General guitar gadgets mini mixer help needed

[doug0147]

I am a relatively new pedal builder. So far I've made a Electro Harmonix Booster, ProCo Rat, and a mini mixer from the General Guitar Gadgets site.

The first two were successfully, but I need some help with the mixer. Here is a link to the mixer project.

http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=68&Itemid=26

I only wanted two inputs so I left two of the inputs out. I ended up using a TL082 instead of the TL072 IC in the bill of materials.

The thing works, the problem I have with it is that it takes out the some treble and sounds a little muddy and reduces volume. Is this just how it is supposed to sound? The plans say you can reduce the value of R10 to incease gain. I did this and swapped it for a 22K. It sounds louder and has more gain, but the tone is still off. Is the TL082 maybe causing this? Any help will be appreciated.

[Ben N]

What are you feeding in? That mixer is unbuffered and has a low input Z, so if the feeds are from high impedance sources, you may need (a) buffer(s) ahead of the mixer. For example, you could not reasonably expect to feed an unbuffered guitar signal through without losses.

[doug0147]

I was actually using a raw guitar signal. Do I need to build in buffers for each input?

My goal with this thing is to be able to split a guitar signal 2 ways, have one line going from the guitar signal to mixer input 1, and have the other side of the split signal go into an effects chain and out of that chain into the 2nd input of the mixer. The whole point is for my bass player to be able to use the ProCo rat I made and have his original signal at the same time.

But as of now I can't get a good signal with only a guitar in one input.

[petemoore]

  ...Any boss/ibanez or box with Jfet switches = line buffer when in bypass mode, great test-tool for trying out if there are impedance differences/improvements.
  ~high impedance in put and ~low impedance output there.
  Otherwise a buffer is a pretty easy thing to whip up.

[doug0147]

so you're telling me I need buffers right? I saw 2 buffer projects on General Guitar Gadgets. One is IC based and one is called "discrete". Does it make a difference which one I use?

[Ben N]

Discrete just means it uses discrete, or separate, transistors. Generally speaking, the most "transparent" amplifier you can make would be op-amp based (i.e. the IC), although I'm not sure it really matters for a unity gain device like a buffer. Some ICs can also offer a better combination of high input impedance and low output impedance. For your purposes, probably the simplest thing is to build a dual IC buffer using a dual opamp, like a TL071/081--very simple, very few parts. If your mixer uses an IC socket, you might try pulling the TL081 (which has a jfet input and therefore a very high input impedance) for use as the buffer, and selecting a low-noise bipolar IC like an NE5532 for your mixer.

As Pete suggested, Boss or similar pedals, with their 470k or better input impedance buffers, are quite handy for testing the hypothesis that you are suffering from an impedance mismatch.

[Mark Hammer]

The solution to your problem could be as simple as using a higher value pot on the mixer.

Think of it this way.  Imagine that the circuit omitted inputs 2, 3, and 4 so that it was just one unit-gain inverting stage followed by another.  It wouldn't really "do" anything, but we'll ignore that for the moment.  Okay, now let's imagine you were feeding it with a volume pedal, and that volume pedal that you got an amazing deal on has a pot in it that is, ummmmmm......let's say 10k.

"No treble", you say.  Immediately, people here reply to the thread and say "The problem is that 10k and similar low-value volume pots are intended for use with low output impedance devices.  If you are feeding the guitar directly to the volume pedal, yu need to either a) stick a buffer before the volume pedal, or b) get yourself a volume pedal intended for use with guitar directly so that it doesn't load down the signal.  Something on the order of 250k to 1M."

So, what's the difference between a 10k pot installed in a passive volume pedal then fed to the input jack and C1 of the Mini-Mixer, and the 10k pot at the input of the mini-mixer?  Nothing.  Swap out the existing pots for higher-value units, like 250k-500k (log).  These will behave themselves when feeding the mixer with pedals, but will also restore some of the sparkle lost when feeding your guitar to it directly.

Incidentally, this is the classic problem and fix for the Boss CE-1.  It came with a 50k input pot in anticipation of being fed from a keyboard, and when people plugged a guitar in, they lost a lot of top end.  Replacing the 50k pot with a higher value solved the problem for thousands of players.

[doug0147]

thanks for all the great advice. first thing I'm goingo try is changing the pot

[doug0147]

Mark - I replaced the pots with 500K's. It added back alot of high end, but it's still not quite there yet. I think I'm going to try adding buffers at the inputs. I'll let you guys know how it goes.

[ashcat_lt]

I was going to say that you're likely going to want to buffer the signal before the split anyway, so why not save yourself some noise floor by using the lower value pots? 

[doug0147]

Im a little confused on where to put the buffer. It first I thought I needed to put one after each input jack. Ashcat - what would be the advantage of the 10K pots if I use a buffer? I have 500K's on there now and they are way better than the 10k's were. I drew a diagram of how I think you are saying I should use the buffer. Is this what you are talking about.

[David]

In this thread, http://www.diystompboxes.com/smfforum/index.php?topic=54816.0

look at reply #6.  There's your answer.

[doug0147]

so it looks like I should put a buffer at both inputs. but what about the split? should I put one where the signal splits?


Does this make sense?

guitar-->signal split-->one side to bufferd input-->other side to bufferd output-->into effects chain-->out of effects chain back into second bufferd input

If i did that I would need 3 buffers.

[Ben N]

Now that I see your diagram, it occurs to me that you need to also make sure that your fx chain is in phase with the straight signal--if not, that could also be a source of signal loss. It also occurs to me that what you really need is a Splitter-Blend--everything you need, nothing you don't. (There are other similar designs out there; Robert Keeley posted one in his magazine column a few years ago IIRC that was functionally equivalent.) That will do exactly what you need--buffering, mixing, phase switching. The only thing it wont do is use your GGG board  . In order to use it with just a straight signal in one channel, you could (a) hard-wire that channel buffer-to-mixer (skipping the second buffer on that channel and the send and receive jacks) (b) follow the schematic and patch the send directly to the receive; or (c) internally hard-wire the send and receive jacks using shorting/switching jacks--just follow the wiring in the schematic of any amp that has a series fx loop. Even if you don't go that way, it is worth studying the schematic, reading the article and following the links to Geofex and GGG to get some perspective on your project.

[doug0147]

Wow! Thanks Ben. I wish I would of seen that thing in the first place!