iefes

Help with simple 2 Ch Mic preamp with Mix, 9V Battery
« on: November 05, 2018, 06:26:37 AM »
Hi everybody, I'm fairly new to the forum looking for some help from you experienced people.

I'm trying to build a simple two-channel mic preamp, powered off a 9V battery to go with the accordion of a friend of mine. His accordion has two built in mics with a  trs jack for each. My plan is to build a small box which can be mounted to the belt/strap of the accordion. I would like to be able to control the volume of each mic independently and mix the signals to a mono-signal OR leave them separate and use two mono outputs, one for each channel.

I thought I'd use the simple mic preamp presented by ESP (https://www.abcelectronique.com/annuaire/montages/cache/1034/preamplificateur-pour-microphone-faible-bruit.html), one of those for each channel.

How can I mix the two channels with their own volume control? My idea was to put a 10k volume pot to the output of each channel and mix them using an opamp. What would be the best way to do so? I have tried some configurations and simulated them in spice but the results don't look very promising. I think one problem is that the volume pot interferes with the feedback-network used to set the gain of the preamp.

The configuration that can be seen in the attached schematic is what seems to work best so far. I think I would have to reference the non-inverting input of the opamp to half the supply voltage, right? But this gives weird results unfortunately.

I would like the output of the whole circuit to be able to drive a long unbalanced cable going to a mixer or something.

Are there any other ideas on how to mix the two signals with each having their own volume-control? The gain will be set by a trim-pot once we have figured out what works best with those old mics.

Any help would be highly appreciated. Thank you very much!


L´Andratté

Re: Help with simple 2 Ch Mic preamp with Mix, 9V Battery
« Reply #1 on: November 05, 2018, 07:28:40 AM »
One important thing first: do you need phantom power?
Strictly amateur since 1973...

iefes

Re: Help with simple 2 Ch Mic preamp with Mix, 9V Battery
« Reply #2 on: November 05, 2018, 07:38:49 AM »
No phantom power needed. The mics seems to work nicely. We did a quick test on my guitar-amp and it worked. So I assume these are dynamic microphones where no phantom power is needed.

L´Andratté

Re: Help with simple 2 Ch Mic preamp with Mix, 9V Battery
« Reply #3 on: November 05, 2018, 08:42:33 AM »
Ok iirc there´s a version on the ESP site of this circuit using an opamp follower as output, that should drive
your faders alright.
Maybe TRS jacks mean balanced signal, have you checked that or ruled that possibility out? It has advantages, e.g for longer cable runs, btw. what did you mean with long, some wouldn´t consider 10m long...

There are two 220u caps unconnected on your schem. The reference voltage could need another 10-47u cap for filtering (yes even with a battery). You could also put two batteries in series for +/-9V and  tap 0V from the middle. Just some ramblings until the experts chime in  ;D
« Last Edit: November 05, 2018, 08:58:59 AM by L´Andratté »
Strictly amateur since 1973...

iefes

Re: Help with simple 2 Ch Mic preamp with Mix, 9V Battery
« Reply #4 on: November 05, 2018, 12:07:13 PM »
Thanks for your suggestions!
I have simulated the version including the opamp and got a smaller voltage swing before clipping, that's why I discarded this idea but maybe it just needs a few value-tweaks.
With the design including the opamp follower are 10k pots okay as volume controls? And should I still put another opamp stage behind the volume pot for mixing or just mix the signals passively?

The jacks are mono so it should be unbalanced. This is quite an old accordion so I think there's nothing balanced in there.
More filtering will be included, yep, and I might go with two 9V batteries depending on how much space the circuitry is going to occupy. But best would be only one 9V battery.

iefes

Re: Help with simple 2 Ch Mic preamp with Mix, 9V Battery
« Reply #5 on: November 06, 2018, 03:36:45 AM »
So, I simulated the opamp follower version again and found that changing R4 to 2k and R5 to 330 gives good results. However, after re-reading ESP's article where he mentions that the discrete setup should be able to drive a tonestack or faders, I changed everything back to the two-transistor design (per channel).  I like this version because of the slightly smaller footprint.

The circuit attached has been simulated with spice and seems to work well. The output will be out-of-phase from the input but this shouldn't hurt because it'll always be the very first device is the signal chain.

However, I'm a novice and just know some stuff about effects-pedals and guitar-amps. That's why I would like to ask you for further tips and suggestions. Is there anything in the circuit which is not necessary? How about the 100k at the output? Is it needed?
Would you suggest some high-frequency roll-off? And maybe a low-cut as well?

Some general aspects of a good preamplifier regarding the frequency response would be really nice.

Any ideas regarding the switching between 1 channel out and 2 channels out? It would be cool to have it switch to mono when there's no plug inserted in out2. Any ideas on how to achieve this?

Thanks a lot!
« Last Edit: November 06, 2018, 04:02:37 AM by iefes »

moamps

Re: Help with simple 2 Ch Mic preamp with Mix, 9V Battery
« Reply #6 on: November 06, 2018, 04:45:27 PM »
If nothing else, you need electrolytic blocking caps at the outputs as well.

iefes

Re: Help with simple 2 Ch Mic preamp with Mix, 9V Battery New
« Reply #7 on: November 07, 2018, 04:12:22 AM »
Okay, thanks! I've included them now. Additionally I have used smaller coupling caps after Q2 to cut a bit of the low-end below 30Hz. Those frequencies are not necessary anyways. Same for the very high-frequencies which I cut around 50k using a simple LPF.

I think I'll put it together in the next days and see how it performs. If you have further suggestions please fire away!
« Last Edit: November 07, 2018, 07:53:13 AM by iefes »


 

Related Topics

  Subject / Started by Replies Last post
29 Replies
12290 Views
Last post November 27, 2005, 03:04:28 AM
by PRR
7 Replies
5323 Views
Last post May 02, 2014, 01:09:15 PM
by carlmart
5 Replies
1948 Views
Last post September 27, 2011, 07:08:47 PM
by ncoak
2 Replies
2460 Views
Last post June 18, 2013, 07:55:28 PM
by joaquins