Whistle Rock Audio - New 312 PCB with a twist!

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mikefatom

Well-known member
Joined
Apr 5, 2009
Messages
640
Location
Toronto, Canada
Ladies and Gentlemen,

Everyone seem to have made their own version of the classic 312 pre. This is mine and this is what I've been working on during my time off from work. It is nothing mind blowingly new but I wanted to share and get feedback on it :)

Description/features:

- Straight up basic 312 pre circuit, no fancy build option, but a couple extra features added.  ;)
- Relay switchable HPF made up of a simple RC network before the 2520. Simple but very effective for proximity effect bass correction. I tested many options including active circuits but settled on this more subtle effect.
- Minimum signal path from the mic input to the input TX, nothing in between!
- Automatic relay-switched DI input option. The DI signal goes straight to the op-amp. I will start working on an add-on FET DI board for it.
- Relay switched phase reversal done between the op-amp and output TX.
- Relay switched TX output ratio, from 1:2 (normal) to 1:1 (-6db attenuation) to pad the output a bit and give a slight tonal variation. I like that feature, but I am debating changing this to a switchable T-Pad. opinions?
- Phantom power capacitor gets discharged through the indicator LED (when disengaged), to avoid undesirable POPs when plugin in a different condenser mic.

Schematic is HERE.

More info to come...

but for now, it's picture time! Click on the image for more and hi-res pictures.



Comments? thoughts?

Mike

 
Some nice additions there , no pad ?
going to need it if you do drums
can always use a hpf , what freq ? how steep ?
you could put two , one after the opamp , maybe
if that doean't screw with the impedance ganged
on one switch  [ two pole filter ? ] for a steeper slope

Nice work
 
Thanks guys.

babyhead: I tried to make it extra sexy ;)

Greg: No input pad. I decided I would rather have a HPF and an output pad instead. No one seem to like using the input pad anyway... Maybe in a future revised version  :)

The HPF is @ 90Hz with a -6db/dec slope. Just enough to tame down the boomyness of close miked sources. I chose this frequency very carefully to my liking. One could choose any frequency he/she wishes though. It's just a matter of selecting the proper R and C of course.

cheers

Mike
 
Mike,


  I don't know about nobody seeming to use an input pad! Pretty much essential for a 312 if you ask me! Especially as 312 excels on drums, gtr amps bass amps, etc.  . . . You could always build a 20dB pad into a very short mic lead and insert inline when necessary. That's what I have, 'cos I have a load of pre's with no pad.


  I actually sometimes prefer the sound with the pad in and 20dB more gain, and swallow the extra noise as an acceptable evil with quieter sources. I am not sure why. it seems to be "faster" with higher gain settings. Maybe the slew rate increases with gain(?) - or is it just that i prefer it with less transformer saturation? This effect is more marked with some pre's than others. Noticable with 312. but VERY noticeable with Helios'es!


  Kindest regards,


      ANdyP
 
Andy: I hear you. I don't record much drums, but when I have in the past, the problem was always the hot level hitting the A/Ds. once I padded them down, they would sound :D. That's why I am seriously thinking of putting a switchable t-pad at the output. I have to let my serious AE friends experiment with it and see what they think of the circuit as is.

Jay: Thanks :)

tv: It needs to be authoritative to power up 8 of those pres  ;).

Thanks for he feedback guys,
Keep them coming, good or bad :).

Cheers

Mike
 
Hey Mike!

Looks extremely well tought and crafted.
Believe me a PAD is extremely useful on the 312 preamps.
You just need it there, is not an option....i don t record drums but still use it.

You better switch ratios on the input transformer rather than output...(just my toughts)

Personally i'd stick with a simplistic aproach...but thats just me!

Good luck

Jorge A.

 
Looks like sockets for the filter components? Nice idea.  8)

Looks pretty darned slick Mike!

Cheers, Jeff
 
Jorge: Thanks for the compliments. It seems like the inclusion of an input pad on a 312 pre is considered a must :p. I think the next version will see some changes...

Jeff: Thank you sir! As you can see, your EA2622, EA2503 and the C&K pushbuttons were put to good use ;)

I did a few listening tests and so far so good. It seems a LOT quieter than my previous 312 build which is from a Fabio board. It may be due to the ground planes that aren't there on Fabio's boards. Everything works a charm! I'm quite happy with the performance so far.

Cheers

Mike
 
hey mike,

just saw this... looks nice.  Glad you got the HPF working.. I didn't give it enough time back then, but I'm glad it works. 

I also say to add an input pad, it's almost 100% necessary.

