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AWG Buffer Amp
mawyatt:
Agree a simple feedback network is in order which is what we have on the designs we've done. Used the classic R and C in series and the combo in parallel with the feedback R. Might consider doing this with the Juntek if stability was an issue.
At first we thought the high frequency content of the waveform at power down was due to the XL6012 but the frequency is ~ 2MHz which indicates it's coming from the LM1875. What's likely happening is the power supply voltages are falling unequally at power down, and the LM1875 internal bias is getting messed up and changing the characteristics allowing this burst oscillation. With a carefully controlled power down this should be avoided. The LM3886 has a mute function which could be employed to mute the output during power up and down, and has a more uniform open loop response without the quirkiness of the LM1875 around 1~4MHz as shown in Figure 12 Open Loop response.
I'm a retired IC designer so interested in the chip details, and if you study the designs you can see that the LM3886 employs local internal feedback to help keep the output well behave during the mute process which is a current controlled gain function as shown in Figure 44 of the LM3886 data sheet.
If you plan to power these amps from a SMPS be sure to pay attention to the PSRR, especially the negative supply PSRR. At the XL6012 cycle rate this is only 20~30 dB rejection. The DPA-1698 uses a dual inductor (4.7uH) power supply filter on both the + and - supplies for this reason, however you have the benefit of the lower audio frequencies so maybe not an issue.
Anyway good luck with your headphone amp design and build as I'm sure you'll find the LM1875 a good device.
Best,
precaud:
Yes, the -V PSRR is a red flag waving, for sure, but it's typical of most op amps. For audio, not much to concern oneself with.
A little OT, but I wonder how much of the 1875 distortion is common-mode, and if it could be lessened by the usual tricks; balancing the impedances at the inputs, and/or operating it in inverting mode (it's very easy to swap the speaker leads to restore polarity...). The data sheet only shows it used as a non-inverter.
precaud:
I've built the amp, running at 16dB gain into a nominal 16 Ohm load, made a few mods to address specific issues, and I have to say the result is underwhelming. Distortion spectra is particularly ugly. The most significant improvement came from adding a 100pF compensating capacitor across the 6k65 feedback R (-3dB @ 240kHz). This lowered 3rd harmonic distortion by over 12 dB, making the sound much less edgy and brittle (see attached). But it still isn't an impressive amp for high-quality audio by any measure.
BTW mawyatt, I think you were reading the Fig 12 gain-phase plot wrong. Phase margin is 45º at 10X gain, which is probably why they recommend it as minimum...
mawyatt:
--- Quote from: precaud on January 23, 2021, 06:19:40 pm ---I've built the amp, running at 16dB gain into a nominal 16 Ohm load, made a few mods to address specific issues, and I have to say the result is underwhelming. Distortion spectra is particularly ugly. The most significant improvement came from adding a 100pF compensating capacitor across the 6k65 feedback R (-3dB @ 240kHz). This lowered 3rd harmonic distortion by over 12 dB, making the sound much less edgy and brittle (see attached). But it still isn't an impressive amp for high-quality audio by any measure.
BTW mawyatt, I think you were reading the Fig 12 gain-phase plot wrong. Phase margin is 45º at 10X gain, which is probably why they recommend it as minimum...
--- End quote ---
Not having any experience with the LM1875 is why we included the LM3886 and TDA7293 for evaluation.
Don't think we got the gain-phase plot wrong (likely you are not using dB for the Gain axis and why @ 10X shows 45 degrees which is incorrect, 10X is 20dB), included as Fig 12 from the LM1875 data sheet. As you can see the phase is ~45 degrees when the gain crosses 12dB (4X), at 10X which is 20dB gain, the phase is ~65 degrees.
Best,
precaud:
Ooops, you're right, my bad, I was reading the gain as mag ratio, not dB.
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