I have a question to Peter (or anyone else who feels confident on the subject).
I am contemplating reducing the input impedance of my amp, so I can take advantage of some of my new resistor discoveries that are only applicable at lower resistance values. At the moment, I use 10K. So here is the question:
How low can I take my amplifier input impedance before I overload the TDA1543 chip, or run into some other problems?
Normally, I would look at the max rated current in the datasheet and calculate the required impedance, but firstly, in this DAC, the chip is running outside its specification; and secondly, I admit I do not thoroughly understand how the I/V conversion works, and how it might affect rated current... So I decided it is best to ask the elders
I am contemplating reducing the input impedance of my amp, so I can take advantage of some of my new resistor discoveries that are only applicable at lower resistance values. At the moment, I use 10K. So here is the question:
How low can I take my amplifier input impedance before I overload the TDA1543 chip, or run into some other problems?
Normally, I would look at the max rated current in the datasheet and calculate the required impedance, but firstly, in this DAC, the chip is running outside its specification; and secondly, I admit I do not thoroughly understand how the I/V conversion works, and how it might affect rated current... So I decided it is best to ask the elders
Here's the link explaining how to calculate I/V resistors: http://www.diyaudio.com/forums/digi...ng-dac-complementing-cd-pro-7.html#post173483
Your best bet is most likely trying few different lower resistor values in the amp first to see how it works, and then making your own.
Your best bet is most likely trying few different lower resistor values in the amp first to see how it works, and then making your own.
The TDA1543 is a current output device and hence would not normally distort with reduced load resistance, rather the linearity could improve. This is because loading reduces the output voltage swing as the resistance seen by the TDA1543 reduces, causing it to operate in a more linear region. In other words, the effect of reduced resistance is only a reduction in amplitude. The current swing of the TDA1543 does not change.
Perhaps a more critical problem with loading is that the low frequency response becomes limited by the RC time constant of the output capacitance (the capacitor on the output of the DAC) in combination with the load resistor you choose on the output. In testing I would focus on any loss of low frequency extension, loss of detail or veiling. This could be countered by increasing the size of the coupling capacitor.
Thanks Peter, that's what I wanted to hear!
@Hierfi
By reduction of amplitude you mean lower volume, or lower dynamic range? The former should not be a problem, but the latter I want to avoid at all cost.
As to RC filtering, I intended to counter that by swapping the output caps for a pair of 47uF/6.3V Black Gate Nx. Am I correct in my calculation that for a TDA1543 powered with 8V power supply, maximum voltage seen by the output capacitor will be 5V (i.e. the voltage swing component: 8V - 1.8V - 1.2V = 5V)? I would hate to blow such rare caps by overvolting them.
@Hierfi
By reduction of amplitude you mean lower volume, or lower dynamic range? The former should not be a problem, but the latter I want to avoid at all cost.
As to RC filtering, I intended to counter that by swapping the output caps for a pair of 47uF/6.3V Black Gate Nx. Am I correct in my calculation that for a TDA1543 powered with 8V power supply, maximum voltage seen by the output capacitor will be 5V (i.e. the voltage swing component: 8V - 1.8V - 1.2V = 5V)? I would hate to blow such rare caps by overvolting them.
Thanks Peter, that's what I wanted to hear!
@Hierfi
By reduction of amplitude you mean lower volume, or lower dynamic range? The former should not be a problem, but the latter I want to avoid at all cost.
As to RC filtering, I intended to counter that by swapping the output caps for a pair of 47uF/6.3V Black Gate Nx. Am I correct in my calculation that for a TDA1543 powered with 8V power supply, maximum voltage seen by the output capacitor will be 5V (i.e. the voltage swing component: 8V - 1.8V - 1.2V = 5V)? I would hate to blow such rare caps by overvolting them.
If the output of the TDA1543 can`t get closer than 1.8 volts from the 8 volt supply the maximum voltage would be approximately 6.2 volts. This is if the output gets permanently digitally set to maximum. The other factor is that for any reasonable size of external resistor you select after the capacitor will cause the breakdown current to be limited by that resistor, probably in the uAmp region. In other words more leakage would appear as a voltage drop across your external resistor that in turn reduces the remaining voltage across your capacitor. If you have a voltage drop of any significance across your external resistor this will indicate you have some leakage issues.
