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Old 21st August 2013, 08:05 PM   #541
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Thumbs up Did some measurements....

Quote:
Originally Posted by Ken Newton View Post
Hi, Torchwood,

You may want to use some caution in placing capacitors across transformer primaries. This creates a resonant tank circuit. By my calculation, placing a 4.7nF cap. across the 3H primary of that Edcor transformer may produce response peaking around 1.3KHz or so.
Hi Ken, Got around to measuring a few Edcor transformers last night. Was rather interesting. Measuring just the primary without terminating the secondary gave weird results. Measured anywhere from 3 to 13 H. However when you terminate the secondary we (my Cousin and I) measured 30 mH. After some thought this makes sense. It also clearly shows why the secondary should be terminated so that the dac doesn't see any odd reflections from the secondary. We measured a XSM 600/600 and a WSM 2.7K/1.2K with the same results. So what the dac sees is 4.7 nF across about 30 mH. My Cousin did extensive sweep testing with the aforementioned WSM transformer with a CS4397 dac no peaking was seen. Given my druthers I'd pop for Cinemag transformers for my future projects, however the Edcors are great bang for the buck. Dave
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Old 21st August 2013, 08:09 PM   #542
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Correction WSM 1.2K/2.7K 1:2 ratio....
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Old 21st August 2013, 11:22 PM   #543
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Quote:
Originally Posted by PreSapian View Post
I just realized pin3 is being unused in the circuit. For differential input wouldnt i just have to connect it to the dac?
Indeed yes, but show me the circuit first, just be safe.
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Old 22nd August 2013, 05:59 AM   #544
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Click the image to open in full size.

hi, sorry it's too amateurish

all good for 1541 but what about 1543? where would i connect the vref pin to?
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Old 22nd August 2013, 05:35 PM   #545
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Quote:
Originally Posted by georgehifi View Post
Skouliki, did you ever get to try the 3 stack on one of your channels?

Cheers george
Hi george. I did try 3 stacked AD844 in one of the channels. The 3-stack channel seems to have more depth in the sound but that could be my subjective impression. I cannot say for sure. I will order more Ad844 so i can evaluate properly both channels
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Old 22nd August 2013, 07:27 PM   #546
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I saw it on the old pedja thread that stacking does lower the input impedance. I wonder, tho, even with 50ohm halved by stacking how ad844 can compare to something like this that has zero? input impedance to begin with. currently i wish to build both to compare....

'Super common gate i/v converter for 1543' is the thread which i cant seem to link on my android phone...
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Old 22nd August 2013, 09:26 PM   #547
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What's the input impedance of the inverting input of the AD844 if you use a feedback resistor as usual for an opamp virtual ground?
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Old 22nd August 2013, 09:42 PM   #548
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Has it got feedback, from what I understand an i/v with feedback can get it's knickers in a knot with the glitches/noises that come out of dacs

Cneers George
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Old 22nd August 2013, 10:37 PM   #549
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Quote:
Originally Posted by georgehifi View Post
Has it got feedback, from what I understand an i/v with feedback can get it's knickers in a knot with the glitches/noises that come out of dacs

Cneers George
The datasheet for the 844 shows I/V converter with usual feedback resistor. In general, the input impedance of the virtual ground circuit would be the value of the fb resistor divided by the open-loop voltage gain.

The claim is that, since the instantaneous input voltage arising from a current step is much lower than that caused by an opamp with the more usual very high input impedance, the glitches will be much smaller.

PreSapian was wondering how the non-feedback use of the 844 might compare with a near-zero impedance circuit. One logical comparison would be with the 844 arranged to have near-zero input impedance.

Such low impedance can only be achieved with lots of gain somewhere, and gain always has a down side.

I've just stuck a couple of 844 in an Arcam Alpha and, once I'd stopped them fidgeting and corrected Arcam's strange choice of offset nulling current, it sounds and looks a bit better than it did. It takes me so long to do, however, that I can't be sure I remember what it sounded like before.

I've been scouring this thread to find the bit where it is demonstrated that the AD844 can suffer current starvation when used singly with, say, a TDA1541A. Help, please

The Arcam is quite unusual, btw, for having no cap in the signal path until after the I/V stage.

Last edited by PlasticIsGood; 22nd August 2013 at 10:41 PM.
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Old 22nd August 2013, 11:19 PM   #550
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Quote:
Originally Posted by PlasticIsGood View Post
I've been scouring this thread to find the bit where it is demonstrated that the AD844 can suffer current starvation when used singly with, say, a TDA1541A. Help, please :
Here is a post by Supra with a TDA1541, as it has so much higher output than the PCM1704 (1.2mA) I'm using and it was much better with two than one, he found 3 stacked to be much better, looking at the output of the TDA1541 (4mA) I think 4 stacked may/maynot be even better.

Using the AD844 as an I/V

Cheers George
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