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Old 6th July 2012, 09:37 AM   #971
EUVL is offline EUVL  Europe
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I have no need personally to do balanced-> SE conversion, as everything I build is balanced.
Well, almost everything.

And most certainly not with an opamp.


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Old 6th July 2012, 11:06 AM   #972
regal is offline regal  United States
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Quote:
Originally Posted by EUVL View Post
I have no need personally to do balanced-> SE conversion, as everything I build is balanced.
Well, almost everything.

And most certainly not with an opamp.


Patrick
Not to be argumentive but at some point to reap the benefits of balanced you do a subtraction/summation. Now some wait till the speakers/drivers I guess the voice coil provides the some CMRR & distortion cancellation, it just seems a poor choice, why amplify distortion? Just curious what the alternatives out there are, I was considering possibly going balanced up to the push-pull mofsets on the power amp outputs.
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Old 6th July 2012, 11:27 AM   #973
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The speaker coil sees no Gnd, only +Vout and -Vout.
So there is no such things as CMRR at the voice coil.


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Old 6th July 2012, 12:14 PM   #974
regal is offline regal  United States
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Well you agree the voice coil is a poor choice to sum the differential signal.

I am no fan of opamps either but they are good at addition and subtration. We share the opinion that they are inferior for I/V. But if they are suitable for servoing dac offset with the Sen, they are even more suitable for differential to unbalanced conversion, that's what they have been optimized for the last few decades (in different aplication.)

Obviously the sim shows an advantage to a balanced Sen I/V then summed, so where do you propose the conversion to unbalanced, sound in air is not differential so has to be done somehwere prior.
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Old 6th July 2012, 12:22 PM   #975
EUVL is offline EUVL  Europe
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> Well you agree the voice coil is a poor choice to sum the differential signal.

No I do not agree.

As I said, I only build balanced circuits.
That means that I connect my speaker voice coils to +Vo and -Vo of my power amp, by choice.


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Old 7th July 2012, 08:28 AM   #976
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I ordered a hard copy of Linear Audio Vol #2 so I can read about this design. I did some simulations of my own based on the links provided early in this thread. I was able to get an input impedance of about 1.2 ohms using bipolar transistors in the sim. The distortion is higher with bipolars though noise is about the same.
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Old 7th July 2012, 10:05 AM   #977
regal is offline regal  United States
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Originally Posted by dirkwright View Post
I ordered a hard copy of Linear Audio Vol #2 so I can read about this design. I did some simulations of my own based on the links provided early in this thread. I was able to get an input impedance of about 1.2 ohms using bipolar transistors in the sim. The distortion is higher with bipolars though noise is about the same.
This is a FFT of a pair of paralel PCM1704's with a passive I/V of 40 ohms, a terrible scenario if you believe high input impedance (voltage compliance on the dac i-out pin) is a significan issue (in other words same a a single pcm1704 with 80 ohms i/v input resistance.) This ~100mV voltage is amplified with a discrete Jfet stage and then followed by the filtering and a diamond buffer.

This is at full 0dbs (I haven't calibrated the sero point). As you can see there is no distortion above -90 dB. I am sure your BJT sims show much higher distortion, best I have ever seen with a BJT open loop I/V is -80db . In real life with an imperfect i-dac output (the chips have inherent distortion and low output impedance) you won't come close to designing a lower distortion stage than this 40 ohm input impedance I/V design with BJT's without using feedback to the i-out pin.

I guess what I am saying is stick with the j-fet version of the SEN especially if you sim higher thd with BJT's, because the "non" linearities caused by the j-fet's 7 ohms input impedance is neglible and unimportant.

Click the image to open in full size.

So this is proof that for the king of R to R that I/V input impedance (voltage compliance) really doesn't matter.

The goal is lowest open loop distortion without nfb on the i-out pin of he DAC chip. That most of us agree on.

I disagree with Evil and others that NFB and opamps are bad to use post I/V conversion, but we all have our opinions.
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Old 7th July 2012, 06:10 PM   #978
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The jfet version I simulated had 15 ohms input impedance and the distortion was about 0.0005% vs. 1.2 ohms and 0.0006% thd for the BJT version. Each had S/N of about 135dB (@ 1kHz) and the same gain of 53dB. I haven't read the article so I can't comment any further. I'm not expert enough in DAC's and I/V design to say much either. Thanks for the information.
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Old 7th July 2012, 06:48 PM   #979
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Of course the input impedance effect of distortion depends crucially on the DAC. But, as well, the circuit distortion is not due only to it. The JFET approach has the great advantage of essentially no loss of current, unlike bipolars.

Fed from a true current source that circuit works about as well as it can. IIRC Patrick tested his with Didden's AP driving a 20k resistor, which is a pretty decent approximation to a current source.

For further improvements in measured performance (whether audible or not) a bootstrapped cascode structure can be used. But then you need more voltage overall, and one has to wonder if it is worth it.
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Old 7th July 2012, 06:48 PM   #980
EUVL is offline EUVL  Europe
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You can download the article online free of charge by following the link at post#969.
Courtesy of Jan Didden.


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