My SC-1 Mic Preamp and PSU Kit

[owel]

It's finished... I decided to put 4 channels of my SC-1 mic preamp kits in a half-rack case.

The preamp is a THAT 1510 based pre, with DC servo using Burr Brown chip and THAT 1646 balanced line driver. (See my sig for more info)

It's got soft-start phantom power... so no loud thudding/popping when you turn on/off phantom power. Using THAT chips, Burr Brown chips, Grayhill switches, Bourns pot, C&K switches, gold-plated machined IC sockets, Panasonic caps, C0G NPO, metal film, RFI input and output protection, diode/zener input and output protection, Neutrik XLRs, etc...

Frequency response I got using TrueRTA software is here.
http://www.fivefish.net/diy/sc1/images/SC1quicksweep.gif

I have a kit builder compare it's sound to the DAV, and find this pre more open and clearer. Another calls it with "miles and miles of gain"... at +72dB (i.e. +66db from preamp + 6db from balanced driver). It's basically clean, crystal clear, lots of gain, quiet, low noise.

Some photos:

The SC-1 Kit:

4 channels in one 1/2 rack case.

The PWR-TRAFO and PWR-CONTROL Kit in use... 25VA transformer assures plenty of power to power up 8 channels!

Top View. Pretty tight.

Back panel, showing the XLR input and output jacks. Decal was also used for the back panel.

The Front view... I'm trying out different knobs... the front panel was also created using decals.

 

[okgb]

Looks Great ! boards & the build

happy recording

 

[Greg]

Great build !!! Hope you enjoy them !!!

I think I see a ground from the IEC going to the side of the chassis. Just curious, did you tie the input XLRs pin 1 to the chassis as well?

 

[mikka]

Lovely build ..... :thumb:

 

[owel]

[quote author="Greg"]Great build !!! Hope you enjoy them !!!

I think I see a ground from the IEC going to the side of the chassis. Just curious, did you tie the input XLRs pin 1 to the chassis as well?[/quote]

Electrically, yes.

From pin1 XLR there's a short, maybe 4mm extra wire connecting it also to the case terminal of the XLR jack. The XLR jack is conductive so it makes good contact with the case. So effectively, pin 1 of both input and output XLR jacks are also grounded to case.

I've tested it for RFI/Hum/Noise using the above grounding scheme at the following conditions, and happy to report no noise... (well, very slight noise at +72dB gain setting).

At +72dB gain, 100% volume:
- with 20ft cable connected to condenser mic (no phantom),
- with 20ft cable and no mic,
- with hanging XLR inputs and no cable,
- with 150-ohm terminated inputs (pin 2 and 3)

No significant or loud humming/popping/RFI/crackle/fizz/fuzz detected.

 

[Bluzzi]

Beautiful work! Good pricing!

Could your power supply PSU-1848 be made to put out +- 24vdc if we substitute the regulators?

jim

 

[owel]

[quote author="Bluzzi"]Beautiful work! Good pricing!

Could your power supply PSU-1848 be made to put out +- 24vdc if we substitute the regulators?

jim[/quote]

Bluzzi, thanks!

No need to change the regulators for the PSU-1848 kit. As is, it can be adjusted to the following voltages:

Output 1: 20V to 48V+
Output 2: 1.2V to 21V+
Output 3: -1.2 to -21V+

By replacing R4 and R5 from the original 120R to 100R, you'd be able to achieve +/-24V for Outputs 2 and 3.

There are 3 Bourns top-adjust (multi-turn trimmers) for very fine adjustments too.

 

[owel]

Closeup photos of my PSU-1848 power supply kit...

PCB measures 3.5" x 1.9", Rohs lead free, double-sided with plated-through holes, top and bottom solder mask, with top white silk screen printed with component values and part #s.

There are (2) sets of solder pads for the (+48), (+), (GND) and (-) so it will be convenient to wire 2 channels of pre to the psu board.

There are also 2 on-board LED indicators on the PSU board. You can wire both to your front panel as power-on indicator, or leave one soldered to the PCB, and wire the other one to the front panel. (It's a good visual reminder when working that your PSU is still energized when you see that LED still lit up.)

Using (3) Bourns top-adjust multi-turn trimmer pot. (more expensive but also nicer).

The regulators are on the edge of the PCB for heatsinking it to your case (use TO-220 insulators) or other heatsink.

Very small... measuring 3.5" x 1.9" and low profile... will fit in a 1u rack. The (2) 1000uf/80V caps are snap-in type, low profile height.

 

[SonsOfThunder]

Owel...wow, there is a name I haven't seen on the board in a long while. Nice to see you again.

Nice project you got here, very pro look. Nice work! :thumb:

Lots of projects on yer site there. Good luck with those!

Peace!
Charlie