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Old 7th January 2013, 01:21 PM   #1591
opc is offline opc  Canada
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Quote:
Originally Posted by analog_sa View Post
Let me guess, transformers are not your favourite coupling devices

Still, the voltage issue you mention is generally a non-issue - there is no dc potential across the windings as the two drains are at the same potential. Not sure if anyone can say exactly by how much this will deviate at different temperatures but i don't anticipate any issues. As for bandwidth and distortion, 80kHz is more that fine with me and i don't really see much point reducing thd below -80db.

Will certainly try it both ways, just have to find some space for the caps.

And thanks for the heatsinks specs.
That's correct, there is no DC across the +/- terminals, and the net DC to GND of each leg is roughly 23V IIRC.

You will see a few mV depending on how accurately you set the voltages, and it will vary by a few mV over temp. If you have matched transistors, and you set everything carefully, you might be able to get away with it sans DC blocking capacitor.

Just for the record though, I fully agree with qusp... I don't know why anyone would ever opt to use a transformer when a capacitor is an option. There are applications where transformers work just fine, but in this case you will see a significant reduction in BW, some phase anomalies, much higher THD+N, and a possibly some issues maintaining low enough DC across the outputs.

I'm working on a fully DC coupled variant right now, but there are some issues with turn on and turn off transients, and you have to live with an op-amp in the circuit

Cheers,
Owen
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Old 7th January 2013, 01:25 PM   #1592
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you might be able to get away with it sans DC blocking capacitor.
I gathered thats the only format he is considering.

I mentioned the unbalanced reflected impedance on the other side of the TX, most amps have pretty unbalanced impedance +/- inputs, I would expect this imbalance to be reflected in the DC across the windings? thats a question, maybe there is something i'm not considering. this would be the case no matter how perfectly you have matched the D1

youre right though, I wasnt thinking re the actual voltage not being the full rail voltage, rather than just the 23v

doh, dont try to be clever at midnight

Last edited by qusp; 7th January 2013 at 01:32 PM.
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Old 7th January 2013, 01:28 PM   #1593
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Originally Posted by opc View Post
If I were you, I would look closely at the superb linear voltage regs from TI. They make a +/- 1A pair in the TPS7A series

Indeed very impressive published data. For some reason i am unable to find a positive regulator in TO-220...
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Old 7th January 2013, 01:34 PM   #1594
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it doesnt exist, even the neg one is a large SMD part that looks similar to to-220. its only the DFN under discussion afaik, certainly they are what i'm considering
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Old 7th January 2013, 01:41 PM   #1595
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Originally Posted by opc View Post
there are some issues with turn on and turn off transients, and you have to live with an op-amp in the circuit

Transients are unavoidable in a dc circuit. If a parallel/shorting muting relay is allowable this shouldn't present any problem. Is the opamp only a servo?

I am not dead set on transformers. In fact there are very few transformers whose sound i like. Among other things they provide ground isolation and filtering - worthwhile properties in a dac plus BAL-SE_BAL conversion, which in this particular case seems quite handy.
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Old 7th January 2013, 01:53 PM   #1596
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Don't get me wrong... I fully encourage some experimentation with transformers, I'm just making it clear that they are not my cup of tea.

There are two more specific applications I can think of where I actually would encourage the use of a transformer:

1. If you need more output voltage but don't want to change the circuit.

2. If you must have SE output.

Number 1 can also be accomplished by increasing the resistor values, but that will reduce idle current and impact THD+N. If you needed 4V out then increasing resistors is probably the better option, but if you need more like 8V out, then you'd probably be best with a transformer.

Number 2 can be accomplished with an op-amp (which is what I would do), but I can understand someone being drawn to the simplicity and passive nature of the a transformer to accomplish the same thing.

When the application only requires DC blocking like it does with the NTD1, then I find it hard to make a good argument for transformers. As soon as you throw impedance conversion or BAL/SE conversion into the mix, then the argument for using a transformer becomes a lot stronger.

