Hey guys.
I recently purchased an SMSL THX phones amp, and while I mostly love it, it does have a few serious faults.
I want to point them out, and see if maybe there's a way to mod the circuit to fix at least the worst problem.
Background:
I need a bunch of amps for both my recording studio (250 ohm - 600 ohm cans) and my band's live cue system (low impedance IEMs.) I already have two extremely high-end tube amps for the studio, but they don't play nice with low impedance, obviously. I also have a bunch of Massdrop O2 desktop editions. Being a lifelong pro recording engineer, I've spent my life doing audio comparisons, and I have well-trained ears. - So please no one tell me I need to do careful double-blind tests, etc. That will just waste time.
OK, so the O2's are great for the money, but I wanted something better for me, and something with hopefully more solid low end for the drummer. I decided to try a THX, and took a chance on the SMSL version.
Here are my thoughts, and questions: ( This will just concern the SMSL THX vs my Massdrop O2's
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First the good: (the great)
The SMSL seems even cleaner than the O2, with a more open (wider?) presentation. I’m tempted to say that the stereo perception is wider, but I’m not sure if that’s technically correct. The difference here, vs the O2, is not subtle. I love it.
There is also just a tad more bottom end fullness, which is what I had hoped for.
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A possible negative:
The upper mids seems a little recessed. Or possibly, the upper mids in all of my other amps are too forward, but I think it’s the former.
I don’t know what could cause this, since all amps test flat, but it’s absolutely the case. For instance, I find it slightly easier to sing through the O2 than through the SMSL, because my voice is more “in my face” so to speak. (Yes, I checked the levels.) - Even though the SMSL is nice to use just for listening to music. In this case, the difference IS subtle, and not enough to get hung up about, but it’s definitely there.
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A definite negative:
Even with proper gain structure, this amp drives 32 ohm IEMs with a slightly wimpy & somewhat distorted LF. I’m being picky here, but the problem exists.
I read somewhere that the true measured output impedance of the SMSL is 1.3 ohms. If so, well there you go.
# Is there any chance the output driver could be improved?
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Finally, the BIG negative:
(One I’m hoping a circuit mod might fix)
With this amp, if I switch it to "low gain" and then compensate with the volume knob to get back to the original level, the sound COMPLETELY falls apart.
It sounds massively compressed, and maybe even distorted. Horrible. Hot garbage. You get the idea….
I’m pretty sure that what's happening is that the low gain setting must also be lowering the input headroom. I'm running a fairly hot signal to the amp. Not quite +4, but more than -10. Probably around 0 dBu. I tried lowering that level "significantly, and then doing the same test as before, except with the volume knob in its second half of travel. With 250 ohm cans, I no longer hear that compression & distortion, so it MUST have been the input overloading.
If I understand correctly, many headphone amps vary gain this way, including the Objective2, but it strikes me as a foolish design. Also, even though the Objective2 also does it this way, my O2's (which are Massdrop desktop versions with low & mid gain) do NOT run out of headroom the way the SMSL does. I have to hit them with a full +4dBu signal for that to happen, and even then it's subtle.
So, obvious final question:
Does anyone have a mod to increase the input stage’s headroom?