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Hi George,

When you go to substitute with a current-feedback opamp, you almost certainly will need to change the passive components to keep it stable. In addition to careful power supply bypassing as already mentioned, the stability of current-feedback opamps is critically dependent on the feedback impedance. For instance, you cannot normally use a capacitor in the feedback network without making the opamp unstable (in contrast with standard voltage-feedback opamps), and the feedback resistance must be above some minimum value (I didn't read those datasheets yet - but they will mention the minimum value you can use). Note that the bandwidth of the stage is dependent on the feedback impedance... lower Z = higher bandwidth, until you've exhausted all your phase margin and it starts to ring.

I suggest you use a feedback impedance higher than the minimum value by a decent margin, and be careful to minimize stray capacitance at the (-) input. If you have a good oscilloscope to check for ringing and oscillation, then you can push the boundaries a bit more.

Read this article to see how to properly use current-feedback amplifiers to do DAC I-V conversion. It covers the subject in greater detail...

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- Chad.
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"Fashion is mistaken for good design; moral fashion is mistaken for good." -- Paul Graham

[quote="carlosfm

You need at least 100uF bypass caps from each rail to ground, on the PSU pins (really, as close as you can). Panasonic FM 100uF/25V are almost mandatory, you won't go back.
And a very small cap across the rails (1nF MKT ).
You can also try latter adding snubbers from each rail to ground(0.22R+3.3nF).
This way, there's no lack of dynamics and bass slam.
IME it's an opamp that doesn't seem to add or remove anything. It has very tight, correct bass (sometimes incorrectly confused with lack of bass), but in a dynamic sort of way.

Well, anyway, it's up with you if you want to try it again.
But have in mind that the THS4031/2 are very very picky with the PSU bypass arrangement. They can sound very different, very easily.[/quote]


Carlos,200uf for each side bypasssed on the pins under the board with 6.8uf Tantalum and .1ufMTK of the supply and it's got choke reg as well, the only thing I didn't do is the 1nf across the pins, thats not going to bring the bass dynamics up to the AD825.
Don't get me wrong the THS4031 has the most transparent cleanest 3d sound all the chips but it can't match it in the bass and rhythm department of the AD825, it is more of a microscope looking into the music where the AD825 a bit rough it may be and not as 3d, plays music. This is why I wanted to try current feedback opamps, as there is in my opinion no voltage feedback opampthat does everything right, maybe the current f/b ones will have other problems I don't know no one seems to be able to tell me this is why I'd like to try them, Pedja Rogic seems to like them, http://www.pedjarogic.com/1541a/rev0.htm
Below is the circuit of my power supply.

Cheers George

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

A shot of the guts, if anyone is interested.

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

George, the AD825 may work better for you indeed.
One can't generalize.
You are using the PCM1738 dac, which is delta-sigma. Some of these dacs sound anaeminc and some opamps that are not neutral can be used to 'compensate' for the lack of dynamics and slam.
It's a question of synergy, but done the wrong way.
The problem IME most of the times is not the dac chip. It is the 'recommended' datasheet values, which are just copied by most CDP manufacturers. They don't even care to listen.
Looking at the PCM1738 datasheet is no different from most others. Namely the cap at pin 19 (Vcom1) to ground. It's too small. You need low impedance at low frequencies and it's not a 10uF cap that will give you that.
Please try a good, low ESR 220~330uF cap there. Like a Panasonic FM.
Really, try it.

I prefer R2R dacs and the THS4031/2 sounds wonderful (and no short of dynamics) with the TDA1541A and the AD1862.

Anyway, your bypass arrangement is very different from what I use.
No 100nF bypass, no tantalum, just 100uF on the pins. The 'snubber' arrangement is the bypass.

_________________
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

He is using a trick that makes it operate the way my stage does.

(I wonder if Sander is reading this! Snicker, snicker.)

