May i suggest other DAC.
AD1853 3mA pp 30pf
and it's brother
AD1955 8.6mA pp 100pf
i was told both are way better sounding than PCM1794.Especially AD1853.
I'm looking forward to build this I/V with something similar to this
USB DAC 2 - Hi-Resolution System
I like the AD1955 on paper, but the one thing that irks me somewhat is how the digital filters precision turns to crud when used with 192kHz. Now TI don't show their digital filters response for different sampling frequencies, so one only assumes that they are all the same, I wonder if this is a valid assumption. The trouble with the PCM1792/4 chips is that they are of old design and the engineers currently working at TI aren't totally familiar with how it works inside. At least that's the impression I got when talking to them.
As dirkwright wrote, you can use a single phase of the differential outputs. There are both positive and negative consequences of doing this.
Some positive consequences: When used with a current-conveyer I/V (such as is the circuit under discsussion in this thread), or with a current feedback transimpedance I/V stage, reducing the magnitude of the signal current will usually reduce the distortion produced by the I/V stage. In addition, the amount of circuitry is reduced.
Some negative consequence: Slightly increased even order distortion and reduced SNR due to elimination of push-pull operation. In addition, the output impedance of a zero feedback I/V circuit, using a high impedance node for the output, will double versus the differential circuit. For example, with smms73's circuit, the I/V resistor effectively sets the output impedance. If both output phases were utilized, the I/V resistor could be reduced by half (and with it, the output impedance) and produce the same output voltage amplitude.
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Hi,
other current output DACs may be used alternatively. It seems to me that the actual DAC-cores of modern sigmadelta DACs are not the sonically decisive part but the periphery around that DAC-core. I found the mentioned ADIs sonically on par with the TIs, though an exact evaluation is very difficult, since small differences in clocks, PCB-layout, digital filter, etc. are of larger sonic influence than the DAC-core itself. The analog stages of the eval-boards of the ADI-DACs for example, equipped with OP275 OPAmps, simply sound terrible.
A interesting feature of the TIs and the AD1955 is the possibility to bypass the digital filter and to drive the DAC-core from a external digital filter at rates of up to 384kHz (see Anagram or PMD).
As to my knowledge the PCM1792/1794A feature a more powerful arithmetic machine for the digital filter than the (therefore cheaper) PCM1796 which may lead to small sonic differences.
jauu
Calvin
other current output DACs may be used alternatively. It seems to me that the actual DAC-cores of modern sigmadelta DACs are not the sonically decisive part but the periphery around that DAC-core. I found the mentioned ADIs sonically on par with the TIs, though an exact evaluation is very difficult, since small differences in clocks, PCB-layout, digital filter, etc. are of larger sonic influence than the DAC-core itself. The analog stages of the eval-boards of the ADI-DACs for example, equipped with OP275 OPAmps, simply sound terrible.
A interesting feature of the TIs and the AD1955 is the possibility to bypass the digital filter and to drive the DAC-core from a external digital filter at rates of up to 384kHz (see Anagram or PMD).
As to my knowledge the PCM1792/1794A feature a more powerful arithmetic machine for the digital filter than the (therefore cheaper) PCM1796 which may lead to small sonic differences.
jauu
Calvin
I like them because
- cheap ;-)
- low noise, low Rbb (thanks to Joachim who pointed that out some time ago)
- high beta (the ...-40 types), up to almost 100mA flat
- can carry quite high currents (although you of course are limited with a TO92 which has a thermal resistance junction - ambient of around 250K/W)
- up to 200-300 mA Ic almost flat Ic vs Uce curve (low early voltage)
So, are you saying that we should use 15.6mA for the current generator in our simulations when it is connected in differential mode as shown for example in the attached figure?
Attachments
An issue with using DAC chips such as the AD1955 or AD1853 with a grounded-base I/V circuit is that the differential current output pins on these DACs want to see a common-mode input voltage that's offset above D.C. ground. For those ADI DACs, the figure is near +2.8VDC. The PCM179x series, on the other hand, wants to see a common-mode input voltage that's at D.C. ground. This issue can be addressed by level shifting the grounded-base input bias reference level from ground to the required D.C. offset, with a resistor biasing network and A.C. ground coupling capacitors.
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How the simulators handles differential signal source generator modeling might make giving an simple answer problematic. The most clear advice I can think of is to use whichever of the two current values for the source gen. which produces a 7.8mA peak and 15.6mA peak-peak signal current swing through the I/V resistor of the I/V stage being simulated - assuming, of course, that both output phase currents are being differentially utilized.
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One other point. When modeling a two-terminal differential signal source you should set the internal source impedance equal to twice that of a single DAC output pin impedance. For example, if the impedance of a signle DAC output pin were 1K, then set the modeled generator internal impedance to 2K. That is because the DAC differential outputs are effectively in series with each other with respect to a common, differentially driven load. Which means their individual output impedances are also in series, again with respect to a common, differentially driven load.
