Little Dot MK VI / Little Dot MK VIII SE Owners Unite - Page 31

I just checked again with LD and David said the power tubes should be matched but "not too picky" to quote him, sorry I can't be more precise.

Thanks for checking  The Little Dot might well have some circuitry to compensate for variance but usually in such balanced designs closely matched tubes can really improve  performance. Sadly there is very few professional tubes seller that can properly matched tubes and it also cost money 

Still I would love to know more, especially if current draw matters at all. Does anybody know if Sword Yang reply to individual's email? ( I don't have it, does he speaks English?)

The DC servo control that is provided by the op-amps will control the idle current of the output tube in order to have zero DC offset within reason. This is important for a push pull amp and especially for the LD Mk6+ since it has no output coupling cap to block the DC. Each power tube has its own DC control op-amp. What is important is that each  triode section in the same tube is to be fairly close together. That is usually not a issue with NOS tubes. For used tubes I do sometimes find the triode balance is way off in the tube because previous that tube was used in a circuits that only one side of the tube is being used. My sencore TC28 is very good is telling me what tube are NOS and what is slightly used by the life test. The GM is not as important for power tubes as it would be for driver tubes. My tester is just a dynamic tube tester that has no GM reading capabilities but has a very sensitive shorts and leakage test. There is no way for a tube tester to perfectly match the tubes since a tube tester does not know the circuit or amp that the tube will actually be used in. I can see the DC servo action in operation on my LD Mk6+ when the right meter turns on the needle will overshoot a bit and then undershoot and then re-balance steadily. I do find that tubes with high leakage can mess up with the sound and there not many testers with the senstivity range of my Sencore.

Thanks for the detailed answer John. What do you mean by each section of the same tube to be fairly close? Gm or current draw? I bought quite a few supposedly matched quads but moving the tubes around in the amp lead to different bias reading on the meters so I suspect those are not very well matched if at all. I understand matching tubes is a kind of black art as it is rarely done at operating voltage anyway. At the end of the day it is probably worth buying tubes from a professional sellers that actually attempt to match rather than off fleebay.

Do you know  how tubes with higher Gm such as  7236 or 5998 affect Zout and current delivery. The bias end up being quite  lower (40mA) and  the amp run a lot cooler with more gain suggesting better efficiency 

The B&K and Jackson circuits are also excellent in detecting leakages and shorts.

The Hickok circuit is not very good as it only tests at 0,25Mohm.

 

If a tester can read the plate current during testing and the current of each section is the same or close then you have balanced sections.

 

Otherwise you'll have to plot the plate current versus voltage curves and compare them.

Not many sellers have that kind of equipment.

zenpunk

 

Both of the 6SL7 and the 6AS7/6080 tubes are what we call dual or twin Triodes. Some tube amps will use a single driver tube for both channels. Since the LD Mk6+ is a balanced amp I suspect since I do not have the amp schematics, is that both sides are being used.

7236 or 5998 tubes are compatible but not identical electrically. It should have different bias levels for best results. The same is true with my Quick Silver M135 mono tube amps. Different compatible tubes have different bias levels requirements and I have to manually adjust the bias levels on my Quick Silver amps depending on tube type that I am using. Otherwise the tubes will not operate in its more linear range.   If you moving the same tubes around in the amp leads to different bias reading on the meters then I would suspect that the tube(s) is not well matched depending on how much difference you are indicating on the LD meters. Currently my meters are reading 70mA on both sides with the Bendix tubes.

I have completed my small project. This may not be common knowledge but the Sennheiser HD598, HD558, HD518 headphones are very easy to convert to balanced mode. The replaceable cable already have the four color coded wires in it. No need to take the headphone apart. I did order a replacement cable from Custom Cable in UK but it is not easy for a vendor to get the replacement cables from Sennheiser for the above headphones. I brought the ‘helping hands” with Magnifier from Radio Shack at 50% off. While I have three soldering irons I decided to use a soldering gun for this project. I brought mine from an Auto parts store. The cheapest place to buy a soldering transformer gun.  The wires color codes were not as I expected it to be.  I did know that the Red is the right+ and white is the Left+ but did not know what is the corresponding - wires were. So I use my meter on resistance setting and put on the headphones and connect one meter lead to the red wire and touch the other meter lead to the other two wires possibilities and when I heard static on the right channel I know I got the right pair for the channel.

