Using the AD844 as an I/V

[dirkwright]

Here is the bode plot for the AD844 TDA5141 circuit of post 63. Bandwidth is just fine with very little phase shift. The distortion is huge. Something is wrong. I did not include the DC biasing circuitry in my sim.

[MiiB]

to be honest.. distortion has nothing to do with audio performance...I mean we measure Distortion in speaker by the full percentage, who cares about 0.00 something digits...??

Confident the AD844 sounds terrific.. for use with higher current output DAC's, it may have too much gain.

[dirkwright]

Originally Posted by MiiB

to be honest.. distortion has nothing to do with audio performance...I mean we measure Distortion in speaker by the full percentage, who cares about 0.00 something digits...??

Confident the AD844 sounds terrific.. for use with higher current output DAC's, it may have too much gain.

Sorry, my mistake. I ran it on +/- 5 V instead of 15VDC.

Distortion is nothing to worry about.

[abraxalito]

Originally Posted by Ken Newton

It would be interesting to know the reasons why Ayre stopped utilizing the AD844 in the output stage of one of their earlier CD players, I don't recall the model.

From what I recall Charles Hansen saying (and my memory is rather hazy), ADI moved (shrunk I guess) the original process and as a result some of the newer chips turned out noisier than the originals. Perhaps the earlier parts were beating the DS specs comfortably and the newer ones had less margin? He didn't sound too impressed with ADI's customer support anyway - their response being (as you'd expect) 'it meets the datasheet spec'.

[MiiB]

Dirk.. Do you have an LT spice model of the AD844..??

[dirkwright]

Originally Posted by MiiB

Dirk.. Do you have an LT spice model of the AD844..??

I used the one from them:
AD844 datasheet and product info | 60 MHz, 2000 V/µs Monolithic Op Amp | Operational Amplifiers (Op Amps) | Analog Devices

[georgehifi]

Here is a little from the now obsolete AD846 datsheet also can be used as current feedback opamp, seems it was the forerunner to the AD844, also had a TZ type pin 5. but was mainly used for bandwidth limiting compensation or as a low mA output point but they don't say what the output impedance is at that point either, looks to be similar architecture though.

Cheers George

[georgehifi]

Quotes from Charles Hansen designer of the Ayre CDP's
"Those are not op-amps. An op-amp (by definition) operates by using feedback. Our 7-series products use Analog Devices AD844's (designed by the great, but very stubborn, Barrie Gilbert) in the analog signal path, but with no feedback. So while the part was designed to be used as a current-feedback op-amp, we don't use it that way. Just one reason that they are in the "Class A" rating in Stereophile."

Quote from Jan Didden
"Yes, I agree, the AD844 are one of the best kept secrets in audio. I used them in my error correction amp as precision gain-of-one-without-feedback.

You can set the gain with a single resistor and get an open loop buffer to boot.
You remember the AD846? Almost the same except that the max lin current for the transconductance output was 10mA in stead of 1mA for the '844. Discontinued, of course.

Barrie sort of confided that he had a much better part on the shelf but unless there was sufficient demand, they would probably not produce it.
If I only had a 100 grand!"


Cheers George

[abraxalito]

Barrie Gilbert should be regarded as something of a hero by all DAC designers - not only has he given us the the AD844 but also the AD830 (and subsequently AD8129/30) which have used in my DAC designs. I plan to use the AD8129 in a future design. Bottom line is - his parts sound great

[georgehifi]

Originally Posted by Ken Newton

It would be interesting to know the reasons why Ayre stopped utilizing the AD844 in the output stage of one of their earlier CD players, I don't recall the model. I believe they used the chip in the manner Pedja Rogic does, open loop with the transimpedance node connected to ground via a resistor.

The Ayre player/s were the 7 series, I believe he used 4 per channel as I/V, filter, and two for the balanced and single ended outputs all used as the zero global feedback way. I wished they had old school multibit dacs though, then I would look/search for them s/h.
Ayre CX-7 CD player | Stereophile.com

Cheers George