Acoustic guitar preamp data?

[Morocotopo]

Hi guys. Today a friend asked me if I could build him a preamp for his acoustic, in a box format (not internal). It has a, I believe, magnetic pickup and a mini condenser. There´s very little about acoustic pres in general in the net. Lots of Tube Screamers but few clean pres. I guess it should have a stereo in, independent EQ for each channel, phantom power capability, maybe a balanced out, a parametric mids control, FX loop...
So, have any of you made something like this? The basic building blocks are nothing too complicated. But before starting to design/patch things together, I thought maybe someone already did this.

Any schems/pages/advice you can give me, will come in handy.

Thanks.

[Morocotopo]

Ok, it´s pretty clear that everyone is busy building Fuzz Faces, Tube Screamers and Big Muffs...



No one ever built an acoustic preamp?

[tempus]

It has a magnetic pickup (not  piezo)? If it has a piezo, you absolutely need a buffer before anything else happens - the output Z of piezos can be 1MEG at 100 Hz.

It sounds like you're going to be pretty adventurous here, but we could use a bit more information.

Is the preamp going to be onboard?

If you're going to have a balanced out, are you planning on plugging directly into a mic input on a mixer or amp? Before you get into sweepable mids and FX loops, you need to consider how this'll be used in a real world situation, and you might want to do a little experimenting with levels. For instance, if it is a magnetic pickup, it'll likely distort the input of a mic pre on a mixer (typically where phantom power comes from).

Are you going to somehow blend the 2 signals into one (stereo but not balanced) output or have 2 separate outs?

At this point, I can give you some useful info. I use a condenser mic in my classical and have for years. It doesn't need any preamplification if I plug directly into a mixer (which I typically do) but it does need to have its output Z lowered. This is easily done with an opamp or a simple transistor buffer. I don't know about your buddy's rig though, and it may well be that it doesn't need anything at all if you're going from condenser right to mixer.

You need to find out and disclose a little more about your design before I can help you any more with your plans.

[Morocotopo]

Tempus, thanks for the answer.

The idea is to make a "swiss army preamp", I mean, it has to have two inputs configurable for mic/mag/piezo, so I guess 10 Megohms input impedance in at least one of them. The guy has a Marshall acoustic amp, but could eventually plug into a mixer directly. It has to have phantom power capability, but also battery power (two 9V batts I guess). It will be an external unit, not onboard the guitar so as to be able to use it with several guitars. The output will be a blended one, but including separate ones wouldn´t be too difficult and very useful, specially with a mic. Gain/level in each channel. Then comes the extras: EQ, notch filter, loops.

So, as you can see basically the idea is to make the most versatile unit possible. I´m thinking op amps for the actives, I can get NE5532, TLC2262, the TL family, LF353, will have to see wich one works best for the job.

There´s not much more I can tell you right now, I´m still in the planning stages. Next I should at least make a block diagram of the main parts I guess. But any info you can give me will be useful.

Let´s say I have a two input pre, both with 10M impedance. Will both be good for the mic/mag/piezo? Is there any problem putting a mag or a mic into such a high impedance?

[tempus]

The problem with the high input Z is more that it will add noise to the overall sound. It's good practice to keep the input Z as low as is practical. If you have a 10 meg input Z on a preamp, and you want a gain of 50, you have to use a 500meg negative feedback resistor (that's going to be a lot of added noise), unless you use a buffer first and then go to the actual amplification stage. So I'd design 1 channel for a mic input, and another for a mag/piezo. Maybe use a TLE2071 (which is a lower noise version of the 071) for the piezo as a buffer and then go to a low noise BJT like the 5532 for amplification, and just use a 5532 for the mic section. You'd need to find out the output Z of the condenser to set the input Z on the pre, but typically modern mic preamps use 1500 - 2K for their input Z. As I mentioned earlier though, if the mic is going into a mixer, it won't need any amplification. You could design your box to just deliver line out signals, but that'll rob you of balanced ins and phantom power, which you mentioned in your original post. If you do want these things, you'd have to look at possibly attenuating the mag/piezo output so that it won't slam a mic input on a mixer.

The other issue you may have is that the piezo really benefits from having some type of buffer onboard, since that extremely high output Z makes it difficult to run a cable of any length without losses. That might not be your problem though, if you're just planning the outboard interface between the guitar and the amp.

Hope this helps. Keep the information coming and I'll help out where I can.

[Morocotopo]

The problem with the high input Z is more that it will add noise to the overall sound. It's good practice to keep the input Z as low as is practical. If you have a 10 meg input Z on a preamp, and you want a gain of 50, you have to use a 500meg negative feedback resistor (that's going to be a lot of added noise), unless you use a buffer first and then go to the actual amplification stage. So I'd design 1 channel for a mic input, and another for a mag/piezo. Maybe use a TLE2071 (which is a lower noise version of the 071) for the piezo as a buffer and then go to a low noise BJT like the 5532 for amplification, and just use a 5532 for the mic section. You'd need to find out the output Z of the condenser to set the input Z on the pre, but typically modern mic preamps use 1500 - 2K for their input Z. As I mentioned earlier though, if the mic is going into a mixer, it won't need any amplification. You could design your box to just deliver line out signals, but that'll rob you of balanced ins and phantom power, which you mentioned in your original post. If you do want these things, you'd have to look at possibly attenuating the mag/piezo output so that it won't slam a mic input on a mixer.

The other issue you may have is that the piezo really benefits from having some type of buffer onboard, since that extremely high output Z makes it difficult to run a cable of any length without losses. That might not be your problem though, if you're just planning the outboard interface between the guitar and the amp.

Hope this helps. Keep the information coming and I'll help out where I can.

Tempus, very very helpful. Yes, I know the higher the Z the higher the noise, you provide an interesting alternative though: buffer-->gain stage. Nice idea. Might even make a FET buffer for the piezo...
Yes, the guitar should have some sort of buffer. I was thinking maybe a short cable (no longer than, say, 3 meters) could be used without too much loss, but we´ll see.

I´ll define the whole system more clearly, so as to have a firm base from where to start designing. Thanks for the help!

[PRR]

> high input Z is more that it will add noise to the overall sound. It's good practice to keep the input Z as low as is practical.

The 10Meg bias resistor is _in_parallel_ with the source. Source impedance will divert the noise power. Super-high Z is fine, even good.

*Except* when the plug comes loose. Then a lower Z catches less buzz from the air.

> It has to have phantom power capability

Power management becomes an issue. Two TL072 input amps is easy, but a capable low-Z line-amp is hard, and adding frills tends to suck all your power out.

Find a commercial product. I'm not saying to buy it, but looking at what others have done is often good plagia**** research.

[tempus]

Thanks for the clarification PRR. And I forgot about the phantom power issue; the standard phantom power arrangement is 48v through a 6.81K resistor (2 legs for balanced), which only provides a maximum 7mA of  current. This doesn't give you a lot of room to play with.

[Morocotopo]

OK, so phantom power is a low current situation. Maybe just the ability to PASS phantom to the mic, but not for self power...

Paul, I´m currently plag... er, looking at the Rane ap13 schem. That´s a full featured dual pre with all the bells and whistles you might ever want in a pre. It´s the only commercial schem I was able to find on the subject.

[JohnL]

There is some good info at the Jensen Transformer site.

http://www.jensen-transformers.com/apps_sc.html#general

[PRR]

> It´s the only commercial schem I was able to find

Also look at sales-sheets. Count knobs, jacks, power options. Specs if they have them, but mostly I'm wondering about the overall shape not specific chips.