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Old 20th August 2012, 03:46 PM   #11
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Quote:
Originally Posted by Ken Newton View Post
Three op-amps in parallel could reduce Johnson noise, but by no more than 9.5dB relative to a single such op-amp. However, you would need to isolate the low impedance output of each op-amp from that of the others with a resistor, so not the have them buck each other, which would increase distortion. Not what we want. Output isolation resistors will contribute their own Johnson noise, reducing that theoretical 9.5dB figure.
I was only half serious Ken. I also meant using one LME49600 in place of each of the buffers for three total in the entire circuit. I should try different emoticons to convey my silliness. This is supposed to be fun after all.

I'm surprised that the SK170 in source follower mode wasn't used for the buffer... everyone's favorite transistor! (I got an idea - an entire hifi made with nothing but SK170's! )
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Old 20th August 2012, 03:47 PM   #12
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I think he means one LME49600 250mA buffer on the output of each op-amp; like in post #3 but IC's instead of discrete.
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Old 20th August 2012, 03:47 PM   #13
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Lme49600 has a quiescent current of only 13 ma the buffers I use for u1 have 27ma. I think the lme will work in class b in this circuit because of the 6.2ma offset of the dac plus the 21ma swing.
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Old 20th August 2012, 04:02 PM   #14
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Thanks for sharing your opinion about noise ken. but Unfortunately professional audio is a specs war , and wins the lowest noise. that's the reason the original circuit is so bad, with low resistors they have the low noise specs, and at 1khz the distortion seems tolerable . This is what the average consumer wants.
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Old 20th August 2012, 04:29 PM   #15
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I thought this was DIY? what war?
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Old 20th August 2012, 05:15 PM   #16
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Originally Posted by dirkwright View Post
I was only half serious Ken. I also meant using one LME49600 in place of each of the buffers for three total in the entire circuit. I should try different emoticons to convey my silliness. This is supposed to be fun after all.

I'm surprised that the SK170 in source follower mode wasn't used for the buffer... everyone's favorite transistor! (I got an idea - an entire hifi made with nothing but SK170's! )
My mistake, dirk. I'm sometimes slow on the uptake.

On top of which my 9.5dB figure was wrong anyhow. The figure should have been around 4.7dB.
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Last edited by Ken Newton; 20th August 2012 at 05:23 PM.
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Old 23rd August 2012, 10:57 AM   #17
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Originally Posted by Ken Newton View Post
This is an interesting topic
unfortunately not everybody thinks like you, i was hoping that more people would share their experience in using op amps and buffers.
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Old 23rd August 2012, 01:18 PM   #18
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Originally Posted by smms73 View Post
unfortunately not everybody thinks like you, i was hoping that more people would share their experience in using op amps and buffers.
I don't have much of any experience so I have nothing to share, sorry.
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Old 23rd August 2012, 02:11 PM   #19
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Not too long ago I started a topic on looking how to improve the PCM1794 I/V stage as well. It was looking into the op-amp implementation of the circuit by using different ICs.

ADA4637 Opamp IV Stage Simulation - PCM1794 (some help and wisdom needed)

IC selection plays a big role as well in the circuit. I was focusing more on optimal IC selection at the I/V buffer stage because that's seems to be the weakest link to me.

I do lack the decades of experience of most on this forum but just by reading a bit on Johnson Noise (thermal noise of components?) it seems that it is relatively low and the IC and it's complementing component selection is key to great results.

Currently I have stopped my simulations (and DAC design) due to school and work but I plan continuing in the near future and coming up witth two new IC selections for the pcm1794 IV stage and a modifyable IV opamp stage so we can all experiment. Hope I can get to it sooner :/
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Old 23rd August 2012, 10:52 PM   #20
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smms73, seeing your so handy at running sim's, could you do me a favour and run the sim on this, circuit.
As so far it has sounded so much better than anything I have ever heard, esspecially in dynamics from top to bottom, but what I have noticed is that it is very unkind to bad recordings, but if the recording is good it is very sweet sounding even with the massive dynamics it has.
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