Quote:

modmax said:

I followed the case templates from the MOSFET-MAX site and the heatsinks are 1.5" and I bought the HiQ Heat Sink Mounting kits from Beezar.com.  Given this info, what would you recommend for a max DB bias?

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What I said above.  There are heat sinks that are larger than the ones the PCB allows - some as large as an entire case or larger.  That's all I meant from that statement - that there's a physical limitation based on the PCB and case design that prevents you from biasing them further.

First listen is not good.
 
MOSFET-MAX setup:

1. V+ = 27 VDC
2. 12FM6 Tubes = ~15 VDC
3. MOSFET Bias = ~225 mV

 
Audio chain:
Foobar 2K (WASAPI: Speakers (USB Audio DAC)) --> grubDAC (internal) --> MOSFET-MAX --> Grado SR60
 
I have both 1/4" and 1/8" jacks on my amp.   With both jacks, there is a humming / static sound and music sounds very tinny and faint.  This is with MOSFET-MAX volume @ 100%.  Also, strange thing, physically touching and moving the L/R/Gnd wires connecting the grubDAC and also the RCA inputs to the MOSFET-MAX changes the humming sound I hear during playback.
 
I grounded the case to the MOSFET-MAX both through the RCA inputs and through the center standoff.  I also connected 2 ground wires from the grubDAC to the single ground input at SW2.  I also connected 2 ground wires, one from each RCA input, to the single ground input at SW1.
 
Anybody have any ideas on what might be the problem? 

Wait just a minute....  okay, I flipped the input select switch and suddenly I hear my music - a lot of it and it sounds pretty damn good.
 
Chain is still Foobar 2K (WASAPI: Speakers (USB Audio DAC)) --> grubDAC (internal) --> MOSFET-MAX --> Grado SR60.
 
I am confused though.  This position of the input select toggle switch on my front panel breaks the connection between IS-1 and IS-2 on the MOSFET-MAX.  I thought breaking this connection would turn on SW1 (which my RCA jacks are connected).  But apparently IS-1 --> IS-2 break turns on SW2.  Does this sound correct?
 
Also, with the front panel input select switch flipped to the position which connects IS-1 to IS-2, which turns on SW1, should I hear any sound coming from my grubDAC (connected to SW2)?  I do hear some sound which as I mentioned in my post above has a humming / static noise in the foreground and very faint sound of my music in the background.  And the humming / static sound is affected by touching and moving the L/R/Gnd wires coming from the RCA inputs and grubDAC L/R/G connections.
 
Should I not have grounded the MOSFET-MAX PCB to the case bottom via center standoff and the rear panel via RCA jacks?
 
Do I need to ground the volume pot to the PCB via the set screw on the top right corner of the volume pot and ground pad near the pot on the PCB?

Quote:

modmax said:

Wait just a minute....  okay, I flipped the input select switch and suddenly I hear my music - a lot of it and it sounds pretty damn good.
 
Chain is still Foobar 2K (WASAPI: Speakers (USB Audio DAC)) --> grubDAC (internal) --> MOSFET-MAX --> Grado SR60.
 
I am confused though.  This position of the input select toggle switch on my front panel breaks the connection between IS-1 and IS-2 on the MOSFET-MAX.  I thought breaking this connection would turn on SW1 (which my RCA jacks are connected).  But apparently IS-1 --> IS-2 break turns on SW2.  Does this sound correct?
 
Also, with the front panel input select switch flipped to the position which connects IS-1 to IS-2, which turns on SW1, should I hear any sound coming from my grubDAC (connected to SW2)?  I do hear some sound which as I mentioned in my post above has a humming / static noise in the foreground and very faint sound of my music in the background.  And the humming / static sound is affected by touching and moving the L/R/Gnd wires coming from the RCA inputs and grubDAC L/R/G connections.
 
Should I not have grounded the MOSFET-MAX PCB to the case bottom via center standoff and the rear panel via RCA jacks?
 
Do I need to ground the volume pot to the PCB via the set screw on the top right corner of the volume pot and ground pad near the pot on the PCB?

Click to expand...

The switch energizes both relays.  However, the input connections to the pot go through the energized poles on SW1, but through the de-energized poles on SW2.  IOW, the input block closest to the right side of the PCB (looking from the front or volume pot) is connected with the relays de-energized or OFF.  The input block on the inside or left (when viewed from the front or volume pot) is connected when the relays are energized or ON.
 
