UCD400 boxed

Just a picture of my just finished amp. Still a lot of fine tuning to do. The box is from audiokit/Italy. Rather small size. Cooling block is from schuro like the caps. Transfo. is a 300W(2x40V) toroid. No snubbers anywhere.
Just a quick scope measuring is 100V at 8 ohms both channels driven. Not bad.
[IMGHTTPDEAD]http://www.xs4all.nl/~grotel/Photo's/UCD400boxed.JPG[/IMGHTTPDEAD]
 
well, after taking the input caps out and changing the gain smd resistor from 560 to 180 ohms, I find the gain far to low for any preamp system, I fired it up and found an unproper grounding of the heatsink where the UCD400's are mounted on. Humm all over. After making a hard connection to the zero of the cap. PCB its OK now. My tuner(aircaps.) was completely freaked out from the 400kc. Now...dead silent.
Sound is very nice. The only things that get burning hot are the 12V step down transistors. Hope this stays OK.
Channel separation at 15kc is a tad lower for the module where the input section is nearest to the power output coil. Would be better if they were separated a bit more. Ah well...have to built another one!
 

ghemink

Member
2004-06-14 2:26 pm
Japan
Bgt said:
well, after taking the input caps out and changing the gain smd resistor from 560 to 180 ohms, I find the gain far to low for any preamp system, I fired it up and found an unproper grounding of the heatsink where the UCD400's are mounted on. Humm all over. After making a hard connection to the zero of the cap. PCB its OK now. My tuner(aircaps.) was completely freaked out from the 400kc. Now...dead silent.
Sound is very nice. The only things that get burning hot are the 12V step down transistors. Hope this stays OK.
Channel separation at 15kc is a tad lower for the module where the input section is nearest to the power output coil. Would be better if they were separated a bit more. Ah well...have to built another one!


Are those transistors getting that hot??? I`ll try to measure it sometime, these guys do not have to handle that much current so expect it to be OK. Anyway, I`ll check sometime with my thermocouple.

Best regards

Gertjan
 

ghemink

Member
2004-06-14 2:26 pm
Japan
Bgt said:
well, after taking the input caps out and changing the gain smd resistor from 560 to 180 ohms, I find the gain far to low for any preamp system, I fired it up and found an unproper grounding of the heatsink where the UCD400's are mounted on. Humm all over. After making a hard connection to the zero of the cap. PCB its OK now. My tuner(aircaps.) was completely freaked out from the 400kc. Now...dead silent.
Sound is very nice. The only things that get burning hot are the 12V step down transistors. Hope this stays OK.
Channel separation at 15kc is a tad lower for the module where the input section is nearest to the power output coil. Would be better if they were separated a bit more. Ah well...have to built another one!


Hi Bgt,

I measured the temperature of those two transistors in my experimental setup. They get indeed quite warm, around 70C while the temperature in my room was 30C. This was for the UcDs with the OPA opamp. I have nor measured it for the AD opamp version.

I guess this is still well below what is safe for these transistors.

ABout the gain, I would say that the standard 26dB gain should be high enough for most preamps. In my case I have more than enough gain. If you have a preamp that gives 2V RMS out, then you can already drive them to max power.

Best regards

Gertjan
 
Gertjan,

Thanx for measuring it. I did also and found 69C (with a small laser measuring device, forgot that I have 1) after warming up a bit. So that should be OK. Its a 8620 Type UCD. But it warms up the housing a bit which is a bit of a pitty with such a "cool"amp. :xeye:
The housing is a bit smallish but that's what I like.
What I've found with the gain is that for 100V peak out you need 5v peak in. Well my tuner output is only 0.5V rms. My DAC(Elektor2000withAD8620's) is about 2V peak. Combined with the passive preamp, which has a gain of about 0.9x that leaves me with a very soft amp. if I don't change the gain. With the 180 ohm resistor I only need 1.4 V peak for 100V peak out.

KR, Bert
 
Mauricio,

Concerning the position of the PCB, I tried to put it vertically but then 1 side of the T bone piece got too hot. The way it is now it is more evenly hot/cool on the PCB. There are cooling vents in the top and bottom cover just where the heatsink is positioned. Just look at the pic. Here you get your convection cooling airflow.
[IMGDEAD]http://www.xs4all.nl/~grotel/Photo's/UCD400kasttop.JPG[/IMGDEAD]

Anyway, after having it on for 4 hours on average level(classical music) the 12V stabilizer transistors are about 74C and the output coil is 70C. The T bone piece is 50C. Inside the box 37C. Room temp is 24.

[IMGDEAD]http://www.xs4all.nl/~grotel/Photo's/bottom2.JPG[/IMGDEAD]

It is very easy to tune now. Very easy to reach the parts.
 
Bgt:

Yes, very elegant solution.

<<Concerning the position of the PCB, I tried to put it vertically but then 1 side of the T bone piece got too hot. The way it is now it is more evenly hot/cool on the PCB. There are cooling vents in the top and bottom cover just where the heatsink is positioned. Just look at the pic. Here you get your convection cooling airflow.>>
But for the cooling issue I wouldn't be concerned about T bone piece asymetric warming; that's its function. For the T bone piece you have more than enough with your heatsink. I understand that convective cooling will only benefit the heatsink-Mosfets, not the secondary transistor-opamps-coils mounted on the PCB.

Am I wrong?

Best wishes
Mauricio
 
not the secondary transistor-opamps-coils mounted on the PCB.
In the top lid there are also venting slots and because the PCB's are mounted on a quite low position in the housing there is a lot of air around the parts and this air comes out on top. The heatsink is mounted on the low side of the cabinet so there is still some space for the hot air to go out on top.
I measured the heat everywhere and the biggest heat comes from the heatsink/Tbone, not the 12Volt transistors and output coil. Yes they feel hot but have no heatsink. These are just minor heat generators. The output Fets have a idle current of about 60ma x 120 Volts which is about 7.2Watts to burn for each amp. Maybe more. Bruno will know.
If you feel the amps. temperature after playing for 8 hours you will feel that the heatsink has the biggest chunk of heat from the output fets. The heatsink will reach 55C.

I had them vertical in my first try of the UCD's in a housing used for my Crescendo's. But this was not the best solution. The housing was bigger and had a lower volt toroid(2x35V) so it did not get that hot.

[IMGDEAD]http://www.xs4all.nl/~grotel/Photo's/UCD%20vert2.JPG[/IMGDEAD]