The idea of switching the output trafo 1:1, 1:2, is pretty clever. Congrats on originality. But unless the tonal variety you mention is very cool, I'd say go with an output pad so the user has control over the attenuation (I´m using a 10dB pad, as suggested by DrPat in the API thread).  If you decide to add an output pad, you shouldn't need a balanced T pad, just a U pad will do.

Power supply caps could also be a bit smaller (100uF is plenty for a 2520).

That's about all the constructive criticism I can offer, I'm dead tired. Good luck!
 
gar381 said:
Very Nice Mike  :)

Looks GOOD and I bet it Sounds GREAT !!!

BTW.. Nice use of relays  + Phase reverse IS in a
better place.  Like it Like it.

(How is that little black DOA sound with it ?? ;D)

GARY

Thank you for the kind words Gary. I REALLY like the sound of your gar1731. Sounds gorgeous!  8)
I am so excited about all the possible tonal variations this preamp + a ML2520, a gar2520 or a gar1731 can give me.

If the music ain't happenin, I only have myself to blame now!  ;D
 
mitsos said:
hey mike,

just saw this... looks nice.  Glad you got the HPF working.. I didn't give it enough time back then, but I'm glad it works. 

I also say to add an input pad, it's almost 100% necessary.

The idea of switching the output trafo 1:1, 1:2, is pretty clever. Congrats on originality. But unless the tonal variety you mention is very cool, I'd say go with an output pad so the user has control over the attenuation (I´m using a 10dB pad, as suggested by DrPat in the API thread).  If you decide to add an output pad, you shouldn't need a balanced T pad, just a U pad will do.

Power supply caps could also be a bit smaller (100uF is plenty for a 2520).

That's about all the constructive criticism I can offer, I'm dead tired. Good luck!

Thank you for the constructive criticism mitsos. I really appreciate this. I have already modified my layout and schematic to include an input pad in the next version. I'm considering using a variable T-pad attenuator like the one Jeff (Classic API) and Mike (Hairball Audio) use for the output trim. It's easier and more flexible I believe. It can be added to  any API type output TX after the fact anyway.

hmmm... choices choices....  :p

I agree about the PSU caps being larger than needed. I like over doing things  :-[.

Thanks,

Mike
 
Ladies and Gents,

I already posted this in my WM thread, I thought I would put it here as well as a continuum to what this thread was about.

---------------------------
Here is a little preview of what I'll have available for DIYers soon :). It's a 312 PCB for 1U format but with a small twist, a proximity effect compensation filter... I mean... and simple RC HPF.  ;D
It has relay switched input PAD (I know I know... ;)) , HPF and polarity reversal. Gain is step controlled with a 12 position Grayhill switch. There is provision for an optional DI board as well with automatic input switching (straight into the DOA). The whole thing is meant to be powered up by the ML12 PSU I've been selling.

I wanted something simple, elegant and very easy to assemble for friends who aren't the most experienced at soldering.
I think I got something pretty decent. :)

ML12v2.0%20Snapshot.tiff


I'll have the 500 series version next as well as an EQ to match! 8)

More to come!
Cheers

Mike
 
looking good so far. If you ask me input pads are non conducive to making a loud record that will distort, and have to compete with other loud recordings that distort, in a market place where people buy the louder more distorted album because louder is better, in the minds....

If you really want it right, make the input pad variable... ALA JLM

Variable%2020dB%20PAD.gif



 
pucho812 said:
looking good so far. If you ask me input pads are non conducive to making a loud record that will distort, and have to compete with other loud recordings that distort, in a market place where people buy the louder more distorted album because louder is better, in the minds....

If you really want it right, make the input pad variable... ALA JLM

Variable%2020dB%20PAD.gif


lol. Are you saying that loud, distorted records are good? 

I'm not sure i understand.  :)

Personally, i can't stand the sound of a smashing SSL. Maybe the odd time...
then i go back to 'Brothers in Arms'.

Preamp looks awesome Mike. Pad is another good feature.

Darrick

 
This looks great Mike.

I thought I was done with preamp building and then something new comes along!

Very much looking forward to giving this a go. Does this version have 48V? You didn't mention it.
*Edit - looks from the trace like R1 & R2 are phantom resistors, so I think so.*

I absolutely love the sound of the API A2D, but the tiny knobs irritate me.
 
Thanks guys,

pucho812: The dual pot looks like a good idea. How does that affect CMRR?

desol: Thanks :)

zebra50: Yes! 48V phantom included ;)

Cheers

Mike
 

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