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???? 
(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.