With the INA134s installed as the pre-driver I decided to look at 10 kHz THD.
The open loop THD performance of the DCAO2 (below 100 mW) isn't influenced much by the bandwidth of the pre-driver.
A THAT1240 or INA134 as pre-driver doesn't make much difference.
Closed loop however (for power levels from 100 mW to 2W) the bandwidth of the THAT1240/INA134 pre-driver
does make a difference.
The THAT1240 has a typical 8.6 MHz unity gain bandwidth; the INA134 is 3.1 MHz typically.
For the INA134 to be as stable as the THAT1240, the local feedback capacitor around the error op amp/gain stage has to be increased from 22 pF to 100 pF.
As Jim Williams points out in his series on op amp boosters, the error amp needs to be slower than the output stage. (See part two:
https://www.proaudiodesignforum.com/for ... ?f=12&t=13)
The THAT1240/INA134 stage is inside the feedback loop and is part of the output stage.
If the pre-driver stage is slow, the error amp must be slower.
The added compensation required reduces high frequency open loop gain and the amount of corrective feedback available.
The 10 kHz THD at elevated power levels show this: The THAT1240 has significantly lower THD because it has about 2.75X the bandwidth.
After discovering rising THD vs. frequency I decided to re-visit the use of op amps to act as pre-drivers.
THAT1240s were chosen over op amps because they have accurate gain.
The signal gain of the NPN/PNP pre-drivers should be closely matched to reduce even-order distortion.
This is particularly important when the DCAO2 is being used open loop.
THAT1240s eliminated the requirement for a trim that an op amp circuit based on 1% resistors would require.
The motivation to try INA134s instead of the THAT1240 was to obtain higher output current when the DCAO2 was used in 8 Ohm applications.
The lower bandwidth of the INA134 however requires the error amp to be more heavily compensated and the THD at HF not as good as the THAT1240.
The use of op amps as pre-drivers - something I originally tried - seemed to offer the best of both: High output current and higher pre-driver bandwidth.
The only downside seemed to be the requirement for a trim.
But this is DIY and trims aren't avoided like they would be in a production environment.
Since I was going to try using op amp drivers one more time I rebuilt one side of the protoboard and plugged in a couple of NJM2114s.
More Fun...
Stabilizing the NJM2114s - which are turbo charged NE5532s - turned out to be an interesting exercise.
The NE5532 was equally interesting.
The NJM2114 has a 15 MHz unity gain bandwidth and 15 V/µs slew rate versus the NE5532's 10 MHz BW and 8 V/µs SR.
Both have higher bandwidth than the THAT1240 or INA134s internal op amps.
But they performed
worse.
To make a long story short it took a lot of tuning and the resulting Cfb turned out to be 100 pF + 1K.
The reason Cfb had to be so large is because the NE5532/NE5534/NJM2114 family have an interesting "resonance" in their out-of-band response around 500 kHz.
If you sweep slowly from 500-600 kHz there can be a peak in the response up to 6 dB.
Small-signal and large-signal square waves also reveal this.
With there being three NJM2114/5532 stages in total, with all three in the feedback loop, keeping overshoot and stability tamed required that the error amp be heavily "slugged."
The resulting 10 kHz THD with this resolved was still higher than the INA134s.
You can see the open loop response "hook" in several published 5534/5532 datasheets:
The above appears to be drawn with the hook above 10^5 and several other datasheets also show the discontinuity.
Some do not.
When you sweep past this frequency the response looks like a snap oscillation but its actually a very narrow Q response peak.
It looks like the nested compensation internal to the 5532 has one stage where compensation seems to end and the next stage picks it up.
In most applications this discontinuity is of no consequence.
As John usually points out, out-of-band material should be filtered out at the input.
But because the feedback loop of the pre-driver are nested inside the DCAO2's global feedback the hook creates unnecessary problems.
Based on the results with the NE5532/NJM2114 I decided to try a different op amp family. My first choice was the OPA1612.
The OPA1612 is a Silicon/Germanium wonder.
The small-signal BW is 40 MHz with a SR of 27 V/µs.
It has 40 mA outputs.
The results are very, very good.
With the OPA1612 I didn't have to work around the 5532's 500 kHz weirdness.
I could reduce Cfb back to 10 pF and maintain stability into capacitive loads of 30R||1nF.
This is the DCAO2/OPA1612 THD at 1.5W, 10 kHz into 30 Ohms 1 nF:
Dual Class-A DCAO2 using OPA1612 pre-drivers at 1.5W into 30 Ohms and 1 nF.
Red is Gen/Mon shifted by +12dB, Green is THD of the DCAO2
What I discovered is that several "fast" op amps worked well as the pre-driver. The key was to not use a 5532/2114 as the pre-driver.
For all but the last FFT the error amp and pre-driver are the same op amp type and the Cfb is 10 pF.
DCAO2/OPA2604 THD at 1.5W, 10 kHz into 30 Ohms 1 nF:
Dual Class-A DCAO2 using OPA2604 pre-drivers at 1.5W into 30 Ohms and 1 nF.
Red is Gen/Mon shifted by +12dB, Green is THD of the DCAO2
The OPA2604 used a Cfb of 10 pF.
Curiously, the even-order null point was different for the OPA2604. The OPA2604's own even order may be getting nulled by the trim.
DCAO2/OPA2134 THD at 1.5W, 10 kHz into 30 Ohms 1 nF:
Dual Class-A DCAO2 using OPA2134 pre-drivers at 1.5W into 30 Ohms and 1 nF.
Red is Gen/Mon shifted by +12dB, Green is THD of the DCAO2
The OPA2134 with its 8 MHz BW is still a respectable performer.
The NJM2114 works great as an error amp. This is the FFT of the NJM2214 driving an OPA2604 pre-driver:
Dual Class-A DCAO2 using NJM2214 error amp and OPA2604 pre-drivers at 1.5W into 30 Ohms and 1 nF.
Red is Gen/Mon shifted by +12dB, Green is THD of the DCAO2
This particular configuration is stable with no Cfb because the input error amp is slower than the pre-driver.
When the NJM2114 is used both as error amp and pre-driver, the THD performance is worse due to the required heavy compensation of 100 pF:
Dual Class-A DCAO2 using NJM2214 error amp and NJM2114 pre-drivers at 1.5W into 30 Ohms and 1 nF.
Red is Gen/Mon shifted by +12dB, Green is THD of the DCAO2
A 20 dB reduction in THD at 10 kHz is available simply by making the "correct" op amp choice for the pre-driver.
The overall performance improvement by using high-bandwidth op amps in the pre-driver have caused me to re-think the use of line receiver ICs.