Cheers,
Owen
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Old 7th January 2013, 02:09 PM   #1597
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but why would you put it after the NTD1? if you were going to use a TX and -80db THD is good enough, you should just connect the TX directly to the DAC, by putting the tx after the D1 you have just thrown away all the goodness you have gained in the first place with great effort/expense and chassis realestate

in the case of direct IV with TX vs NTD1 with caps its just a matter of taste, but putting it after the D1 just doesnt compute for me i'm afraid. you get all of the benefits mentioned above by connecting directly, but a lot cheaper and in a lot smaller space.

Last edited by qusp; 7th January 2013 at 02:12 PM.
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Old 7th January 2013, 02:15 PM   #1598
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2. If you must have SE output.
wouldnt you get better performance just by using half the D1 output? normally I wouldnt say that, but with transformers you are going to lose a lot more than 3db

anyway good luck with that analogue_sa sorry for my fervor here, the main appeal TX ever had for me was the simplicity of just connecting directly with a transformer-> IV resistor maybe a simple jfet buffer after it. but in the end I just didnt like it

Last edited by qusp; 7th January 2013 at 02:19 PM.
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Old 7th January 2013, 03:20 PM   #1599
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Originally Posted by qusp View Post
but why would you put it after the NTD1? if you were going to use a TX and -80db THD is good enough, you should just connect the TX directly to the DAC, by putting the tx after the D1 you have just thrown away all the goodness you have gained in the first place with great effort/expense and chassis realestate

in the case of direct IV with TX vs NTD1 with caps its just a matter of taste, but putting it after the D1 just doesnt compute for me i'm afraid. you get all of the benefits mentioned above by connecting directly, but a lot cheaper and in a lot smaller space.
The main reason, and it's a big one, is operating the DAC in current mode rather than voltage mode. If you connect a transformer directly to the output of the DAC you'll be running in voltage mode which, quite frankly, sounds like crap.

With the NTD1 transformer coupled on the output, at least you're running the DAC in current mode into the NTD1. You're also driving the transformer from a low impedance source which is better than driving it off the DAC directly.

As for the SE output thing, you really do give up a lot deriving the signal from only one leg of the D1. THD+N increases above -100dB and I actually don't much like the way it sounds either. You're much better off feeding the differential output to an op-amp to get to SE, or as I mentioned above, running a BAL-SE transformer. At least that way you get some of the differential cancellation advantages which impacts both noise and distortion.

Again, I'm not arguing with you overall... I would exhaust pretty much every other option before going to a transformer, but not everyone feels that way. Where I tend to strongly disagree with the use of transformers is when people resort to using them simply out of an irrational fear of having a capacitor in the circuit. People tend to associate negative things with series caps, and positive things with transformers, when it's very much the opposite. In a blind AB test, I cannot detect a good capacitor in series with the signal chain, but I can definitely detect a transformer.

Cheers,
Owen
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Old 7th January 2013, 03:33 PM   #1600
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As for the SE output thing, you really do give up a lot deriving the signal from only one leg of the D1. THD+N increases above -100dB
Quote:
The main reason, and it's a big one, is operating the DAC in current mode rather than voltage mode. If you connect a transformer directly to the output of the DAC you'll be running in voltage mode which, quite frankly, sounds like crap.
hmm but weve established that hes happy with -80dB and we both agree that thats probably a good thing because it wont be too far wrong . do you think you could still pick ESS current mode and ESS voltage mode, or ESS single phase mode, once its been reduced to -80THD?

that was my point, there is a pretty severe leveler in the circuit that dominates all the other factors

as for reality of what I would consider or use myself we are totally on the same page. but if using a device that knocks the THD down that low, burning 50W and taking up more than 10x the space to get the THD to burn (most of the reason to take those extreme steps in the first place).. thats nuts IMO

Last edited by qusp; 7th January 2013 at 03:39 PM.
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