Millett "Starving Student" hybrid amp
Feb 17, 2015 at 8:17 PM Post #6,916 of 7,269

tomb

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Hello again. I've been thinking for some time now and looking for answers in this thread, but i can't find what I'm looking for.
My question is if there is any chance that this amp(12au7 build) will fry any low impedance headphones? The only thing I've seen is that you can protect your
headphones during the startup sequence by adding diodes. Is there any good tutorial on how to do this since I didn't really understand how it's done.

rolleyes.gif

Thanks in advance!


Buy a Millett Hybrid MiniMAX.
 
Seriously, I don't often make a statement like that because I sell the MiniMAX.  I also sell the current kit offering of the Starving Student.  However, it's a valid statement.  If you are really concerned and don't trust yourself to wait until the amp is warmed up before plugging in your phones, then you need to build something else that has a true, delay-relay circuit (or transformer output) that will protect headphones under most circumstances.
 
Search this thread.  There have been numerous discussions about relay-delays - both early on in this thread and relatively recently.  As with everything else, it will take more effort and money.  At some point, you enter into diminishing returns about whether it's a better decision to upgrade the Starving Student, or build something else that was designed to include more features at the outset.
 
Seriously, the Starving Student offers fantastic performance at an extremely low cost (Thank you, Pete Millett!).  However, there are certain limited expectations that you should temper as a result of that low cost.
 
EDIT: Sorry - it seems like Groundhog Day for some of these questions and I am perhaps over-reacting a bit.
 
Feb 18, 2015 at 1:57 PM Post #6,917 of 7,269

KimLaroux

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Hello again. I've been thinking for some time now and looking for answers in this thread, but i can't find what I'm looking for.
My question is if there is any chance that this amp(12au7 build) will fry any low impedance headphones? The only thing I've seen is that you can protect your
headphones during the startup sequence by adding diodes. Is there any good tutorial on how to do this since I didn't really understand how it's done.

rolleyes.gif

Thanks in advance!

 
It's simple, and to be done with any tube amp: Only plug the headphones in once the amplifier has warmed up and unplug them before powering it down.
 
Feb 18, 2015 at 7:34 PM Post #6,919 of 7,269

tomb

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Well, not any tube amp ...  a true, transformer-coupled amplifier doesn't have capacitors that are needed to block the DC.  The transformers do that already.  You can leave a pair of headphones plugged in all the time.
 
Feb 27, 2015 at 4:19 PM Post #6,920 of 7,269

fuens

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Hello mates.
It is a technical question, about Dsavitsk 12au7 additions highlighted.
If I install a toggle switch (two ways) in the input signal, switching R16 & R17 with values 10K and 50k (using 50k potentiometer)
Does this would work as high and low gain?
Or signal become unstable? :O
 
Thanks on advance.
 
Cheers
 
Toni
 
Feb 27, 2015 at 10:27 PM Post #6,921 of 7,269

tomb

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  Hello mates.
It is a technical question, about Dsavitsk 12au7 additions highlighted.
If I install a toggle switch (two ways) in the input signal, switching R16 & R17 with values 10K and 50k (using 50k potentiometer)
Does this would work as high and low gain?
Or signal become unstable? :O
 
Thanks on advance.
 
Cheers
 
Toni

 
Should work fine.  The Starving Student that was sold long ago at Whiplash Audio did almost exactly what you describe.  I haven't had personal experience with the 12AU7 version, so maybe the gains are different.  If it was the 19J6, you should be switching 50K and 100K, not 10 and 50.
 
Feb 28, 2015 at 3:07 AM Post #6,922 of 7,269

slimzim

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Hello mates.

It is a technical question, about Dsavitsk 12au7 [COLOR=636466]additions highlighted.[/COLOR]

If I install a toggle switch (two ways) in the input signal, switching R16 & R17 with values 10K and 50k (using 50k potentiometer)

Does this would work as high and low gain?

Or signal become unstable? :O

Thanks on advance.

Cheers

Toni


I completed my build a week ago, and I used 150k ohm resistors and I also used a 50k ohm stereo pot. I also read that capacitor C7 & C8 increases gain so I did not use them. The gain is more than enough this way. Turning the volume up more than 1/3 makes it way to loud for me.
The point is you may want resistors with higher value than 50k. I hope this is to any help! :D
 
Feb 28, 2015 at 3:14 AM Post #6,924 of 7,269

fuens

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I completed my build a week ago, and I used 150k ohm resistors and I also used a 50k ohm stereo pot. I also read that capacitor C7 & C8 increases gain so I did not use them. The gain is more than enough this way. Turning the volume up more than 1/3 makes it way to loud for me.
The point is you may want resistors with higher value than 50k. I hope this is to any help!
biggrin.gif

Hello Slimzim
The matter is adjust the gain for high an low impedance headphones.
What kind of headphones do you use?
 
