Small mic pre/mixer - now with pics!

[mcs]

Here's a PCB layout.

I had forgotten the 100R resistors accross the input choke (recommended by Samuel) - I hope the rest is OK...

Best regards,

Mikkel C. Simonsen

 

[mcs]

Here's a schematic of the mixer section. I don't know what happened to the pot symbols...

Best regards,

Mikkel C. Simonsen

 

[flaheu]

Why not adding a line input by switching 2 10K in on each input ? (see 1512 datasheet )

I'm looking at your schematics and find it very nice, I'm looking myself for a long time for a mixer like that.

The 1512 seems to have a very good reputation, I think it will grab these for my mixer.

:thumb: :thumb:

 

[mcs]

[quote author="flaheu"]Why not adding a line input by switching 2 10K in on each input ? (see 1512 datasheet)[/quote]
I did that in my "bigger" version of the circuit, which can be seen at the bottom of this page: http://www.groupdiy.com/index.php?topic=10417&postdays=0&postorder=asc&start=30

In this case I need a small box, so I'm not sure there's space for 20 input connectors on the back panel...

I'm looking at your schematics and find it very nice, I'm looking myself for a long time for a mixer like that.

I looked through some catalogues also and found nothing - that's my I'm building one myself instead.

The 1512 seems to have a very good reputation, I think it will grab these for my mixer.

It works well in my experience at least.

Best regards,

Mikkel C. Simonsen

 

[mcs]

I have (almost) assembled a pair of boards now. Here's the mic pre board:

And here's the mixer/headphone amp board:

But I still have a couple of concerns with the RFI filter. One thing I'm wondering about is the 100R resistors accross the input choke. Wouldn't they effectively short circuit the choke? The impedance of the 1n cap is approx. 160R at 1MHz (if I remember my formulas). With less than 100R in series and 160R to ground, that doesn't sound like a very effective attenuator... Or am I missing something here?

Another concern is the 1n filter caps. Will the mic be able to drive a capacitance that high, when you add the capacitance of 5-10m of cable also? The mics in this case are either dynamic mics or Audio Technica lavellier mics (the type that can run on a 1.5V element or phantom power).

Best regards,

Mikkel C. Simonsen

 

[mcs]

If anybody has comments on the EMI filter bit of the last post, I would love to hear them.

I need the preamp on Friday, so I have to finish it tomorrow :shock:

Best regards,

Mikkel C. Simonsen

 

[Samuel Groner]

I have no idea what inductivity your CM choke got so it's difficult to answer your question. Basically you are right that parallel resistors do reduce effectivity at some high frequencies but as I've pointed out it is likely that you'll get serious peaking otherwise and hence increase sensistivity to RFI rather than decreasing it. Run a few simulations with source impedances of 20 ohm to 300 ohm and you'll see what I mean.

That's what I'm currently using as RFI filter: A_r1.pdf

Samuel

 

[mcs]

[quote author="Samuel Groner"]I have no idea what inductivity your CM choke got so it's difficult to answer your question.[/quote]
I use one of the types you mentioned some posts back - the Farnell 926-5783 (2*40µH).

Basically you are right that parallel resistors do reduce effectivity at some high frequencies but as I've pointed out it is likely that you'll get serious peaking otherwise and hence increase sensistivity to RFI rather than decreasing it. Run a few simulations with source impedances of 20 ohm to 300 ohm and you'll see what I mean.

I'll try.

That's what I'm currently using as RFI filter: A_r1.pdf

Looks interesting!

Thanks for your reply.

Best regards,

Mikkel C. Simonsen

 

[Samuel Groner]

I quickly run the simulations myself for three different values of CM choke shunt resistors (100 ohm, 220 ohm and 470 ohm) and the load configuration given in the schematic posted above:
[removed]

Each simulation is for three different source impedances (20 ohm, 150 ohm and 300 ohm).

The 100 ohm figure seems to be rather conservative. On the other hand, some microphones have inductive source impedance which again increases peaking, so perhaps 220 ohm is the highest value I'd recommend.

Just to prevent misunderstandings: these frequency responses apply for common-mode signals only, so a bit of peaking is not as serious as if it were for the differential response.

Samuel

 

[mcs]

Too bad you removed your results, as I wanted to compare them...

But here are my results. I think they are similar to yours. I don't remember how to combine plots of different values, so I had to make a plot for each.

