Another Whammy came into this world yesterday. For now, I have no complaints, the device is completely silent and plays well. LME49720 was used for the first test, and there will be better OPAs on the test. I enriched it with DC voltage protection for headphones and a Mute circuit for a delay of 5s from switching on. The transformer is 30VA custom made, with a blind thread and a magnetic shield. It is also excellent as a Line preamplifier, although it is slightly overkill for that purpose. 
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Oscilloscope testing revealed oscillating problems. The first opamp I tried the LME49720 oscillated slightly with any compensation value (C2&C7) so I removed them and add a 100nF decoupling caps to the opamp supply and to the 7815/7915 input pins. The oscillation in operation stopped, but a strong low-frequency oscillation appeared when it was turned off (just for a few seconds).
The next test was with the OPA2604 in the same configuration, there were no problems.
Then the OPA1612, it oscillates like crazy no matter what you do to it.
I was then able to get the Sparkos SS3602 to work nicely, it needs 68pF compensation to reproduce proper squares without overshoot.
After switching off, the amplifier outputs about 1VDC for quite a long time, over 30 seconds, after which the voltage drops for a few more minutes. DC at the output is solved by mute relays that disconnect the output immediately after switching off.
The work continues. The next plan is to try a Zobel circuit to add to the output to see if it will be more stable.
The next test was with the OPA2604 in the same configuration, there were no problems.
Then the OPA1612, it oscillates like crazy no matter what you do to it.
I was then able to get the Sparkos SS3602 to work nicely, it needs 68pF compensation to reproduce proper squares without overshoot.
After switching off, the amplifier outputs about 1VDC for quite a long time, over 30 seconds, after which the voltage drops for a few more minutes. DC at the output is solved by mute relays that disconnect the output immediately after switching off.
The work continues. The next plan is to try a Zobel circuit to add to the output to see if it will be more stable.
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Continuation of the story.
I successfully tuned the amplifier for the OPA1612 by adding to the output a Boucherot cell (Zobel network) 10ohm+47nF. Everything is perfect now, regular squares without any compensation. The frequency range of the amplifier is quite wide, it goes up to 650kHz -3dB.
On the lower side, the limiting frequency of 8Hz -3dB is determined by C26 and C27 (22uF). I think that is too high and at least 100uF should be put there.
Then I return the Sparkos opamp, because that is the ultimate goal. Now he oscillates like a fool. I take off the Zobel, the oscillations disappear.
The device is a real challenge, it has to be tweaked for each opamp separately.
I successfully tuned the amplifier for the OPA1612 by adding to the output a Boucherot cell (Zobel network) 10ohm+47nF. Everything is perfect now, regular squares without any compensation. The frequency range of the amplifier is quite wide, it goes up to 650kHz -3dB.
On the lower side, the limiting frequency of 8Hz -3dB is determined by C26 and C27 (22uF). I think that is too high and at least 100uF should be put there.
Then I return the Sparkos opamp, because that is the ultimate goal. Now he oscillates like a fool. I take off the Zobel, the oscillations disappear.
The device is a real challenge, it has to be tweaked for each opamp separately.
The amp above arrived at my home, and its sound profile is truly crafted for enjoyment. It delivers a smooth, dynamic, and vibrant audio experience that feels lifelike. There's a subtle tonal character with a touch of warmth and a bit of heft, making it undeniably inviting. The bass is slightly elevated, whether it’s natural drum beats or synthetic bass lines, which is perfect for a bass enthusiast like me. Despite this, it doesn’t overshadow the midrange, which is intense, emotionally rich, and has a tube-like realism. The midrange is slightly forward but maintains a natural feel with its volume and detail. The treble is lively and energetic, with a hint of silkiness, as @Zen Mod describes it.
I forgot to mention that the Muses02 was selected because it sounds more refined and aesthetically pleasing than the Sparkos. Additionally, we were unable to achieve proper square waveforms on the oscilloscope with the Sparkos when the volume was increased. It only functioned correctly with 300pF compensation (C2 & C7). In contrast, the Muses02 operates from 2 Hz to 350 kHz with proper square waveforms at -3 dB with 68pF.
This circuit is designed to achieve high stability using JFET opamps, which have significantly lower input bias currents compared to bipolar opamps. The low bias current minimizes DC offset and drift, enhancing the opamp's stability and reducing sensitivity to variations in the input signal or power supply fluctuations. Avoiding complications at higher frequencies is advisable, but personally, I have consistently preferred bipolar devices and discrete circuits. Bipolar opamps tend to offer more aggressive performance with greater detail. They excel in providing a natural perspective with realistic depth, superior focus, and accurate tonal and textural reproduction. Additionally, their bass quality is outstanding.
In my experience, Burson opamps often sound artificial, as if they were on steroids, and can become fatiguing over extended listening periods. I chose the Muses02 opamp because it delivers a softer, bassier sound, making it more suitable for prolonged listening without causing fatigue. Alternatively, the Sparkos SS3602 opamp offers a more dynamic sound but is also more aggressive and has slightly less bass, at least subjectively. While measurements show both opamps to be completely linear, each has its unique strengths.
