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Aiyima 04 Op-Amp Problem

scott wurcer

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Hmm, I had no problem with AD797 at 20 dB gain when I replaced the LT1082 in my MC phono stage a few years ago. However I've read in Douglas Self "Small signal Audio Design" that early versions of the AD797 were difficult to stabilize and later versions not. Maybe the designer @scott wurcer can comment on this.

There were never any changes to the circuit. One must be careful many phono circuits end up closing the loop at unity gain at high frequencies and gain of 5 stable op-amps will oscillate. I never heard of a customer ever making a claim like the OP37 is not stable below G =100. This would actually be absurd.
 

LTig

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There were never any changes to the circuit. One must be careful many phono circuits end up closing the loop at unity gain at high frequencies and gain of 5 stable op-amps will oscillate.
Stage 1 of my MC phono preamp uses the AD797 with gain fixed at 20 dB (linear FR) so no unity gain at high frequencies. If I ever need to go lower in gain (but not lower than 10 dB) I might need a small cap to stabilize though.
I never heard of a customer ever making a claim like the OP37 is not stable below G =100. This would actually be absurd.
According to the datasheet the OP37 is stable at gains of 5 or above. I tried it in a similar configuration like the AD797 and had no problems.
 

MagentaAudio

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I've just purchased an AIYMA A04 amplifier with the 32V 5A supply and did some measurements of THD. Attached are the curves for 40W into 8ohm. THD is 0.5%which is not exactly HiFi. At least its mainly 2nd harmonic so not easy to detect. Does anyone know why it's so much higher than the manufacturer of the Class D chip claims or have measured results that are significantly better for this amp.
 

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MagentaAudio

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Correction to my previous post. I've discovered that the output of the amplier is biased at about 16 VDC above earth potential (half supply voltage). Both speaker terminals are at the same potential so no problem for the speaker. However the x40 pad I'm using has protection diodes and the 12V bias was pushing these just into conduction and causing the 2nd harmonic distortion. I've now put 0.2 uF DC capacitors in series with the inputs of the x40 pad to block the DC and redone the measurements. Note that after doing the previous work I have also replaced the op-amps with BB OPA2134PA type and I get the distortion vs freq. as shown at 1W into 8ohms.

Also attached is distortion vs voltage output at 1 KHz into 8 ohm for information. This is with 32V DC supply.

I'll put the original op amps back and redo the measurement for comparison.
 

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  • AIYMA A04 THD at 1W 8 ohm OPA2134PA op amps..jpg
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  • AIYMA A04 THD vs output voltage into 8 ohm at 1 kHz with 32VDC supply and OPA2134PA op amps.jpg
    AIYMA A04 THD vs output voltage into 8 ohm at 1 kHz with 32VDC supply and OPA2134PA op amps.jpg
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