If you have a signal going through the GrubDAC at the same time you are connected to signal through the other input block, yes - you will hear some bleed-through.  The main reason this happens is that the volume travel/gain on the amp is really over 100% at 12 o'clock on the volume pot.  There is not a source nor headphone in existence that will require a volume setting much beyond this.  So, you are really into the >120dB regime if you are turning up the pot enough to hear the music bleed-through from the Grub.  Still, the input relays are for convienience, only - not to fully shut off any chance of active signal between the two input blocks.  So, if you switch from one to the other and then listen for extended periods, I would go ahead and shut off the other music source.  This still gives you the convenience of not having to connect/disconnect RCA cables.. 
 
Note that isolation for the relay contacts is about 95dB at 1MHz and increases exponentially at frequencies below that.  In the audible band, it should be well above 120dB.
 
I'm not sure about hearing differences when you move the wires around.  That sounds like bad connections or at least, bad connections with the ground wires.
 
Yes, you should ground everything you mentioned, but truthfully, you shouldn't notice an ill effect just with the bare PCB - except for the signal ground not making a good connection at the input blocks.  With that, you will definitely notice noise - sometimes very bad noise.
 
The pot ground is simply an irritant issue, IMHO, although to be perfectly correct - it should be grounded.  9 times out of 10, the pot will ground itself through the shaft and the case endplate.  However, if it doesn't make this connection or you don't install the ground wire, then you will get some hum if you touch the volume knob (assuming you use a conductive knob).

Any chance I should expect to hear some ground hum when the input selected is RCA but there is no source connected to the RCA jacks?

Quote:

tomb said:

[...]
 
The pot ground is simply an irritant issue, IMHO, although to be perfectly correct - it should be grounded.  9 times out of 10, the pot will ground itself through the shaft and the case endplate.  However, if it doesn't make this connection or you don't install the ground wire, then you will get some hum if you touch the volume knob (assuming you use a conductive knob).

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I used a large plastic knob on my Starving Student, and every time I touched it the amp hummed. Grounding the shaft did the trick.
 
In tube amps, you can make them hum just by passing your finger close to a wire, without even touching it. It can be really annoying when you try to debug a running amp.

Can someone try an experiment where they set the input to their Millett to RCA without any source connected to the RCA and let me know if they hear any hum in their headphones? And if they do, at what % volume they begin to hear the hum?

Whilst I don't have this amp, in my experience it's very rare for an amp with open RCA jacks to be silent.
 
If you connect a source does the hum reduce in volume?
 
If it does then I'd expect that it's completely normal behaviour. On my SS CKKIII (on which during one normally shorts the audio pads to ground during the setup process) with open RCA I get noticeable noise by 15% on the pot, if I have a decent source connected then it's silent until 75-80%.
 
Should the RCA jacks be grounded to the case on this amp? (As far as I'm aware on most amps the RCA should be isolated with plastic washers from the case)
 
Hope you get these issues sorted,
Chris

Quote:

goobley said:

Whilst I don't have this amp, in my experience it's very rare for an amp with open RCA jacks to be silent.
 
If you connect a source does the hum reduce in volume?
 
If it does then I'd expect that it's completely normal behaviour. On my SS CKKIII (on which during one normally shorts the audio pads to ground during the setup process) with open RCA I get noticeable noise by 15% on the pot, if I have a decent source connected then it's silent until 75-80%.
 
Should the RCA jacks be grounded to the case on this amp? (As far as I'm aware on most amps the RCA should be isolated with plastic washers from the case)
 
Hope you get these issues sorted,
Chris

Click to expand...

My millett, with the RCAs in use is pretty close to silent. Your RCAs should be grounded to the case (well, assuming your case is grounded elsewhere, it doesn't really matter). I use the plastic on the outside, as they are colored, and no plastic on the inside, fyi.

Quote:

goobley said:

Whilst I don't have this amp, in my experience it's very rare for an amp with open RCA jacks to be silent.
 
If you connect a source does the hum reduce in volume?
 
If it does then I'd expect that it's completely normal behaviour. On my SS CKKIII (on which during one normally shorts the audio pads to ground during the setup process) with open RCA I get noticeable noise by 15% on the pot, if I have a decent source connected then it's silent until 75-80%.
 
Should the RCA jacks be grounded to the case on this amp? (As far as I'm aware on most amps the RCA should be isolated with plastic washers from the case)
 
Hope you get these issues sorted,
Chris

Click to expand...

 
Thanks Chris.  I will connect a source to my RCAs tomorrow after I get a cable and see if the hum is reduced.  I don't recall hearing any hum when listening with the grubDAC selected as input.
 
Quote:

scootsit said:

My millett, with the RCAs in use is pretty close to silent. Your RCAs should be grounded to the case (well, assuming your case is grounded elsewhere, it doesn't really matter). I use the plastic on the outside, as they are colored, and no plastic on the inside, fyi.

Click to expand...

