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Old 19th May 2013, 02:32 PM   #441
Coris is offline Coris  Norway
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Default Low resistor on I/V continue....

I have experimented more accurate/systematic on this subject...
The goal is to find the best compromise when the residual HF noise from the DAC is minimum, reasonable offset on outputs, and no audible noise/hiss on speakers.
Some results here (with scope screenshots). Original I/V op amps and LM6172 as final. No any HF filtering on I/V, or further. Just straight signal path! DC coupling on outputs.

I have started (from an initial 750 ohm - x1 gain - ref voltage 2,2V, offset -0,5mV on outputs) with 330 ohm resistor on I/V and a gain on final of x3. I have no screenshots for this.
I've got an -1mV/-1,2mV (L/R) offset on outputs, I/V ref voltage = 1,4V. The residual HF noise is 130mV. Of course no any audible noise floor.

Next, lowered the I/V resistor to 75 ohm, and increase the gain to x12. The offset outputs goes to -2,9mV/-3,2mV (L/R) with a 0V reference voltage on I/V stage. I've got 46mV HF noise for accordingly usefully audio signal, as is to be seen in the picture.

Next step, lowered the I/V resistor to 40 ohm with a gain of x22 on final. The offset on outputs increase to -4/-6mV for ref=0V. The HF noise get lower to 28mV for the outputted audio signal showed in the picture.

Last step, 20 ohm on I/V, and x75 gain (ref=0V). The offset on outputs get -10/-15mV (L/R), and the HF noise is now 18-20mV for a 11Vpp max audio out.

In all those steps I could not notice any audible noise floor on speakers. I have observed that with lowering the resistor on I/V, there is improvement in sound stage. Best for 20 ohm.

I could get lower with that resistor, but I just got tired this time... I have a feeling that the noise floor it will begin to rise with lowering further this resistor... But I will verify this one day...

With a resistor on I/V as recommended in Ess9018 datasheet (680 ohm) or even more to 750 ohm, the residual HF noise from DAC become hundreds of mV (500 -600mV), for an in may case, 12-15Vpp amplitude of useful signal and a x1 gain on final. In this case I had to use another procedure to lowered the HF noise to 32mV (no filter). As normal, one have to use filters to get rid of this quite high level HF noise on outputs...

I`m not decided yet which way to chose... but I think to take it the lowest resistor value one, or so.
Welcome to comments and opinion about those functional configurations here... Or maybe more/better results...
Attached Images
File Type: jpg 75ohmI-V x12gain.jpg (294.2 KB, 259 views)
File Type: jpg 40ohmI-V x22gain.jpg (289.7 KB, 249 views)
File Type: jpg 20ohmI-V x75gain.jpg (350.9 KB, 222 views)

Last edited by Coris; 19th May 2013 at 02:56 PM.
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Old 19th May 2013, 03:43 PM   #442
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That's quite interesting Coris, thanks for the measurements.

I am quite surprised by the excessive amount of noise you find.
It almost looks like the complete filtering of the DAC output has gone!
Basically there are three sources of noise that could play here:

1. The DAC noise - but knowing the ESS reputation I would expect that to be very, very small.

2. The noise from the I/V resistor.
I did a quick check with my old rule-of-thumb that 1k over 1 Hz gives 4nV, that would be about 600nV for 1 K over 20k bandwidth (multiply by quare root of BW). Even with a gain of 100 that would still be only 60uV!

3. The noise from the post-I/V amplifier. I guess you use an opamp here? I'm sure you selected a low-noise opamp (or use the opamp already present?). There's one more possibly cause - the resistors to set the opamp gain. The resistors also cause a noise which would be amplified by the opamp gain so that could explain the increase in noise with gian. What resistor values are used in setting those gains?

I don't know what's going on here but it is fishy.
BTW I looked at your scope plots, difficult to read the horizontal scale, is it 50m/s?

jan

BTW Did you check that the DAC is in current mode? If it still is in voltage mode, that can also explain some of the excess noise. I know the ESS can be used in both modes, but maybe that is fixed in the Oppo, don't know so this maybe a nonsense question.