The effect of externally loading the output with a lower value resistor is intimately connected with the value of the resistor connected between the output of the TDA1543 to ground. This is the resistor before the output capacitor. For example if both the external resistor and this internal resistor are 1kohm the output will drop to 50%. This is because the current swing of the DAC doesn`t change and the DAC sees the parallel combination of the two resistors as 500 ohms resulting in 50% swing. You could attach 10 ohm on the output, this resulting in 1% swing if the internal resistor is 1kohm. There is only a reduction in volume, the dynamic range actually increases if the DAC could produce more current.
What is interesting to note is that a current source with a 1k ohm load can be equally modeled as a voltage source with a series 1k ohm source resistance.
This means that when you parallel up a number of TDA1543 chips the equivalent source impedance of the DAC drops to the value of the resistor to ground needed to set up the quiescent operating point. This helps in lowering the source impedance of the DAC when directly driving cables and may account for some of the audible differences in using multiple DAC chips.
I have just tested a number of resistors at my amp's input. It has resistor sockets so it was a relatively quick and easy job. I decided to run these tests before swapping the coupling caps.
I started with 3.48K, and haven't noticed any problems at all. Only the volume went up considerably - I use a passive, resistor-based "preamp" so I understand the decrease in DAC output was more than offset by a lower voltage divider ratio at the preamp.
I then tried subsequently lower values: 2.5K, 2K, 1K, and finally 300R. I currently use 4.7uF BG N coupling caps and the bass filtering only became obvious at 300R - admittedly, I only tested it with classical, might have been more apparent on heavier material.
I think the low-level detail became a little clearer with the lower resistors (1K and 300R). I am not sure about the dynamics as the changes in volume made it all hard to judge, but I think it was largely the same. All in all, the results are encouraging, and I will certainly be experimenting further.
Yes, but is it not 6.2V relative to ground?
Please correct me if I'm wrong, but the way I understand it is that when there is no signal present, the output stays at the "voltage midpoint", which for me calculates as 1.8V + (5V / 2) = 4.3V. When there is signal present, voltage can deviate by a maximum of 5V/2 =2.5V either way from this midpoint. So, relative to the output cap, the midpoint becomes a "0V", and the only voltage actually seen by that cap, is the +/- 2.5V signal swing.
I started with 3.48K, and haven't noticed any problems at all. Only the volume went up considerably - I use a passive, resistor-based "preamp" so I understand the decrease in DAC output was more than offset by a lower voltage divider ratio at the preamp.
I then tried subsequently lower values: 2.5K, 2K, 1K, and finally 300R. I currently use 4.7uF BG N coupling caps and the bass filtering only became obvious at 300R - admittedly, I only tested it with classical, might have been more apparent on heavier material.
I think the low-level detail became a little clearer with the lower resistors (1K and 300R). I am not sure about the dynamics as the changes in volume made it all hard to judge, but I think it was largely the same. All in all, the results are encouraging, and I will certainly be experimenting further.
Yes, but is it not 6.2V relative to ground?
Please correct me if I'm wrong, but the way I understand it is that when there is no signal present, the output stays at the "voltage midpoint", which for me calculates as 1.8V + (5V / 2) = 4.3V. When there is signal present, voltage can deviate by a maximum of 5V/2 =2.5V either way from this midpoint. So, relative to the output cap, the midpoint becomes a "0V", and the only voltage actually seen by that cap, is the +/- 2.5V signal swing.
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6.2 volt is relative to ground. My calculations are for worst case as it is usually best to consider what could happen. If all is good under such circumstances any normal quiescent operating point doesn't matter. In any event I don't see a problem under the worst case condition possible.
Some further issues that you could run into is when using high value capacitors in conjunction with low values of resistance after the capacitor. If you turn the DAC off, when the power supplies falls on the DAC a high value current can feed back through the DAC's output into its power supply and damage the DAC itself. Protection against this can be found in some regulators that call for a reverse diode to bypass this current.
The worst case of risk to dynamic overload is in using the highest value of resistance. This generates the highest voltage. This results from the fixed current swing of the DAC feeding into a fixed resistance, or when V output equals (I x R) and I is a constant.
If you are reducing the value of the resistor at the amplifier end you are also reducing the source impedance as seen by the amplifier, or alternatively the output source impedance of your resistive divider preamplifier into the amp. By adding low value resistors this also upsets the natural logarithmic nature of the resistive divider as set by the manufacturer. This doesn't matter if you can still find settings to your liking. The clarity your are finding could be the result of lowering source impedance as seen by your amplifier.
