[SP Wild]

I'm gonna have to agree with you... with the HD-650 - the details are incredible - somewhat on par with the BCL SS amp - and this amp is a detail freak.  But the sound is not as sterile - far more engaging.  The bass compared to the Cayin is not as prominent but is sooooo tight - as tight as most ss amps!!! (the BCL is a smudge more defined but irrelevantly so). 
 
My HD650 plug is now permanently attached to the Ming-da - the Bryston BDA-1 (very detailed and smooth dac) as the source - a $2000 dac paired to a $300 amp and I am loving it!!! 
 
and I have yet to roll tubes!!!
or upgraded my headphone cable!!!

 

[moodyrn]

Stop it. You are going to make me get another one( especially after recently getting another pair of sextetts). The only reason I got rid of it was because of my HE-5's which are only 25 ohms.

 

[SP Wild]

A mini-review of this amp with the K701 here.

 

[Nada]

HUM? What hum?
 
I rolled in some different NOS tube combinations using lower gain driver tubes.
 
With the 25 ohm Denon D7000 they only hum once the volume knob on the amp goes over 11 o'clock, with the volume at zero on Foobar and the DAC on USB or any other input. If I try really hard I might be able to pick the beginning of the hum  at 10 o'clock - I like to think so anyway but Im probably trying too hard. But I cant listen to music playing at 10 o'clock (even with the volume down by -5dB on Foobar), for more then three seconds without pain and usually listen at 8 o'clock.  So no hum can I hear with music.
 
I wonder if hum is secondary to tubes or dirty mains power?

 

[SP Wild]

 
Quote:

HUM? What hum?
 
I rolled in some different NOS tube combinations using lower gain driver tubes.
 
With the 25 ohm Denon D7000 they only hum once the volume knob on the amp goes over 11 o'clock, with the volume at zero on Foobar and the DAC on USB or any other input. If I try really hard I might be able to pick the beginning of the hum  at 10 o'clock - I like to think so anyway but Im probably trying too hard. But I cant listen to music playing at 10 o'clock (even with the volume down by -5dB on Foobar), for more then three seconds without pain and usually listen at 8 o'clock.  So no hum can I hear with music.
 
I wonder if hum is secondary to tubes or dirty mains power?

 
What tubes are you using to eliminate hum on the D7000?  My system is entirely floated on car batteries and power is removed from dirty mains.  I am only using stock tubes and it hums badly with D7000s - worse on the right channel.  I do not know anything about rolling tubes and I would love to know which tubes will eliminate hum for the D7000.  Thanks.

 

[Tigerwoodkhorns]

Moodyrn,
 
This is a great little amp. 
 
I have your Ming Da for sale with the tubes that you put in it.  Let me know if interested (see the for sale section).
 
I am just using y 2 channel system and not using this amp very much. 

 

[SP Wild]

I wanna hook the LCD2s to all its bloody outputs.

 

[Nada]

 
Quote:

 I do not know anything about rolling tubes and I would love to know which tubes will eliminate hum for the D7000. 
 

I don't know if entirely eliminating hum is possible but like I said on my post at my listening levels I cant hear hum. Im on a 240V grid, with a 240V machine. I wonder if that is the reason. Or maybe its the tubes. I think the stock tubes are average. Skylab is the tube guru. If you go to the start of this thread you can see his recommendations. I think the important thing may be to have  low gain driver tubes in the front but I know nothing. As every tube is different I think its possible to get unlucky and get a maverick noisy tube even if its New Old Stock and recommended. The best advice I can give is to follow the suggestions on this site from experienced users. Tube rolling is fun. Enjoy.
 
 
 

 

[Linyanti]

SP Wild, awesome move floating off the grid and on car batteries. Is it an inverter that gets it back into AC and into the amp? Is the hum different if it is hooked straight to wall power? If it is an inverter, is there a ground pin? If so you could do a ground lift. Either the cheap and not really dangerous way (just clip off the ground pin from an adapter) or the HumX way. (I do not know if they have a 240 volt device).
 
