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Old 31st March 2008, 01:02 AM   #1
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Default My result of opa2134 and opa627 comparison

I built an active XO for my new speakers with opa2134 and was not happy with the sound. It seemed that the dynamics and details of the music were gone with the active XO. That prompted me to investigate whether it was the design, implementation, or the opamps, or the caps that damaged the sound the most. I was determined to find out the sound of different opamps and bypass caps once more, this time, specifically, the difference between opa2134 and opa627. Although I have read various comments on this forum, getting first hand information would give me further assurance.

I previously reported that an upgrade from opa2134 to opa627 on the NAD542 gave some audible improvement but not "night and day" difference.

Often subjective listening tests really depend on many factors, including all the components in the entire audio chain, how the opamps are used, the input and output impedance, current demand, noise requirement, whether the rails are quiet enough, whether the opamps are used for the bass or the treble, etc.

This time the source was my upgraded Marantz SA11s1, a very accurate CD player.

The power amp was also very accurate with diminishing distortion figures under many test conditions. These were no caps in the signal path. There was no input cap.

I built a simple opamp unity gain buffer. Before the buffer there was only a volume control. After the buffer there was only a 200R series resistor to drive the cable.

The speaker drivers belonged to the very low distortion group. Tweeter was the top model from Dynaudio Esotar T330D. Midwoofer was the ScanSpeak Revelator 18W8531G00. The XO consisted of simple RCL for the tweeter and LCR for the midwoofer, using Solen Perfect Lay inductors and ICW SA Clarity caps. The drivers were mounted on an open baffle.

Only SACD, mostly DSD recordings, of classical music was used in the evaluation.

This was basically as good as I could have.

The power supply was of the LM317/337 type implemented strictly according to the manufacturer's datasheet. A pair of Rubycon ZL 2000uf was mounted on board with the buffer. A pair of Rubycon ZL 100uF bypass caps was mounted closer to the chip. Initially 0.01uF polystyrene bypass that was even closer to the chip.

The opa2124 was first mounted. Comparing to my 12B4 tube preamp, the 12B4 preamp sounded far more refined.

In other words, I would not be happy with the opa2134 sound at all if used as a buffer / preamp.

Since the opa627 datasheet recommends 0.1uF ceramic + 10uF Tant bypass, I added a 0.1uF polypropylene for bypass. I was still using the opa2134.

The sound was improved. It was obvious. But still, nowhere close to the 12B4.

I then replaced the opa2134 with opa627. WOW, what a difference! The music was back again.

Comparing to the 12B4, the opa627 had simular presentation. There was only a slight difference. As a matter of fact, if I did not listen carefully, I would not know which was which.

The very well executed 12B4 (choke input, shunt reg, etc) sounded slightly smoother and rounder on the top end and more "romantic" (looser) on the bottom end. The opa627 sounded slightly sharper (or harsher depending on how you look at it) on the top end but tighter on the bottom end. Overall, the 12B4 was slightly smoother but the opa627 was slightly detailed. This may well be due to the effect of the output cap (ICW SA Clarity) used in the 12B4 and absolutely no caps in the opamp buffer signal path.

I could not tell which was better. My 12B4 could still be improved slightly in which case it would be better. But then I could also try to bypass the rails for opa627 with a, say, 10uF MKP that may improve the sound?

This would be great news for those who want to build a 12B4 preamp - why be bothered! My 12B4 preamp was expensive and weights a tone. Just build the opa627 buffer! The 12B4 may still have a slight edge if the ultimate components and construction methods are used, but at the price of 20 times more costs for the improvement of less than 2% on sound?

I later added a 0.1uF MKP to bypass the two rails. i.e. a single cap connected the POS rail to the NEG rail. I have not had the time to make a detailed comparison yet but the result obviously did not improve the sound, at least not immediately audible.

So my conclusion was that on a lesser system, like the NAD542 I used to use (I still have it and still like it), there was improvement by replacing the opa2134 with opa627. But when the system is further improved and becomes more revealing, the opa627 beats the opa2134 by far. Also, I concluded that 0.1uF (and I will try larger values) MKP to bypass the rails are necessary. 0.01uF is not sufficient.

I will do further tests on this in the coming a few days, but mainly on the different types of MKP caps.

I am very interested in comparing the opa627 to the ad825 and the new LM????

Again, this time I ordered 4 pairs of opa627 from Chris who sells in the eBay (seller: ca2832 email: cdaly@netspace.net.au Web: http://my.opera.com/opamp/blog/). The much improved sound simply indicates that the parts Chris supplied were genuine BB products. His prices are very good and he delivered them very promptly. I have no connection to Chris. I am just a happy customer.

