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Old 29th May 2004, 08:46 PM   #1
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Default Anyone know this audio opamp?

Hi everybody,

I stumbled across the MC33078 from ONSEMI. This is an intriguing opamp, ostensibly developed specific for audio. For one thing, it has an output stage that appears to be class A, or at least non-switch off.

Although the open loop gain shows the familiar fall off from 100Hz for a run-of-the-mill opamp, the THD curve is almost flat to 100kHz! Also the CMRR is flat. This combination is unusual.

If you look at the representative circuit, there is a complex output stage. Some feedforward capacitors I think, and the diodes have some function in the output stage biasing.

Two questions I have:
- Can anyone explain how this output stage functions?
- Any experiences? (I'm listening to a pair as preamps now, sound not half-bad).


Jan Didden
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Old 29th May 2004, 09:06 PM   #2
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I know them, they're nothing special soundwise. Most modern known audio-opamps sound better. I replaced them often in gear, I forgot what gear. Maybe Rotel preamps.

Can't comment on the output stage although it seems no more or less complex than found in most opamps. At least what the manufacturer wants us to see in the simplified diagram
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Old 29th May 2004, 09:34 PM   #3
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Yeah, it always says 'simplified diagram' or 'representative diagram'. But this output stage does look a bit more involved than the standard totem-pole or quasi-complementary stage. It does intrigue me.
So, you don't think it is too hot. What do you normally replace it with?

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Old 29th May 2004, 09:44 PM   #4
moamps is offline moamps  Croatia
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Hi,
Some expensive AMEK audio consoles use this chip (probably because very low noise) mixed with 5532 and 072.
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Old 29th May 2004, 11:51 PM   #5
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Interesting op-amp.

It looks like the reason the distortion is nearly independent of frequency is they're using Cherry's trick of enclosing the output stage in the Miller loop via C2. So the local feedback from C2 increases at the same rate the global feedback is decreasing, giving nearly constant distortion vs frequency much like VAS distortion in a power amp. Then at higher frequencies input stage distortion starts to dominate I guess.

It looks like Q7 performs current limiting, as does Q3. I can't figure out what C3 is all about though, or how the bias currents in Q8, Q10, Q11 and Q12 are set. I guess if Q10 and Q12 have the same emitter area, they'll have the same emitter current. Maybe R6 determines how the DC current of Q9 gets split between Q8 and Q10? Also, it looks like the DC current in R7 is zero, assuming the D3 drop and Q11 Vbe are the same. It looks like D4 is normally off.

It is pretty baffling though, isn't it?
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