The pcm1794a datasheet I/V converter and how to improve it. - Page 11 - diyAudio
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Old 31st August 2012, 09:05 PM   #101
smms73 is offline smms73  Portugal
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Originally Posted by dirkwright View Post
Can your buffer drive 150 ohms cleanly? That's what I need to know.

Thanks for the reminder about the output opamp. Will do chief!
Yes of course, the current is what's the most important . The buffers most have a quiescent current high enough for the buffers don't leave class a operation. Start with the circuit in post #3 and go playing around with the values . You can chose other opamp. But dont use a very fast ones. You can use BC327 if you want.
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Old 31st August 2012, 09:29 PM   #102
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Originally Posted by smms73 View Post
Yes of course, the current is what's the most important . The buffers most have a quiescent current high enough for the buffers don't leave class a operation. Start with the circuit in post #3 and go playing around with the values . You can chose other opamp. But dont use a very fast ones. You can use BC327 if you want.
Is the LME49990 ok with you? It's really awesome on paper.

GBW = 110MHz
Full Power bandwidth = 291kHz
PSRR = 144dB
CMRR = 137dB
Slew rate = 22V/usec
Input current noise density = 2.8pA/Hz^1/2

I can get to this later.
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Old 1st September 2012, 01:07 AM   #103
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Hi Ken .
Can you give us details about the dac chip you use, and the value of the iv resistor?
I am now trying new possibilities in my discrete balanced iv. And I am thinking in using a 5ohms resistor iv. Do you think is to much ?
If I use the pcm1794 in mono I can use a 2,5ohm resistor.
Sergio,

I'm also presently using the PCM1794A, except in stereo mode. I use 150R carbon film MELF type resistors on each channel for passive I/V duty. I don't make use of the differential ouput capability of the 1794A chip - instead taking a single output phase of each channel - only because my linestage only features single-ended inputs. In this configuration, the +/- 3.9mA peak pin current develops 0.41VRMS across the I/V resistor while generating 0.0022% THD, acording to my PC card based spectrum analyzer. This signal level only requires moderate linestage gain to deliver normal listening levels.

The PCM1794A seems to be quite happy with passive I/V values of at least 200R in stereo mode, and 100R in current-doubled mono mode. Yes, you read that right, 200 ohms and 100 ohms. There are at least two very well regarded high-end components of which I'm aware - one is the Audio Research CD-8, and the other is the Neko D100 - that utilize passive resistor I/V of such seemingly high values along with the PCM1794A. In either case, the quiescent voltage on any of the four output pins would be +1.24VDC. The signal amplitude would then be 0.5VRMS single-ended, and 1.0VRMS balanced.

As I had indicated, this combination of passive resistor I/V routed directly (A.C. coupled) to my FET linestage is synergistic, and has provided subjectively the most transparent and naturally dynamic sound I've yet obtained from my experimental DACs. I had before used this same configuration with an AD1865 DAC chip to similar, if not quite as impressive, result as with the PCM1794A. Although, I must say that I'm highly intrigued by your uniquely behaving grounded-base discrete circuit.

One other tip about using the PCM1794A DAC. It features an input pin selectable sharp/soft digital reconstruction filter impulse response. T.I. suggests that the soft setting is intended for applications requiring low latency throughput of the FIR filter. What it also provides, however, is a much improved impulse response having much reduced ringing. Although the FIR filter does remain linear-phase (having both pre and post ringing) in either setting. The soft filter repsonse of the PCM1794A used to be the 'listen' seting on Ayre's digital players before they implemented their own proprietary minimum-phase filter via FPGA chip.
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Last edited by Ken Newton; 1st September 2012 at 01:36 AM.
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Old 1st September 2012, 02:12 AM   #104
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Wow. thanks Ken. My mind is slightly blown.
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Old 1st September 2012, 07:50 AM   #105
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Thanks very much my friend...
Thanks for sharing with us your experience.
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Old 1st September 2012, 02:36 PM   #106
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Dang. I want to print out that response and frame it. I had no idea.
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Old 1st September 2012, 03:01 PM   #107
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I suppose I should perhaps point out that when I talked about the performance being degraded by increasing the impedance that the DAC sees, that I was meaning that 0.002% distortion is a degraded figure. That to me, when the DAC is capable of much lower is bad thing, but then it depends on what your goals are.
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Old 1st September 2012, 03:42 PM   #108
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Yes, not the figure the PCM1794A datasheet says is possible. Keep in mind, this was measured via a PC soundcard, so it also reflects the distortion of that card's AKM based ADC section. In addition, my DAC features only a single pole analog output filter, plus it only utilizes one phase of the chip's differential output. Even so, that 0.0022% THD figure is still about -93dB from the FS signal, and provides more support for the notion that there is more to subjectively transparent sound than obtaining the very lowest possible THD.
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Last edited by Ken Newton; 1st September 2012 at 04:02 PM.
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Old 1st September 2012, 03:50 PM   #109
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It seems there more than one route to ascend to audiophile heaven .

My solution is to build the two IV converters, and decide for my self.
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Old 1st September 2012, 04:16 PM   #110
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Dang. I want to print out that response and frame it. I had no idea.
We are all adventuring together on the same quest.
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