Regards,
Bill
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Old 31st March 2008, 01:57 AM   #2
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Good test and very interesting. I'm a big fan of the LME49710, as it beats the 134 and 627 by a decent margin on most of the datasheet parameters. But, and it's a significant but, the LME is also a 55MHz GBP part. I suspect the people who have been unhappy with it haven't had sufficiently good layout and bypassing. It also needs isolation resistance to drive more than 100pF. You should try it- it's only a few dollars compared to things like the 627. I used to use the 627 in a scientific application, and it's a super part. It used to be cheaper and more available. It's really suited to electrometers and such, and I'd guess the high price is due to the low input current, much testing, and higher fallout than most op-amps. It has very low overshoot and actually slews faster than the LME part. It needs guard rings on the pcb to take advantage of its low input current, otherwise it's kind of wasted. I'd be very interested in how you think they compare- in my phono preamp I liked the LME better, but I haven't tried them in my active crossover yet.
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Old 31st March 2008, 02:16 AM   #3
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I think the AD825 has a much higher spec and is much cheaper. It compared well to the opa134. I have not compared it to opa627 but I will, because I still need quite a few more opamps and opa627 is too expensive to my pocket. I have never tried the LM4562.

Also, I will try a shunt reg in a couple of weeks on the opa627 buffer and see if it makes any difference.

By the way, what are guard rings? where can I get them?
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Old 31st March 2008, 04:06 AM   #4
KP11520 is offline KP11520  United States
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Hi Bill,

Have you thought about biasing the OPA627 to Class A? I have been a pest about this for about 9 months around here and I have MY answer on this (It can certainly be done).

If you need, I can tell you what I have learned.

I think you might be just as surprised as when you removed the 2134 and replaced it with the 627.

Regards//Keith
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Old 31st March 2008, 04:18 AM   #5
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Guard rings are sold in the same stores that sell buckets of prop wash ;-) No, seriously, they're just very fine traces on the pcb that fully surround the input pins. Without those traces, surface leakage on the pcb will far exceed the input current of the part. For most audio circuits they probably don't matter, but in a high impedance circuit you don't get the performance the part is capable of without them. They might also matter with active filter circuits. I believe the OPA627 data sheet shows how to do them.
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Old 31st March 2008, 05:11 AM   #6
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Conrad,

Thanks for the information. I used matrix type varo board. Does it matter for audio signals up to only 20kHz?

Keith,

I heard of biaing into class A but never tried it. My class B (or AB) amp sounds as fine as class A amp, I suppose, having own a Musical Fidelity A1000 pure class A 60W amp for a couple of years and built a number of my current class B amps. Given that these tiny chips are mounted on a browndog heat dissipation may be a problem with class B, let alone with class A. I guess these chips are designed to be used in class B.

I am not surprised you may achieve better sound with class A bias but unless the heat dissipation is not an issue I would hesitate to do so. I guess you will tell me that you have never had a chip overheated (IC heatsink?).

I guess the trick is to add a few resistors. I am happy to give it a try and report back here. What is your fomula?

Regards,
Bill
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Old 31st March 2008, 05:57 AM   #7
EUVL is offline EUVL  Europe
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Or maybe try one of these :

Pass Discrete Opamp in DIP-8 Package

Apologies for the advertisement.

I guess there are probably better (or rephrased "more audiophile") unity gain buffer circuits than an opamp with over 80dB loop feedback, discrete or otherwise.


Patrick
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Old 31st March 2008, 08:02 PM   #8
KP11520 is offline KP11520  United States
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Hi Bill,

I hope you have been using Dip Sockets. Attached are 2 files of how to take 2 JFET Transistors to make a JFET Cascode. This is the best way to bias as it is the most consistant regardless of the variation in signal passing through the op amp. It employs two 2N5484 JFETs per channel.

Notice on one of the diagrams I said pin 4 -Vs and pin 7. Pin 7 is wrong because it is the +Vs so substitute that for pin 6 in the diagram. Pin 6 is the output and that is the correct pin to connect the D from Q2 to.

Buy 10 or 12 of the JFETs. They are cheap and I know at least Fairchild makes them.

Then check for Idss. You need two categories, High Idss and Low Idss. Then find two in the High Idss category that are a match and two in the Low Idss that are another match. When I say match, I mean as close as possible. These will be the 4 that you make the two Cascodes with. The two Low Idss ones will be Q1 and the two High Idss will be Q2.

As far as heat, everything I read said it is not an issue and shouldn't even need a Heatsink but you can certainly employ one if desired.

Build them and do one channel first to see if you like what you hear in that channel! If so, make it permanent in both channels!

Regards//Keith
Attached Files
File Type: pdf class a bias jfet cascode pcb for op amps.pdf (57.7 KB, 388 views)
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Old 31st March 2008, 08:03 PM   #9
KP11520 is offline KP11520  United States
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Second daigram:
Attached Files
File Type: pdf jfet cascode to bias opa627ap on pcb of cd player.pdf (95.8 KB, 304 views)
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Old 31st March 2008, 08:48 PM   #10
sandyK is offline sandyK  Australia
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Default My result of opa2134 and opa627 comparison

HiFiNutNut
It has been my experience that despite what the manufacturers
may say about ceramics and tantalums used for bypassing,and this applies to the AD8066 as well, that they usually make the treble sound metallic, and give a hard edge in general. I find it far better SQ wise,to wherever possible , use voltage regulators ,e.g. 78L12 and 79L12, closer to the audio I.C. and use 100nF poly. bypass capacitors at the I.C. The typical
47-100uF at the output of the close by VR is normally fine. I guess that the manufacturers specify those particular components to guarantee stability?
The OPA2134 is not a bad sounding I.C. , it is the way it is implemented. The LM4562 can also sound excessively detailed, and fatigueing in some arrangements.

SandyK


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