MC33078 Op Amp Upgrade Recommendations

[jmproductions]

Hey guys I have an Anthem D1 preamp that I would like to upgrade the 33078 (2AVXR) op amps in. It appears that there are 2 gain stages of these chips after the DAC before the volume control chip. Space is limited so I would not be able use any fancy discrete chips but I can fit any DIP8 or SOIC8 chips in there so I'm considering something like:
MUSES01
MUSES02 (or MUSES8820E)
OPA1612 Q1
AD ?
But I'm open to any suggestions you might have. Luckily I have 2 of these preamps so I can experiment on one of them to see what happens. I can take any voltage measurements that may be required but I do not have a scope. Any help from the experts here would be much appreciated and I am willing to learn!

 

[Mark Tillotson]

What impedance circuitry is involved - current noise specification may be crucial, in which case the MC33078 are fine - similar to NE5532, or you could go to JFET opamps. The OPA1612 has ultra low distortion, but more current noise than the existing MC33078.


Different opamps have differing decoupling requirements too, so that might need checking.

 

[jmproductions]

Good questions I'm sure, is there a way to measure the impedance with a standard multimeter? I believe the 33078 are bipolar as well as the Muses02, so should I stick with bipolar? I could take a picture of the board if that would be helpful. Could you read the current noise specs on the Muses? Here, scroll down: MUSES Official Website

 

[abraxalito]

You really need the schematic to figure out what the working impedance is around the opamp. Sometimes its determined at AC so a multimeter won't necessarily help.


OPA1656 has lower voltage noise in the mid-band (but higher at LF) than MC33078 so might be worth a look. Its FET input so won't give you any current noise issues.

 

[jmproductions]

You really need the schematic to figure out what the working impedance is around the opamp. Sometimes its determined at AC so a multimeter won't necessarily help.

Yeah that's what I was afraid of, and no schematic available unfortunately.


Would there be any way to determine the approx decoupling requirements and impedance performance by examining the spec sheet of the 33078? http://www.ti.com/lit/ds/symlink/mc33078.pdf


So would everyone agree that the mfg's choice (other than cost) of using the 33078 would be primarily based on low current noise? Yes that OPA1656 has some impressive specs for sure, has anyone heard what it sounds like?

 

[Panelhead]

I have a dac/preamp that used the MC33079 Quad opamps. I suspected they were a weak link. Had two, so modified one, left side the other stock.
Tried several nice quad opamps, both FET and bipolar input. None were better than the MC33079. Many sounded worse. Spent a month on this, soldering the quad SOIIC opamps in and out.
It was a total wastes of time.
The MC3307x family are cheap, and work well. This must be why manufacturers select them as OEM.

 

[jmproductions]

I have a dac/preamp that used the MC33079 Quad opamps.

Just took a quick look, so the 33079 is the 4 circuit version of the 33078? Interesting... can you list some of the ones you tried?


These 33078s are everywhere in this preamp, must be 40 of them in there with the exception of 8 or so OPA2132/2134s in the RCA output buffers.

 

[abraxalito]

Yes that OPA1656 has some impressive specs for sure, has anyone heard what it sounds like?

If getting the best sound is your primary aim then the lowest hanging fruit for me in every single piece that I've modified has been power supplies, not opamps. Even the relatively humble TL072 can sound very good when its fed the cleanest, lowest impedance power.

 

[Kay Pirinha]

I have a dac/preamp that used the MC33079 Quad opamps. I suspected they were a weak link. Had two, so modified one, left side the other stock.
Tried several nice quad opamps, both FET and bipolar input. None were better than the MC33079. Many sounded worse. Spent a month on this, soldering the quad SOIIC opamps in and out.
It was a total wastes of time.
The MC3307x family are cheap, and work well. This must be why manufacturers select them as OEM.

Wisely said! Despite of it's age, the MC33078/LM833 still is a very good dual opamp for audio purposes. Rolling it will worsen the appliance's performance with some non negligible chance.
Best regards!

 

[Mark Tillotson]

Good questions I'm sure, is there a way to measure the impedance with a standard multimeter? I believe the 33078 are bipolar as well as the Muses02, so should I stick with bipolar? I could take a picture of the board if that would be helpful. Could you read the current noise specs on the Muses? Here, scroll down: MUSES Official Website

At that price only a brainwashed idiot would buy them! I bought 100 NE5532's for the same money, and the MUSES02 is basically about the same as an NE5532 without the high current handling I think.



You need the circuit schematic really to figure out the noise impedance of the various stages, and any existing decoupling. Any bipolar-input opamp without current noise specification is rather an unknown quantity above 1k impedance or so. JFET and CMOS input opamps are very low current noise, so for them only quoting voltage noise is understandable. As well as noise you need to check what the load impedance is, many opamps can drive down to about 2k loads, but some can handle heavier loads like 500 ohms.

