Making a multi pre-amp box, questions and tips?

[deadlyshart]

Hi guys!

So I'm gonna be wrapping up my pedal-making for a bit, because it's been a ton of fun, but I've made most of the main pedals I want, and I've been spending too much time on it  (if there is such a thing! I'm sure someday the addiction will come raging back.)

The last pedal I want to make is a multi pre-amp box. I've been recording my band live (in a practice space, not concerts). My audio interface is the Scarlett 18i8 (manual here, pdf). Just to give some background for what I'm trying to accomplish, it has 4 XLR/quarter inch combo jacks on the front, and those have gain knobs, and a button to supply phantom power to them in pairs. However, on the back, it has 4 inputs, but they only accept quarter inch jacks, don't have phantom power, gain knobs, and apparently only accept line levels.

When we were recording, most of our mics had XLR outputs from the mics, and several of them were condensor mics that needed phantom power. So what I'd really like is a device that makes those four inputs on the back more like the inputs on the front. I'd like (4 of each):

-XLR or quarter inch inputs (doesn't have to be a combo jack, just wired internally. Also I don't think I ever actually use balanced inputs, so any reason not to just solder them internally?)
-48V phantom power switch
-quarter inch output jack
-Gain knob
-Multiple outputs?

The box I'm planning on using is fairly massive, so I think it can handle all these components.

I think I know most of the basics for all this, but there are a lot of specifics that I could mess up and want to check  . To give an overview of what I'd do if I were to start right now:

For the op amp, I'd probably use a pretty standard circuit like this (from http://www.rason.org/Projects/opamps/opamps.htm ):



and for the phantom power, do something like this (from http://sound.whsites.net/project96.htm ):



But I have so many questions...

-Is the TL082 a fine op amp to use for this?
-Does inverting/non inverting matter at all?
-How much gain should I go for? Should I use one or two gain stages?
-What's the easiest and best way to get a 48V source for the phantom power stage? A buck boost convertor or something? In fact, does it even need to be 48V, or could it be less?


It would be pretty good to have the option to do a thing where I split the input signal so that one output goes to the audio interface, and one goes to the normal amp/speaker, so we could record all together without bleed... is that really as simple as just splitting the input wire into one that goes to a "bypass" output, and one that just goes to the op amp input, or will one affect the other significantly?

Any advice would be much appreciated, or schematics of other people who have done this! Thanks in advance, sorry for the long post/naive questions.

[PRR]

That is a poor "preamp" for any purpose I can think of. Unbalanced, heavy load on the source, too much gain not adjustable. I'm sure you can run a $2 dynamic to tape and talk through it. But for music performance??

TL07x is a great guitar preamp and a poor low-Z mike preamp.

"Phantom" rather implies Balanced. (We could quibble, but still...)

You can do much better.

What you want is Four Good Mike Preamps. You can buy these in a box ready-made. You may find you can buy cheaper than you can build. (Chips etc FAR cheaper in 10,000-crates than in 4-packs.) There is a "DIY $10 mike preamp" plan floating around which is fault-free, though will be more than $10 by the time you are done.

[deadlyshart]

That is a poor "preamp" for any purpose I can think of. Unbalanced, heavy load on the source, too much gain not adjustable. I'm sure you can run a $2 dynamic to tape and talk through it. But for music performance??

TL07x is a great guitar preamp and a poor low-Z mike preamp.

"Phantom" rather implies Balanced. (We could quibble, but still...)

You can do much better.

What you want is Four Good Mike Preamps. You can buy these in a box ready-made. You may find you can buy cheaper than you can build. (Chips etc FAR cheaper in 10,000-crates than in 4-packs.) There is a "DIY $10 mike preamp" plan floating around which is fault-free, though will be more than $10 by the time you are done.

Hi, thanks for the advice and sorry for the (very delayed) reply, I've been very busy and am just getting back to this.

I'm trying to understand the things you said but unfortunately there are some pretty big gaps in my knowledge, if you could clarify a couple of the things you meant...

When you say the preamp is unbalanced, do you just mean in a mono/stereo sense? If so, I don't mind it being just mono. What exactly does it mean that it's a heavy load on the source? (source meaning, the source signal, like the guitar?

What do you mean by

TL07x is a great guitar preamp and a poor low-Z mike preamp.

I kind of get the concept of impedance matching, but I'm still definitely missing something... So I know that when impedances are matched you have the best signal transfer. A guitar or mic has a set impedance inherent to it, right? This article I'm reading says you usually want to have a lower source impedance than the impedance of the thing the source is going to... So when you said that it's a poor low-Z mic preamp, do you mean that it's a low-Z preamp, and that's bad, or it's bad at being a low-Z preamp? In the schematic I posted, is its input impedance just 1kOhm cause of that resistor?

ahhh, sorry for my confusion and ignorance. I guess I just don't get why I can't use the same gain stages I've used in these pedals (that used TL072/4558/LM358's or whatever), or rather, why they won't be good... thank you for any advice.

[PRR]

> I know that when impedances are matched you have the best signal transfer.

NO!! Put that right out of your head!  It is "correct" in specific circumstances we almost never meet in audio.

> This article I'm reading says you usually want to have a lower source impedance than the impedance of the thing the source is going to

That is how we usually do it.

> why I can't use the same gain stages I've used in these pedals

This gets complicated. You can, but they will hiss much more than a preamp designed for low-Z sources.

