Little Dot MK8SE / MK6 Super Mods (All verified mods are on first page)
Jan 9, 2019 at 11:56 PM Post #3,571 of 4,154
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Hi guys, my first post here at this forum.

I am using my little dot Mk6+ with the ESS SABRE DAC that is built into my LG G7 phone. I added a non-balanced to balanced adapter to my amplifier as shown. Being a noob, I learned that even though my headphones are balanced, when using the RCA inputs, only one side of each amplifier push-pull is used...what a waste!! So I did what the manufacturer should have done.... install the proper converter.
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I am using Texas Instrument's DRV134 precision audio balanced driver chip and powering off the +15v AND -36v rails from the amplifier. The DRV134 is actually powered off +12V and -12V rails provided by the regulators as shown. The chip also offers additional gain so the conventional non-balanced RCA signal is raised to that of balanced, allowing me to keep the gain of my Mk6+ to a minimum. I will be adding a DPDT switch so when I use the balanced input, the signal will not back-feed into the output of the DRV134.
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After 8 hours of use, my power resistors for the regulated +15v and -36v rails were very discolored. So that part of the faulty design of the amp had to be upgraded to handle the additional current draw of the DRV134.

The 4.2K resistor I replaced with a 10 watt 1.5K resistor and increased my zener diode to a 1.5 watt. The -36V rail resistor I replaced with a 1.2K 10 watt resistor and upgraded to a 5 watt -36V zener. Now I have sufficient power for the DRV134 and more stability for the MK6+namplifier's op amps. I have chassis mount 20W versions to mount on the base plate on order to clean things up.


One problem I am encountering regardless of tubes I install.... my right channel drops all the way to ZERO..... still plays...but voltage drop across the .27 ohm resistor for the mA meters verifies my meter is working. I have bias and it makes no difference. This was a factory problem before my modification. Any ideas? Moving tubes around, even trying my Tung Sols.... nothing changes on the right. It hovers between 0-20mA. The left channel is 60-80 mA. The meters themselves are good, so this is something in the circuitry.

On order are all new .22uF capacitors in case one is leaky... my capacitors in my amp appear to be pulls...the are all different lot numbers, even the WIMA housings vary from unit to unit. They do appear to be genuine WIMA, just a mishmash of manufacturing dates.?. Replacing with new and also installing PPS (Polyphenylene Sulfide) capacitors for the 1uF coupling stage.

Also, being that I am not a tube expert, what are the bias voltages supposed to typically be from the Op Amp to get me in the ballpark? I can go from one extreme to the other on the trimmer and current drain does not change, but at some point the audio does drop out. Tube Noob here and I have yet to see left and right meters agree.
 
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Jan 11, 2019 at 7:56 AM Post #3,572 of 4,154
After 8 hours of use, my power resistors for the regulated +15v and -36v rails were very discolored. So that part of the faulty design of the amp had to be upgraded to handle the additional current draw of the DRV134.

The 4.2K resistor I replaced with a 10 watt 1.5K resistor and increased my zener diode to a 1.5 watt. The -36V rail resistor I replaced with a 1.2K 10 watt resistor and upgraded to a 5 watt -36V zener. Now I have sufficient power for the DRV134 and more stability for the MK6+namplifier's op amps. I have chassis mount 20W versions to mount on the base plate on order to clean things up.


One problem I am encountering regardless of tubes I install.... my right channel drops all the way to ZERO..... still plays...but voltage drop across the .27 ohm resistor for the mA meters verifies my meter is working. I have bias and it makes no difference. This was a factory problem before my modification. Any ideas? Moving tubes around, even trying my Tung Sols.... nothing changes on the right. It hovers between 0-20mA. The left channel is 60-80 mA. The meters themselves are good, so this is something in the circuitry.

Hi gavincurtis!

I used to use my ipod as source single ended and it sounded pretty good, but a lot of bass and treble boost!

I'm not familiar with that type of adapter you're using but were those power resistors discoloured before you used the adapter because this is a common problem. Did you smell burning?

The upgraded wattage resistors is what we've done on this thread but why did you replace the 4.2k with a 1.5k?

