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SC-1 Mic Preamp NOW ON SALE!!!!

After many months of development, testing, troubleshooting, frustration, problems, and accomplishments... the SC-1 mic preamp kit, PSU-1848 power supply kit and power transformer kits are now available for sale!

Click here to ORDER

Photo of a finished SC-1 mic preamp kit! (Note: XLR jacks and wires shown on picture not included in the kit.)

Features:
- Low-Noise, transformerless, High-Quality Mic Preamp
- All solid-state design, using chipsets from THAT Corp and Burr-Brown
- Soft-start, slow ramp-on +48V phantom power
- Crystal clear Red LED indicator for phantom power
- Electronically balanced input and output stages
- 12-position Grayhill gain selector switch
- Gain range from +6 to +72dB, in 6dB increments
- Input RFI protection
- Input clamping protection
- Output surge protection circuit
- Output RFI protection
... read more...

Download PDF file of Assembly Guide/Kit Instructions

Related products like the PSU-1848 Power Supply Kit, Power Transformer Kit, Power Control Kit are also available for sale. Click here.

Thursday, December 20, 2007

DCC-1848 Prototype Testing... SUCCESS!!!!!!!!

Thursday, December 20, 2007 10:16:23 PM (Central Standard Time, UTC-06:00) ( )

Woohoo! Success. My DCC-1848 PSU is working. DC-to-DC Power Conversion from a single 9V battery (or wall wart).

Okay, what happened? Yesterday, I was moaning that everything was a disaster...

First of all, don't work on your building your prototypes at 1:00 in the morning.

My Mistakes:

1. The biggest mistake of all... I sent the wrong PCB gerber files to the PCB fab house. So I got the wrong PCB back. The hole spacing, and layout are wrong, so components are inserted crooked. This made working on the prototype PCB harder.

2. I have a silkscreen mistake on the PCB. During the building of the prototype, I followed the silkscreen on the PCB... instead of my schematic. Result? I inserted the wrong value of capacitor on the (-) Inverting section. But still... this should have not prevented the circuit from working.

3. And the real cause of the problem.... <drum roll>.... I inserted a diode backwards. Stooopid mistake. My silkscreen shows the correct orientation but I didn't noticed it, and I inserted the diode backwards.

As soon as I noticed that problem, desoldered the diode, and used a brand new diode to put into the location, and VOILA!!! IT WORKS!!!! SUCCESS!!!!

Some pics...

Here, you can see +18Volts output from a single 9V battery.

And now... the problem section... the -18V Inverting section. But as you can see, it's now working! Smile

And the biggest question of all... can it do 48Volts for phantom powering?
Sure can! Check it out... +48V output from a 9V battery source.

Just for kicks... let's see what's the highest voltage we can achieve...

NOTE: The PSU is regulated and adjustable. So just because I'm calling it a DCC-1848, it doesn't mean it can't go higher than 18V, or 48V.

And here's a picture of my ugly prototype power supply...

This shows the +48V section that I'm scrapping. Instead, I'll use the same design I used for the +18V section for the +48V section.

It's an ugly piece of work... BUT IT WORKS!!! This is so cool.

BTW, I checked the PSU DC output on my oscilloscope... it's clean. I thought my oscilloscope was not working, but there was very very very tiny ripple, no oscillations, or any nasties.

Of course, the real test is .... when connected to my SC-1 preamp, how does it sound? i.e. will I hear any whining, nasty, artifacts leaking into the preamp?

That's the next phase of my testing.

For now, I'm really pleased with the outcome of this testing.

Okay, did more testing...

The above voltage readings were UNLOADED. So what happens when it's loaded?

I tested the DCC-1848 PSU prototype loaded with (1) channel of SC-1 Mic Pre and (1) IO-module.

My goal here is to keep the output voltages at +18, -18 even with a running load.

It seems that a 12VDC input is the ideal input voltage for the DCC-1848. (which is what the original intent was anyway... powered by a car battery or wall wart)

When the input voltage drops below 9.8Volts, we get +18.2, and -17.8VDC outputs. Weird. And get this... the PSU PCB emits a high pitch sound (later on this...)

I set the gain of the SC-1 preamp all the way to the max... +72dB, max volume, and listened at the output on my KRK monitor. Wow! You wouldn't believe it... It's just hiss, only hiss, no RF junk, no nasty interference that you would expect from a switching PSU, just plain hiss. In fact... and I hate to say this... the output IS cleaner than a linear power supply powered by the AC mains! I can't believe my ears. Totally *unexpected*! I would expect that switching PSU to emit more junk and be more noisier than my linear PSU. But I'm very surprised.

BUT...

Of course, there is a big BUT...

Right now, I am having this high-pitch vibration coming from the PSU PCB.... no, the noise is not coming from the preamp, nor is the noise travelling from the PSU to the preamp... The high-pitch sound is coming from the PCB itself!!!

I narrowed the source of this high-pitch sound down to the Inverting Section. And further narrowed it down to the Trimmer and IC sockets. It seems the mechanical contraptions inside that trimmer is vibrating at a high pitch that you could hear it. When I touch the trimmer, the pitch changes and if I squeeze the legs of the IC in the IC socket, the sound disappears or lessens.

Also, I could hear what I thought was an AM radio... coming from the Trimmer! kinda like a "crystal radio" From some listening, I found out it's WSM radio station. We live only a few miles away from it.

This WSM tower is gigantic-enormous. See this writeup.
http://www.oldradio.com/archives/stations/ccs/wsmtwr.htm

The question is... will other people hear this weird high-pitch sound coming from the PSU PCB? Or is it just me, since I'm pretty close to this AM tower?

Anyways... I'm also thinking of maybe replacing that trimmer in the Inverting section with a different kind of trimmer... and see if the sound/vibration goes away.

Another thing is maybe I shouldn't use IC sockets and just solder the IC directly to the PCB. This will remove any vibration (within the IC socket contacts) that can be amplified.

Well... so far, right now I'm looking at this with a magnifying glass, scrutinizing it and I'll try to solve any anomaly, issue, noise, design problem before I call it a final version.

In summary...

** the noise performance of the switching DC-DC psu is excellent. No penalties, nor does it affect the performance of the SC-1 preamp.

** a 12VDC input seems to be the ideal/sweet spot. This won't be a problem since 12VDC wall warts are easy to find.

** Need to find a fix for the high-pitch vibration coming from the PSU PCB.

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