An increased source impedance has beneficial THD implications for the I/V stage. Although, the doubling of the signal current magnitude then degrades the I/V stage THD.
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OK, thanks.
The outputs from the current generator in the image I posted above are 180 degrees out of phase and half the value of the generator, so it appears to be functioning as a differential current source. I've done this with the voltage generator in other circuits in Tina also with the same effect.
(edit: whoops, made a mistake. Setting the current generator to 7.8mA produces that amount on BOTH pins of the generator, not half like I thought)
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Hi,
in Mono mode the DACs currents just double to -12.4mA | +-7.8mApp for the PCM1794a, or -7mA | +-4mApp for the PCM1795.
Now the outputs of L+ and R- must be connected together as noninv current summing point. Same applies L- and R+ for the inv sum. The DAC must be set to mono mode and the appropriate channel Left or Right selected via setting of Pin1 and Pin2 of the Chip.
As mentioned before low THD requires that the signal current stroke remains small against the current conveying transistor´s bias current. As such a PCM1795 in Mono mode has similar current values and this results in similar IV-distortion figures as a PCM1794a in stereo mode. On the other hand will a PCM1794a in mono mode lead to increased THD of the IV-stage.
Higher DAC output currents are not neccessarily a improvement.
If for example one bypasses the digital filter section of the DAC chips a PCM1795 in Mono mode may be better than a PCM1794a in stereo mode.
jauu
Calvin
in Mono mode the DACs currents just double to -12.4mA | +-7.8mApp for the PCM1794a, or -7mA | +-4mApp for the PCM1795.
Now the outputs of L+ and R- must be connected together as noninv current summing point. Same applies L- and R+ for the inv sum. The DAC must be set to mono mode and the appropriate channel Left or Right selected via setting of Pin1 and Pin2 of the Chip.
As mentioned before low THD requires that the signal current stroke remains small against the current conveying transistor´s bias current. As such a PCM1795 in Mono mode has similar current values and this results in similar IV-distortion figures as a PCM1794a in stereo mode. On the other hand will a PCM1794a in mono mode lead to increased THD of the IV-stage.
Higher DAC output currents are not neccessarily a improvement.
If for example one bypasses the digital filter section of the DAC chips a PCM1795 in Mono mode may be better than a PCM1794a in stereo mode.
jauu
Calvin
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Good circuit
I have built quite a few circuits based on these principles using mirrors to
sum the +and- phases then that current modulating cascoded OP stage.
DAC's , front ends of open loop SS power amplifiers and even a mic pre that
have a bal IP and SE OP (fully complimentary).
I have also implemented the same type of OP stage with those cross
coupled darlingtons. They do work very well to linearise non linear base currents and capacitances.
It's really a design twist of the 'Baxandall pair' connected BJT's, using the
'error' current from the bases of the main OP BJT's as + feedback. Your sim
numbers tell the story.
It all works just like it should in real life too with one gotcha - stability.
You may or may not have stability issues. It depends on layout etc. C1 will
certainly help but you may need additional base stoppers in either the main
OP BJT's or the 'helpers'.
Once you get the whole thing on a board and prototyped, it really is worth
having a good listen with and without all the Baxandall collector connections
to up/down labelled points. With these connected, the distortion will
certainly measure a lot better, however it may not actually sound better.
IME, with all these small + feedback loops, the sound can be worse. Without
them it is often more natural, but you won't get the super low dist numbers.
I'll be interested if you have similar results with this design. Keep us posted.
BTW, your choice of 327 / 337 in most locations is a pretty good one for 2
reasons - who can guess what they are?
T
Nice to hear that someone has already test the concept, and it work.
I have thought of this idea a long time ago when i needed a high voltage amplifier for a valve circuit (the input was a valve), but it was never build and it remain forgotten until last week, when I remembered that could be a good idea for a i/v converter. I never heard of 'Baxandall pair' but i will investigate.
STABILITY:
stability is one of the biggest problems in audio circuits, and for me is a very important matter.
Is perhaps the number one reason as otherwise very good concepts sound bad,. And is also one of the reason people tend to prefer a simple circuit versus a more complex one, as normaly, in many schemas there is no attempt to use stability measures
I do not know which methods you use to stabilize the circuit but rely only in base stoppers and good pcb layout is important but not enough.
this topology is very fast, so there are many possibilities for instability to build up. the
solution is to slow it down.
Today i have spend some time analysing possible nodes where stability can be a problem, and i will post soon the results.
After the circuit is rock solid stabilized there will be no reasons to sound bad.
Thanks for the tip about ZDS1009 and the interesting literature about current mirrors , i will see if i arrange time to read it.
In the last circuit i have post the distortion is smaler with the -6.2ma off ofset that without them.
about the stability, i have talk about that in my last post, and the final circuit will not have stability problems.
I have been posting the evolution of this project, i start to doubt if this is the best thing to do in this forum, but ok.
the idea of using a small current output dac as pcm1795/96 is a very good one, i will see the datasheet on this dac.
The input snuber will have very low interaction in the audio frequency, and is not a burden for the dac.
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