The burnt resistors in the power stage could be a cause of the bad meter readings on the right channel. Was it like this from new?

Were the burnt resistors just on the right channel or both?

The other meter reading of 60mA would be right for 6AS7G's.

Also when does the audio drop out, after a while or after you adjusting the trimmers?

Sounds to me like a bad connection somewhere on that right channel, has it been soldered recently?
 
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Jan 12, 2019 at 5:07 PM Post #3,573 of 4,154
Hi gavincurtis!

I used to use my ipod as source single ended and it sounded pretty good, but a lot of bass and treble boost!

I'm not familiar with that type of adapter you're using but were those power resistors discoloured before you used the adapter because this is a common problem. Did you smell burning?

The upgraded wattage resistors is what we've done on this thread but why did you replace the 4.2k with a 1.5k?

The burnt resistors in the power stage could be a cause of the bad meter readings on the right channel. Was it like this from new?

Were the burnt resistors just on the right channel or both?

The other meter reading of 60mA would be right for 6AS7G's.

Also when does the audio drop out, after a while or after you adjusting the trimmers?

Sounds to me like a bad connection somewhere on that right channel, has it been soldered recently?




The resistors were discolored before I did any modifications (I also had the bad reading on my mA meter BEFORE my mod as well). The sound is definitely superior with the addition of the unbalanced to balanced converter. Correct me if I am wrong, but if you are using your amp strictly on RCA inputs alone aren't 2 of your 4 output tubes are doing nothing? You have to use the balanced inputs and have balanced wired headphones to take full advantage of your investment? Problem with some RCA non-balanced to XLR balanced coverters out there is they use cheap electrolytic capacitors in the signal path and fail at subsonic frequencies, which is what the Mk6+ is all about being able to do. The DRV134 is a DC coupled device and has no capacitors of any kind in the signal path. Just the power rail capacitors for stability. I tried to attach photos, but I don't understand how that works here.

The 4.2 K resistor was the one that discolored as well as the other one used for negative voltage rail of -36 volts. They used 2 8.4 K in series in my amp. But when you run 3 watts of resistors at 3 watts like the Chinese love to do in their designs.....you get very hot resistor that will not be long for this world. Those resistors are not part of the sound, they are part of the power supply for the op amps that bias the outputs. You need +15V and a -36V. To power my DRV134, I need more current. When using the stock resistors and zeners, the voltage dropped down to 5V and -3V.... so I upgraded the zener diodes to higher power units and lower value resistors to increase the current delivered to the more capable zeners. That way the voltages are stable at +15V and -36V, even with the DRV134 at full load in the system. Also the heat of the resistors is away from those adjacent electrolytics.

No other resistors in my amplifier are discolored, just the ones near the zener diodes in the power supply section. The big 330 ohm units are perfect as are the 300 ohm. The solder is not terrible, but the WIMA capacitors are all from different years and seem as if they were used or pulled from something else as well as a couple being deformed where the leads enter (too hot of soldering). Just in the slight even that one is compromised, I have brand new WIMA replacements as well as 1.0uF PPS audio coupling capacitors.

My left channel audio drops out after the amplifier has about 30 minutes of use. The mA meter doesn't change on that side...just no audio. The right channel seems to sound okay, albeit a little thin and meter shows 0-30 mA and moves around over time...often hovering around 10mA. Swapping tubes does absolutely nothing! The needle will be completely at zero at times, yet the channel works.... the meter itself and its shunt resistor resistance/voltage drop did collaborate they are working correctly and the problem is in the circuit.

What is the typical bias voltage for these output tubes? Just a ballpark... and can I damage the output tube if I wander too far off the correct bias voltage during adjustment?

Anybody know what the chip is in the center of the circuit board? Output protection? The schematic provided at this forum was a life saver, but is there any additional of the power supply, power up delay and protection circuits?

Thanks!
 
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Jan 13, 2019 at 2:36 AM Post #3,574 of 4,154
nothing changes on the right. It hovers between 0-20mA. The left channel is 60-80 mA. The meters themselves are good, so this is something in the circuitry.
Yes, the meters corresponding to the bias setting only.

