AKSA's Lender Preamp with 40Vpp Ouput GB

[Monk55]

😜 No problem…thanks. Those babies are cheap…..at least for now.

 

[Monk55]

I was just going to warm & bend.

 

[Kokanee]

Hi fellow builders,
A little help please. The SK104 heatsinks I have for the ALP mainboard do not have a rod/pin to insert through the pcb. Is the transistor enough support for the heatsink once soldered in the pcb.

Also, C124 is installed on the bottom side of my pcb facing inward, and will interfere with the 3 pins from the DB. Is it OK to trim the 3 pins close to the pcb, or, bend the 3 pins on the DB so they are flat to the pcb ?. I will place a piece of Kapton tape over them.
Regards,
MM

 

[xrk971]

Hi Kokanee,
The heatsink is probably ok with out pins if you don’t move it around a lot. You could add some RTV silicone blobs to secure it to the PCB. Make sure the heatsink touches the board mechanically and RTV secures it.

The bent capacitor and Kapton tape is fine.

Good luck!

 

[Kokanee]

Thanks X,

Heatsink: Thanks for letting me know about the mechanical contact between heatsink/pcb. I may look around for some pins/solid wire that I could insert into fins on the heatsink.

DB pins: Is it OK to bend the 3 DB pins flat to the underside of the pcb, or, can I trim the 3 DB pins so they do not interfere with the C124 capacitor. This will allow the C124 cap with the bent leads to lay fairly horizontal, rather than hanging at a downward angle due to the length/interference of the DB pins.

Since I will only ever use the ALP as a preamp, I could consider soldering the DB to the mainboard, then I would be able to trim the DB pins a bit shorter and solve the above situation.

Thanks for the help, will await your response to the DB pin situation.

MM

 

[xrk971]

Soldering is a good option if you don’t plan on changing the board out. It’s more secure and better contact quality.

 

[Kokanee]

Thanks X, solved the heatsink situation with a couple of trimmed nails which fit perfectly, so should be able to solder to the board.

Going to solder the DB in place after testing if they work.

MM

 

[PierreQuiRoule]

IMO it is preferable to trim the DB pins than bending them, in case you want to unsolder them to pull out the DB in the future.

FWIW, another option for the heatsink might be to enlarge the PCB holes and screw the heatsink from underneath the PCB. Choose screw size just large enough to self-tap into the heatsink aluminum.

 

[Kokanee]

Hi PQR,

Thanks for the advice on both accounts. Using my trimmed nails with 1 wrap of Kapton tape, I have a nice tight fit in the body of the heatsink. The nails fit the pcb holes nicely and should be no problem to solder.

 

[Kokanee]

I soldered in the DN25440 transistor and thought I would test it to see if I had it in correctly. My chinese tester tested it as a D-Mos when tested in free air G,D,S, and tested it as a resistor with a resistance reading for 1,2,3 when soldered in the circuit. Is this normal?

I did not think it was normal, and desoldered it. Tested again and tested as a D-Mos. Unfortunately, I wrecked a couple of pads around the holes in the pcb when de-soldering. Thanks for any help with this.

 

[xrk971]

The DN2540 is very sensitive to static discharge. It’s one of the few MOSFETs that I have damaged from not careful handling. Discharge your fingers or hand before handling. Wear an ESD wrist strap, etc. It’s a depletion mode MOSFET which means that it conducts when there is no gate voltage present. If the gate burns out from ESD, it measures as a resistor.

 

[Kokanee]

Wow, thanks for the info. I was careful, but will be more careful.

It still does not sit right with me: The transistor before de-soldering, came up as a resistor and then after de-soldering and testing, the transistor measured properly as a D-Mos transistor, giving I=2.3mA and Vgs = 2.3V.

I tested another transistor I had, and the I=2.2mA and Vgs=2.2V.

My question is why would soldering the transistor in the pcb, cause the transistor to test like a resistor?

Thanks for the help with this,

MM

 

[Vunce]

If the rest of the daughter board is populated with parts, testing a transistor "in circuit" will often give false results. Probably nothing wrong with DN2540 in the first place.

 

[xrk971]

Vunce is correct. You can only test a bare transistor, not one installed in circuit other than to check for a dead short between drain and source.

 

[Kokanee]

Thanks guys, understood, just like testing a cap or resistor while installed with other parts. Will wait for my package from X, and then continue.

 

[Kokanee]

Hi X,

Package arrived safely. Thanks for the extra DB's. I may try those pretty purple one's at some point. While I have you here a quick question: " What preamp for HPA use only do you recommend if using the 25VA / 15V Talema transformers and Yarra format.

Note: I will have the ALP and Hakuin/Yarra for preamp use only.

Thanks

 

[xrk971]

I’m not sure I understand your question: what preamp to use if setting it up to use as a headphone amp? No preamp needed - it serves as both the preamp and headphone amp because there is built in gain of circa 12dB. What makes it a headphone amp is the extra current capability to drive lower impedance loads. You may not need as much high voltage swing on a headphone amp.

 

[Kokanee]

I guess I did not explain the question very well. Let's try again.

If I use the 25VA / 15V Talema transformers and Yarra psu, I estimate I will have about 17 or 18 VDC for use. Is this enough for using the Melbourne DB or Waynes 2018 DB for a HPA, or do I need more voltage? I have both of these DB.

If I do need more voltage, I would imagine that their are other PSU's that could be used that drop less voltage, so I might be able to squeeze 20 VDC from the Talema transformers.

 

[Kokanee]

Hello AKSA Lender Pre builders,

I have some SK95 heatsinks that I would like to use for the transistors on the ALP. They are Fischer and the dimensions are 25mm L x 12mm W x 7mm T. They fit the transistors well as they are TO-220.

I am wondering if they are enough sink for the DN2540 transistor. I think OK for the SA1381 transistor. See attached pic.

Thanks for the help.

 

[xrk971]

That should work - it will run pretty hot but as long as the heatsinks are below 55C you won't burn yourself, but they can run 85C fine as long as you don't touch them.