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New Millett Hybrid Maxed Amp - Page 440

post #6586 of 6600

I believe there is a problem with my Millett MOSFET-MAX after 2 years of excellent service.

 

Every now and again, when listening to music/movies, with music/movies paused, or very quiet passages in music/movies, I will hear a buzzing or "bzzzzzt" sound on the left.  Today, I heard it again while watching a movie on my PC while the movie was paused.  I shutdown the movie to investigate as follows:

 

Specs & Equipment:

- HifiMan HE-400s

- Power supply @ 27 V

- Tube Bias at 14.0 - 14.1 V

- Mosfet Bias at 275mV

- Tubes 12AE6 (matched from Beezar)

 

Boutiques:

- CA2L/R Elna RFS Silmic II 1000uF, 35V (Handmade Electronics)

- CA7L/R Elna RFS Silmic II 1000uF, 35V (Handmade Electronics)

- CA8L/R VitQ 0.22uF 100V PIO Cap (Beezar)

 

Debug steps so far:

 

1.  I turned up the volume from min to max - I hear a buzzing from the left earcup between 2 o'clock and 4 o'clock.  Buzzing gets louder progressing from 2 to 4 o'clock then cuts out after 4 o'clock.  No buzzing from right earcup.

 

Is it a bad driver in left earcup?

2.  I swapped cable connections on the bottom of the HE-400 earcup terminals between left and right earcups.  Now I heard buzzing on right earcup.  No buzzing on left earcup.

 

Is it a bad headphone cable?

3.  I plugged in a pair of Grado SR-60 and some Shure IEMs and I hear buzzing from both only on left side.  No buzzing on right.

 

Is it a bad tube?

4.  I swapped left and right tubes.  Buzzing was still only in left earcup.  

 

Is it a bad 1/4" Neutrik jack?

5.  I plugged all headphones into the 1/8" jack and heard buzzing only in left earcup between 2 o'clock and 4 o'clock.

 

Is it a bad integrated grubDAC?

6.  Buzzing still audible with grubDAC USB disconnected and only in left earcup..

7.  Switched over to RCA inputs and buzzing still audible only in left earcup.  Nothing plugged into RCA ports.

 

 

Can someone help with further recommendations for debug to root cause this problem?  Or, better yet, does someone know what the problem is due to past experience?  :-D

post #6587 of 6600

Well, until I got to #4., I would've instantly said, "bad tube."  However, you have admirably performed the steps necessary to remove the most likely suspects.

 

I hesitate to mention this, because in my time on Head-Fi I have not read of this happening.  However, relays are known to go bad after extended use and age.  Very often, if they are in the signal stage, a bad set of relay contacts can cause an awful buzz, depending on which channel's relay contacts go bad, first.

 

If you read a lot of threads in AudioKarma's vintage sections, etc., you might see where this happens with some regularity with equipment using relays that are vintage.  I have a couple of Sony cassette decks where the relays are known to cause problems.  There is a huge, intermittent buzz that occurs - perhaps exaggerated by the huge amplification that takes place in a tape deck among signal, recording, and playback.  Bottom line, it's the relays and it is a known and documented condition.

 

Again, I have not heard of this happening to any headphone relay-delay circuit on Head-Fi, but if you can maybe place some alligator clips leading to your headphone connections at the right place, you might confirm whether this is the case.  An ideal location would be on the right lead of RB14L before it feeds into RA4L and the relay, and the bottom lead of RB14R, before it feeds into RA4R.

 

If the buzz still exists after checking your headphone response from these points (maybe you should use a cheap pair of headphones/earbuds), then it has to be a bad transistor in the buffer, most likely.  If the signal is clear when connecting to those points, then replace the relay.

 

Let us know what you find ...

post #6588 of 6600

Thanks, Tom, for the excellent (as usual) advice.  I have not yet given it a try but I will this week and will follow-up with my findings.

 

On another note, I am having a case of upgrade-itis and I was wondering if my Millet MOSFET-MAX amp is capable of driving the Hifiman HE-500 headphones to their full potential?  I have seen a posting on 6moons that claims these headphone require 1W of power.

 

Has anyone paired the HE-500 with the Millett MOSFET-MAX and if so, what are your impressions?

post #6589 of 6600
Quote:
Originally Posted by ModMax View Post
 

Thanks, Tom, for the excellent (as usual) advice.  I have not yet given it a try but I will this week and will follow-up with my findings.

 

On another note, I am having a case of upgrade-itis and I was wondering if my Millet MOSFET-MAX amp is capable of driving the Hifiman HE-500 headphones to their full potential?  I have seen a posting on 6moons that claims these headphone require 1W of power.

 

Has anyone paired the HE-500 with the Millett MOSFET-MAX and if so, what are your impressions?


Just crank up the bias if there's any doubt - it won't be an issue.  I've had it up to 150ma bias before - it can go higher than that if you can stand to run it hot.  The existing bias recommendation (125ma max) is very conservative.

 

I've successfully driven modded vintage Fostex's and modded Yamaha's that were very, very inefficient - with a MOSFET-MAX.  The only thing you need to be concerned about are the center heat sinks.  One of those cheapo Extech meters with a thermocouple can help you check this.  You can even mod it to use a 30V power supply, depending on the caps you used.  Granted, that's quite a mod because it would require a transformer, but still ... the MOSFETs can take it.

post #6590 of 6600

I have an integrated grubDAC within my MOSFET MAX enclosure with front panel switch to select between RCAs and grubDAC.