. Replacing with new and also installing PPS (Polyphenylene Sulfide) capacitors for the 1uF coupling stage
Not sure where on the board yiu see 1uf unless its PSU stage.

can go from one extreme to the other on the trimmer and current drain does not change, but at some point the audio does drop out. Tube Noob here and I have yet to see left and right meters agree.
There is first mistake.
The trimmers by the opamps are NOT for any bias settings.
They are only for DC servo, so you won't get any small DC offset at the output of your headphones, because since this is a "OTL" (output transformerless) output, the tubes ouput going direct to your headphones without any caps or Transformers in between.

I am getting away from the point that you messsed up and must now adjust the trimpots (trimmers) to get zero DC offset between pos &neg output of each channel..
Get DC meter and adjust. Both trimmers can be elevated or lowered equally to cause the corresponding meter for that channel to move up or down slightly by a few points.
A final test for zero dc is having your headphones on and listening for any tiny click/pop sound as you insert into the balanced socket (not the single-ended as that by default always pops in any unit).


The resistors were discolored before I did any modifications (I also had the bad reading on my mA meter BEFORE my mod as well).
The resistors are usually cheap made china parts so its normal to see a burn which also meansthe resistance probably not the same anymore, but have not to do with your meters anyways.
If They are by the diodes, they are in series and taking current because of opposite what your thinking, not enough of a load on them(!)
Due to either the low bias of one side, or if you ever turned the unit in without the power tubes installed.

When there isn't enough load, the diodes acting as zener try to Control the higher voltage drop and so they take on more current which burn resistors.

Correct me if I am wrong, but if you are using your amp strictly on RCA inputs alone aren't 2 of your 4 output tubes are doing nothing?
Sorry wrong.
You basically went thru alot of trouble to make that front end balanced, but it doesn't matter to the driver stage.

It is ONLY at the single ended output jack that you are getting half the tubes output.
Yet it IS true that the balanced inputs give a different stronger better sound anyways.


The 4.2 K resistor was the one that discolored as well as the other one used for negative voltage rail of -36 volts. They used 2 8.4 K in series in my amp.
You mean they were side by side so in parallel, correct?
Cheap way for them to solve the wattage issue without using larger sized resistors that take up space.

No other resistors in my amplifier are discolored, just the ones near the zener diodes in the power supply section.
Yeah just replace them.

My left channel audio drops out after the amplifier has about 30 minutes of use
It is probably because of your opamp DC servo circuit.
Check the setting of the trimpots and then check sockets of the opamps, as sometimes they loose connection.
Also check your tiny dip switch gain setting by flipping up and down a few times to clear that.


The right channel seems to sound okay, albeit a little thin and meter shows 0-30 mA and moves around over time...often hovering around 10mA. Swapping tubes does absolutely nothing! The needle will be completely at zero at times, yet the channel works..
The issue here is your meter is telling you your bias is off.

The main culprit are the anode and cathode resistors, as they set the bias in this type circuit.
You need to replace those anode resistors especially as when they burn the value goes up, decreasing the current, so decreasing your bias, so posing less load on the PSU, so causing the zeners to overwork, to cause the resistors there to burn, to give more heat and burn to then give you stress and grey hair, to you not getting sleep, to affecting your personal life, to affecting those around you, wich may add to global warming, which then I get as a snow storm approaching next weekend...
:)


Anybody know what the chip is in the center of the circuit board? Output protection? The schematic provided at this forum was a life saver, but is there any additional of the power supply, power up delay and protection circuits?
No chip in middle.
There is a relay in middle to cut sound, and another relay by bottombot beginning of PSU stage to delay power to ouput stage when first turned on.
 
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Jan 13, 2019 at 9:08 PM Post #3,575 of 4,154
Thank you for all the help. No problem for me to readjust the zero set point. I have all the tools and knowledge for that. I have built and designed solid state amplifiers of many kinds, but tube theory I am not overly familiar with. Now that I know what the op amps are doing (I figured either bias or voltage offset), I will proceed with setting them using proper equipment that I have and zero out the voltage offset that I have obviously messed up.

