[Miller-8]

Hello chaps I now have an Alps Blue to fit to my 708B, am I best to remove the little circuit board thing that the original pot it attetched to and solder the wires straight to the Alps? Also you know the little nodule on the Alps do you cut that off or make a wee hole for it to fit into?

 

[dcheming]

Quote:

Originally Posted by Miller-8 /img/forum/go_quote.gif Hello chaps I now have an Alps Blue to fit to my 708B, am I best to remove the little circuit board thing that the original pot it attetched to and solder the wires straight to the Alps? Also you know the little nodule on the Alps do you cut that off or make a wee hole for it to fit into?

I don't think the pins of the ALPS will fit in the little PCB of the stock pot so you pretty much have to solder directly to them. I wonder if someone offers a little PCB that works with the 27mm ALPS pots? That would definitely make it much easier.

The tab keeps the pot itself from rotating if the mounting nut comes loose so it might be worthwhile to drill the little hole, but it's not imperative that you do so. There's some pics of how I did my ALPS in the mod thread about 1/3 of the way down in my first post.

 

[Miller-8]

Need help wiriring up Alps blue pot. The pot is marked 1, 2 and 3 is 1 the ground connections? and is 2 the output and 3 the input?

 

[dcheming]

Quote:

Originally Posted by Miller-8 /img/forum/go_quote.gif Need help wiriring up Alps blue pot. The pot is marked 1, 2 and 3 is 1 the ground connections? and is 2 the output and 3 the input?

My RK27 isn't numbered like yours, so assuming that you have pin 1, then 2, then 3, and then a fourth spot with no pin, then going by the pin numbers you said above you are correct.

 

[Miller-8]

ok thanks. I noticed in a picture someone posted that they had the two ground connections linked with a copper wire. Should I do this?

 

[dcheming]

Quote:

Originally Posted by Miller-8 /img/forum/go_quote.gif ok thanks. I noticed in a picture someone posted that they had the two ground connections linked with a copper wire. Should I do this?

Yeah, you should do the same also. The ground is shared between the two channels throughout the amp anyways which is why it's okay to link those two pins together like that. Also you'll need to ground the ALPS to the chassis of the amp to prevent induced hum. You can see how Chops did this in post #5 of the mod thread. He just ran a short little jumper wire from the ground link of the ALPS over to a nearby screw holding the face plate on. If you do it the same way be sure to scrape a bit of the paint off to get a good connection.

 

[Miller-8]

Thanks. Havent yet connected the ground to the chassis but i've plugged in in to try it. A few things I've noticed:

1. There is a low hum that wasnt there before, not very loud but noticable when no music playing. Perhaps this will go when I connect the ground wire to the chassis?

2. I can now use a lot more range on the volume knob! The original one must have not been a log pot!

3. Sounds more detailed and crisper.

 

[dcheming]

Quote:

Originally Posted by Miller-8 /img/forum/go_quote.gif There is a low hum that wasnt there before, not very loud but noticable when no music playing. Perhaps this will go when I connect the ground wire to the chassis?

Yeah it goes away completely when you ground it.

 

[Chops]

Here you go, just to keep things easy.

 

[Miller-8]

LOL mine isnt as neat as that, mine is a mess. I've only just started soldering. Anyway i tried to undo that chassis screw that you connected the ground to but it wont budge, so I've just slotted the ground wire into the crevice in the corner where the metal folds over.

Hum is greatly reduced now and volume of the music is louder again, hmm.

I think next I shall attempt to replace the output and input resistors, and also the input caps. Bit worried about touching things though will doing any of that put me at risk of a shock from the power supply caps?

Also the input caps I notice some people put jumpers in their place is that safe to do? How can I tell if my DAC is putting out any DC? What happens if I put in jumpers if there is DC leakage? Would I get a hum noise or something more serious?

 

[Chops]

Quote:

Originally Posted by Miller-8 /img/forum/go_quote.gif LOL mine isnt as neat as that, mine is a mess. I've only just started soldering. Anyway i tried to undo that chassis screw that you connected the ground to but it wont budge, so I've just slotted the ground wire into the crevice in the corner where the metal folds over. Hum is greatly reduced now and volume of the music is louder again, hmm.

Well I can't take all the credit for the solder job on the Alps. My father did all of the connections on it. However, I am the one who soldered the ground wire after I pretty much taught myself how to solder on PCBs. It's as simple as pie now!