 

I want to add output RCAs to the grubDAC so that I can use the grubDAC with other amps.  Can I just connect a couple RCAs to the grubDAC outputs which are also connected to the MOSFET MAX PCB, or do I need to put another switch in place to select between grubDAC driving MAX PCB vs RCAs?

post #6591 of 6600

Wiring them in parallel is just fine.

post #6592 of 6600

HELLPP!  My beloved Mosfet Max is misbehaving.  One channel cut out, so I went looking for the problem.  (It turns out the problem had nothing to do with the amp!)  First I rechecked all the biases, which I hadn't done since I built it a couple of years ago.  They were all pretty high, so I reset the tube bias and then started reducing the mosfet bias in one channel and the whole thing went dark.  (At this point I noticed that my cat had chewed through the headphone cord.  Anyone want a free cat?)  Well, I figured the mosfets were fried because of overbiasing, and that a blown fuse made it go dark.  I replaced the mosfets, turned the biasing pots all the way down, and turned it on again - and blew another fuse.  The V+ was OK, still the same 27V.  Any ideas?  What should I look for?

post #6593 of 6600
Quote:
Originally Posted by schubert View Post
 

HELLPP!  My beloved Mosfet Max is misbehaving.  One channel cut out, so I went looking for the problem.  (It turns out the problem had nothing to do with the amp!)  First I rechecked all the biases, which I hadn't done since I built it a couple of years ago.  They were all pretty high, so I reset the tube bias and then started reducing the mosfet bias in one channel and the whole thing went dark.  (At this point I noticed that my cat had chewed through the headphone cord.  Anyone want a free cat?)  Well, I figured the mosfets were fried because of overbiasing, and that a blown fuse made it go dark.  I replaced the mosfets, turned the biasing pots all the way down, and turned it on again - and blew another fuse.  The V+ was OK, still the same 27V.  Any ideas?  What should I look for?


1. Check the resistance in the biasing pots with the power turned off.  Ensure that they're being turned in the right direction.  A fully "closed" buffer trimmer on the MOSFET-MAX should measure 1K ohm with your DMM.  If it's a lot less, you may have over-biased the buffer.

2. Check and re-check those mosfets.  Did you switch one from the other?  Are they mounted with the proper shoulder washer?

 

The 9Z34's go on the inside heat sinks, the Z24's go on the outside heat sinks.  All the mosfets should be on the left side when facing the PCB from the volume pot/headphone jack.

post #6594 of 6600

There's no question that I overbiased - I had it set right at first, but the bias drifted significantly with time.  The mosfets are in correctly, for it worked ok for quite a while before this happened;  and they're shoulder-washered too.  Question - between which two pins on the biasing pots should there be 1000 ohms when "closed"?  I couldn't tell from the schematic.  I just turned them clockwise as it says in the guide.

 

Anyhow, I think I'm on my way now - I found that QB1R/L were shorted, so I replaced them and this time the fuse didn't blow.  

post #6595 of 6600
Quote:
Originally Posted by schubert View Post
 

There's no question that I overbiased - I had it set right at first, but the bias drifted significantly with time.  The mosfets are in correctly, for it worked ok for quite a while before this happened;  and they're shoulder-washered too.  Question - between which two pins on the biasing pots should there be 1000 ohms when "closed"?  I couldn't tell from the schematic.  I just turned them clockwise as it says in the guide.

 

Anyhow, I think I'm on my way now - I found that QB1R/L were shorted, so I replaced them and this time the fuse didn't blow.  


Well, for future reference, two of the pins are tied together on the trimmers.  So, you either read zero ohms or something else.  I would try to post a pic, but seriously, it's pretty easy to see the trace connecting two of the pins and as I said - if two of the pins measure zero ohms, then put one of the DMM probes on the unused pin. ;)

 

If QB1R/L are shorted, that can really mess things up.

 

Good luck in fixing it!

post #6596 of 6600

Well, duh - don't I feel stupid - I see what you mean now that I look more closely at the schematic.  Anyway, success - it's all put back together, well biased, and making beautiful music again.

post #6597 of 6600

does anyone here need parts for their millet hybrid builds? i have some leftover parts that someone could use...contact me if you're interested and i can tell you what i have.

post #6598 of 6600
Quote:
Originally Posted by tomb View Post
 


Just crank up the bias if there's any doubt - it won't be an issue.  I've had it up to 150ma bias before - it can go higher than that if you can stand to run it hot.  The existing bias recommendation (125ma max) is very conservative.

 

I've successfully driven modded vintage Fostex's and modded Yamaha's that were very, very inefficient - with a MOSFET-MAX.  The only thing you need to be concerned about are the center heat sinks.  One of those cheapo Extech meters with a thermocouple can help you check this.  You can even mod it to use a 30V power supply, depending on the caps you used.  Granted, that's quite a mod because it would require a transformer, but still ... the MOSFETs can take it.

Looks like the issue I have been having is the 1/4" Neutrik jack failing.  The additional 1/8" jack I have installed sounds nice and clear.

 

Neutrik 1/4" has static.  I think I might have to stick with the 1/8" jack for a while.  


Edited by ModMax - 10/13/14 at 9:41am
post #6599 of 6600
Quote:
Originally Posted by ModMax View Post

Looks like the issue I have been having is the 1/4" Neutrik jack failing.  The additional 1/8" jack I have installed sounds nice and clear.

Neutrik 1/4" has static.  I think I might have to stick with the 1/8" jack for a while.  The 1/4" Neutrik on Mouser is $1.46/each, but they want $8 shipping!  triportsad.gif
Yeah, shipping always gets you. Sorry, but I don't have a spare 1/4" jack in my spare parts bin. Hopefully you'll be able to find one...
post #6600 of 6600

The price plus 1st Class US domestic shipping might be cheaper at Beezar.  In no way do I try to compete with Mouser, though. ;)

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