[Wing]

Lately for the last 2 months or so, I've pretty much dropped off headwize and headfi. The reason for this is I have been spending time pursuing a profound audio revelation from a simple tweak I made to my MG Head. To put it simply, I have discovered true Single Ended Triode amplification without global negative feedback. All I did was remove the negative feedback by desoldering (and insulating) the green wires leading to the contact on circuit board with the label NF. I had noticed the negative feedback from the supplied circuit diagram in the owners manual and had reasoned that the musical signal which has been phase shifted through a cap and transformer, could not be fed back without somehow damaging the output. Time coherence is all screwed up, resulting in lost details.

The music I hear is now in another dimension altogether. People who have been raving about the MG Head ain't heard nothing yet. Details, imaging, palpability. The music is REAL. In fact, in a head to head comparison with KurtW's Microzotl, all I could do was laugh. Kurt himself admits that the Zotl sounded flat in comparison and the modified MG Head had more texture. This is no fault of the Zotls ZOTL innovation but it's push pull topology. Single ended will kick butt on PP in terms of imaging and detail most, if not all of the time. I have gone through my CD collection and now can't seem to stop listening to music once I start.

There is a advantages and drawbacks to removing NF. ALL is revealed, including the character of the tube used, the quality of the audiophile recordings you spent big bucks on, details in CDs you own you never knew about, as well as, the poorer quality of the $7 transformers in the MG Head. For one, the frequency extremes are rolled off (Kurt brought his really cool hi tech Neutrik audio measurement devices, which measures everyting except the most important measurement, real music). Subjectively, the rolloff does not detract too much from musical enjoyment. The biggest problem would be the noise created by the power transformer inducting some low level hum (not too audible) in the audio OPTs as well as amplifying any kind of ground loops in the power lines, for some reason in the left channel (quite annoying). Fixing the ground loop in the chassis itself helped quite a bit but did not remove the buzz. Gain went up by quite a bit, the low/hi impedance switch now makes a difference with my HD 600, with hi Z sounding louder and better. The stock tube sounds pretty good but is a little overbright and not as refined as the Mullard. It is also quiet, unlike my Sovtek 12ax7LPS which hissed and sounded brittle. Too bad I sold my Telefunken and other NOS or I would roll em again.

I have been researching tube audio esp. SET, hanging out at the Tube DIY forum at Audio Asylum, reading the Morgan Jones book, the Rosenblitz audio myths book and I have a long way to go. What I have heard has finally shown me why those craze SET tube heads are raving about (I guess I'm one now too). My next steps are replace the transformers with power torroids and name brand OPTs (Magnequest, Electra-Print etc). I will keep you guys updated and if I'm satisfied with what I got, show it in the next BA headwize/headfi meeting.

Try it for yourself and listen, especially if you own the HD 580 or 600, a Mullard CV4004 driver tube and an MG Head. You may also see the light and become a true believer. There, I have finally stopped procastinating on my report and said my peace. Let him who have good ears, hear.

PS. Just in case the improvements were hallucinations, I have another MG Head with another Mullard for baseline.

 

[clifford]

Wow!
Can not wait to try! But have no idea how to it.

Wing, can you post how to do it in detail? I am sure it will cause lots of noise here.

 

[bootman]

Wing, have heard of the new MG Head OTL amp?
It also lets you remove the feedback!

 

[KurtW]

Wing, weren't you going to convert this to OTL? That along with the no negative feedback would be the way to go I would think. The initial (non-negative) feedback from other posts on the MG Head OTL seems to indicate this.

Just to add my comments on the difference between the non-modified MG Head and the modified one (Wing, it's great that you have a control amp to compare against...very scientific!) the modified amp with no feedback did have a much airier, three dimensional soundstage. It also had an audiable rolloff in the bass and the high end treble, and double the measurable distortion. Wing is correct that these shortcomings don't detract much from the overall musical experience, even if they're not ideal. Compared to the ZOTL which is push pull but doesn't have the traditional audio output transformer, the modified MG Head still had more three dimensional soundstage but gave up ground in overall speed and frequency extremes. I would guess that a better output transformer or better yet none at all would help things even further in the MG Head.

I should also mention that I did a blind test switching amps when Wing was listening and he always could pick out the one with no negative feedback. It took me a little while to learn the sonic characters of each amp but after awhile I could also pick them apart, so it was pretty obvious that the differences are really audiably distingishable.

 

[Wing]

I have not heard the new ASL OTL headphone amp nor have I followed the threads but if I remember correct, it has a different topology as well as tube set from the MG Head DT.

All that is needed to mod the MG Head to no NF is to remove the bottom cover and desolder (or snip) the 2 green wires leading to the board. The area on the board has the labels NF on it. You do this at your own risk. I will not be held responsible if you family gets to collect on your life insurance when you forget to unplug the AC cord before doing this or if your amp makes smoke instead of music if you don't insulate the bare wire ends and it shorts.

