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Old 21st January 2013, 02:10 AM   #141
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Quote:
Originally Posted by abraxalito View Post
What was wrong with the buffer to begin with?
I don't know it sounds great to me, just that some here said it wasn't the best design they've seen, but then how can that be seen without all the missing bits that are not shown on the simplifed diagram.

Ho! and BTW the dc offset direct coupled all the way is only a nice stable 10mV at pin 6, I could null this out with your dac offset nulling circuit that you gave me because that's adjusted for zero at TZ, and I figure it's better to have zero there than at pin 6.

Cheers George
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Old 21st January 2013, 02:13 AM   #142
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From studying the datasheet - the OL output impedance being 15R, the buffer's output quiescent is probably around 1mA. You could experiment with adding a current source (try to both rails) drawing say 4mA (an LM317L would work) to bias one or other of the output transistors into classA. Be sure to feed the LM317L from a very clean power supply though.
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Old 21st January 2013, 02:17 AM   #143
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Doh, you guys just missed my previous post, go back a page. The buffer adds more distortion but as noted it doesn't sound too bad.
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Old 21st January 2013, 02:21 AM   #144
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No, I read it. I don't consider full scale distortion to be a significant contributor to SQ. Pedja quotes the figure at 2VRMS, it would have been much more interesting to learn of the 50mVRMS figure.
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Old 21st January 2013, 02:34 AM   #145
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Pedja is asked:

"Talking more about IV conversion I note that you did some tests previously with AD844 that found some poor distortion performance in the output stage of the 844 and I wondered was this test with feedback to the negative input terminal or open loop?"

And says:
"Once I actually posted AD844 performances both with and without feedback... Without feedback and with inverting input loaded by TDA1541A output current and impedance, a THD is at about or a bit above 0.1% (this includes both AD844 current mirrors and its output buffer, and each of these two stage are contributing about the same to this figure). With feedback it performs so good as good opamp does."

CG describes the issue:
"The AD844 is a special case. It's a current feedback type amplifier. The transimpedance node is connected to a pin on the package, which allows the part to be used open loop (actually, as the AD797 can be) if you want to run it that way. There may be good reasons to do so... Thank Barrie Gilbert for following Wyn Palmer's lead on making this pin available. (Wyn was a serious DIY audio guy at one time.) There was a thread about this very topic about 2 1/2 years ago."

CG continues with some great description:

"Back to the AD844 thing for those who didn't read the previous thread. Or threads.

The AD844, like the AD846 before it, use a classic CFB topology. The non-inverting input is a complementary pair of bipolar devices driven at their bases. The collectors are connected to current mirrors that ultimately connect to the Tz node. This Tz node is available at one of the pins. From the Tz node to the output is a complementary buffer stage. Or stages. The inverting input is at the connection of the emitters for the complementary input devices. It's shown in the data sheet.

So... If you connect the inverting input to ground through a resistor and add a resistor to ground from the Tz pin, you can build a pretty nice open loop amplifier. The devices inside are nicely matched because of the IC fabrication process. You can use the buffer as is or add your own externally.

If you instead want to build an inverting amplifier like you might want for a DAC I/V section, you connect the DAC current output to the inverting input. The non-inverting input is then connected to whatever voltage the DAC output wants to see at it's output. Maybe it's ground, maybe it's something else.

Or, if you want to build a differential amplifier, you use two AD844s and, well, this exercise is left to the reader.

The upside is that the devices are really well matched. The downside is that you don't have control over the individual transistor currents and you might have to deal with effects like thermal modulation across the device and substrate effects. ..."

Tidbits to feast upon ...
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Old 21st January 2013, 02:35 AM   #146
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Quote:
Originally Posted by abraxalito View Post
From studying the datasheet - the OL output impedance being 15R, the buffer's output quiescent is probably around 1mA. You could experiment with adding a current source (try to both rails) drawing say 4mA (an LM317L would work) to bias one or other of the output transistors into classA. Be sure to feed the LM317L from a very clean power supply though.
Thanks, but it's so good at the moment I'll just sit on it for now and bath in it's beauty, beside I've never done that before.
What I cannot understand is that I am the only one here playing around with this, what's everyone doing, get on board and do it, it's not hard to implement into a cdp that already has an opamp for an I/V.

Cheers George
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Old 21st January 2013, 02:39 AM   #147
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I'm busy getting highly decent sound for way, way cheaper than a PCM1704 since you're curious
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Old 21st January 2013, 02:59 AM   #148
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OK, what is, when are you going to show us?

Cheers George
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Old 21st January 2013, 03:16 AM   #149
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Its a work in progress over here : The Ozone layer - modding the Lite DAC-AH

The idea behind it is on my blog - use a timed array of very cheap DAC chips. I'm using passive I/V and and a steep passive filter before the (wideband) amp stage. Total parts cost under $250 but a lot of labour involved in modding and building.
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Old 21st January 2013, 07:22 AM   #150
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Quote:
Originally Posted by AudioLapDance View Post
The devices inside are nicely matched because of the IC fabrication process. You can use the buffer as is or add your own externally.

If you instead want to build an inverting amplifier like you might want for a DAC I/V section, you connect the DAC current output to the inverting input. The non-inverting input is then connected to whatever voltage the DAC output wants to see at it's output. Maybe it's ground, maybe it's something else.

Or, if you want to build a differential amplifier, you use two AD844s and, well, this exercise is left to the reader.

The upside is that the devices are really well matched. The downside is that you don't have control over the individual transistor currents and you might have to deal with effects like thermal modulation across the device and substrate effects. ..."

Tidbits to feast upon ...
This is good to know thanks AudioLapDance, this is probably why I'm getting little and stable dc offset, not a coupling cap insight.
I just read that's short circuit protected, maybe I should just let the 15ohm output do the driving and get rid of my 47ohm series resistor,
Cheers George
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Last edited by georgehifi; 21st January 2013 at 07:27 AM.
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