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Old 2nd February 2007, 04:17 PM   #1
arupg is offline arupg  Latvia
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Default Best OpAmp for Audio Alchemy DITB

Hello

I have an Audio Alchemy DITB (dac-in-the-box) with a PS1 power supply. I have done the Kalman Robinson mods which were
1. replace 5 V supply regulators to 1A
2. replace 12 V supply regulators to 1A
3. increase the raw supply caps
4. use caps on bypass supply

The above did improve the DITB considerably but I am curious if the stock OpAmp OP275 can be substituted to something better.....

Question is which one?? OPA627/637, AD varieties, or the Burson discrete OpAmp.

Inputs from fellow members would be most appreciated.

Cheers!
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Old 2nd February 2007, 04:37 PM   #2
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I have gotten the best sound by using the B-B OPA2134PA. Major improvement also by adding as much electrolytic storage on the supply lines in all areas, esp. right close to the output chip, as will physically fit in the tiny little guy. I've been able to make the lowly DITB totally smoke a Denon DVD3910, Benchmark DAC1, and even equal my fabulous sounding, very, very upgraded Pioneer CLD95(which has very well implemented AD1862 dac chips).
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Old 2nd February 2007, 05:03 PM   #3
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The primary differences between the '627 and the '637 is unity gain bandwidth:
vailable Channels S S
CMRR(Min)(dB) 106 106
>> GBW(Typ)(MHz) 16 80 <<
IIB(Max)(pA) 1 1
Iq per channel(Max)(mA) 7.5 7.5
Number of Channels 1 1
Offset Drift(Typ)(uV/C) 0.4 0.4
Open Loop Gain(Min)(dB) 112 112
... etc. ... from: http://focus.ti.com/docs/prod/folders/print/opa627.html

Personally, I would not bother changing out the Analog Devices' OP275 op-amp unless you just want to see if you can improve this particular A/A DITB. (You might gain some on the op-amp unity gain bandwidth question, but the DITB has bandwidth limiting components in other parts of the circuit. Possible but small improvements in slew rate(s) ... and there might be an overall improvement of the THD+N numbers = 0.00x% v. 0.000x% [the exact quantification without lab quality equipment / analisys = unknown and possibly specific to the your A/A DITB].

The addition of larger voltage regulators was a very good idea = less heat and more PS power "headroom", etc.
The use of larger or better caps on the PS rails was also very smart = better noise floor close to the chips, etc. You might examine the possibility of putting a reasonably sized plastic cap right across the power pins on the output op-amps (pins 4 & 8). Close coupling of a "snubbing cap" with a fast response time often makes a significant difference to any op-amps' performance, sometimes more so than all the other PS and PS rail work other than adding the larger PS caps. (I'm a believer in the generous use of plastic snubbing caps in parallel with electrolitic caps on PS rails = as written by Bob Pease in Electronic Design Magazine.)

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Old 2nd February 2007, 05:13 PM   #4
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stephensank's recommendation for the TI / B-B OPA2134 is a good one, although any improvements over the A/A DITB Analog Devices' original will be hard to quantify.

Also: " ... Major improvement also by adding as much electrolytic storage on the supply lines in all areas, esp. right close to the output chip ..." =

" ... I've been able to make the lowly DITB totally smoke a Denon DVD3910, Benchmark DAC1 ..."

One would wonder if this were possible, but sometimes we get an appreciation of the "unaccountable changes caused by an unknown variable" = got lucky?

