filters...
PostPosted: Fri Aug 04, 2006 9:14 pm 
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Location: Buenos Aires
Yes, Carlos. I know that too much filtering is a bad thing.
The AKM dac has switched capacitors filter inside, some 6dB/oct filtering. So your advice is good.

I think that I´ll do some experiments with opa602 and some minimal filtering.

Is a nice opamp that opa602; I reccomend it very much: not so overwhelming on paper, but when you note that all the specs are measured with 500pF and 1k at the output, the device makes more sense.
Well suited to drive capacitive cables, low impedances, etc.

Is very good on vocals, with a smooth quality difficult to describe.
I prefer it to opa627 -even with heavy decoupling, etc-
I know that this is a entirelly subjective appreciation, but the difference is very noticeable. Maybe I´m pursuing this kind of sound...

Quote:
How many times did I come back to my bench, because what I thought was "technically" a better solution didn't sound good?


Yes, is common...
I´m thinking seriously in obtain some oscilloscope to do more measurements, and to avoid ¨tocar de ouvido¨ (playing by ear, an expression to signify ¨improvisation¨).

You make me laugh a lot with that ¨Thick as a Brick¨ joke...
It´s difficult to express some very technical things in a forum, more so for the amateur. And I`m new here, so I don´t know whether express myself in some decontractè way -you know, telling jokes and so-, or in a more solemn manner :? . For what I have seen, the tone of the postings here is not very stiff; more the other way around: funny.

Alas, sorry for my rawness.

Obrigado, and cheers.

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 Post subject:
PostPosted: Fri Aug 04, 2006 9:39 pm 
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Dog
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Just a few days ago I tried the LT1363 op-amp as I/V, on an old Philips CD160 of mine that I'm working on.
Well, let's say that it's there to stay. :rock:
I'm using them at +/-18V.
There's nothing after them, no stinkin' buffer, just direct output from the I/V op-amps.
I modified the circuit for single opamps instead of duals (as it was originally).
I'm keeping the 4x oversampling on this player, with that stinkin' SAA7220, but I'm reclocking the BCK line.
Anyway, the only filter I have here is a very small 56pF styroflex cap across the 1.8K feedback resistor.
This is very far from the 2.2nF value recommended on the TDA1541A datasheet :shock: but what the heck, the more I listened the more I concluded that a MUCH lower value sounds much better, and I was still not happy with 330pF.
Hey, this is an R2R dac, it doesn't wistle like a delta-sigma. :finga:

No candidates for the "Tick as a brick" award?

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 Post subject:
PostPosted: Fri Aug 04, 2006 9:54 pm 
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Who you calling "Thick as a brick?" . Not Moi I hope?

BTW, that's fine to relieve the FB filter as carlos said. You may wish also to investigate the PSRR of the I/V chip - if one supply is substantially worse then bias into Class A on that side. Check waveforms superimposed on the supplies with a FFT. :finga:

Cheers,
Greg

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 Post subject:
PostPosted: Fri Aug 04, 2006 10:50 pm 
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Dog
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Greg, PSRR is very similar on both rails.
Doesn't look bad at all.


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 Post subject:
PostPosted: Fri Aug 04, 2006 10:59 pm 
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Hi Carlos,

Yep.

Cheers,
greg

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PostPosted: Mon Aug 07, 2006 4:53 pm 
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Sheep

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Location: Buenos Aires
Quote:
Hey, this is an R2R dac, it doesn't wistle like a delta-sigma

-originally posted by Carlos-

Nice whistler is my AK delta sigma dac... I was doin´experiments w/opa602 without lpf -saturday afternoon-...

Once I connected everything, I switched the amp and... surprise! whistles!
In a first instance I supposed that the cd inside the cdp has some Moog or Theremin music, supposition that was obviated inmediately when i saw the first fumes emanating from the amp... :shock:

Oscillation, for shure. And a burned 10 Ohm resistor in the RCh Zobel -I think is a Zobel, cause there are some inductor and caps too-
I repaired the amp -unwelcomed extra work- , put some filters around opa602 (exactly what is recommended by the manufacturer of the dac)
and now the cdp is singing like a soprano -i´M not saying ¨like a bird¨, ´cause birds do whistle and I´m fed up with that-
So be careful, Carlos...
To be honest. I don´t know exactly if then problem was for the filterless dac, or for some other cause: I reflowed all soldering in the cdp output stage PCB, cause I detected a bad trace -intermittent- in the V+ of the right channel opamp.

Any of you knows what happens when a rail in a +/- 12v PSU feeding an opamp is dead? :?:

That was my first ¨burning¨ episode in all my DIY career... I have been lucky, and I have been very careful about everything, but this ¨near miss¨ is some sort of advice to me. Better to get some good test equipment when fiddling with some things.

Carlos, by the way, a question:
Do you know some online program to draw the circuit that I´ve done around TPA6120, so I can attach some image to my postings?

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 Post subject:
PostPosted: Mon Aug 07, 2006 5:48 pm 
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Cow
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Quote:
some online program to draw the circuit


you can use Paint and parts from enclosed bmp file (copy and paste)


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 Post subject: schematics
PostPosted: Mon Aug 07, 2006 7:36 pm 
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I`m no Picasso with paint, but this are the schematics...

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PostPosted: Mon Aug 07, 2006 7:40 pm 
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Sheep

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your tip was a good one; at least I`ve managed to post the circuit

Cheers

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 Post subject:
PostPosted: Mon Aug 07, 2006 9:30 pm 
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Dog
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Aqualung, you should have used at least a first order lowpass filter on the OPA602 too (on the inputs, not across the feedback resistor), I didn't recommend filterless with a Delta-Sigma dac.
Probably the OPA602 was oscillating too.

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Carlos Filipe

"Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius - and a lot of courage - to move in the opposite direction." Albert Einstein


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 Post subject: sorry Carlos, my fault
PostPosted: Mon Aug 07, 2006 10:29 pm 
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Sheep

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Location: Buenos Aires
Quote:
Aqualung, you should have used at least a first order lowpass filter on the OPA602 too (on the inputs, not across the feedback resistor


I was at work when I posted that :tease: so I couldn´t afford to explain myself with acuity.
I was using some form of vestigial LPF at opa602 input -4k, 1000pF-, so the whole issue left me perplexed. And the amp oscillation and fumes really scared me...
I don´t know really what happened: maybe opa602 wasn´t happy with some rf at the inputs, or maybe the ¨intermitent¨ v+ in the right channel opa...
Or maybe some contamination in the PCB -hairlines, flux, whatever-, or a cold joint... who knows.
Some reflowing and cleaning with isopropanol later -plus the addition of a textbook filter, recommended by the dac makers- , left me at least with some peace of mind :angel1:

The strange thing about all this is that I´ve managed to put in work TPA6120 with no issues at all and then, for so much fiddling with the thing, almost blowed an amplifier dealing with a much more stable and limited opa...

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