[tomb]

I don't know - I think this still proves that the buck converter is causing the issue.  Just because you could hear no hum through the Pi that you had connected to the buck converter, that doesn't mean it wasn't causing noise - because it wasn't amplified.   Once you connect the buck converter so that it's going through the MAX, you hear it.  "Hum" may be a relative term here, too.  Is it really 60Hz hum or is it something higher like 120Hz ripple or oscillation on some other frequency?  I've had an amp design that was causing a spike at 180Hz.  It sounded for all the world like ripple, but it was actually fast Schottkys oscillating as the rectifier.  Some snubber capacitors around the Schottkys removed the noise.
 
There could be a lot of different types of noise coming from the buck converter.
 
I am not sure your measurements mean anything.  Ground at the wall outlet (your "earth") is not connected to your project, because of the two wire AC output from the walwart.  Output ground on the amp, on the other hand, is referenced to the DC-negative ground plane on the MAX board, which is also not connected to your "earth."

 

[scootsit]

Hey tomb, I'm sorry, I can't seem to find the biasing ranges. I found the ones on diy forums, but I also recall that there was a tweak of increasing the tube biasing to about 15V and the mosfets closer to 285 or 290mV. Am I making this up? I just moved and my amp seems to be biased to the values I'm remembering.

Thanks!

 

[tomb]

Hey tomb, I'm sorry, I can't seem to find the biasing ranges. I found the ones on diy forums, but I also recall that there was a tweak of increasing the tube biasing to about 15V and the mosfets closer to 285 or 290mV. Am I making this up? I just moved and my amp seems to be biased to the values I'm remembering.

Thanks!

 
This page has all of the bias points for a MOSFET-MAX:
http://diyforums.org/MOSFET-MAX/MOSFET-MAXsetup.php
 
It shows that the MOSFETs are biased around 220mV to 275mV.  However, that is a very conservative bias for the MOSFET-MAX.  If you are willing to keep an eye on the heat build-up, then yes - you can try to bias them even higher.  I think an increase to 150ma (330mV) is probably do-able under most circumstances.  Higher than that, you probably need to make sure the heat sinks have good ventilation.  If I remember correctly, there is definitely a difference in sound quality going from 80ma to 125ma.  I'm not sure I noticed much of a change beyond that, but it's been a long time.  Obviously, if the higher bias is not doing anything for you, then you would want to let the amp run cooler.
 
The 15V tube bias I think is limited to the 12FM6 tubes, specifically.  I believe someone discovered a long way back that the tube curves for that tube are more optimum at 15V.

 

[scootsit]

I found that page, but couldn't figure out where I got the 15v bias. I'm using 12FK6 tubes, do I'm not sure why I biased so high. Thanks!

 

[tomb]

I found that page, but couldn't figure out where I got the 15v bias. I'm using 12FK6 tubes, do I'm not sure why I biased so high. Thanks!

 
Yes - for the 12FK6 and 12AE6 tubes, the best bias is 13.5V.  This assumes that you have the power supply output adjusted to 27VDC.  So, that gives you an equal voltage swing up and down around 13.5V.
 
The 12FM6 - as stated - seems to run better at 15V.  Although that gives you unequal voltage swing with a 27VDC power supply setting, there are really very few times one would use that entire swing.  So, it seems to work a bit better overall.  Obviously, if you could set the voltage supply to 30VDC, then it would work even better.  However, years of testing with very high current 24VAC walwarts (up to 2 Amps), 29VDC seems to be the absolute limit before the LM317 loses regulation (and thus, the PS gets noisy).  We settled on the conservative 27VDC setting recommendation to account for worldwide variances in line voltage and walwarts.
 
You might be able to get 28-28.5VDC power supply voltage if you try, but you run the danger of losing LM317 regulation and making the amp noisy.  You can see this yourself by adjusting the PS trimmer.  Simply keep turning the voltage up until it stops increasing.  To be absolutely certain you haven't exceeded the regulation capability of the LM317, dial it back by about 2VDC.  The LM317 needs to burn about 1.5VDC to maintain regulation, so if you dial back 2V to be absolutely safe, it often ends up at 27.5 - 27VDC.

 
The primary basis of the voltage increase from 24VAC to 27-29VDC is through rectification and cap smoothing.  While an over-abundance of current capability will buy you some more voltage, I think it's in the fractions of volts or less.  All that pretty much means you need to implement your own transformer if you want 30VDC on the PCB, or - accept a very slight chance of clipping on the positive side of the voltage swing from 15V to 27VDC.
 
The discussions on the 12FM6 occurred only through forum posts, either in here somewhere or on diyforms.org back with the original revMH Millett Hybrid.  I can't remember who originally discovered the optimum tube curves for the 12FM6, but Pete Millett even mentioned higher biases to reduce tube distortion in his original Audio Xpress article:
"Build A Low-Voltage Tube Hybrid Headphone/Line Amp"
Look at pages 28-29 (pages 9 and 12 of the PDF excerpt) for Pete's discussion on raising the bias to reduce the distortion, at the expense of lower output (less room before clipping).