The other consideration is that the resistive load seen by your DAC is also variant because of adding a low output resistor on your preamp. In other words if you set the preamplifier to its maximum output this transfers the 300 ohm resistor to the input of your resistive divider preamp, this then appearing as a load on the output of your DAC. Under such circumstances when using 300 ohms the low frequency extension becomes limited in relation to the coupling capacitor seeing this variant load. Ultimately your low frequency becomes limited as the gain setting on your preamp is increased. If this isn't at absolute maximum this shouldn't be a problem either.
I just wanted to let you guys know that I have replaced the coupling caps, and moved to low-ohm input resistors (I am still experimenting, but I will probably settle on 1k - 1.5k).
The new caps were a very nice upgrade in its own right, the Nx series Black Gates are certainly a class above the ordinary N series.
Quite the opposite was true for the resistors - the change in the value itself has left the sound virtually unchanged (my initial impression of better detail was just a product of higher loudness). The type of resistor on the other hand has significant influence on the sound.
The new caps were a very nice upgrade in its own right, the Nx series Black Gates are certainly a class above the ordinary N series.
Quite the opposite was true for the resistors - the change in the value itself has left the sound virtually unchanged (my initial impression of better detail was just a product of higher loudness). The type of resistor on the other hand has significant influence on the sound.
hi im sorry to be reviving an old thread but i've so far built two of peter's latest designs on a veroboard. i think they're the best thing happened to 1543, btw. the problem i have though is when i try piggybacking 2 1543 together, the volume starts chocking out on heavy passages. if the music gets busy, the volume literally drops down, only to come back up when the things calm down. it's as if it's pulsating. do you think it's the circuit being extra sensitive to vref changes as the temperature increases on the piggyback setup? they're ofcourse ran at 8v, fed from lm317 reg and resistors are peter's values.
and i bolted a large to220 heatsink on top. besides the crippling issue, piggyback setup sounds a lot like 12 chip setup at 6v but slightly better at everything.
and i bolted a large to220 heatsink on top. besides the crippling issue, piggyback setup sounds a lot like 12 chip setup at 6v but slightly better at everything.
I suspect the voltage output of the DAC's have risen considerably to the positive rail when you paralleled another DAC. I suspect clipping. You can reduce the 2k7's to something like half the value. Check out the quiescent voltage prior to paralleling the DAC's and measure the voltage after reducing the resistor.
Remember that the DAC's are current output devises with half the full scale current as the quiescent point. In other words it is (I x R) equal set point.
By the way, what are you using for the coupling transformer? What you are doing appeals to me as well.
Remember that the DAC's are current output devises with half the full scale current as the quiescent point. In other words it is (I x R) equal set point.
By the way, what are you using for the coupling transformer? What you are doing appeals to me as well.
ih hierfi, i figured out that the pin 1 of the dac feeding the negative signal was disconnected, so in a way you were right. (excellent!)
but after repairing this i realize that indeed single chip solution sounds better, as peter stated. piggyback smears the sound- which i'm guessing is caused by phase distortion? also the frequency extension is better with single chip, most noticeable in the bass. right now i have altec 15356a and jensen je-10kb-c which i'm planning on connecting it in reverse for 1:4 out. you can see my build here. http://www.diyaudio.com/forums/digital-line-level/231606-discrete-output-single-tda1543-4.html
but after repairing this i realize that indeed single chip solution sounds better, as peter stated. piggyback smears the sound- which i'm guessing is caused by phase distortion? also the frequency extension is better with single chip, most noticeable in the bass. right now i have altec 15356a and jensen je-10kb-c which i'm planning on connecting it in reverse for 1:4 out. you can see my build here. http://www.diyaudio.com/forums/digital-line-level/231606-discrete-output-single-tda1543-4.html
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Hi
I finally dared to connect the power to test my take on the "pushed" TDA1543. As you see i took the sissy road and used PCB's, looked several times at Peters hard wiring but decided that i would not be able to fix that. This sounds so good, airy, vivid, smooth and powerful all at the same time. I am so happy
Now i just need to finnish and get the enclosure that it deserves
/Anders
PS sorry for the crappy picture quality DS
I finally dared to connect the power to test my take on the "pushed" TDA1543. As you see i took the sissy road and used PCB's, looked several times at Peters hard wiring but decided that i would not be able to fix that. This sounds so good, airy, vivid, smooth and powerful all at the same time. I am so happy
Now i just need to finnish and get the enclosure that it deserves
/Anders
PS sorry for the crappy picture quality DS
Hi Peter,
As I understand, you are not connection anything to GND, but are going directly on the pins 9+10?
Ref: Picture in post #5
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