Here are the results for a 20R source impedance:

http://stiftsbogtrykkeriet.dk/~mcs/RFI/RFI-20Rs-100R_par.gif
http://stiftsbogtrykkeriet.dk/~mcs/RFI/RFI-20Rs-220R_par.gif

And the same for 100R source:

http://stiftsbogtrykkeriet.dk/~mcs/RFI/RFI-100Rs-100R_par.gif
http://stiftsbogtrykkeriet.dk/~mcs/RFI/RFI-100Rs-220R_par.gif

I also made a plot with no parallel resistors:

http://stiftsbogtrykkeriet.dk/~mcs/RFI/RFI-100Rs-no_par.gif

When the source impedance rises, the bump disappears - I guess that's correct. This is what I get with 600R source and no parallel resistors:

http://stiftsbogtrykkeriet.dk/~mcs/RFI/RFI-600Rs-no_par.gif

There are a couple of other files in the folder also.

This is the code I simulated:

ThatMic indgangsfilter

R1 2 5 2.7k
R2 2 0 6.81k
R3 5 0 6.81k
R4 2 3 4.7R
R5 5 6 4.7R
R6 3 0 10k
R7 6 0 10k

* parallel
R10 1 2 100R
R11 4 5 100R

* source
R8 4 0 20R
R9 in 1 20R

C1 2 0 1n
C2 5 0 1n

L1 1 2 40u
L2 4 5 40u

* Input signal
Vin in 0 AC 1

* Literals
.OP
.AC DEC 100 10 10meg
.TF V(3) Vin

.WIDTH OUT=80
.OPTIONS LIMPTS=10000
.PROBE
.END

Best regards,

Mikkel C. Simonsen

 

[mcs]

Here's the almost complete mixer (I ran out of sockets at channel 8...):

Front:

Back:

Inside:

The front panel is from Schaeffer - I was too cheap to get the back panel done there also.

The mixer was used yesterday at "Kløften Festival", and it worked very well. We were the opening act! Not exactly at the biggest stage though :green:

Best regards,

Mikkel C. Simonsen

 

[skipwave]

Wow! Excellent work, so neat and clean. I've been using the 1512 preamp a lot. A little mixer full of them would be so useful. :thumb:

 

[rafafredd]

Man, there´s a very nice collection of kits and the prices also looks really nice. How haven´t I seen it before??? Michael, I know you hate using the forum as a market place, but please, let us know that you are offering things like these, and let us know once n a while when you ave new kits comming. It eally can help people to make higher quality DIY stuff...

 

[Eric H]

Beautiful.
Screwing the standoffs together looks to have been very entertaining.

-Eric

 

[mshilarious]

Regarding the input impedance, the electret capsule mics are typically happy with impedances as high as you like. The phantom resistors alone would be good for them. If you're still doing a few more boards as your pictures indicate, you could do some "electret" boards, and then go back and do a few "dynamic" boards with a value lower than 2K.

 

[mcs]

[quote author="rafafredd"]Man, there´s a very nice collection of kits and the prices also looks really nice.[/quote]
Thanks!

How haven´t I seen it before???

Don't tell anyone - they are a big secret! :shock:

Michael, I know you hate using the forum as a market place, but please, let us know that you are offering things like these, and let us know once n a while when you ave new kits comming. It eally can help people to make higher quality DIY stuff...

I think most of the stuff I have isn't really suited for recording use, but some is of course. I also have a few things on the way that could be interesting to people here...

[quote author="Eric H"]Beautiful.
Screwing the standoffs together looks to have been very entertaining.
[/quote]
That was quite easy actually. I discovered that those two standoffs have the exact right length when they are connected and tightened :grin:

[quote author="mshilarious"]Regarding the input impedance, the electret capsule mics are typically happy with impedances as high as you like. The phantom resistors alone would be good for them. If you're still doing a few more boards as your pictures indicate, you could do some "electret" boards, and then go back and do a few "dynamic" boards with a value lower than 2K.[/quote]
The electrets I'm using are not "raw" capsules. They are connected to a phantom-powered box the size of a cigarette pack, so I guess that contains an output buffer of some kind. Since there is a balanced output it must contain something...

Best regards,

Mikkel C. Simonsen

 

[carlmart]

Any suggestion on how to implement this design with a single supply, likely using a -V inverter and a DC-DC 48v phantom generator?

On considering options for a two channel portable preamp, and THAT's chip might be an option. But power has to be some kind of battery up to 12v.

Ready made DC-DC supplies, for say +/-12V are not really an option, as cost is an issue. An inverter at most.