Edit: Had a few friends over yesterday to show off my Oppo - they were suitably impressed ;-)
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Last edited by jan.didden; 19th May 2013 at 03:45 PM.
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Old 19th May 2013, 04:18 PM   #443
Coris is offline Coris  Norway
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That noise you can see in my plots is high frequency (in Mhz range) residual noise which it came out from the DAC. This is normal, and its pattern, as one see in the pictures here, is also characteristic for ESS9018. This noise is to be filtered out by normal filtering stages placed in the signal path. As usually, the circuit designers place such filters into the I/V, between I/V and final and after final stage. Even though some designers use ferrite beads in the signal path to minimise this noise out on cables. But all those filters in my opinion affect quite much the quality of the resulting audio signal. There is huge difference when you hear the analogue out from the DAC with/without those filters. I chose to remove those filters. So, your observation that it may be no filtering in this case, is right.
Else, the noises you refer to, as I/V resistor noise or chip noises, are existing there in the pictures, but are extremely low comparing to this HF noise from the DAC....

The disadvantage of increasing audio signal quality (post DAC processing) is to have this HF noise out from the DAC at quite high levels. But those noises are not audible at all... Anyway is not very fortunate to have out on cables a such radio frequency in hundreds of mV... So one may find a way to not post filtering the audio signal, and in the same time get the lowest possible HF noise out of the box. It seems that using a lower value I/V resistor it is a benefit to reduce this HF noise.
It is my tring here in those above experiments, to find a solution to this issue...

PS. In one plot is about 50µ/s on horizontal. But here is more important what is on vertical scale.... The noise is measured at the output in absence of the useful (converted) audio signal.
As you may see in a earlier post here, Dustin (the designer of this chip) is describing what about this "current mode" of the DAC. It seems that people misunderstand a little this concept. In this case here, using a high value resistor for I/V stage it make the DAC work in "voltage mode". The opposite (lowering that resistor) it should "open" the current mode... But this aspect it still be discussed... I think that I understood right what Dustin try to explain in its (long time ago) intervention.

Last edited by Coris; 19th May 2013 at 04:46 PM.
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Old 19th May 2013, 05:30 PM   #444
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I have found the relevant posts; the Zout at each output appears to be 781 or so ohms. That explains why it can be used in voltage mode (high Z load) or current mode (low Z load). Very unusual, but apparently it works.

The disabling of the filtering is a personal judgement call - I would not do that, knowing what havoc these HF signals can wreak on the rest of my system, and I also am of the opinion that it detracts from the audio quality. But, as I said, personal judgement, to each his own.

Do you use the original opamp for I/V conversion, what type is it?

jan
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Old 19th May 2013, 06:01 PM   #445
Coris is offline Coris  Norway
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LM4562
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Old 19th May 2013, 06:16 PM   #446
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Ahh the good old LM4562 - can't find anything wrong with that one!

BTW I still have problems with your scope shots. I'm pretty sure the horizontal scale is in 50mS per division, putting the noise square in the audio band, which I would expect anyway. It also corresponds with the mS vertical scale (same indication) and the frequency cursor.

Ohh well.

jan
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Last edited by jan.didden; 19th May 2013 at 06:20 PM.
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Old 19th May 2013, 06:45 PM   #447
Coris is offline Coris  Norway
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Yes, is right. I double checked with a higher resolution plot. Is 50ms scale.
As you can see there is about high frequency bursts, and those have a lower frequency which is in low audio range. Even though is not possible to hear out of the speakers such residual noise, at that level.
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Old 20th May 2013, 07:15 AM   #448
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Just got a reply from Oppo about the crossover in the 105. The xover between the 'small' speakers and the sub is 12dB/oct. Which is pretty tame as xovers go.

I suggested a software update where the customer can select say 12, 18, 24 dB/oct slopes and, ideally, a few different filter types like Butherworth or Linkwitz/Riley.

That would allow people to chuck their active crossovers and just use the Oppo for that, and that can only be a Good Thing for Oppo!

But don't hold your breath

jan
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Old 20th May 2013, 08:20 AM   #449
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Can someone summarize, which mods have been tested by now in this thread, and what was the result?
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Old 20th May 2013, 10:23 AM   #450
dazzz is offline dazzz  Israel
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Quote:
Originally Posted by Coris View Post
LM4562
LM4562
Recommended for all Mod
He is really OP AMPS
thet ALL AROUND
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