 

[jmproductions]

Well I will agree it must be a pretty good sounding op amp, my Lexicon pre had all 33078s and it was quite articulate. So my burning question now is if the 33078 is such a great op amp, why is no one mentioning them? I've read just about everything I could find on the net about op amp impressions and you will never see this model come up, good or bad.

 

[Mark Tillotson]

Well its on Doug Self's list of audio opamps in several of his publications, up there with about 20 others. Perhaps the lack of USP, and not being cheap enough to undercut 5532's?

 

[jmproductions]

Looks like the LM833 has the same specs except they talk more about audio applications on the datasheet. So yeah the LM833 actually comes up quite a bit. I found one post where someone referred to 33078 as a 5532 with a better spec sheet, although there are differences in general specs. But mostly what I'm finding is that people say this chip is drives low impedances well.

 

[sgrossklass]

With just two stages, I would try reverse-engineering one channel. If you're lucky, it's not too dissimilar to a typical application in the DAC datasheet. It would be really good to get a feel for what kind of impedances and how much potential high-frequency content we're dealing with. (BTW, AK4395 Fig. 13 is excessively low impedance, AK4396 Fig. 17 seems more sensible.)

Given that MC33078s typically were a cheaper 5532 alternative, one of those may not be a bad bet... though there are some applications at low to unity gain where a cheap 5532 (read: TI) may actually be inferior due to elevated levels of high-frequency common-mode distortion, assuming output loading is not the limiting factor. LM4562 / LME49720 would be another, more modern option (less voltage noise, even better load driving).

Check out Samuel Groner's excellent opamp measurements, he tested all of NE5532, LM833 and MC33078 (and LM4562 and a range of others, too - notable exceptions being any JRC types and the OPA16xx which only appeared in later years).

 

[jmproductions]

I would try reverse-engineering one channel.

It's tricky because it seems to be at least a 3 layer board. I did look at Groner's impressions, thanks. Doesn't seem like he's real impressed with the 33078 and would favor the 5532.


The DAC is an AK4395VF, does that tell us anything useful?

 

[Mooly]

...................These 33078s are everywhere in this preamp, must be 40 of them in there with the exception of 8 or so OPA2132/2134s in the RCA output buffers.

That number of devices means you must consider current consumption of any substitutes and the capabilities of the supplies they run off.

The LM833 is available under two versions, one with the old NPN/PNP output stage and the later with a new and faster quasi complementary stage.

Post #72
About op-amps use.

Post #78
About op-amps use.

 

[jmproductions]

Good info, thanks! I'm not tremendously worried about the supply because this is a multi-channel surround preamp (powering many, many op amps) with quite a beefy power supply that looks something more like a power amp would have, and I'm only considering swapping 2 of them. There could be another gain stage in there but I could not find the L/R channel signal present at anything else I tested. The unit was mfg in 2004, when do you think these changes occurred?

 

[Kay Pirinha]

Looks like the LM833 has the same specs...

According to the TI datasheets, LM833 and MC33078 are equivalent.
Best regards!

 

[sgrossklass]

According to the TI datasheets, LM833 and MC33078 are equivalent.
Best regards!

That would be because they are literally selling the same chip under both names. TI's LM833 is a rebranded version of their MC33078 (which in turn is a clone of the original Motorola part with some differences in performance).

It gets super confusing because TI are also selling the different, original NatSemi LM833. LM833P would be the TI one, LM833N is the NatSemi.

BTW, has anyone else noticed that the TI NE5532 seems to be cheating on its slew rate spec? Looking at Groner's high frequency linearity graphs, the steep increase in distortion linked to slew rate limiting occurs lower in frequency than e.g. in the LM833(N) and about on par with the MC33078, both rated 7 V/µs. The old Signetics NE5532 is slightly faster than the LM833 though.

It's tricky because it seems to be at least a 3 layer board.

Well, then what sort of resistor values are you seeing there at least?

The DAC is an AK4395VF, does that tell us anything useful?

That was a first-rate DAC chip in those days. 120 dB of dynamic range still is nothing to sneeze at, digital filter passband ripple is very low, and I would imagine a good implementation would sound as good as most anything out there. Take a look at the datasheet. Its pin-compatible successor AK4396 only improved performance slightly but offered DSD operation and reduced power consumption in return, it remained popular for many years.

 

[jmproductions]

Well, then what sort of resistor values are you seeing there at least?

Ok I will post a pic of the board and measure some values tomorrow... really appreciate your help!

That was a first-rate DAC chip in those days.

Yes and I believe they continued to use it all the way into the production of the D2v which is still highly regarded.


I got a deal on a set of Muses8920Ds and threw them into the output buffers, also freshened some caps and thing does sound real nice.