My microphone preamp (Finally!)

[Flatpicker]

Here?s a proto mic pre I?ve been working on for the last year.

http://mywebpages.comcast.net/flatpicker/Preamp1.jpg
http://mywebpages.comcast.net/flatpicker/Preamp2.jpg

Of course it didn?t take that long to layout and build - most of my time was spent waiting for money to come in that didn?t have to go right back out, if you know what I mean. Anyway, I finally managed to assemble one channel of it over the holidays, but my digital camera was broken so I could not post pictures until now.

My intention was to layout a board that allowed me to try different op amps, input transformers, and output configurations. The one in the pictures is configured basically like the ?JFET Mike Preamp using Jensen JE16 Input Transformer? circuit shown on Fred Forssell?s schematics page. The 12 position switch sets the gain in 5dB steps and the pot is a 5dB gain trim. The 3 switches are pad, phase and phantom power.

For the record, I am NOT using the API type output transformer in this configuration ? I just threw it in there for ?looks? while taking pictures. The chassis is from Par-Metal and I punched the holes using a manual press-punch at work.

Except for one or two schematic screw-ups (that ended up being pc board screw-ups, hehe) it works great. Sounds great too. I can crank the gain up to 70dB and it?s extremely quiet. In comparison the pres in my Aardvark Q10 sound muddy, and with the gain set to 70dB using the same monitoring levels, the noise sounds like Niagara Falls!

I wanted to try something different and put the transformer immediately after the XLR, hoping that the extra gain would improve the signals S/N a little before traveling across the board to the switches. Also, since I?m using the LL1538XL and plan to mostly record acoustic music, I reasoned that it couldn?t hurt to try it this way.

In hindsight, I should have kept the pad and phase switching on the primary side of the transformer using relays and silver DPDTs instead of 4P4T gold push buttons. The relay version would have actually been cheaper, plus you can?t find the needed make-before-break pushbuttons anyway. To be honest, I can?t remember why I was so hell-bent not to use relays. Ah well? learning ? that?s what it?s all about, isn?t it?

Needless to say, a new ?improved? version is in the works. Now, if I just had some more money? :wink:

 

[Chuck78]

Have you put audio through it yet?? If so how does it sound? Maybe some audio clips would be in order?

Looks great by the way.
:grin:

 

[Category 5]

VERY nice work Tim. It looks great! Congrats.

Now PLEASE tell me where you found the single deck Grayhill! I need a pair for my Bauman A-P-eye preamps. Grayhill's website just locks up when I try to do the search.

Shane

 

[Flatpicker]

Hey, thanks for the complements, guys.

Chuck - Yes, it sounds great (and will sound even better when I replace the 992 with a Forssell 993!). I'll try and put up some samples when I get a chance.

Shane - Here's the Grayhill and Newark part numbers:
Grayhill #71BDF30-01-1-AJS
Newark #91B8747

I also laid out the board to use this Grayhill PCB mounting accessory for the switch listed above:
Grayhill #71C2053
Newark #93B5778

I've ordered them from Newark on two separate occasions and they always quoted me long lead times, but I ended up getting them within a couple of weeks.

 

[Marik]

Tim,

Finally!!!! :green: :green: :green:
Looks very nice :thumb:
Now, your task is to start recording. :guinness: :guinness:

 

[Rossi]

Great work, Tim. Very nice!
Beautiful case, too!

One question: I see that C23 has such long legs while all others are short. Is there a reason for that?

 

[Flatpicker]

[quote author="Marik"]...Now, your task is to start recording...[/quote]Hehe, I actually began working on some serious recordings this week. Up until now, I was only recording short clips to test it out. I need to get the other board assembled and put in there.

 

[Flatpicker]

[quote author="Rossi"]...One question: I see that C23 has such long legs while all others are short. Is there a reason for that?[/quote]C23 is the "speed" cap for the bargraph meter. I ended up trying different values after I had bolted in the board, soldering them in from the top. I just happen to leave the leads long on it.

 

[kevinc]

Very nice job, Tim!! And I really like the look of that Lundahl input transformer, too! :wink:

 

[pmroz]

Flatpicker, beautiful job. also thanks for Grayhill info...

on the Q10, do you bypass Q10 pres by plugging your nice pres into the inserts? Will the 1/4in in front bypass pres also?

i'd like to use the inserts for inserts, but that front 1/4in will probably not bypass pres. Too late to ask aardvark... hmmmm...
i see there is a 'support group'...

 

[mkruger]

cool beans!

 

[Flatpicker]

Hey Kevin - I've got a LL1678/adapter-board as well, so that I can plug it into the same spot. I haven't tried it yet, tho. I made my transformer adapter boards with jumpers so I can try different ratios.

 

[Flatpicker]

[quote author="pmroz"]on the Q10, do you bypass Q10 pres by plugging your nice pres into the inserts? Will the 1/4in in front bypass pres also?...[/quote]Yes, use the inserts. If you need an "insert" just patch it in between the pre and the Q10 insert.

A few months ago I ask someone at Aardvark about this and was told that the Line input was transparent. But when I tried it that way it sounded like crap to my ears. Going in through the inserts sounds much better.

 

[pmroz]

Flatpicker, thanks

But when I tried it that way it sounded like crap to my ears. Going in through the inserts sounds much better.

i'm in agreement with this.
Would flip the whole thing round, except for master volume knob.
what to do with 5-8? when i get more than 4 good pres bet i'll go hacking away at it.

that's for tomorrow... for now nice work! congrats!

 

[Bryson]

Very pretty!

 

[giobinein]

:shock: :shock:
Very nice !!
What schem have you followed for the the LED meter ??

 

[Flatpicker]

[quote author="pmroz"]...when i get more than 4 good pres bet i'll go hacking away at it...[/quote]I?d like to try that as well. Let me know when you do so we can collaborate. I sure wish we could find a schematic for that thing!

 

[Flatpicker]

[quote author="giobinein"]...What schem have you followed for the the LED meter ??[/quote]It's similar to circuit that Peter C has on his website (there's thread about it here that someone just made current) except mine is an 8 LED version. It's a fairly simple circuit where the signal is buffered, rectified, and connected to one of the comparators inputs. The other comparator input is tied to a resistor voltage divider string. The tricky part is calculating the resistor values for the resistor voltage divider string, depending on the reference voltage and the level at which you want it to turn on the LED. I?m still tweaking mine.