 
Thanks.  I did measure the RCA jacks to ground and found continuity for both RCA jacks (outer body) and PCB GND with a value of 0.2 Ohms.  With my 12FM6 tubes in the sockets, I measure 33 Ohms between V+ and GND, and approx. 1 kOhm between RCA L/R and GND.  I am using GND at the center standoff of the PCB.  I also made sure that I have continuity (0.2 Ohms) between the RCA jacks (outer body) and the screw head on the underside of my case that grounds the PCB's center standoff to the enclosure.  So my RCA jacks are grounded to the rear panel which in turn grounds the PCB to the rear panel.  The PCB is also grounded to the enclosure through the center standoff.
 
I also use the color plastic washers on the outside of the rear panel for the RCA and no plastic on the inside.

Quote:

modmax said:

Quote:

goobley said:

Whilst I don't have this amp, in my experience it's very rare for an amp with open RCA jacks to be silent.
 
If you connect a source does the hum reduce in volume?
 
If it does then I'd expect that it's completely normal behaviour. On my SS CKKIII (on which during one normally shorts the audio pads to ground during the setup process) with open RCA I get noticeable noise by 15% on the pot, if I have a decent source connected then it's silent until 75-80%.
 
Should the RCA jacks be grounded to the case on this amp? (As far as I'm aware on most amps the RCA should be isolated with plastic washers from the case)
 
Hope you get these issues sorted,
Chris

Click to expand...

 
Thanks Chris.  I will connect a source to my RCAs tomorrow after I get a cable and see if the hum is reduced.  I don't recall hearing any hum when listening with the grubDAC selected as input.
 
Quote:

scootsit said:

My millett, with the RCAs in use is pretty close to silent. Your RCAs should be grounded to the case (well, assuming your case is grounded elsewhere, it doesn't really matter). I use the plastic on the outside, as they are colored, and no plastic on the inside, fyi.

Click to expand...

 
Thanks.  I did measure the RCA jacks to ground and found continuity for both RCA jacks (outer body) and PCB GND with a value of 0.2 Ohms.  With my 12FM6 tubes in the sockets, I measure 33 Ohms between V+ and GND, and approx. 1 kOhm between RCA L/R and GND.  I am using GND at the center standoff of the PCB.  I also made sure that I have continuity (0.2 Ohms) between the RCA jacks (outer body) and the screw head on the underside of my case that grounds the PCB's center standoff to the enclosure.  So my RCA jacks are grounded to the rear panel which in turn grounds the PCB to the rear panel.  The PCB is also grounded to the enclosure through the center standoff.
 
I also use the color plastic washers on the outside of the rear panel for the RCA and no plastic on the inside.

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That all sounds about right. 0.2r is a tiny bit high, not really high, but perhaps a tad.
I think your build is probably fine, and if you threw a source (even turned off) onto the RCAs, it'd be silent. Just make sure you've scraped off some anodizing wherever you ground your case, because the anodizing is non-conductive. If there is nothing around the RCAs in the holes, then that's a ground. Mine grounds there, and through the standoff, I basically dragged the board in an out a few times to scrape away some of the anodizing under the standoff.

My cheap DMM shows 0.6 Ohms when I touch the leads together. Was this 0.2 Ohms measurement error-corrected? If not, then those 0.2 Ohms were probably just the impedance of the leads and the points of contact.
 
From what I can understand, your enclosure is grounded at two places. This sounds like a ground loop, and could explain the hum you get.

Quote:

kimlaroux said:

My cheap DMM shows 0.6 Ohms when I touch the leads together. Was this 0.2 Ohms measurement error-corrected? If not, then those 0.2 Ohms were probably just the impedance of the leads and the points of contact.
 
From what I can understand, your enclosure is grounded at two places. This sounds like a ground loop, and could explain the hum you get.

Click to expand...

It shouldn't be a problem, the signal isn't using the case as a grounding plane, it's really just a bit of shielding, as far as I know.
 
When you connect a source to the RCAs, the hum will go away, it's no big deal, it happens any time you have RCAs without a source connected. If there is still hum with a source connected, that's different.

Quote:

scootsit said:

 
When you connect a source to the RCAs, the hum will go away, it's no big deal, it happens any time you have RCAs without a source connected. If there is still hum with a source connected, that's different.

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THIS ^

Quote:

kimlaroux said:

My cheap DMM shows 0.6 Ohms when I touch the leads together. Was this 0.2 Ohms measurement error-corrected? If not, then those 0.2 Ohms were probably just the impedance of the leads and the points of contact.
 
From what I can understand, your enclosure is grounded at two places. This sounds like a ground loop, and could explain the hum you get.

Click to expand...

 
My DMM shows 0.1 Ohm with just the leads contacting one another.