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 pftrvlr 10th July 2008 10:51 PM

After having assembled a few DACs, I am thinking to scratch build a 1541A DAC. While researching on the web, I found one implementation using a 317 CCS and an OP Amp as IV.

Is there any advantage using a CCS in place of a resistor? How is gain of the IV stage calculated with a CCS load?

 lcsaszar 11th July 2008 12:13 PM

The CCS does not affect the gain. It represents a constant (that is DC) current that provides the idle -2 mA current of the TDA1541A. No AC current is flowing through it.

 kevinkr 11th July 2008 04:19 PM

I'd probably use something other than an LM317 as the CCS in this application - maybe an IXYS, transistor ring of two, cascode or similar - much better performance, although in a virtual earth situation perhaps it is not that critical...

 pftrvlr 11th July 2008 07:17 PM

Say for example if 1541A output current ranges from 0 to 4mA, when a CCS is used, what is voltage swing we can expect?

 kevinkr 11th July 2008 07:29 PM

Quote:
 Originally posted by pftrvlr Say for example if 1541A output current ranges from 0 to 4mA, when a CCS is used, what is voltage swing we can expect?

Doesn't affect the voltage swing either, just offsets the 0 signal current so that the output of the transimpedance amplifier (I/V converter) can be dc coupled. The CCS set to -2mA results in the transimpedance amplifier seeing an input current range of -2mA to +2mA, but the magnitude of the current it sees at its input has not changed. (still 4mA) Voltage swing is determined solely by the feedback resistor employed in the I/V converter.

 pftrvlr 12th July 2008 05:06 PM

2 Attachment(s)
I should have posted the pix earlier. As far as I see,
the CCS is used in the feedback resistor position, as shown here. I figue the voltage swing should be pretty big, but how big?

 kevinkr 13th July 2008 09:19 PM

The CCS in this circuit is not used in the feedback resistor location, if you look carefully you will notice that it instead goes directly to the +15V rail.

The circuit design is rather unconventional, so I might have missed something else, but I suspect there might be a resistor omitted from the schematic between pins 2 and 6 of the 844 otherwise it is being used in a very unconventional fashion as perhaps a (close to) unity gain transimpedance amplifier with those odd connections to pin 5. I don't know and haven't the time at the moment to analyze further.

A ccs inside the feedback loop is not a real good idea for a variety of reasons not the least of which is if it works properly it should look like a very high impedance to any ac signal which should result in clipping at the output of the amplifier for any signal containing an ac component.

 pftrvlr 13th July 2008 10:01 PM

You're right, I mis-read the schematic. The CCS is not in the feedback loop.

I browse the AD844 info, it is specifically designed for current-voltage conversion. It does not use a feedback resistor in I/V application.

Haven't heard a lot about AD844. Wondering if it is good for hifi.

 kevinkr 13th July 2008 10:25 PM

Quote:
 Originally posted by pftrvlr You're right, I mis-read the schematic. The CCS is not in the feedback loop. I browse the AD844 info, it is specifically designed for current-voltage conversion. It does not use a feedback resistor in I/V application. Haven't heard a lot about AD844. Wondering if it is good for hifi.
I had a Sonic Frontiers dac 1 that used them and it most definitely used the standard I/V configuration with a resistor between pin 2 and pin 6. Look at p.9 of the AD844 data sheet for the standard configuration that shows this, and this even includes a table of recommended resistor values for I/V conversion.

Note that the dac I/V application shown on P.13 uses a dac that has an internal I/V resistor to assure good thermal tracking with the dac's internal ladder. Some older audio dacs have an internal I/V resistor but most new ones do not. (related to dac architecture issues)

This a very old design, it has relatively good ac performance and what can only be characterized as mediocre dc performance by current standards.

They sounded ok, but would not be my first choice.. See all the threads here for recommendations of better op-amps. I like the National LM4562 family which is current sota. If you use dacs with differential current output take a look at the TI THS4131 differential amplifier - works great.

 pftrvlr 13th July 2008 11:07 PM

Thanks Kevin.

A quick search yields these two threads: