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

General idea....

IO-Module ... stands for Input/Output Module.

add "extra" features to the basic SC-1 Mic Preamp Kit. This will basically serve as an add-on to the SC-1 Kit.

Why didn't I include these features in the SC-1 kit.

1. Cost - more parts, more expensive parts, and more development means more cost to the base price of the kit.

2. Need - not all people want or need these features... why pay for things you're not going to use or want?

** Actually, one feature that I earlier planned to include in the IO-Module got included in the base SC-1 Kit. That's the +48V soft-start phantom power.

So what will be in the IO-Module Kit...

some ideas:

1. DI - Instrument Input
2. pad
3. polarity reverse
4. Output transformer
5. Relay controlled switching of pad and polarity
6. Analog VU meter connection

The DI circuit I'm thinking of doing a Hybrid DI... i.e. it's an Active DI with a transformer output stage... kinda like an active/passive DI. Maybe even add a tone control? What do you think?

The Pad and Polarity Reverse will need to be relay controlled, so audio signal paths are kept short and "ground guarded."

The output transformer should be *optional* and the user may add it later... that means, I need a way to "jumper" it if it's missing. Also, would be neat will be a way to put the transformer IN or OUT of the circuit... maybe by switching relays.... hmmm.

Analog VU... not a lot of people want/need this... but those that do want it... want it BAD. And I don't blame them. These things look cool and very retro! I like it. Possibly need a buffer circuit just to be *safe* and avoid overloading the output of the preamp.

Now, before you get excited because you see *transformers* written all over on this IO-Module, I just want to encourage some "cautious exhuberance."

Transformers are EXPENSIVE. Even the cheapest transformer may cost $50. Man... that's a lot of high-grade opamps you can buy for that... hmmm, like 10-12pcs. Let's see... 10-12 IC chips or (1) Transformer... which would you pick?

Also, if all you need/want is some pads and polarity reverse... I can show you how to do those things for a few dollars worth of switches, some wires and soldering. So don't buy the IO-Module if that's all you want. Now, if you believe in minimal signal path and relay controlled switching for these tasks, then by all means, go for the IO-Module. But what I'm saying is, I can show you how to do pads and polarity reverse for FREE if that's all you want.

But if you need a DI Box, want to be able to plug a guitar/bass/keyboard into your SC-1 preamp, want some DI transformers and output transformers, then the IO-Module is for you.

*Note: You also don't need to buy the IO-Module if all you want to do is connect an analog style VU meter to your preamp. But if you want a buffered output so you have some peace of mind that the VU meter is not affecting the output of the pre, then the IO-Module is also for you.

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I'm all for the buffered output for the meter and the D.I. Although personally I would prefer a flat active D.I. as opposed to using transformers. I do most of my instrument "sound" manipulation with software like Gearbox and Amplitude so the cleaner and flatter my recorded instrument the better. Those are my primary interests for the IO Module. I'm sure others will have other opinions.

I've ordered a couple of transformers to test the Hybrid DI circuit...

Scott, I'd probably make the transformers optional, so you can jumper them if you don't want it.

Now the question is discrete FET DI or chip-based FET DI. Decisions....

Advantage of the chip-based FET DI
- low power
- split psu requirement (which already exists on the PSU)
- low noise
- I can use the same part# that I used on the SC-1 mic pre kit, which just simplifies life (sourcing, inventory, etc)
- easier to assemble for the kit builder

Advantages of discrete FET DI
- old-skool analog design
- can't think of anything else much

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I'm also checking out some output trafos and consulting with the transformer company.

I haven't decided yet... but I'm looking at (2) options for the output trafo. Either one I think will work with the SC-1

1) The first trafo option will give a mellow sound and cut down any harshness in the sound (I know, it's subjective...). I would think this will add some obvious iron flavoring to the SC-1's clean output.

2) Second trafo option has dual secondary winding... very capable, clean and clear, low THD, but costs 4x as much as the first trafo option. (Used in the last Academy Awards live feeds). The dual secondary can be used to provide (2) outputs of the same signal... i.e. route the signal to (2) different gear or places at the same time, or use the other output for the VU meter, totally isolated.

Decisions, decisions....

Is there really a need for feeding (2) gear at the same time in the studio??? I would think in a live setting this is a requirement... not sure about recording in the studio.

Or should I just give the user the option of ordering either one of the output trafo?

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I think you already made the case for the chip-based FET DI.

As far as the trafo, unless you think the output transformer is going to be the big draw, I would just make it optional like you said, and provide part #s or links for people to order what they want. There are so many different transformers and they are all more or less expensive I can't see stocking any unless you think they are going to really move. Of course I'm biased since I probably won't use the trafo..