Since it is mostly on the right side, have you swapped the tubes from side to side? Also does it matter if you are using the CD or PC input when only hooking up one source at a time? I found that the PC input hums more than the CD input. The PC headphone input combines the right and left neutrals and that may do something to it.
 
I notice that most people hear the hum at the 11 o'clock position. probably a control pot issue. I rotated it back and forth lots and that also helped. I was tempted to spray some tuner cleaner in it but that could really mess it up.
 
Skylab suggested the EL84 Mullards which I am using and also 12AX7 RCA's. The hum got better (and the sound) with these tubes but what cured it was lifting the ground with the Ebtech HumX. And disconnecting the PC input and only using the CD input.
 
If you are completely floating, have you tried running an ipod on battery into the PC input and not having anything else connected? If you still have a hum then it probably is somewhere in the amp. I took my tubes out and used contact cleaner with Q tips to scrub away any tarnish on the pins. It cleaned up the sound a little but did not change the hum I had been experiencing.
 
Good luck.

 

[SP Wild]

Thanks,  I have auditioned numerous inverters - I stayed away from fast switch mode pure sinewave inverters and ended with a $1100 slow switching pure sinewave inverter.  I assume the ground must be lifted as the whole battery system is fully floating - fibre optic from computer goes into source, so there is no shared ground there either.
 
I have switched the tubes around, but have noticed that the right side EL84 tube does not sit as tight on it's socket as the rest of the tubes - how could I tighten the clamping force of a tube socket?  But rocking that particular valve doesn't alter the hum at zero o'clock which is more audible on the right side, I have no hum issues with 650 or 701 only 27ohm Denons.

 

[Linyanti]

Others know more about tube sockets than I do but the more that I used the amp the tighter the fit got. At first it was real wobbly but the heat must have changed the shape and molded the pins. also I gently straightened some of them. Now it is the opposite. When I take the tubes out I am afraid they are going to break since they seem stuck. I ran my amp for weeks to get it burned in.
 
Your electrical power system sounds nice. I always enjoy tweaking and even if the ground is isolated you still may want to experiment with getting rid of it completely. It does violate electrical code but you could make an outlet/plug combo from parts from the store and then snip off the ground pin on the plug. then plug the power cord into that and then that into your power supply. it would only cost a few dollars and not require modification of your current equipment.
 
As far as the impedance goes I purchased a 75 Ohm and then went to a 300 Ohm (which is better) headphone adapter from awwan on e bay. He is great. 22 dollars and I got it in less than a week from Hong Kong.
 
Is your DAC connected to anything else? Like using the balanced lines for your stereo and the unbalanced outs for the Ming Da? If so then you could try unplugging the other component. I had to unplug my preamp from the Ming Da even though the balanced lines from my preamp out to my stereo amp were performing perfectly.

 

[Nada]

Quote:

                          I have no hum issues with 650 or 701 only 27ohm Denons.

  Ive read some have had success silencing hum in low impedance headphones like your Denon's by using an adaptor to bump up the impedance. Which Denon's do you use?
You might want to check ebay seller awwan who can make up an adaptor to your specific needs for about $25. Check out other posts to get an idea of how well they work:. Cheers.
 

 

[SP Wild]

I am researching NOS tubes and decided I will go for Raytheon or Sylvania NOS 5751 for the 12AX7 (for lower gain and hopefully reduce hum for the D7000).  The best EL84 are EL84M and 7320 - Siemens NOS looks like the make to go for.  For 12au7 (not for mingda but for the Cayin) it looks like CBS/Raytheon 7730 is the best.
 
These tubes are dearer than the actual amp when outfitted with the best tubes.  Am I going overboard with these tubes, or will a better amp warrant the best tubes?  Should I just look at matched non NOS pairs?  Any suggestions appreciated.  Cheers.
 
Also an earlier post in this thread stated that this amp is indeed transformer coupled, not OTL???

 

[Skylab]

I used Mullard EL84's and CV4004's in the Ming-Da for a long time - they cost about the same as the amp did

 
Sure sounded good, though.

 

[BmWr75]

Skylab,
 
Were these Mullards NOS or reissues?