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Old 3rd May 2013, 12:16 PM   #101
pilli is offline pilli  France
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Quote:
Originally Posted by sidiy View Post
The filter is a 2nd order Butterworth, Fc=288.6kHz.
So I've been looking around to try and learn about filters...
Didn't learn much, but it seems that the topology used in the 0404 and also suggested in the AKM datasheet is a "Multiple Feedback Filter".
A good description is for example here, where they also show the differential version, that interests us.

Our values would give a cutoff frequency of 206 KHz.
It seems like the Bessel/Butterworth/etc "response shape" applies somehow to MFB filters as well... is it in the relative values of R-s and C-s ?

In fact other V-out DACs, like Wolfson and NPC, show the more usual Sallen-Key filters. They too, come with or without the coupling capacitors...


This doesn't change much to our previous findings, just wanted to document it.

The wiki is still blocked on a pending moderation...
I try to attract the attention on this, but everybody seems busy elsewhere...



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Last edited by pilli; 3rd May 2013 at 12:26 PM. Reason: Correctetd statement on response shapes
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Old 12th May 2013, 03:23 PM   #102
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Hello everybody. I just updated and fixed some stuff in the wiki.
Mainly:
  • 470uF and even 220uF filtering caps will potentially make the board unstable. I'm trying to get some feedback out of 100uF.
  • Found the purpose of OWNK N-JFET trannies. They mute outputs at immediate turn on, so that speakers won't bump.
  • Added some infos about changing opamp rail supplies
  • Added some thoughts about be-or-not-to-be of DAC decoupling caps
  • Fixed image size, links, post DAC schematic and other minor stuff

Best
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Old 12th May 2013, 07:59 PM   #103
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I'm having a strange problem. After some modifications related to the post DAC, the inputs don't work correctly anymore. They sound like impedance mismatch is happening, plus a lot of hiss is present. I haven't ever played around input sections so I'm just wondering how the hell went wrong. Any ideas? :\
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Old 18th June 2013, 10:35 PM   #104
pilli is offline pilli  France
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Default Op Amp comparison

Been off of this thread for a while, but I got to a point where the next thing to do is to test "external" output stages, not just to tweak the card. And I am not very fast at building things...

To start, I wanted to compare different op amps used as "summing filters" like in the basic schematic (but without coupling caps out of the AKM4396). So I built two identical circuits with sockets, each with its own "local" regulators (78/7912) and some decoupling. These can be A-B switched: I have these ITT telecom relays with a 6-poles, and that's quite convenient for in+, in-, out, on 2 channels.

In terms of op amps (duals only... too complicated otherwise) what I had at hand are LM833, C4570, JRC4560, OPA2604, OPA2134 and a thing marked OP271.
To make the test blind, I had an "assistant" (my 10-year old son...) put a piece of tape over the chips, and mark them A to E (the OP271 was excluded, since the package makes it too easy to recognize - it didn't sound right anyhow, in a non-blind comparison...)

Then I ran "each against each".
The results are somewhat different from what I expected...:

Ok, the winner is OPA2134, that consistently beats all others.
But then, second place is C4570. Unexpected to me...
Honorable third is 4560, whose full bass deserves to be mentioned.
Then LM833.
Disappointing was the OPA2604, that sounded harsh and narrow.
(But this chip hadn't been used much, maybe it has to burn in?)

Each step of this ranking is a noticeable difference on the one below, in terms of "width" of the soundstage and precision of the instruments, like nicer decays, or more silky (realistic) string instruments.
So for this experiment, despite non-optimal conditions (sockets, relay, long connectors from the DAC) I can report that replacing op amps does make a difference. And OPA2134 is indeed good value.

Just thought I would report.


Next: extract the 2608 and 5532 from the e-mu and A-B compare these with some of the others. But they're smd, so it may take another while...



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Last edited by pilli; 18th June 2013 at 10:53 PM.
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Old 19th June 2013, 09:26 AM   #105
AndrewT is offline AndrewT  Scotland
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Opamp amp rolling is not recommended by anyone who understands opamps.
Read some of the pit falls of opamp rolling and then come back and tell us you were not listening to opamps that were misbehaving.
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regards Andrew T.
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Old 19th June 2013, 01:18 PM   #106
pilli is offline pilli  France
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Yes, I am aware of that.

No general conclusions can be drawn from this cheap experiment, which has a limited scope.
The circuits I compared are at the quality level that I can implement myself, which I know is not extremely high.


Nevertheless:

- The op amps I compared are not really very fancy, so the risk of misbehaving seemed low.
Only the OPA2604 datasheet describes specific optimization;
indeed I cannot exclude that it is the reason of its disappointing behaviour.

- I did apply some basic measures, like regulators very close to the op amp (< 1 cm), caps from supplies to ground and a cap across the supplies.

- I paid attention to signs of instability, like overheating or noises; didn't detect any.



So my conclusion should be
"I should not use in this cheap circuit the op amps that deserve better optimization".
That's still valid, isn't it?
But it's clearly not the opamps' fault.

I chose to not go for very high performance components, since I wouldn't really know how to use them at their best.
Maybe I will try to build a better circuit, especially for the OPA2604.
And then compare it with the gross one.


By the way, this whole playing around we're doing on the eMu 0404 is really not "unconditional highest end".
We're on the cheap side of things.
I still think some small improvements can be reached with a relatively small effort.


You are right that we should be lucid about these limitations.



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Old 18th August 2013, 09:00 PM   #107
jjhaz is offline jjhaz  United States
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Default EMU 0404 Headphone mod gone wrong

I replaced the opamp and two caps in the headphone section and now I have very low output. Both channels work but they can be driven into distortion very quickly.

I changed the opamp out trying a couple of different opamps just to see if I had a bad opamp but it yields no difference. Any ideas what I may have done???

Thanks

JJHaz
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Old 20th August 2013, 12:21 AM   #108
pilli is offline pilli  France
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Quote:
Originally Posted by jjhaz View Post
I replaced the opamp and two caps in the headphone section ...
Which caps did you change, what is their marking on the PCB?

Sorry if it's obvious, but some first things to check would be orientation of the op-amp (pin 1 on the correct side) and polarity of the caps, if electrolytics.
Also check that the supply pins (4 and 8) of the opamp get the correct voltage, should be around -6V and +6V respectively.


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