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Old 26th November 2007, 09:11 AM   #1
Skorpio is offline Skorpio  Denmark
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Join Date: Mar 2004
Location: Denmark
Default I/V stage question...

I have a very good I/V stage, single-ended based on a grounded base stage with a feedback-pair. This keeps the input resistance below 1 Ohm.

I am thinking of trying this:

http://www.pedjarogic.com/1541a/pdf/discrete_I-V.pdf

But the input resistance seems to be 3-5 Ohm....

Does anybody knows the importance of this? What is the sonically improvement by lowering the Rin? Opamps have VERY low Rin but don't sound as good as my current stage.....so perhaps the more stable, but higher Rin is better?
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Old 3rd February 2008, 02:32 AM   #2
tedr is offline tedr  United States
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Join Date: Jul 2006
Default I'll try

this applies to DAC with current output only. The output of the DAC can be understood to be approximately a current source, that is, a relatively high impedance, probably a few kilo Ohms. What is wanted is to convert this to a voltage. Sometimes there is a DC voltage on the DAC current output, sometimes not. Best performance is obtained when both AC signal and DC conditions are met by the IV converter stage.

The DAC output is a current usually a few mA. Let's say it is 2mA maximum, with a sine wave 0dB FS. Now 2mA flowing in a 1 Ohm load produces 2mV. This is a voltage caused by the current flowing from the DAC into the load. Now with 5 Ohm load the voltage produced is 10mV. If the load impedance could be made infinitely small there would be infinfitely small voltage produced.

It is not clear whether this change from 2mV to 10mV causes degradation that is audible. It depends on the effect on the DAC when there is a little bit of voltage on the output. Some effects that can be calculated and understood have effects that are below the measurable threshold and inaudible. You may do listening tests to reach a conclusion in this case. Also the data sheet from the manufacturer will show a recommended load for the IC, you can be guided by this.

I'm not sure what you are thinking of when you say opamps have a very low input impedance. The traditional voltage feedback linear opamp has high input impedance at both inputs. Sometimes the opamp is configured as an inverter and in this situation there is a virtual earth dynamic low impedance at the summing point, the inverting input terminal where feedback is applied. This low impedance is a property of the feedback loop. It is common to believe that the impedance there is zero Ohms but it is truthfully not zero but some value that depends on the open loop gain. And it is a dynamic low impedance that depends on feedback action, the summing point is not stable until the feedback loop has stabilised, settled. Measuring the signal amplitude at a summing point to verify the impedance using Ohms Law is usually not reliable, the signal is an extremely small current not a voltage. I have seen voltages between 10uV and 1mV at summing points but this does not define the impedance there.

Consider also the out of band signals that are present at the DAC output, the images of the baseband signal at multiples of the sampling frequency. These images are amplified with the wanted signal and can require more bandwidth than some opamps can provide, this may be part of the sound quality differences that are heard.

Ted
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