Jocko

Sounds like a poor PSRR to me, better make sure your rails do not have too much capacitance on them or you'll be listening to the caps rather than the opamp.

Cheers,

Sander.

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Indoctrinated in the ways of the dark side

I am, but it is getting late, my eyes are starting to droop, I fail to see your point, maybe in the morning?

Cheers,

Sander.

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Indoctrinated in the ways of the dark side

Sander, whatever the PSRR of a circuit, whatever it is, PSU decoupling always makes difference.
Most circuits are sensitive to the PSU impedance, and in the case of an opamp working at audio frequencies, some capacitance is needed on the pins. Not too much, just the right amount.

In the end, you really have to listen. What you though that would be right may sound bad.
It took me some time to get to the PSU decoupling values that I recommended here in this thread, for the THS4031/2.

Small bypasses alone on the pins and larger caps far away (more than 1cm is too far) is something that only works for RF/video applications. Indeed what many opamps were made for. The datasheets just reflect the typical application.

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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

Carlos,

I'm not debating whether you need decoupling, or where, I'm merely stating that on an opamp that's that sensitive to how much capacitance you use you should be careful. PSRR for a standard miller opamp is near zero if not for loop gain, hence the GBW sets how much PSRR you'll get on the 'bad' rail.

Ps. anyone care to correct the monumental spelling error in the topic title?

Cheers,

Sander.

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Indoctrinated in the ways of the dark side

Hmm. Without trying to start a food fight. I haven't met an opamp yet that didn't sound different depending on the bypassing scheme. As Carlos says, each amp is different and the bypassing is different for each to get the best sound.

[quote="hifizenI suggest you use a feedback impedance higher than the minimum value by a decent margin, and be careful to minimize stray capacitance at the (-) input. If you have a good oscilloscope to check for ringing and oscillation, then you can push the boundaries a bit more.

Read this article to see how to properly use current-feedback amplifiers to do DAC I-V conversion. It covers the subject in greater detail...[/quote]

Thanks for the link Chad I missed it first time around, very interesting read by Walt Jung, this was the sort of thing I was hoping to get from the lads here, it's a bit of an excercise to get it to work, I was hoping for something simpler, but one thing really p----s me off, is no mention on how it sounds, isn't that the final reason for doing it in the first place.
Or do people like Walt and co. get off just looking at their scopes, and to hell with the sound.

Cheers George

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

As much as you can read sonic impressions by others, they are not yours.
There are personal sonic priorities in audio, or let's say (I don't like to say this) tastes.
The same way as you can be fooled by reading audio reviews.
Just build it and listen for yourself.

I think that tastes are not discussable.
But IMO there are people with bad taste.

_________________
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

Even if they are your (Walts)own opinions on how something sound, it really does matter to me.
BTW Carlos just removed to the 10uf cap on pin 19 of the PCM1738 and I had a 470uf 16v Black Gate in my kit, it's warming up as I speak, will let you know if I hear a difference, SEE WE ARE TALKING ABOUT THE BLOODY SOUND, of a stupid cap, this is a whole operational amplifier I was wondering about with Walts artical of the transimpedance amps.

Cheers George

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

George
I wouldn't say you're listening to the sound of a cap. But you are listening to the affects of a cap.

After a close listen, I can say that there is virtualy no difference on pin 19 Vcom1 (internal bias decoupling pin) of the PCM1738E, between a 10uf unbranded electrolytic and the 470uf Low ESR Black Gate, you get a very, very small feeling that the Black Gate is tighter in the bottom end, but it's a feeling and no way fact.
The difference between the AD825 and the THS4032 for the I/V however, is large, and could be easily picked in a blid test.
I wish I could have the transparency detail and 3d of the THS with the bass dynamics and rhythm of the AD, the only one I haven't tried yet is the hidiously expensive OPA627, here in Australia they are $60 x4 ouch.
Maybe the Transimpedance opamps if I figure out how to impliment them properly.

Cheers George

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