I would have never gone into the thing, but it stopped working after 8 hours of use. Upon opening my amplifier, all of the WIMA caps were old pulled units....plier marks on them and all different lot numbers/date codes. Even the colors of red varied from deep red to light pink and sizes varied...my amp was apparently built from a used parts pile.

I have replaced them all with brand new WIMA now. The 1.0uF coupling capacitors were 4 variants of wima (all 4 were different sizes) I replaced with 1.0uF KEMET audio PPS now. I upgraded all zeners to 5 watt units to keep their temperatures in the long life zone. Chinese engineers don't care about reliability and always run components way too hot.


"Sorry wrong.
You basically went thru alot of trouble to make that front end balanced, but it doesn't matter to the driver stage.

It is ONLY at the single ended output jack that you are getting half the tubes output.
Yet it IS true that the balanced inputs give a different stronger better sound anyways."


Thank you for that knowledge.

So does the front end tube also double as a phase inverter when using single ended RCAs as the inputs? In the schematic, the RCA only goes to one "input" side of the front end tube which goes to the appropriate output tube. Only the balanced XLR input uses both inputs of the front end tube. So when using only one input of the front end tube, does the other half of the front end tube take on the function as a phase inverter for the 2nd output tube? Otherwise, that 2nd output tube just sits idle wouldn't it? When mine was working and using the RCA inputs, I noted no obvious difference in output between my headphones with their balanced cable (4 pin XLR) and same headphones using the 1/4" unbalanced plug harness.

With my DRV134 modification, there is a significant improvement now between the 4 pin XLR harnes and the weaker sounding 1/4" unbalanced.....as I would expect.

It would have been nice if a $1000 retail price boutique headphone amplifier that people would potentially modify/service came with a real owners manual, proper schematics and theory of operation! Instead, I received a folded pamphlet of useless and upside down printed nonsense. VERY disappointed with my Mk6+ so far and regret selling my Mk4.

"No chip in middle.
There is a relay in middle to cut sound, and another relay by bottombot beginning of PSU stage to delay power to ouput stage when first turned on"

On my amplifier there is indeed a chip in the center of the circuit board. It is a 9 pin SIP device with 3 capacitors around it with its audio cut out relay. I suspect that is the DC offset protection perhaps....?

That reminds me, does your Mk6+ have fuses installed? I have jumper wires....no fuses in mine... completely bypassed with those 2 jumper wires.

I would like to replace them with fuses as I have some pretty expensive headphones. What are the fuses supposed to be? 100mA? 150mA? something other? Any reason why the "factory" would bypass the fuse protection? I got a lemon it seems, unless they are all like this.

One final question, are there any rules regarding powering up without tubes? I am now going to check my bias resistors as I still have the right channel bias current averaging 15mA or so... it goes all over the place actually. Those resistors of interest are the higher power 330 ohm and 300 ohm ones correct?

Thank you again all for your help, time and knowledge. :)
 
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Jan 16, 2019 at 8:33 AM Post #3,576 of 4,154
I would have never gone into the thing, but it stopped working after 8 hours of use. Upon opening my amplifier, all of the WIMA caps were old pulled units....plier marks on them and all different lot numbers/date codes. Even the colors of red varied from deep red to light pink and sizes varied...my amp was apparently built from a used parts pile.

I have replaced them all with brand new WIMA now. The 1.0uF coupling capacitors were 4 variants of wima (all 4 were different sizes) I replaced with 1.0uF KEMET audio PPS now. I upgraded all zeners to 5 watt units to keep their temperatures in the long life zone. Chinese engineers don't care about reliability and always run components way too hot.



:)

Sorry to hear about your quality control problems. None of us to my knowledge have had any complaints about the amp build quality, in fact my amp, an early version had high quality Dale resistors installed. Where did you get your amp?

The amp operates a White Cathode Follower circuit, check out page 1 to see how that works.

On my amplifier there is indeed a chip in the center of the circuit board. It is a 9 pin SIP device with 3 capacitors around it with its audio cut out relay. I suspect that is the DC offset protection perhaps....?

:)

I think that is the headphone protection circuit.

That reminds me, does your Mk6+ have fuses installed? I have jumper wires....no fuses in mine... completely bypassed with those 2 jumper wires.

