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Old 28th November 2007, 06:59 PM   #1
Rufinus is offline Rufinus  Germany
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Default Guitar USB preamplifier/interface

Hello everyone.

I would like to do some home recording, so i had an idea of DIY interface suitable for guitar or microphone connection and better then usual el-cheapo onboard soundcard.

And here is what it looks like:

Mic----------(MicPreamp)-------->PCM2900 channel1

Guitar------(GuitarPreamp)---->PCM2900 channel2

So, its basically a 2-channel (PCM2900 is stereo) ADC connected over USB to the PC.

For the Mic preamp i would use following schematics: http://sound.westhost.com/project66.htm
Click the image to open in full size.

And for the guitar preamp, i had to design a little myself (see attachment).
Now, CRITICS and SUGGESTIONS are greatly APPRECIATED, since i used textbook approach for guitar preamp without audio experience.
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File Type: jpg guitar_pre.jpg (48.8 KB, 680 views)
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Old 28th November 2007, 08:36 PM   #2
Minion is offline Minion  Canada
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I have used that Mic preamp design before and it sounds pretty good but I think you can do better and with less Parts, I would suggest looking at the INA217/THAT1510 Mic preamp IC"s...they are very low noise and can be implemented in a simular way to a regular opamp and the data sheets have curcuit examples Pluss designing a PCB for the mic preamp would be a lot easier than the one you are looking at....

As for the Guitar preamp curcuit I can see a few potential problems....

First you should have the input impedance set to about 1m ohm not 1k5, Guitar pickups are high impedance, so I would switch the 1k5 resistor for a 1m resistor this will also set the frequency of the input filter to somewere arround 20hz...

Also you only have the Gain at about 2x ,You might want more especially for guitar pluss you have no tone controlls or overdrive which are pretty much needed in a Guitar preamp.....
I would maybe suggest increaseing the Gain to at least 10x and to enable a switchable overdrive you can simply add a couple Opposite Faceing Diodes from the output to ground....

You can also use the Second opamp as your tone controll curcuit by adding a couple filters to the feedback loop in an Inverting config.....


Good luck
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Old 29th November 2007, 05:15 AM   #3
Rufinus is offline Rufinus  Germany
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Hi Minion, thanks for reply.

It is definitely worth looking at INA217/THAT1510. But unfortunately they are not easily accessible here in Germany. What i found is SSM2019, which is even pin-compatible with THAT1510. I can buy those SSM2019 for about 6 Euro here.

Otherwise, i would substitute TL071 for NE5532 (about 0.80 Euro) and build Project66, as intended.
What do you think about SSM2019?

Now, for guitar preamp there is a 1500k resistor there. Which makes 1.5M, should be good enough. It's just a weird notation that i used (1500k).

I am not sure about distortion, i have recently built a tube amplifier with nice tone, so for recording it will be micked (here comes in that microphone preamp). Guitar preamp is only for clean, no overdrive.
Tone stack is a good idea, probably will be implemented.

Also, how much gain should i have, i dont want to overdrive the input of PCM2900. Some pickups are quite hot, circa 1.2Vpp and consumer line level is about ~0.3Vrms, if i'm not mistaken.
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Old 29th November 2007, 07:20 AM   #4
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Why not do it all discrete?
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Old 29th November 2007, 11:15 AM   #5
Rufinus is offline Rufinus  Germany
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What do you mean by that? Instead of opams -> discrete elements?

Or mic preamp and guitar preamp in different boxes?
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Old 29th November 2007, 08:08 PM   #6
Minion is offline Minion  Canada
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By discrete he means only useing Passive components (Like Transistors and caps and resistors and no opamps) the front end of the Mic preamp schematic you posted is Discrete....

When going discrete you usually have to do a more complex circuit layout than when useing Opamps......

The SSM2019 is a Good Chip for a Mic preamp ,They are drop in Replacements for the THAT 1510 and the INA217 preamp chips and have simular noise specs.......

I guess if you are going into a ADC you don"t want the levels very hot so a Lower gain is probably a better idea but you will only get a very clean sound which I guess is ok for acoustic type stuff.....

Good luck
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Old 29th November 2007, 08:19 PM   #7
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Any interest is doing a completely discrete design?
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Old 29th November 2007, 09:27 PM   #8
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The front end of your amplifier is built using discrete transistors.

The following stage is an operational amplifier (not discrete).

You are half way there to buiding an all discrete amplifier.
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Old 30th November 2007, 04:23 AM   #9
Rufinus is offline Rufinus  Germany
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Well, discrete design would be interesting from the theoretical point of view and perhaps for other DIYers here. Problem is, i'm not that good in advanced circuits .
So, for this time i would have to use chips.
Then the configuration would be following:
Mic preamp ---> SSM2019 (perhaps also tone controls?)
Guitar preamp ---> above schematics with implemented tone controls (baxandall??), no overdrive, based on NE5532 opamp.
Regarding ADC, PCM2900 is sort of a ready soundcard chip with USB interface and Line level inputs. Uses standard driver and requires very few extra components. The question is to select correct gain in this case, as info about line levels are confusing and not always follows specs.
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Old 30th November 2007, 04:32 AM   #10
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So IC it is. Great!
Any questions regarding line levels?
They differ from consumer to pro-audio.
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