Sennheiser PC37X randomly goes bad after disconnecting the cable ?
Greetings, Yesterday I was using my headset like normal with my macbook, just listening to music and on a call with people like usual, and the headset was perfectly fine. The stock wire that came with the headset is extremely long and yesterday it annoyed me very much that it kept getting tangled with itself, so I decided to see if the cable is replaceable. I pulled out the cable from the headset and saw the adapter, and looked online for a replacement. Upon plugging it back in, the audio sounded extremely muffled and washed out. Im not sure what I did wrong to make it mess up like that as I've always taken good care of it, ive had it for about 2 years and its always just been chilling on my desk, but anywho I thought the cable just went bad and ordered a replacement. The replacement came, and the issue is still persistant, so I am not sure what the issue is I've tried multiple different headsets and the issue is not with the port, and I also tried it with my windows laptop and...
Apr 23, 2024
So, what do I think of this Bravo V2?
1) Its output impedance is too high. It loads down trying to drive my Fostex T50RP headphones with 60 ohm impedance. Driving those 'phones, the Bravo gets 'gnashy' sounding. Harsh highs, rubbery bass. It sounds kind of 'electronic.' However, driving my Sennheiser/Massdrop HD6XX 300 ohm headphones, the sound from the amp becomes acceptable. It's still a bit more harsh and electronic sounding than if I use the O2 instead. But at least it sounds on par with a typical PC laptop's headphone output when driving higher impedance headphones. That's a pathetically low bar to clear...
2) I'm not having any noise or channel imbalance problems. Mine seems good.
3) I expected the Bravo to sound mushy and 'tube-y.' Instead it sounds 'harsh,' even when driving high impedance headphones. I haven't tried any different 12AU7 tubes in it, which I will do. But unless the supplied Chinese-made 12AU7 is an utter piece of crap, I don't expect a different version 12AU7 will fundamentally alter the annoying sonic character of this amp.
I figure the amp is worth the cost of its constituent parts. At some point I'll try some of the mods people have suggested. But if you're shopping for a 'keeper' of a headphone amp, ignore this.
I'm still happy with my O2, so if an Objective 2 comes up as a drop again, I'd recommend that. This Bravo V2 doesn't sound anywhere near as good as the O2. Not even close. Sorry.
- I don't understand why people spend dozens of dollars (or Euros) on any ECC82 or 12AU7 to 'tube roll.' It's a waste. The provided Chinese tube is adequate. If you want to improve the sound slightly, try swapping in any American 12AU7 or similar, or any European ECC82. You won't notice much sonic difference, but you may feel better--especially if you can find a pretty 'Bugle Boy' ECC82 or a nice Mullard. Those are pretty. (I have a few 12AU7s lying around, but had to reject two of them because they're noisy. I did find a good GE 12AU7A, and that's what I'm using.)
- The capacitors in the amp look OK. The ones in my amp look like they were made by Rubycon and Nippon Chemicon. If those are really made by those companies, then they're decent quality parts. They are 85degC rated parts. Upgrading to 105degC rated parts could be a little helpful. Also, the installed caps are 25V-rated parts. Upgrading to 35V-rated parts might make for a tiny improvement. Changing those to higher quality parts might make a tiny difference, but I would not expect any major difference. Probably nothing audible in the end.
- The one big attainable difference would be to replace the IRF630 output MOSFETs (source followers) with IRF510, IRL510, IRF530, or IRL530. Why? Because the input capacitance of the IRF630 is a whopping 800pF (picoFarads), while the IRF510 (and IRL510) input capacitance is MUCH lower at 180pF. High input capacitance in the MOSFET is likely to cause an audible high frequency roll-off when driven by a 12AU7/ECC82 run with only 16V on its plate and very low plate (anode) current. That means the 12AU7 will be running with high internal resistance (rp), which would appear to be in series with the output MOSFET gate (input). That series resistance (R) with parallel capacitance (C) will make a low pass filter (LPF). If the 12AU7 rp is 15k ohms, and the input capacitance of the MOSFET is 200pF, the -3dB down point of the resulting LPF will be about 50kHz. That means its -1dB down point would be within the audible frequency range. In other words, it will have slightly muffled highs. Maybe worse than slightly muffled, depending on the quality of the IRF630 used.
Now if we replace that IRF630 with a (known to be good-quality) IRF510, the input capacitance should be reduced to somewhere around 50pF. R = 15k ohms and C = 50pF, so f3 (-3dB down point) = 212kHz. The -1dB point will now be above 20kHz. That's a BIG difference in the highs!
One thing I've seen is that people say the IRF510 or IRL510 will run much hotter than the original IRF630. The current to that MOSFET is set by the LM317 voltage regulator IC, which is being used as a current sink (current regulator). The only schematic I could find for this amp (http://bilder.hifi-forum.de/max/415727/bravo-v2-schematic_691080.png) shows an IRF510 used as the output source follower, with the LM317 set to 167mA current. If there's 15V across the MOSFET, with 167mA (0.167A) drain-source current, that means the MOSFET would be dissipating 2.5W of heat. The supplied heatsink should be able to deal with that. It could be that the people who are replacing the IRF630 with an IRL510 are not adjusting the value of the current setting resistor for the LM317 (R7 7R5 in the schematic linked above). Perhaps it's the LM317 that runs much hotter after IRF510 MOSFETs are installed? Perhaps R7 can be increased in value (to 10 ohms?) so that there's less current being drawn by the IRF530, and less heat needs to be dissipated by the heatsink on the LM317.
I'll try to replace the IRF630 MOSFETs with a couple of IRF510 I have in a drawer somewhere, and report back. If I can replace the 1000uF 25V caps with better ones that will fit, I'll do that too. That's the output (DC-blocking) cap from the MOSFET source (C4 in the linked schematic), which is directly in series with the signal to the headphones. A better part there should make an audible difference, but might not. The only better caps I have are Nichicon MUSE, which are bigger than the ones supplied, and might not fit physically. We'll have to see...
Electrolytic capacitors are pretty generic, so unless the originally installed parts are terrible-quality fakes (which is possible), I don't see why replacing one set of electrolytic caps with another pretty much equal-quality set would make any audible difference. Perhaps if you used really good quality, low-ESR, high pulse current-rated caps...
3) I expected the Bravo to sound mushy and 'tube-y.' Instead it sounds 'harsh,' even when driving high impedance headphones.
So... It seems the actual sound of the amp agrees with what the circuit simulator predicted. Thinking that the simulator might help me improve the circuit, I tried some things. Nothing helped. I'm sorry, this circuit appears to be beyond help. It's not worth the effort to modify it. I'm not sure what this thing is good for, other than looking kind of cool on my desk at work . Unfortunately, it doesn't sound any better than the headphone jack on my work computer.
Oh well. Moving on now...
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