I would like to replace them with fuses as I have some pretty expensive headphones. What are the fuses supposed to be? 100mA? 150mA? something other? Any reason why the "factory" would bypass the fuse protection? I got a lemon it seems, unless they are all like this.

:)

Yes someone on the thread had jumper wires also. The fuses are 2 fuses in the middle are 0.5amp.

One final question, are there any rules regarding powering up without tubes? I am now going to check my bias resistors as I still have the right channel bias current averaging 15mA or so... it goes all over the place actually. Those resistors of interest are the higher power 330 ohm and 300 ohm ones correct?

:)

Not really. But you definitely have a problem with the circuit that's showing the 15mA, possibly down to the resistors you mention, especially the 330 ohm ones. I would do a thorough check on the connections in that circuit.

Hope that helps!

Edit: Voltages at the anode 330R's should be about 101v and 76v at either side of the resistor, and at the cathode 330R's should be about -104v and -80v.
 
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Jan 24, 2019 at 10:17 AM Post #3,577 of 4,154
Even the colors of red varied from deep red to light pink and sizes varied...my amp was apparently built from a used parts pile.
Those caps are not critical to be same as they there to prevent any high freq oscliation.

So does the front end tube also double as a phase inverter when using single ended RCAs as the inputs?
Sadly no, thats why you noticed a louder better sound with balanced input.
Probably a decision to leave out in order to compete in price with market.

On my amplifier there is indeed a chip in the center of the circuit board. It is a 9 pin SIP device with 3 capacitors around it with its audio cut out relay. I suspect that is the DC offset protection perhaps....?
Yes I forgot about this as well.


That reminds me, does your Mk6+ have fuses installed? I have jumper wires....no fuses in mine... completely bypassed with those 2 jumper wires.
I didn't remember see this on mine but do remember buying three replacement fuses .
the main fuse on mine is inside the AC socket.

Yes someone on the thread had jumper wires also. The fuses are 2 fuses in the middle are 0.5amp.
I need to see pics its been so long I haven't looked inside mine lol.
 
Jan 29, 2019 at 4:52 PM Post #3,579 of 4,154
Hi Guys,
I changed out my fuses 2 x 500mA and 1 x 3A (3.15A actually) to SR 20 Quantum fuses and its really very good. I have a Simaudio HAD 430 as well and it too uses this fuse. Immaculate, I love what they do. Thanks for reading.

I've got cheapo fuses lol!

Hey, Sage Encore how are you getting on with your LD, are you still using those Russian Melz tubes and what do you think about them? Are they the cheap ones?
 
Jan 30, 2019 at 5:42 AM Post #3,580 of 4,154
I've got cheapo fuses lol!

Hey, Sage Encore how are you getting on with your LD, are you still using those Russian Melz tubes and what do you think about them? Are they the cheap ones?
Hi sir,
I love my LD, sometimes I do think of getting something "Branded", I turn her on and wonder why the hell I would want to, she is so lovely, especially with the SR 20 Quantum fuses now after the upgrading I did with the Caps and resistors. I am rocking some Melz 6H8C and 6H9C tubes now. Looking to get the hole version of the 6H9C Metal base (Melz factory). These Russian babes play in a league of their own. My Ken-rad and Sylvanias are collecting dust now, that's how good they are. Exceptional value for money. They are not exactly cheap, these MELZ ones, the Hole version goes for about USD200 a pair and the metal base ones go for USD60-80 a pair made in the 50s". Foton ones are the cheapo ones, but they are not bad, not bad at all.

Get the MELZ factory ones, I tried the Foton ones as well and they are not bad, but the MELZ metal base is really very good, best 6SL7 I have heard thus far. You should give it a try, and ya, change those fuses out, u will be mightliy impressed I am sure.
 
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Jan 30, 2019 at 11:54 AM Post #3,581 of 4,154
Wow i totally overlooked these Melz tubes!
The metal base ones from the 1950s just has to be very good (!)
Reminds me of the rare Sylvania metal base(!)

