DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

Owel,

Well, the PSU powers up & the LED lights, nothing seems to be smoking..so far so good.

When it came to testing the voltages, the 48V pad read c22V...I wound up the trim pot but it goes no higher than 32V. Obviously I've done something wrong...but what? I can see no bridged pins or dry joints. Could you suggest a start point for exploration?

Oh - the -18V rail read c30v too, but I was able to adjust that down to exactly 18V. +18V was fine.

Thanks!

So you got -18V fine, and +18V fine.

But your 48V only reads 32V?

What kind of transformer are you using? How do you have the secondary of the transformer wired up?



Do you have 220 ohm resistor for R2???

Is your R3 the 5K trimmer? And you didn't accidentally swapped it with the other 2K value?

What's your voltage measurement at the positive (+) terminal of the 1000u/80V capacitor below pictured above?

Is it possible that this effect could be caused by mis-placing the trim pots? 15 minutes with a magnifying glass shows me that R3 is where R7 should be and vice-versa...doh!

Yeah, that would do it...

You said



So that would explain it.

Desoldering this will be a pain... it's 3 legged terminal. You need to heat all 3 terminals at the same time and pull it out from the other side.

Do you have a good desoldering pump or desoldering iron?

DON'T DO ANYTHING YET..... let me see if I can come up with a workaround for your problem.

So right now, your R3 is a 2K trimmer instead of the 5K right????

bead...

Here's an easier solution for your problem. Since you swapped a 2K trimmer for the R3 resistor (instead of the normal 5K trimmer), I want you to remove resistor R1 (which is 3K3 ohms) and replace it with a 6K8 (6.8K) ohm resistor.

That will put you right back on track, and let you achieve 48V for the phantom power supply section.

So for your case,

R1 = 6.8K
R3 = 2K trimmer

You just have to replace and desolder one resistor. Easier solution that replacing and desoldering the trimmer.

Hope this helps.

Wow, Owel, that's stunning service! I'll hie me to RatShack and see if they have one. Otherwise I'll keep working on Preamp 1 and make an order with Parts Express on Monday. Funny how this stuff always happens at the w/e, no?

I have the regulation RadioShack blue soldersucker but it has an end the size of a dime (relatively speaking) it's good for clearing solder out of board holes but I don't think I could get an iron & it onto one of these trimmer legs at the same time. Looks like I need an iron with a T-shaped bit.

In reality, phantom power mics will run at 32V. Heck, they'll even run as low as 18Volts.

So if you decide to leave your 48V phantom power only at 32V, that is fine, and I don't think it will be an issue.

(Thought FET microphones produce little output at 18V, but most condenser mics will run happy at 18V. So even at 32V, I think it will be fine and will be enough to power majority of mics out there.)

Success - all working as normal. Definitely my bad here, I must learn to switch off the phone while building pre-amps!

Great!

I'm building another SC-1 kit... I'm putting 4 channels in a half rack width case. It's going to be pretty cramped but from my initial testing, no hum even if the 4th channel is right beside the huge transformer.

I concur - my first preamp is crunched into a corner of the 1U rack with the PSU and trafo, no noise problem so far.

Which is a good thing - I'm using an old ART power strip rack case with some very basic noise filtering and a circuit breaker. It's just over 6 inches deep, so space is at a premium!

If I can get 2 preamps built and working, I'll have to choose between 2 more or the X modules...of course, I could always upgrade the chassis at that point.

Alright its my turn to play the troubleshooting game.

So my PSU appears to be turning on... LED 1 lights up. I am also getting power at the secondary coils of the transformer. So i go to check for output voltages, but with no luck. Also, LED 2 is not lighting up. I thought it might be backwards and thus causing some output problems, but i am looking at it now, and the long leg is oriented at the bottom of the board. Anybody have any suggestions? Thank you very much in advance

Correction: LED1 was lighting up... i just popped the fuse by shorting something trying to measure the current at the secondaries of the transformer haha. Oops. Anybody have any specs on the fuse? Screw it, i'm going to bed. I'll play more tomorrow haha.

Well, i spent some time fiddling today. Replaced the fuse. Took some readings at the secondary of the trafo:

AC1 and CT: 106mv

AC2 and CT: 162mv

AC1 and AC2: 182mv

For some reason, when i touch just one of the leads from the multimeter to the board, it jumps up to like 200mv and i don't know why. I get the feeling i am doing something wrong with the multimeter.

When i measure across the outputs and ground, i get no reading, DC and i get around 30mV AC. LED 1 lights up but two does not. I have no idea whats going on. Thanks.

Oh, by the way. I have spent some time inspecting the board too. Its... well its not my best work by any stretch haha. But it isn't that bad... i would think it should still work. Nothing appears to be bridged or broken. Theres a few minor scratches and burns on the back of the board, but again, nothing major... the pads are still intact and everything. Bottom line is i don't see anything that would be causing a short or open circuit.