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George
What value R are using for the feedback resistor? The same value for both the AD and THS parts?

Yes the same 620ohms with 3300pf filter cap across it -3dB@78k why? gain should stay the same.


Cheers George

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Can't play any more, I think I hear my mother calling me.

The THS parts don't like any C on the inverting input pins. Every time I've tried it it kills the dynamics and the low end.

I'd have to check the data sheet again too, the THS is also picky about what value the feedback R is. Carlos, what value did you use for fdbk R, any C at all?


On my THS parts I like a 3.3n cap across the rails, 100u Pana FMs for bypass literally on top of the chip and .1 ohm/3.3n for snubber values right under the 100u caps. These values are different from what Carlos suggested, it's a personal preference thing.


It's late, I'll have to think about it tomorrow.

boy am I slow - I was going to reply when there was only one other and now there are 3 pages!
AD844 is a special ic - I played with its sibling which I believe is no longer available many years ago, in much the same way I believe that Pedja does (as Jocko was alluding to) - the data sheet for its sibling (AD846) is a good read.
If you try this different approach let us know what you think of the sound - for though we all have 'tastes' I for one am interested in what different people hear. And of course if you say it is bad others can always conclude that it truely is or that you did not futz with the implementation (caps/layout etc) enough to make it work. And if you like it maybe you could use Sander's circuit with a mod or two and maybe it would be even better.

One advantage of using the AD844 in the way that I'm alluding to is that as far as I know it is the only CFA currently available that will work (hint, hint) so no more swapping ICs to see if the sound gets better you would have to be content with trying different caps in different places and ...

1.8K+100pF styroflex for the TDA1541A I/V, 3.3K+200pF styroflex for the AD1862 I/V.
I prefer to use a small cap across the resistor.
It tames the high treble, which is a little 'hard' without the cap.
It has to be small, or it starts killing the harmonics, immediacy, speed, transparency.
Notice that it's a much smaller value than typically used for I/V.

George, keeping with a voltage feedback opamp, you can try the AD827.

_________________
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

Stone me, isn't it a little early to start modding the thing. Heaven forfend, but it may well be ideal as it is.

[quote="rfbrw Heaven forfend, but it may well be ideal as it is.[/quote]


It is 99%, but nothing is 100% perfect, I want that 1%


Cheers George

_________________
Can't play any more, I think I hear my mother calling me.

Hello George

You would be waisting your time with the OPA627, it sucks in the bass, I dont know why.

To get the improvements you are after you cant just replace one part like the opamp, you need to look at the power supply caps types and size, resistors, and and opamps and power supply circuitry.

The improvement might come about by changing caps on the input of the regulator to good sounding ones, reducing them in size on the ouput of a regulator also improves sound.

Get better regulators, try discrete Poogee circuit gives good sound when made with good parts.

Try different resistors, like try carbon resistors with the AD825. Resistors do effect the sound quality.

Regards
Arthur

Ever noticed how every time you go after that last 1% you loose 2% in getting it. Anyway, if you are prepared to do away with opamps, this will may do the trick.

Hi George,
you could try the AD8599, seems good in I/V and not too fussy.
Sound is sorta BB warmth with AD slam and precision if you get my drift.
Cheap too...

Perhaps I should have not said mod since what I'm beating around the bush about is using the circuit in a different way - like the infamous Jocko's IV (has anyone ever seen it, the real one - note I'm saying Jocko should share the circuit, but that when I say 'Jocko's IV' I and most people reading this do not know exactly what we are talking about!) that is with no global fb.

rtbrw said:

Just followed your link 'this' and yes use Sander's circuit in a similar manner - though I must say that I would probably not build such a complex circuit and use it in such a way, but then who knows it might be excellent, I would be more inclined to build something similar to your link. As I said I have played with AD846 years ago in such a manner, but not as an IV at the time.

Bill

Also if I was to go to the trouble of building 4 discrete I/V's, I definatly would go for one that is direct coupled, as I found by putting a N/P Black Gate N/X 100uf 6.3v back into the output of this dac, that the sound deteriorated much much more than doing these opamp swaps (softened the sound). So it's definatly dc coupling for me.

Cheers George

_________________
Can't play any more, I think I hear my mother calling me.

Hi George,

I generally do like CFB amplifiers but I am not really that enthusiastic about any CFB I/V I tried so far, and definitely not about the AD844. It is the possibility to use it without ANY feedback what makes AD844 sing. Used classic way it is IMO inferior to many others, including other CFB opamps. It sounds a bit congested and without much passion.

If you however give up of feedback and load its transconductance node to the ground to perform I (buffered from the inverting input) to V, and tap this V buffered at the pin 6, you get many things that classic digital audio misses (texture, transparency...... naturalness). As a shortcoming I can spot certain dirtiness which one can easily assign to its relatively poor harmonic distortion performance without a feedback (about 0.1% for the buffered output and with 4mA p-p current of TDA1541A). Hopefully I won’t turn you off by this statement since it actually sounds cleaner than the most of the other solutions... And you can still try tapping its unbuffered output (pin 5) and get about 0.05% (again with 1541) and this indeed does sounds a bit cleaner, but in the most applications it also lacks the dynamics of the buffered output. please note that the output current of this PCM is lower which makes things better...

And yes, I agree, omitting coupling capacitors makes a major sonic breakthrough.

Yep, one way to express it.

Jocko