I have tried OTL with a pair of 47uF Black Gates from the plate but I find that although all the frequencies are there, it is not as coherent and palpable as transformer coupled. From what I understand it is an impedance mismatch. The plate output from the el84 is a high impedance output in the order of 5K maybe. How this is supposed to drive 300 ohm headphones properly, I don't know. If I were to try OTL again, I may try taking output from the cathode as a cathode follower which is supposed to have lower impedance but also lower voltage swing.

 

[chych]

Just tried it.... and

holy smokes! You guys were really right about the 3Dness of the amp. Imaging /soundtaging is much better! Such an easy mod too, definetely worth it.

Though yeah, damn, the distortion is higher, hiss and all. Very microphonic too, time to get some sorbothane for it. Sure beats buying a new amp

... gotta tube roll some too... hmm...

Wing, what did you do to reduce the hissing? And god, the gain is insane, time to build some attenuators... I'm running it off a DI/O which provides a hot 7V signal already, sheesh!

 

[chych]

Hmm I just plugged in my CD3000s...

All that hiss is... gone... back to normal... Maybe this is good for driving lower impedance phones? (and my god this sounds so much better)

 

[Wing]

Muhahaha, the first convert. Come one, come all to the temple of SET :>.

I don't know if the mod shows it's best side with low impedance phones. I always use the impedance switch set to hiZ. Warning, this position is not always correct, I think the other MG Head I have has this switch reversed. Listen for a louder more dynamic sound (with you hiZ phones), that's the position you want.

The hiss is probably your tubes. My Mullard and stock is quiet but the Sovtek hisses. Buzzes are a different thing. That's the ground loop. You might want to tie the AC ground directly to the star signal ground. In my amp the AC ground was not properly coupled to the chassis as the paint was insulating it. I noticed in my baseline amp that the amp assembler had scratched up chassis where it contacts the AC ground tab for better contact, but not in my original amp. This will help the buzzes but not eliminate it, my guess is, due to lack of power transformer electrostatic shielding.

You should not be able to hear any distortion unless there was some sort of really bad clipping (umm.. 7 volt DI/O, 0.6 volt 12ax7 driver bias, yup) or your ears are built by Neutrik, which is unlikely. I've tried the mod with the Beyerdynamic DT 831 (120ohm) and Sony 7506 (32ohms??) without noticeable distortion. I hated the 7506 sound compared to my other phones through with the mod, because the midrange on the 7506 sounds way recessed. My impression is, the soundstage is the best with my 300 ohm HD 600. It might be the phones or the fact that the impedance is better matched.

Yeah forgot to mention about the microphony. You only detect it if you scratch or tap the tubes.

In regards to the hiss, maybe there is some sort of tube warmup/burn in here that removes it. I'm not sure.

 

[chych]

Hmm, yeah there really is no distortion actually...

But the main problem is with the Hiss, on my etymotics, same with the microphonics. The microphonics are INSANE with my etys... plugging the thing in creates noise, tappint the MG Head anywhere makes noise, even changing tracks on the CDP (right under the amp) makes noise. Oh well, I'll have to live with it.

No buzz either too, except on this one stupid outlet in this room of mine. The other outlet is fine (ironically the outlet that is fine reads as a ground fault from my APC surge protector while the buzzing one does not).

Going to really need to tube roll though... time to look at NOS, since I will be keeping this amp for a while.

Also, whats up with the soldering? It seemed to me that hte wire was held up with way too much solder, I wonder how many connections there are that are like this, in this "floating" solder thing. I wonder what kind of improvements one would get buy resoldering a lot of the connections.

 

[chych]

Hmm Wing, you are right, just took out my Svet 12AX7 and put in my Sovtek 12AX7WA and the hiss is gone.... I'm going to have to go on a tube hunt then.

 

[Wing]

Like I said, ALL is revealed, noisy, microphonic tubes or good sweet NOS tubes, bad power or good power (use it to test your balanced power/powerplants/line filter, the truth comes out), bad soldering/layout, cheap transformers or brand name OPTs. It also makes glorious music and shows what's truly coming from the source. It's an ecstacy from another dimension. That's why I'm going DIY SET. I can get the good components, build it right and experiment with different circuits and tubes. One of my future projects is an ultra simple single tube headphone SET amp. That may sound even better than a modded MG Head.

Happy tube rolling.

 

[bootman]

Humm, I wonder how the two (modded DT and new OTL head )would sound head to head? (pun intended!

)
They are both the same price so would one be better off geting an older DT and modding it or getting a new OTL?

 

[lextek]

I've been listening to the new OTL Head for two days now. I must say I am becoming a tube believer. Hell I just like looking at the glowing tubes. Vocals in the OTL mode with no feedback is just "liquid'. I really didn't think a $300 amp could sound this good. I'd like to try some different tubes in it. Suggestions?

Bob

 

[Hardhead]

See this tread although it does not address the OTL version.

http://www.head-fi.org/forums/showth...8891#post58891

 

[chych]

Ahh ok I see... this mod IS available on the MG Head OTL that DiverTech is selling... it is just the NF switch off but transformer coupled. So here we have a SET amp, OTL amp, and a transformer coupled amp, all for $330...