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Total Posts: 87
This Year: 24
This Month: 0
This Week: 0
Comments: 9

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.

Friday, December 28, 2007

SC-1 Preamp + IO-Module

Friday, December 28, 2007 10:21:06 PM (Central Standard Time, UTC-06:00) ( )

Took some photos today.

Did some more tweaking on the IO-module PCB design... getting ready to pull the trigger for another prototype PCB run.

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Sunday, December 23, 2007

Oscilloscope photos and audio samples ... DCC-1848

Sunday, December 23, 2007 10:19:52 PM (Central Standard Time, UTC-06:00) ( )

Using the DCC-1848 prototype PSU, loaded with my SC-1 preamp, the IO-module and the VU meter board.

0.2V/DIV

0.1V/DIV... you can see some noise/spikes occuring at regular intervals.

Not bad...

And audio samples.
The Setup:
My SC-1 preamp... with an IO-module and VU Meter board. Everything powered by my proto DC-DC Switching PSU... (12Volts input to +/-18, +48V). Preamp outputs going to a patchbay, then going to a MOTU 828mk2 input. Recorded using Digital Performer. MP3 set at 256Kbit sample rate. Preamp, IO-module, VU meter board and PSU unracked, no case, no shielding, no grounding... just PCBs sitting on my workbench.

First sample:... preamp gain set to 54dB, *unterminated XLR inputs*, max volume
http://www.fivefishstudios.com/audio/DCDC54dB0R.mp3

Second sample... preamp same 54dB gain as above, but XLR inputs terminated with 150R resistor.
http://www.fivefishstudios.com/audio/DCDC54dB150R.mp3

Third sample.... preamp gain set to 72dB, *unterminated XLR inputs*, max volume
http://www.fivefishstudios.com/audio/DCDC72dB0R.mp3

Last sample... preamp gain at 72dB, 150R terminated XLR inputs, max volume
http://www.fivefishstudios.com/audio/DCDC72dB150R.mp3

OK! I think I found the problem and managed to reduce the spikes in the DC INVERTING section. I don't know if the pictures make it obvious but it's a definite improvement.

EARLIER

FIXED? Definitely better... reduced noise and pikes.

I think I'm ready to do a revision 2 of the DC-DC Converter PCB, so I don't have components hanging on the top and bottom of the board (was trying out different things). I think the rest of the problem is now caused by the PCB layout.

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Saturday, December 22, 2007

IO-mod + VU meter ... mated together

Saturday, December 22, 2007 10:18:37 PM (Central Standard Time, UTC-06:00) ( )

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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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Saturday, December 01, 2007

IO-Module Prototype... Success!

Saturday, December 01, 2007 10:12:25 PM (Central Standard Time, UTC-06:00) ( )

The IO-Module PCB's are here!!!!!

One step closer to bringing some iron flavoring to our SC-1 preamps... and about 2 steps closer to having an API-type preamp offering in the future!

As you can see, the square hole for the output transformer is not cutout because this is just a prototype order. So I'll have to cut it out manually. When I place my production run order, it will be cutout at the factory.

So it's time to test this baby!

And this is a preview of what it will look like.

I can't wait to build this board and test it.

Just imagine your SC-1 preamp mated to this IO board....

- Input Transformer
- Output Transformer
- Hi-Z Input with ACTIVE DI... converted to balanced out, then routed to the input transformer.
- Pad Switch -20dB
- Polarity Switch
- Mic/Instrument Switch

Got some work done on the IO-module today...

I stuffed it with components, and just use jumpers (temporarily) to wire the output trafo.

Then I connected it to my SC-1 preamp and then listened to the sound.... arrrgghhh... something is not right. It sounds horrible. Weak. No bass. Instead of amplifying the sound it seems to be making it weak.

What's wrong? I tested every part of this IO-module on a breadboard so I know it should work.

I reviewed the datasheets for the input trafo and my PCB design. They're all correct.

Then, on a last ditch effort, I tried bypassing the output transformer and WHAM!!!! Loud sound filled the room!!!!

Hmmm.... why would it sound great with the output trafo bypass, and not when it's in the circuit... so I untaped all the jumper wires, and on one of the wiring, I have an open circuit. I grabbed a long nose pliers and made sure the wires are twisted tight and making good contact.

Test again, and whooppeeeee! It works.

Really, really really sounds great. If the stock SC-1 sounds great, this is even better. I can't describe it but it's just beefy while still being musical and all the high frequencies still being there. Beefy without being muddy.

I listened to the sound and compared between the output trafo bypassed and not... and frankly, I can't hear much difference. It's wired right now as 1+1:1+1. So basically 1:1.

The output trafo doesn't really add much to the flavoring. But it does contribute to signal isolation due to the transformer properties.

The input trafo adds about +18dB gain. And not only that... most of the flavor seems to come from the input trafo itself. So if you're looking to save some money, you can skip the output trafo but I would 100% recommend that you use the input transformer. It's where the flavor is!

... but come on... if you're going to spend money on the input trafo, might as well buy the output trafo! Otherwise... what's the point? The point of the IO-module is to add the input and output transformers.

Well.. okay, there's the active DI circuit that's part of it.

Then I have a -20db pad switch.

The output trafo can be wired as 1:2 if you want an additional 6db gain.

So let's see... 72db gain of the pre, +18dB of the input trafo, and possible +6db from the output trafo...hmmmm... 96dB gain total???? Shocked

(PS: I haven't tested the DI input yet... I need to hookup a guitar here )

Photos:

From left to right...

POLARITY --- -20dB PAD ---- MIC/DI INPUT ----- HI-Z GUITAR INPUT

I'm going to order some more switches and do a second prototype using a different input transformer.

But I think I can honestly say the IO-Module is a success! Really sounds great!!!!

UPDATE: Just a correction. I removed a resistor during my testing (when I was troubleshooting) and when I added it back, the input trafo does not add +18dB gain. So we're cool. still +6 to +72dB gain from the preamp.

The output trafo, though, can be wired as 1:1 or 1:2 and if wired 1:2, will give you an additional FREE +6dB gain.

PS2: Also tested the DI input with an electric guitar. I can't play squat but yeah, it works... sounds good too. The output of the Active DI is routed also to the input trafo.

You know what's going to be next, right?

So if we get a breadboard, and put some discrete opamp module (I have some Forsell JT-992 and some Melcor) and just add a few resistors and capacitors and hook it up to this IO-module... why... we basically have an API-type preamp!

So in summary....

SC-1 preamp kit + IO-module = RNP/DAV/API hybrid preamp.

Discrete opamp + IO-module = API-type preamp.

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