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Old 13th December 2012, 01:05 PM   #1
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Default Project: Simple bass guitar headphone monitor mixer

Hi!

I've had this idea for some time now, basically I want to satisfy the following needs in one box with relatively few components:

- Hearing myself (my bass) on stage with In-Ear monitors
- Being able to plug in a second monitor signal
- Silent practice at home to music from my computer (think vox amplug)
- Powered by usb/battery/DC-adaptor
- Low parts count, small box
- Stereo output

Since I want to be able to use different power sources, I thought to use a Voltage Doubler chip. This would give sufficient overhead when using USB as power source. (The reason I like to have USB-power is that I spend a lot of time using an amplug by the computer, and I'm getting tired of buying AAA-batteries.)

This would of course work with guitar as well, but a totally clean signal sounds a bit dull with guitar, so some tone shaping circuitry might be needed in that case.

I still haven't breadborded this design so I don't know for sure if it's going to work like this. Will I, for example, need a buffer for the Lo-Z bass signal? I also haven't used the LT1054 before so we'll see how it turns out. Opamp will probably be OPA2134, I'll use dual pots for line-in and master and a single for bass, so there would be three knobs.

Feel free to comment!

Click the image to open in full size.

/Fredde
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Old 13th December 2012, 02:32 PM   #2
JMFahey is offline JMFahey  Argentina
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LM386 works fine from 5 to 9V if all you have to drive is 32 ohm headphones, no need to raise supply nor make split rails.
You should add an input buffer for the Bass signal, so you can also plug passives there.
A simple source follower FET will do.
Select it for Vp between 2 and 3 V so basically it self biases there with a 4K7 to 10K source resistor.
And your master volume should be a classic passive one at the input of the chip, not an active feedback one.
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Old 13th December 2012, 03:38 PM   #3
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Quote:
Originally Posted by JMFahey View Post
LM386 works fine from 5 to 9V if all you have to drive is 32 ohm headphones, no need to raise supply nor make split rails.
You should add an input buffer for the Bass signal, so you can also plug passives there.
A simple source follower FET will do.
Select it for Vp between 2 and 3 V so basically it self biases there with a 4K7 to 10K source resistor.
And your master volume should be a classic passive one at the input of the chip, not an active feedback one.
Thanks, yes I have some LM386's handy, it would indeed be simpler. But there goes my excuse to try out the LT1054

But I'll definitely add a FET-buffer. And I've been thinking the master vol is pretty much redundant with only two channels. Might just settle on a suitable gain and use the input levels only.
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Old 13th December 2012, 07:42 PM   #4
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I'm having some second thoughts about the LM386. Can it be used as a summing amplifier by itself or would it require an inverting stage first? Also, even if it does work by itself, it's still a single amp chip so it would require two of them which is as much space as a dual opamp plus voltage converter.
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Old 13th December 2012, 08:26 PM   #5
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Originally Posted by Fredde79 View Post
I'm having some second thoughts about the LM386. Can it be used as a summing amplifier by itself or would it require an inverting stage first? Also, even if it does work by itself, it's still a single amp chip so it would require two of them which is as much space as a dual opamp plus voltage converter.
An LM386 is a small power amplifier, not advisable to use it as a preamp anyway - it's NOT an opamp.
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Old 13th December 2012, 08:56 PM   #6
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Originally Posted by Nigel Goodwin View Post
An LM386 is a small power amplifier, not advisable to use it as a preamp anyway - it's NOT an opamp.
Exactly. So it would require a preamp, and the parts count would begin to get out of control...

For now, I think I'll stick to my original plan. Added the input buffer (2SK170 is what I have in my drawer) and removed the master vol completely:

Click the image to open in full size.
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Old 13th December 2012, 10:31 PM   #7
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Just built the LT1054 voltage doubler on breadboard (exactly as this schematic, which is straight from the datasheet), and it works. I had misunderstood it's functionality slightly, it doesn't double as in +5V to 5V, it doubles +5V into 10V (or roughly 9V as there is a slight voltage loss that increases with load). With a (half depleted) 9V battey it measured about 16V.

I got a higher reading on the positive rail (almost 1 volt more than on the negative). I don't know why this is happening, but It's nothing I will lose sleep over. One could regulate but I'd call that overkill in this case.

I also tested it "for real" to power one of my DIY preamps (a JFET build), and it worked perfectly without the slightest hiss or hum. (This preamp has an opamp output buffer, though, so it's not very noise sensitive.)
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Old 13th December 2012, 10:46 PM   #8
JMFahey is offline JMFahey  Argentina
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I didn't say you use it as an Op Amp but as a headphone driver
And you need no active mixer, *at all*.
Simply 2 mixing resistors to the LM386 .
It has lots of available gain anyway.
For 1V RMS at the headphones (realistic given the 5V supply), 20X gain means you need 50mV at the input, or 100mV at the left end of each 10K mixing resistor.
And you still have 20X dB *extra* gain available at the LM386, although you don't need them at all.
And yes, you will need two LM386 ... but nothing else.
Your 2SK170 stage will work happily from 5V to ground.
But of course, if you want to play with LT1054 , go ahead and do it, probably I'd also do the same.
It's just that I'm such a minimalist
Oooops !!! Simulposting

Last edited by JMFahey; 13th December 2012 at 10:47 PM. Reason: Simulposting
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Old 14th December 2012, 07:19 AM   #9
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Originally Posted by JMFahey View Post
But of course, if you want to play with LT1054 , go ahead and do it, probably I'd also do the same.
It's just that I'm such a minimalist
Well I have to, I already bought two of them and they are expensive But mostly because I don't find the LM386 that exciting. I use it pretty regularly, but usually just as a quick tool to get some output when testing something on a breadboard (with arduino for example).
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Old 18th February 2013, 10:38 AM   #10
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I've decided to bite the bullet and make a separate output stage with an LM386. This way there shouldn't be any need for the voltage doubler since the OP2134 should work with +/-2.5V and there isn't need for much headroom when the opamp stage is low gain.

The rather uncommon way of using the LM386 (with a resistor between pins 1 and 5) is to make the gain less than the factory set 20. I got the idea from here: HeadBanger Headphone Amp Construction Kit, but I've seen it elsewhere too.

So this is what it looks like, note that I haven't tested this version on breadboard yet so it's likely there will be some changes:

Click the image to open in full size.
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