Feb 28, 2015 at 3:19 AM Post #6,925 of 7,269

fuens

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Now, I have not on my home my Starving Student. Is in my work. (I'm convalescent of work accident)
With a low impedance 32ohm (I remember) I have a little hum.
 
Feb 28, 2015 at 7:06 AM Post #6,926 of 7,269

slimzim

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  Hello Slimzim
The matter is adjust the gain for high an low impedance headphones.
What kind of headphones do you use?

I'm using 32 Ohms at the moment. Audio Technica M50. I have no hum at all, I used a full alu case tough. I think I'll do a remake on the 12au7 schematic for my build. When it's complete i could PM it to you 
wink.gif

 
Mar 6, 2015 at 3:39 PM Post #6,927 of 7,269

fallen angel

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Years later, it's time I get to building my amps. I'm going to gift one to a bud that is lending me some great equipment. I'll be running a pair of 19j6 tubes that I've been hording for the last 5 or so years :wink:. Just need to find someone with a table saw and perhaps a router for me to cut up the wood I've got set aside.
 
Before I get started, I wanted to ask about grounding. I will be using a central grounding plate (copper psb) and running that to the power adaptor by wire. I believe I've seen places where some grounds should be isolated and run separately to the power jack?
 
Mar 14, 2015 at 2:01 PM Post #6,928 of 7,269

tomb

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Nice to hear from you again, FA!
 
 
On a separate subject, I have a customer who may have experienced great difficulty with his SSMH with regard to the LED resistor.  I'm not certain yet whether that is the cause, but additional warnings come at no price whatsoever.  So, for those contemplating building the SSMH kit, please note the following about the LED resistor and standoff provided in the kit:
 
Here is the standoff assembly. This will be provided in the kit with several washers. At least one washer is required for spacing along with the standoff - perhaps two. The only caveat to this you'll see in a moment. There are some smaller washers you can pick from and you'll want to use those next to the PCB, or you may short the standoff against the tube LED resistor - more on that later.

This is critical!! Note the spacing between the standoff, washer/spacer and the tube LED resistor lead. Be certain that you have some space here as shown. Otherwise, disaster will ensue. Note also that the flat side of the hex standoff is also facing the resistor lead. This is an additional insurance that a space is maintained. Note that a small washer must be used here - a regular sized flat washer will contact the lead, so don't use one. Nevertheless, the spacing with only the standoff is not sufficient, so some additional spacing is needed and the small washer provides that.

 
These quotes are taken directly from the SSMH website, here: http://diyforums.org/SSMH/SSMHconstruct5.php
and the SSMH Build Thread in this very same section on Head-Fi: http://www.head-fi.org/t/437075/starving-student-millett-hybrid-pcb-step-by-step-build-guide
 
Further, here is the photo used to illustrate the situation:


 
Please note the proximity of the resistor lead to the standoff!  If you are not careful to solder the resistor flush to the PCB and bend the leads at the absolute joint at the end-surface of the resistor, you may short the LED resistor onto the standoff.  Since we are always looking for good grounding in our amps, there are also specific instructions for scraping the anodizing off around the standoff mounting hole in the case lid.  So, a short at this standoff can cause significant problems.
 
As you can see in the photo, it should not be an issue and there is adequate clearance if you follow the recommendations.  Further, be absolutely certain you use the smaller washers for spacing beneath this standoff.  A regular #4 washer will definitely short out on the resistor lead.
 
I suppose if someone were really wishing to be extra-cautious, you could literally bend the leads to offset the resistor to the right in this photo.  That would give you some additional clearance, even.
 
Mar 19, 2015 at 5:33 PM Post #6,929 of 7,269

caiafa

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I'm very new at building electronic stuff.
If anybody could give me some hints about troubleshooting my problem with my amp it would be great.
 
I forgot to isolate the mosfets from the heatsinks and I shorted them when powering on. Now I only get sound in the left channel. Any guides on what to check for failures? Both of the tube heaters light up.
 
Mar 19, 2015 at 9:50 PM Post #6,930 of 7,269

tomb

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  I'm very new at building electronic stuff.
If anybody could give me some hints about troubleshooting my problem with my amp it would be great.
 
I forgot to isolate the mosfets from the heatsinks and I shorted them when powering on. Now I only get sound in the left channel. Any guides on what to check for failures? Both of the tube heaters light up.


Well, my guess is that you damaged the MOSFETs.  Replace them with proper insulating kits and see what you get.
 

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