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Jens Rasmussen Leach clone group buy

AndrewT tested this board and found the results I mentioned.

The level of compensation probably depends a little on the output devices used as various types probably have slightly different temperature coefficients.

Yes, Jens intended that the compensation boards be mounted with an isolator of some sort.
 
BobEllis said:


The level of compensation probably depends a little on the output devices used as various types probably have slightly different temperature coefficients.


And the types of diodes may also have different coefficients. Can of worms. Perhaps two diodes couple to transistors and two coupled to heat sink, or some other such combination, could be more accurate.
 
Most of us will build with large sinks and operate at low average power, so thermal c

Bob, If you build a strong setup with large sinks and adequate safe operating area could the Vbe be left out altogether-- for simplicity sake. It is there mainly as a protection for BJT thermal characteristics. Or using the same above criteria could not a preset Vbe be installed.

I just know this is a much talked about aspect in BJT amp design. At this moment Doug Self and Cordell are in debate on Thermal Trak optimization. Something that takes a lot to nail down properly.

And thanks for the input. I will reread AndrewT's remarks from earlier in this thread. It has been quite sometime since I visited the beginnings. Two full GB and all.

I also was not aware that an insulator was needed or recommended for the diode board. It seems to function adequately without any.

Tad
 
Re: Most of us will build with large sinks and operate at low average power, so thermal c

tryonziess said:
I also was not aware that an insulator was needed or recommended for the diode board. It seems to function adequately without any.

Tad

Hi Tad,

The insulator is not needed from an electrical point of view, but a silecon pad will help transfer heat from the heatsink to the small diode PCB.

The pad evens out an uneven heatsink to PCB surface....

\\\Jens
 
Tad -

The Vbe multiplier sets the bias voltage. The diode strings provide temperature compensation, and affects the Vbe multiplier output. You'll be operating in class C without a Vbe multiplier. OK for RF resonant circuit, but not for audio.

Theoretically you might get away without compensation with big sinks and low power, but I wouldn't risk my speakers on it. All I was trying to say was that most of us won't push our amps hard enough that a mismatch in the temperature coefficients won't impact the operating point significantly for most of us.
 
I used 3M or scotch tape or sellotape on the sink. Well squeezed down to get the adhesive film with no air.
Then a dollop of thermal paste around each diode and clamped the assembly lightly in contact with the tape.
This works for both the SMD (MELF) and discrete PCBs.
I suspect the MELF will be faster acting and might perform better.
 
Re: Why insulate at all?

clm811 said:
Guys!

why would we need any mica/kapton/tape, etc at all?

The (mine at least) diode leads do not protrude all the way through the diode PCB!

Why don't we just use a thin layer of thermal compound on the two mating surfaces?

Jens wrote:

While I am not in this stage, I can think of 2 possible reasons:
1. prevent the possibility of the thermal compound being conductive. The compound can go into the hole and contact the leads.
2. the little pcb may not be as flat as it can be, if so, combining the pad and the thermal compound probably have a better heat transfer.
 
I intend to use the SMD PCB directly to the heatsink with a smudge of grease. The PCB is small/low mass, the mount screw will aid in the heat transfer to the top side, and the devices are in close to that VIA - mount hole

Do we really think that this kind of approach won't be well suited to the task :confused: Personally, My only other real option would be ill-fitted mica, as I don't have the Kapton tape. Maybe I'll use a cupped washer and really crank the screw into the heatsink :devilr:

Fred does make a good point about thermal grease squeezing up into the diode holes and creating an electrical issue. I hadn't thought of that...
 
... thermal grease squeezing up into the diode holes and creating an electrical issue...

I had dodged that bullet by putting a tiny drop of epoxy into each bevelled hole with a toothpick. Super glue would do as well. Just be careful not to get glue residue on the surface, or it could interfere with the mating of surfaces.

Tomorrow we're doing the power supply layout for the chassis. I hear WJH has his heatsinks completely drilled & tapped, so he's winning the race so far. More Pics to come!

-Chas

P.s. beginning to sound like borderline OCD... :xeye:
 

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I think OCD might be a very useful affliction in this particular hobby. Especially when working with SMD components.
There are many times I wish I had applied just a tiny little bit more obsession too a project and maybe the magic smoke would not have made such a dramatic entrance.

Of course if you ask my wife she will admit I am anal in this endeavor. It still provides great amounts of satisfaction when things work after all that input. Tapping aluminum with a No. 4 tap is stressful.

Tad
 
Rob,
It might be a good time to start soldering in the little 3 legged TO-92 transistors. I found them the hardest to reach and with stuff in the way almost impossible. I also had to use some flux on some of the bigger pieces. Just wash it off when you are finished with some good circuit cleaner or alcohol. Have fun.

Tad