 Right channel Red+ and blue-

Left   channel White+ and black-

I brought the 4 pin XLR from EBay at $3.12 which includes shipping.

I did this on my HD598 and it was much easier to convert than my Russian Fisher Audio FA-002W

I am just delighted that it worked just fine. For some recordings I turn on the headphone crossfeed with increases spatialization standard on J.River and the realism is breathtaking on the LD Mk6+. That concludes my small project for the moment. There is another bigger thing I am working on and I call it the final realism project and it begins tonight since my equipment came in.  

Is there any reqirement to get the length of the different wires exact for the balanced signals to match up perfectly? I aways wondered about that.

Thanks for that John. More specifically what about the lengths of the 2 wires carrying the 2 push/pull halves of the balanced signal to the headphone. I know the signal travels fast but I would have thought that any slight variance in length would affect the timing of the 2 components of the balanced signal putting it out of phase. The reason I ask is because I wouldn't have the confidence to rewire the connections myself, unless I was certain. (Can't get a new paragraph on edit) Also maybe it's a bit off topic for this thread but which headphones/speakers do you use crossfeed on, and is that on your main amp? I remember a while ago that some headphone amps have crossfeed and I shunned it as being rather artificial sounding possibly. I did think it might improve my Shure SE535 in ear phones which I had at the time because they haven't got much soundstage due to the nature of in ear phones. However when I got my Audio Technicas which are open ear phones I didn't consider crossfeed any more.
Edited by baronbeehive - 11/3/12 at 4:09am

Just to keep this short not to hijack this thread. I only use crossfeed only for headphones not for speakers or Binaural recordings. It depends on the microphones setup. There are many implementations of crossfeed.

I am using a Corda Cross-1 cross feed with one of my setups.

I really enjoy my Sennheiser HD598 transformation in using a 4 pin XLR jack. Before I tried the single ended output connection with the HD598 on the Little Dot Mk6+ and the difference is again remarkable on the XLR ouput. This is unlike of the Woo amp WA22 which sounds the same using the 1/4 jack or the balanced ouput connection.

I just completed my realism project for my Little Dot MkVI6+.

 

I am very happy with my Samsung S23A7000D 3D 23” 120Hz computer monitor using active shutter glasses and Frame Sequential mode for 3D at 60Hz. That is one good reason I got the 120Hz monitor since I am not a gamer person. This 3D Frame Sequential mode gives much better quality than side-by-side mode. I can stand up or below at 30% or at the sides at 45% and still have a very clear 3D presentation. Only tilting my head will "wreck" the 3D effect. It is like watching an IMAX movie in 3D only on a smaller screen. I am just delighted with the quality. I tried four different commercial software 3D players on my computer before I found the one that will work properly with my setup.

 

I bought a few music 3D Blu-Ray from Aix Records, like Mozart from Old City String Quartet and 3D effect is just very powerful and clear. A few nice features of the Aix recording is that they offer a choice of  5.1 Dolby TrueHD from a stage perspective,  5.1 Dolby TrueHD from an audience perspective, 5.1Dobly Digital with both stage and audience perspective and a stereo mix in 96kHZ 24 bit. In addition on the disk you can copy the Enhanced headphone mix in “surround”,  2 channel Stereo MP3 at 320kbs, and a 2 channel stereo FLAC at 96kHaz/24 bit as well. I also have Carman the opera in 3D. I hope that more companies will offer more music in 3D Blu-Ray video.

 

Tip: I heard that Samsung web site will have their “Black Friday” starting on 11/18 and the same monitor I am using will be offered at a deep discount along others.

Built myself a Optimized Morgan Jones Headphone Amp with feedback (earmax pro alike) recently, which is going to be my office rig, and spent the last few days auditing the amp. I dumped many goodies into it and precisely tuning the working voltages, It has dedicated low voltage transformer with LDO for heater (UK made ILP transformer) and Canadian made hammond manufacturing transformer for B+ voltage. So I am pretty sure the amp is as good if not better than the stock earmax pro.

 

While the amp is really musical, it clearly lacks the distinct kick and transparency that the LD MK VI can produce. Being using russian 6N23P til now coz I want to make sure everything works well before I put the exotic tubes. Yet I think the morgan jones amp just lacks in terms of power delivery capability (<100mW@300 ohm), and changing tubes merely chances the sound signature but will not improve in the power delivery side. 

 

even happier with the LD6!