Simple amplifier circuit to boost a weak line-level signal


#1

Let me first apologize for taking up time with his somewhat ignorant DIY question. Please feel free to ignore me, or to tell me to do my research myself. :slight_smile:

Here’s the situation: I have a few Roland Boutique modules that all output at somewhat low levels. The mixer that I’m using (Mackie 1202) doesn’t have gain controls on its stereo channels.

Because of this, proper gain staging is only possible when I connect a Boutique to a mono channel on the mixer. Only on the mono channels can I use the gain control to bring the input level up to about 0dB. I would prefer to use the stereo channels for the Boutiques, but even with the output volume turned all the way up on the Boutiques, the input signal won’t go above -10 to -20dB on the mixer.

What I’d like to build is a simple amplifier that I can put in between a Boutique output and the mixer input that brings the level up about 12dB. There’s no need to be able to adjust the gain using a pot.

Do any of you have a suggestion for a simple circuit that would do this trick in the most sensible way possible using modern and commonly available parts?


#2

How will it be powered?


#3

I’m happy to power it whatever way is most convenient. Ideally, I’d turn this into a 8-channel box with 4 stereo minijacks in and 4 stereo minijacks out, so it would make sense to have a more complex power supply section if that keeps the amplifier circuits simpler.

EDIT: Being able to use a standard +/-12V Eurorack power supply might be convenient.


#4

The availability of a bipolar power supply makes this super easy then!

Just use an inverting or non-inverting amplifier circuit straight from a textbook. 30k / 10k (in non-inverting configuration) or 39k / 10k (in inverting configuration) to get a gain near 4. Add a 220 ohm resistor at the output for protection.

NJM2068 or even just TL072s.


#5

Thanks, that should indeed be straightforward. I’ll let you know how it goes.


#6

Breadboarded this today, works great. Now to think about a way to make this permanent.


#7

Here’s a photo showing the first two channels of the Boutique Booster™ soldered onto an Adafruit perma-proto PCB:

Really happy with this. Now to find a nice case to put it in.


#8

Isn‘t your labeling of the 220R and 10K resistors mixed up?


#9

Yes, you’re right. Thanks for catching that mistake! :slight_smile:

I’ve fixed the photo in my comment above; the labels should now be correct.


#10

what is the power supply you are using there?


#11

It’s the PCB from https://www.tindie.com/products/dhaillant/mini-dual-power-supply/


#12

thanks!


#13

You’re welcome. It‘s a neat little linear power supply.

I’m actually thinking of trying a Mean Well switching DC-DC module for this one to keep the whole package as small as possible and to enable the use of a standard 9V DC wall-wart.

Something like a DCW05A-12 (http://www.mouser.com/ds/2/260/DCW05-spec-1109513.pdf) or a DLW05A-12 (http://www.meanwellusa.com/productPdf.aspx?i=87#1) should probably work.

Or do you think I’m overlooking something here?


#14

I’m actually using this kind of converters in the test fixtures for the new modules. They work great!

You won’t need 5W of power though: just a handful of op-amps won’t even consume 1W. So maybe you could consider smaller alternatives like Traco Power TMR1-1222 or TRN1-1222.


#15

Thanks. Just ordered a TRN1-1222. Was filtering on Mean Well only initially because of good experiences with them when I still had my modular. :slight_smile:


#16

I was not entirely prepared for how tiny these modules actually are! :slight_smile:

The wiring has gotten slightly messy, because I did not planned to put power onto the same board, but it’s really nice that it all fits on there:

Thanks @pichenettes, that was an excellent suggestion.

Now to find a nice compact case to put it all in.


#17

This is a cool thread - reminds of me of the shruthi days, when there was more talk about DIY and figuring things out for yourself. Not that I have an issues with the forum nowadays, I just enjoy DIY problem-solving!


#18

…except that this is infinitely simpler, of course. :wink:


#19

Would anyone like to have a nice PCB for this?


#20

Found a pretty decent ’90s style semi-transparent case!