You just have to make sure to keep a little fresh solder on the tip of the iron so it will transfer the heat to your work efficiently, and also wipe the tip off on a wet sponge or paper towel every once in a while.

For that ground connection, try to use a slightly larger screw driver with a larger handle and brace the face plate in the palm of your other hand, push into the screw really good, and try to unscrew it. It's a bit of a pain, but should come loose.

I'm not sure why the music got a little louder from adding the ground though.

Quote:

I think next I shall attempt to replace the output and input resistors, and also the input caps. Bit worried about touching things though will doing any of that put me at risk of a shock from the power supply caps?

YES!! You will be in danger of shocking yourself!!

Before you do anymore mods, you should FIRST put a 2 watt bleeder resistor on one of the PS caps. I went with a single 68kΩ resistor on one PS cap, but dcheming went with two 42kΩ resistors, one on each PS cap. I think dcheming did his that way just because he likes symmetry. I went with just one because I didn't have any other resistors big enough and because my father said that both of those caps are tied together in parallel anyways, so one is all that's really needed as both caps will get discharged regardless.

This single 68kΩ resistor bleeds off all stored power within a few minutes time, but I usually let it sit for about 10 minutes or so just to be on the safe side. I'm usually getting the soldering iron and tools ready durring this time.


Quote:

Also the input caps I notice some people put jumpers in their place is that safe to do? How can I tell if my DAC is putting out any DC? What happens if I put in jumpers if there is DC leakage? Would I get a hum noise or something more serious?

Dcheming would be best to explain this one. Honestly, I can't even remember if you put the DMM on "mV" or just "V" when testing your source (DAC in your case) for DC output. I lost all my notes in my PMs for this entire subject when dcheming and I were going over it for my amp. I'm not even sure what the effects would be if there was any DC present. It would be straight DC, so I doubt it would be 60 cycle hum. Maybe something that overloads the input and possibly blows your cans? Since it's DC (Direct Current), it would possibly push the headphone's drivers in one direction and leave them there, burning up the voice coils in seconds?? I honestly don't know.

 

[dcheming]

Quote:

Originally Posted by Chops /img/forum/go_quote.gif dcheming went with two 42kΩ resistors, one on each PS cap. I think dcheming did his that way just because he likes symmetry.

I actually used two 82kΩ 2W resistors originally, but being in parallel as you said it's equivalent to 41kΩ. I swapped those out for two 160kΩ 2W back when I did the 6X4 mod, so I now have 80kΩ across the filter caps. The main reason that I went with two 2W resistors is so that I'd have plenty enough power dissipation and still be able to keep them under the PS for a cleaner look, but I do indeed enjoy symmetry also.

Quote:

Originally Posted by Chops /img/forum/go_quote.gif I'm not even sure what the effects would be if there was any DC present. It would be straight DC, so I doubt it would be 60 cycle hum. Maybe something that overloads the input and possibly blows your cans? Since it's DC (Direct Current), it would possibly push the headphone's drivers in one direction and leave them there, burning up the voice coils in seconds?? I honestly don't know.

You have it pretty much right Chops. Without the input caps, any DC coming from your source will be amplified and passed on to the headphones. This will cause the membrane of the drivers to push out constantly and this can reduce the dynamic range of your headphones. The much more serious consequence of this problem is that the voice coils can burn up if the DC is high enough. Here's an article that talks more about this.

 

[Miller-8]

ok so how do i attach the bleeder resister safely? Is it safe to take up the power supply board and just solder the resistor onto one of the caps? Do you have to be careful when doing this, too?

 

[Chops]

Quote:

Originally Posted by Miller-8 /img/forum/go_quote.gif ok so how do i attach the bleeder resister safely? Is it safe to take up the power supply board and just solder the resistor onto one of the caps? Do you have to be careful when doing this, too?

I think my father attached a couple of jumper leads to the resistor, then touched them to the PS cap terminals for a few minutes to drain the caps of all that store energy. After they were down to a safer voltage (about 10v or so), then he went ahead and soldered the resistor on.

Please keep in mind that these large caps will gain back some voltage on their own by just sitting there. Sure, you may bleed them off to about 10v, but in several minutes they can creep back up to 20v or more, so don't wait too long to solder on that resistor after bleeding the caps through the jumper leads.

 

[Miller-8]

Would a shock from them be enough to kill you? Do you know how big the current is? (amps)