A proper "balanced Mike input" can be built of many transistors, or transistors and chips, maybe with a transformer. However the One Chip Mike Preamp is a complete solution at a low price. Do you want extended study and build? Or do you want to play/sing? You certainly can start with the known-good chip and have a working solution very quickly, while studying and experimenting in down-time.

> preamp is unbalanced, do you just mean in a mono/stereo sense?

No.

Guitar plug is two conductors, Tip and Sleeve.

"Balanced" interfacing (typically on 3-pin XLR, though can be TRS) has *two* audio conductor, and usually a 3rd just-shield.

"Phantom" *requires* all three conductors to work. (To get power to the mike and signal back from the mike without mix-up.)

"Balanced" interfaces generally require that *both* "hot" wires see the same impedance.

The opamp plan you show does not even have two inputs. It is for un-balanced mikes (now rare since "Pro-like" mikes are $69).

[antonis]

So I know that when impedances are matched you have the best signal transfer.

True but only for POWER transfer..!! 
(Paul yelled at you enough so I took it more gently..)

You must have in mind that we look for the minimum signal loss between 2 stages..

Consider 1st stage OUT Z₁ as the upper resistor of a voltage divider wherein Z₂ of 2nd stage IN the lower resistor..
The lower the ratio of Z₁/Z₂ the lesser the voltage "loss"..!! 

P.S.
I said "voltage"  because we don't deal with current in small signal variations...

[mth5044]

I had a plan to do the same thing - same interface. Made up the schematics, but never made the box. I will search for the schematics tonight for you. Not verified, but should get you somewhere.

[PRR]

> only for POWER transfer..!!

And then only when source power is VERY expensive.

When you can only afford one vacuum tube, you match.

But when you bring POWER to your house, you wire for less than 0.05 Ohms in the feeder, over 2.4 Ohms load in the house. A 98% "mis"-match.

Audio power is still more expensive than raw power and wire, so we often design more like 90% mismatch. Or the 10 to 1 rule. 200 Ohm microphone should see 2K Ohm loading.

[mth5044]

I haven't looked at this for.. a year? Maybe? Again, don't know if it'll work, but this was on the drawing board. The power section is top left. Think I was going to run 16VAC to it for a +/-15V. There there's a little booster of sorts for the phantom power, built around the LM2577. Everything connected north of R10 is for a VU meter. Everything else is a straight (IIRC) copy of Jensen and the datasheets for the INA217 and DRV134. Switch to go between the IC balanced driver or the cheap EDCOR transformer. I was going to replicate it all 4x except the power section, which should be robust enough for 4 channels.

Good luck! I think there's something funny about the grounding 

[robthequiet]

Man, it would be a great project and I'm following just to see where it leads. In my experience, though, if your band is graduating from the Scarlett, maybe time to look at the Scarlett as as a submixer and buy an 8- or 12-channel mixer with per-channel phantom power built in, along with EQ, EFX busses, meters, faders, and mains/monitors out, especially for recording.

To build individual high-quality channel strips would be an excellent project, with selectable line/mic level balanced inputs, to run into the rear jacks, with buffered outputs to send out to your amps. Or direct boxes, too, another choice.

[tempus]

I've got a 4 channel mic pre with 4 of these, and the datasheet is very helpful:

http://www.mouser.com/ds/2/407/1500data-4823.pdf

[tubegeek]

I've got a 4 channel mic pre with 4 of these, and the datasheet is very helpful:

http://www.mouser.com/ds/2/407/1500data-4823.pdf

THAT has perfect app-note solutions to this set of problems. Definitely NOT a wheel that needs to be reinvented, even if you insist on DIY.

[rankot]

I haven't looked at this for.. a year? Maybe? Again, don't know if it'll work, but this was on the drawing board. The power section is top left. Think I was going to run 16VAC to it for a +/-15V. There there's a little booster of sorts for the phantom power, built around the LM2577. Everything connected north of R10 is for a VU meter. Everything else is a straight (IIRC) copy of Jensen and the datasheets for the INA217 and DRV134. Switch to go between the IC balanced driver or the cheap EDCOR transformer. I was going to replicate it all 4x except the power section, which should be robust enough for 4 channels.

Good luck! I think there's something funny about the grounding 

Those 10u caps between pins 1+2 and 7+8 of DRV134 are needed just in case it drives 500ft cable length. Also noted in datasheet is this: "For applications with large DC cable offset errors, a 10-μF electrolytic nonpolarized blocking capacitor at each sense pin is recommended as shown in figure 29". What I can't get there is what do they consider "large DC cable offset error" and does it cover eventual Phantom power protection? I've read few reports of DRV134 being accidentally fired by PP when plugged into mic preamps, so I want to protect my driver just in case. Shall I use 10u caps like this, or between output and pins 1&2 tied together, for example? Or omit caps completely and use some kind of diode protection scheme on outputs?

[tubegeek]

Question for those who have been considering similar solutions to the OP's question: what's wrong with just using the front panel balanced mic-input connections? Is the problem just that they're on the front? I don't get it.

[rankot]

Well, I need this for some other purpose (mic fx loop) and I'm not always sure if someone else will plug this into mic input with phantom power, instead of a line input, and I don't want my gear to be damaged.

[tubegeek]

Well, I need this for some other purpose (mic fx loop) and I'm not always sure if someone else will plug this into mic input with phantom power, instead of a line input, and I don't want my gear to be damaged.

That makes sense, I was just seeing how there were a few others looking at adding external mic pres to an interface that already had them, and wondering WTF...