I also see there is a 6H7C tube type which has common cathode.
That is basically what this amp does on the circuit board it combines the cathode, so that tube can be used too.
:)

Dam i need to try those tube type next as I have currently settled on some tungsol...
That will be my last upgrade.
I must replace my cathode caps and cathode folllower caps to have last weakness parts replaced.
Hopefully this will elevated it further.
This amp will compliment my current amp, (made by SonicTrance) the "Oblivion" (aa hybrid) amp, which is a highly accurate & almost colorless amp.

Remember our MK6 & MK8 amp design responds & relies tremendously on upgrades, more so than other amps I upgraded, but climbs in performance the most I have seen.
The other amps I worked on also benefited from upgrades, as they also use traditional designs.
I upgraded a stax amp, a tube speaker amp, an EC Aficionado, the small APPJ PA1502A, and I forget what else lol.
Currently troubleshooting a Cary 300b amp, but after that will come back to this amp.
I think this thread still needs updating on last mods done, but the thread starter (Ridge78) is MIA.
 
Jan 30, 2019 at 4:30 PM Post #3,582 of 4,154
Wow i totally overlooked these Melz tubes!
The metal base ones from the 1950s just has to be very good (!)
Reminds me of the rare Sylvania metal base(!)

I also see there is a 6H7C tube type which has common cathode.
That is basically what this amp does on the circuit board it combines the cathode, so that tube can be used too.
:)

Yes, somebody mentioned these way back on the original LD thread, ozaudio I think, whatever happened to him!

Pretty sure these are the type mentioned here: http://www.audiotubes.com/6sn7.htm

I also came across a thread discussing them and there are a few variations some of which may or may not be genuine, I can't elaborate anymore on that other than they have the valve type, a number, 1578, on the glass envelope. The participants were sure that some were genuine and others were not.

Unfortunately these are that other type of Russia tube…. expensive!


Dam i need to try those tube type next as I have currently settled on some tungsol...
That will be my last upgrade.

Those Tungsols are way too expensive for me, I still like my GE's, very transparent, maybe not the last word though. This amp as it is now seems to make any tube sound good

I must replace my cathode caps and cathode folllower caps to have last weakness parts replaced.
Hopefully this will elevated it further.
This amp will compliment my current amp, (made by SonicTrance) the "Oblivion" (aa hybrid) amp, which is a highly accurate & almost colorless amp.

It will believe me, it doesn't really have a weakness anymore, it's lively, transparent, great frequency response, a joy to listen to basically

I think you might need to take a look inside your LD again if you can't remember if it has fuses or not LOL, so go on and dust those cobwebs off. You just might be putting off opening it up afraid what you might see in there….sphagetti wire, hot glue... loads of the stuff, not a pretty sight haha!!
:).

Remember our MK6 & MK8 amp design responds & relies tremendously on upgrades, more so than other amps I upgraded, but climbs in performance the most I have seen.
The other amps I worked on also benefited from upgrades, as they also use traditional designs.
I upgraded a stax amp, a tube speaker amp, an EC Aficionado, the small APPJ PA1502A, and I forget what else lol.
Currently troubleshooting a Cary 300b amp, but after that will come back to this amp.
I think this thread still needs updating on last mods done, but the thread starter (Ridge78) is MIA.

Yep, the recent mods have pushed the envelope tremendously to the point where I no longer am interested if it sounds good.... I know it does so I think we should get those mods posted sometime,

Edit: This is the thread I was talking about where they discuss whether the Melz 1578's are genuine: https://www.diyaudio.com/forums/tub...entic-russian-military-1578-vacuum-tubes.html

Incidently I just came across a quote from skylab where he says that the Melz 1578's are "massively overrated."

…. and if you're interested in the Melz tubes there's also this thread which goes into this subject: https://www.audioasylum.com/cgi/t.mpl?f=tubes&m=212173 …. and a lot of mass hysteria over who is right and who is wrong... obviously they all think they're right.... the usual stuff!
 
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Feb 1, 2019 at 10:55 AM Post #3,583 of 4,154
Thanks for links!
if you can't remember if it has fuses or not LOL,
Oh, I remember because I remember paying alot for the replacement fuses!
20150416_211516-1-1-1.jpg
Lol
 
Feb 2, 2019 at 7:27 AM Post #3,585 of 4,154
Not guilty...!
 

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