 

[ericj]

I suck at drilling holes. Good thing my local maker space has an abrasive waterjet system. 
 

 

[tomb]

Wow!  Your pattern matches the tube protectors, too - very cool.

 

[ericj]

  Wow!  Your pattern matches the tube protectors, too - very cool.

 
Thanks. It's all cut out on the waterjet, from 1/16" aluminum. There's a spacer frame that slides into the rails. Did the front and back panels on the waterjet too, with some slight adjustments from the PDF drilling templates since i want a front switch, no external fuse, etc. 
 
I think i spent $22 in cutting time (at $55/hr) doing this. Laser cut polycarbonate or wood laminate would be much, much cheaper and faster. But that makerspace is an hour away from me. 
 
The holes for the trimpots are sized for the Vishay trimpot tweaker, though i have not tested to see if they are as precisely located as the tube holes are. This is of course the original Max board from 10 years ago. I think i was one of the first builders w/ the mosfet output. I need to update the circuit. 
 
I think it actually runs cooler with the slit vents than with the drilled holes. And yeah, I did the center punch thing, and used a drill press, and it still came out looking very amateurish. 
 
I still have another max board. Debating whether to build it out with sanyo power transistors. 

 

[Muamp]

Hi ericj,
 
Quote:

 I suck at drilling holes. Good thing my local maker space has an abrasive waterjet system.

I guess 'maker space' is an organisation in USA. Do they have any outlets in the UK?
To cut aluminium like that is awesome.
 
David.

 

[ericj]

  Hi ericj,
 
Quote:

 I suck at drilling holes. Good thing my local maker space has an abrasive waterjet system.

I guess 'maker space' is an organisation in USA. Do they have any outlets in the UK?
To cut aluminium like that is awesome.
 
David.

 
It's more a movement than an organization. 
 
The basic idea is to have a place - a space - where people can access tools and supplies and share expertise to make physical things, do projects, etc. Most are run as not-for-profit organizations. Some have an emphasis on incubating new businesses that make products or software. 
 
ProVolt is the maker space that is near me, in Provo UT. It was created out of the remains of Zeni Kinetic, which was a failed 3d printer manufacturer. So they have one working, not-very-good 3d printer, a whole lot of 3d printer filament some of which is defective, and a 2-axis abrasive waterjet system that was previously used to cut out the sheet metal parts for the 3d printers they manufactured. There are also a whole lot of hand tools, power tools, a vinyl cutter, and other stuff. 
 
An abrasive waterjet works by mixing an abrasive (garnet sand usually) with extremely high pressure water through a diamond nozzle. They will cut damn near anything but an hour of cutting uses a $20 bag of garnet and a lot of electricity. 
 
Here's a video of ProVolt's OMAX waterjet cutting titanium: 
 

 
Anyway, in Salt Lake City, about an hour from me, there are two maker spaces (Make Salt Lake and 801 Labs) that have laser cutting systems. If i build another Max, I will probably get some bamboo ply and cut a very similar set of panels and rings with one of their systems. 

 

[ericj]

And yeah, there are some makerspaces in the UK - http://www.nesta.org.uk/uk-makerspaces-data

 

[Muamp]

Thanks for the info ericj.
Even though I have a small CNC, I would love to have access to other tools, like the abrassive water jet cutters, also laser cutters, and I am now one of the few remaiming DIYers that still doesn't have a 3D printer.
I will check out if there are any maker-spaces near me.

Thanks,

David.

 

[ericj]

I know a lot of people who have 3d printers and don't use them. 
 
Some of that has got to be because there are a lot of 3d printers that aren't very good. 
 
i've bought 3d printed items that were pretty good. I've also seen a lot of 3d printed items that were pretty rough and barely useful. 
 
The waterjet is the closest thing this makerspace has to an income stream. Every once in a while someone pays for something to be manufactured on it. 

 

[ultraman31]

So, all of sudden, the left channel went blank... When I put the headphone jack into the insert just about 2/3, I hear the left channel, but it doesn't sound right... I'm suspecting it's the female insert....or any other thoughts on what could it be? I know I'm providing barely any info, but just wondering... I'll probably be able to check in about a month or so.

 

[vixr]

  So, all of sudden, the left channel went blank... When I put the headphone jack into the insert just about 2/3, I hear the left channel, but it doesn't sound right... I'm suspecting it's the female insert....or any other thoughts on what could it be? I know I'm providing barely any info, but just wondering... I'll probably be able to check in about a month or so.

some of my tubes are very old... its an easy place to start...swap them in the sockets.