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Old 19th March 2014, 02:40 PM   #1
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Default Opus output filter

Hi all,

I have Opus DAC for with output filter based on simple schematic shown in attachment. The circuit was drawn in LTSpice, so it features LT1028, but in reality the Opamp is currently NE5534 - will be replaced with LME49710 or OPA134.

I was expecting that this setup will sound equal or better than old TDA1541, but it doesn't. It sounds a bit shallow and bright.

As improvement, I am considering some of the following:

1. Remove the 22uF capacitors on DAC output - this should be safe thing to do; even if + and - outputs from Wolfson do not have 100% same DC level, it shouldn't damage the filter; correct?

2. Increase the supply voltage from +/- 11V to +/- 15V and put Class A PS instead of Low Current Bipolar Supply; does this make any sense?

3. Remove the 4.7uF MKT from the output of the filter - again, even if there is an error on the Wolfson output, given the unity gain of the filter, there shouldn't be danger for the amplifier, right?



Thanks for help.

Cheers,
Boris
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Old 20th March 2014, 11:58 AM   #2
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The resistor values are on the high side - noise will suffer.

Have you tried the IVY-III? It sounds excellent with the Opus.
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Old 20th March 2014, 06:14 PM   #3
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Hi Russ,

Would you recommend to use those 22uF & 100k on Opus output with IVY?

Cheers,
Boris
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Old 21st March 2014, 01:34 PM   #4
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Quote:
Originally Posted by boriss111 View Post
Hi Russ,

Would you recommend to use those 22uF & 100k on Opus output with IVY?

Cheers,
Boris
No, they are not needed when using the IVY.
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Old 21st March 2014, 02:02 PM   #5
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How about capacitor after IVY? Do you recommend one or it is not needed?

Cheers,
Boris
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Old 22nd March 2014, 04:16 AM   #6
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Quote:
Originally Posted by boriss111 View Post
How about capacitor after IVY? Do you recommend one or it is not needed?

Cheers,
Boris
Not needed.
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Old 26th March 2014, 10:06 PM   #7
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Default really?

Quote:
Originally Posted by Russ White View Post
Have you tried the IVY-III? It sounds excellent with the Opus.
hmmm, really? A little looking around here and on the TP support site suggest that the sound of the Opus & Ivy combo might fall short of excellent. The following almost kept me from building the IVY III even though I had the kit in the closet.

Quote:
Originally Posted by pinnocchio View Post
I currently own a Opus DAC from TPA and use it with the IVY III line stage but I don't really like the IVY III...
There are resistor changes in the Ivy manual for Opus because the BOM is for Buffalo but, oddly, no change for the four 15nf caps. I have 3.3nf replacements on order but have not yet tried them. It is my theory that the low-pass filter that results from 15nf affects the upper audible range negatively.
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Old 26th March 2014, 10:24 PM   #8
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I wouldn't let that set you off - there are many more who have tried it and loved it.

It works very well - and is my personal favorite output stage for the WM8741.
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Old 29th March 2014, 08:37 PM   #9
Wolfsin is offline Wolfsin  United States
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My theerory likely is not the issue since the filter resistors between ESS and Wolfson are unchanged. I am using the WM8740 and did not populate or power the ballsie except that the IC had been soldered in place. To compensate I have temporarily replaced my HD600s with a pair of rewired MDR-V6s that IMHO are normally short of bass response until the 3.3nf caps arrive.

I believe this is a popular pcb, built and enjoyed by many. I feel confident that with smaller filter caps it will work well for me too. When I remove it from between the Opus and opc's BAL-BAL the HD600s sound great.
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