I'd really be interested in the I/O module for all the features (except for the DI, I've got a Radial Active DI & happy with it) especially the option to have output trafo's and the ability to bypass them. That would be awesome! Owel, let's go with Neve quality trafo's!

On another note, as far as future projects, I'd personally love to put together a UA 2-1176 clone compressor! (the 2-channel 1176), preferably with stereo/dual mono options, of course.

The neve uses carnhill trafo's right....spendy, aren't they across the pond?

I would say have a couple options for trafos...maybe some of those cheap edcor's to whatever you think sounds good. i think adding the trafo is a good idea because anyone can pick up a transformerless di for super cheap, but a nice DI with iron costs more.

If the vu meters I got don't have lamps, I heard I could just buy some lamps and install them somehow? Any idea?

I wouldn't use incandescent lamps... I'd rather use LEDs... you can have different colors... blue, violet, white, whatever.

If your LED needs only 5mA+, you can use the 3.32K resistor and attach it to the +18V psu line. If your LED needs more than 5mA, or you want it brighter, you can lower the resistor value.

R = E/I where E = 18V, and I= current requirement of LED (in Amps). Resulting R will be in ohms. Adjust to taste in brightness.

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Umm.... yea, so in english.... what do I need to do if I want bright led's?

(also, where do I place the led's? the meters are sealed shut and I don't want to risk taking them apart)

Start with a 2K resistor... and let's see where we go from there... I believe the light should be bright enough to shine through the plastic case. Just tape it behind, or on top of the VU.

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Alright!!! I have a working prototype of a passive/active/hybrid DI on my workbench.

It's kinda like a 2 stage hybrid DI... Passive > Active

Features:

- using a DI Transformer input for excellent noise isolation
- Ground lift switch
- After the DI Transformer, it goes to a High-impedance Active buffer opamp, with a slight 6dB gain just to counter the signal loss from the DI transformer.
- Connects to the mic input XLR of the SC-1 preamp (pins 2 and 3) from the output of the active DI stage.


From my testing, I used a 20 ft cable, connected to a Fender guitar. Set the SC-1 preamp gain to 12dB (2nd position on selector switch).

Results?

Hum free, noise free. Totally quiet. You only start to hear a slight hum when you push the preamp to the 9th gain position (54dB gain).

Mind you, this is with the guitar connected to the preamp via the DI.... none of that humming/buzzing noise you normally associate with electric guitars.

Because of the low noise, you can hear minor nuances in the sound. You can slide your finger up and down the fret and hear every detail in the sound.

No loading down of high frequencies here... you can do "harmonics" style fret fingering and it all comes out alive, crystal clean and clear out of the preamp. You don't even have to pluck hard the string to hear the sound.

I can't play a guitar, but man it sounds good. It's almost like a mic'd guitar amp... except it's totally clear, clean and quiet.

I haven't tested it with a bass guitar or acoustic guitar with piezo... will try to do that soon.

And this prototype is just on a breadboard with wires connected via alligator clips. So when this gets on a proper PCB with ground planes, I would think it will be even better/quieter.

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I hanged another transformer at the output of the DI. The second transformer converts the output of the DI to a balanced output, before feeding the mic input of the preamp. So the DI uses (2) transformers, the 2nd one being optional.

I'm having some weird issues.... if there is no jack unplug to the DI input, it's very hummy. But if the jack is plugged into a guitar, it's quiet. (Kinda opposite, huh?) This is the problem I'm trying to solve right now.

If anyone has an active DI, can you guys tell me how yours behave? I may have to buy my own active DI so I can compare it with something.

Also been playing with values of the resistors and caps. So far I think I've come up with a good combination.

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I use an active DI into my SC-1. Its a Samson S Direct Plus. With it connected to my SC-1 and nothing in its inputs it adds about 10dbRMS of noise at max gain. So instead of -75dbRMS with just the SC-1 I get about -65dbRMS with the active DI connected.

If I plug in my Les Paul (humbuckers) with the volume control on the guitar all the way down it adds about 3dbRMS more noise at max gain.

I don't know if thats good or bad, but the DI isn't an expensive one so it wouldn't surprise me if thats very noisy for an active DI!

Thanks Scott for the info.

> With it connected to my SC-1 and nothing in its inputs it adds about 10dbRMS of noise at max gain.

How do you characterize the noise added by your DI? Is it hum or buzz?

Also, when you connect your guitar to your DI, does the level of noise drop or increase further?

>If I plug in my Les Paul (humbuckers) with the volume control on the guitar all the way down it adds about 3dbRMS more noise at max gain.

What is the volume on the guitar is at max? Does it add more noise?

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