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.
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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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.
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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Dang. I want to print out that response and frame it. I had no idea.
We are all adventuring together on the same quest.
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.
I'm committed to the transformer route, since I just received my two Lundahl LL1517's yesterday. I don't know what else I'd do with them if this doesn't sound good (like I can tell... haha).
I just need to get my test gear together so I can look at the hidden dragons way down there in the -130dB range.
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Did anybody notice that the output current of the PCM179x devices are specified as negative?Since they call it output current,a negative value would go into the pin, but i do not believe it's that way.The current comes out of the pins if it's true that the voltage after the first opamps is always negative.
Am I missing something here or what?
Am I missing something here or what?
The transformer produce mainly 3° harmonic distortion and the higher the frequency the lowest the distortion.
I think that with a transformer you will have lower noise and 2° harmonic but a little bit more of the third.
The best is for you to bild it and let us know your results.
I have a emu1212m sound card, what are the sound card that you are thinking to buy ?
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The 49990 its a very good opamp but with a GBW of 110 Mhz is not a good option for using a buffer at unity gain.
But I have to measure the phase of this buffers, in the simulator it seems very good. I am thinking using the OP2134 (gbw 8Mhz) because I have a lot of them.
Since I have a laptop, I only have USB or Firewire digital connections to external devices. So, I need an external "sound card". I've decided that I'm going to get TI PCM4222EVM evaluation board, which is super duper ADC. The board has a serial connection that can be configured for I2S, so I bought a miniStreamer USB bidirectional 12S <-> USB interface. So, I have the ADC side taken care of once I build the box for the board and USB interface. The PCM4222EVM has been used a lot by musicians on message boards like GearSlutz and GroupDIY. I'm also building Pete Millett's sound card interface.
On the source side, right now I have one of Victor's super low distortion 1kHz oscillators (too many zeros to count).
Otherwise, right now I have a simple Behringer UCA202 ADC/DAC with is pretty good. I use TrueAudio's TrueRTA (1/24th octave real time analyzer). I also have other spectrum analyzer software. With an Express Card USB 3.0 and the Behringer, the loop back shows noise at about -110dB if I'm lucky. I'd like to make it to -130dB, with this new gear. I think I may run the ADC on battery power. Someone posted their results with the PCM4222 here on DIYAudio and the noise floor was extremely low. We shall see, fingers and toes are crossed.
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