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Old 3rd February 2007, 03:39 AM   #5
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I'm getting the impression that you are one of the "if it looks good on paper/insruments, it must sound good" crowd. This is what got the Japanese in trouble into the 80's, resulting in things like the Yamaha M80 power amp. As close to perfect measurements as ever seen on earth, perfect square waves at any sonic frequency, but it is famously one of the most abrasive & awful sounding amps since the Dynaco 120.
I don't mean to insult you at all, if you are of that sort of crowd. It's just that in my 35 years in high end audio service, I have come to the conclusion that if something sounds better despite looking worse on instruments or etc., it's just because we haven't figured out what or how to measure whatever the parameter is that made it sound better.
The OPA2134 is not much better or worse than the OP275 on paper, but I can tell you from using both in many, many different applications that the OPA2134 sounds richer, more natural & more musical than the OP275. The OP275 just sounds a bit "dry" by comparison.
As for the cap issue- the stock DITB does have reasonable size petp film caps at the supply rail pins of every chip on board, so that's not much of a concern. It lacks any significant energy storage near thte output chip, though. Now here is where you might think there's "placebo effect" going on- Adding 220uf at each supply pin of the output chip makes a very nice difference overall. Changing that 220uf to 470uf, which darn well should not affect a low current opamp chip, gives the DITB an absolutely clear advantage in bass extension & punch versus the 220uf. And this is with identical grade Nichicon lytics. I have done blind tests with friends on this & they spot the 470uf equipped DITB every time.
The Denon DVD3910 comparison is quite easily explainable by even just considering power supply issues in the output stage, where Denon did nowhere near the supply work that I did to the DITB. This alone could explain the difference. This is also why most dvd players sound anemic & thin compared to a well done old cd player or 16/44 dac.
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Old 3rd February 2007, 05:27 AM   #6
arupg is offline arupg  Latvia
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stephensank & FastEddy: many thanks for your posts and comments/suggestions. I greatly appreciate it.

I must say that stephensank's recommendation of adding 470uF lytic at each Supply pin of the output seems a superb idea. And indeed, I have always felt the AA-DITB to be not so stellar on the low bass area that my B&W speakers can easily & comfortably reach.

I would appreciate if you can tell me exactly how to go about this mod. I presume I will require 2 x 470uF electrolytic caps (what voltage) and also should solder them where? Sorry for bothering you guys but the idea of strengthening the bass sounds fantastic to me. Hope the 470uF wouldnt hurt the OP275 ........

I have 2 x 470uF 25V panasonic electrolytic caps with me and where exactly should I solder these??

Cheers!
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Old 3rd February 2007, 11:37 AM   #7
arupg is offline arupg  Latvia
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Stephensank:

Do I solder a (470uF 25V across V+ & Gnd) and another (470uF across V- & Gnd) of the OP275?

OR

across V+ and V- of OP275?
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Old 3rd February 2007, 12:10 PM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by arupg
Stephensank:

Do I solder a (470uF 25V across V+ & Gnd) and another (470uF across V- & Gnd) of the OP275?

OR

across V+ and V- of OP275?
I would go with the first V+ to G and V- to G.
You may find a difference (good or bad) by adding your second option of V+ to V-, but since a plastic film cap should already be here then I think this may be too much.

According to the Marantz thread guys/gals,Oscon (digital) BG(analogue) caps should be considered for the electrolytic local decoupling and if there is room, very local regulation of both digital chips and analogue chips.
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Old 3rd February 2007, 05:09 PM   #9
arupg is offline arupg  Latvia
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AndrewT
Thanks for your inputs. As Stephensank recommends, whats your take on using a size like 470uF on the V+ to G and V- to G for the OP275? Do you in anyway feel this is too large a size to spark off things like unstability or oscillations etc.

I am more worried about the side-effects of such DIY work as I have expensive hifi components & cant afford to either mess-up or fry them or the dac or the Opamp itself since I live in India and these sort of things are rather a bother to fix due to complete lack of parts etc.

It has to be a SURE SHOT diy or else all hell will break loose...

Thanks & regards
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Old 3rd February 2007, 05:32 PM   #10
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Stephensank's & AndrewT's advise is well founded and will certainly improve the response of the output op-amp ... Using larger electrolitics very close to the power pins of the op-amp may of course obviate the need of any paralleled plastic "snubbing" additions ... but, as long as the soldering iron is hot and the gadget is open on the bench ...

" ... I am more worried about the side-effects of such DIY work ..." As long as no ground loops are accidently imposed and there are no shorts to the power supply rails (like using underrated caps or solder blobs crossing traces, etc.) ... then confidence should remain high ...
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