[aderici]

thank you so much for the information

i want to ask you one more thing, i want to build a bass guitar practice headphone amp, 3 channels, 1 mono channel for bass and 1 stereo to plug your mp3 player in.

at the end signals will get mixed and you'll get a stereo headphone signal that has a listenable mix of your guitar sound and the mp3 sound. So you will be able to play along with songs.

Can any of you help me prepare a schematic for such a device? I really would talk on msn, my address is :

nicolasimben
@
hotmail.com

 

[PeterR]

Quote:

Originally Posted by aderici thank you so much for the information i want to ask you one more thing, i want to build a bass guitar practice headphone amp, 3 channels, 1 mono channel for bass and 1 stereo to plug your mp3 player in. at the end signals will get mixed and you'll get a stereo headphone signal that has a listenable mix of your guitar sound and the mp3 sound. So you will be able to play along with songs. Can any of you help me prepare a schematic for such a device? I really would talk on msn, my address is : nicolasimben @ hotmail.com

Using that chip?

 

[aderici]

Thank you PeteR i have some questions on my mind.

Will this setup have some mixing effect between left and right channels of the mp3 player?

Since the guitar is a passive device and the mp3 player is active, should i do something about impedances? and should i pre-amplify the guitar?

My input cap is 1000 uf, output cap is 2200 uf, input resistor 9,1k, feedback resistor is a 50k pot(gain pot).

if i use 9,1k in 3 inputs instead of 20k, and that 50kpot as feedback resistor instead of 20k, will it be ok?

And my last question is, how can i add 2 volume pots here to be able to mix mp3 and guitar,

Thanks for your help

Alper,

 

[PeterR]

Quote:

Originally Posted by aderici Thank you PeteR i have some questions on my mind. Will this setup have some mixing effect between left and right channels of the mp3 player?

I theory no, in practice not a lot.

Quote:

Since the guitar is a passive device and the mp3 player is active, should i do something about impedances? and should i pre-amplify the guitar?

You got me there, don't know the first thing about guitars. Any idea about the output voltage, output impedance? Having said that, preamping sounds probably like a good idea.

Quote:

My input cap is 1000 uf, output cap is 2200 uf, input resistor 9,1k, feedback resistor is a 50k pot(gain pot). if i use 9,1k in 3 inputs instead of 20k, and that 50kpot as feedback resistor instead of 20k, will it be ok?

Yes. That's one big input cap, though...

Quote:

And my last question is, how can i add 2 volume pots here to be able to mix mp3 and guitar,

Easiest thing would be to put them at the inputs, you then could (or would have to, depends on how you look at it) vary the volume of the stereo source and guitar independently.

 

[aderici]

This is what i found on the internet:

Guitar Output Voltage:
-----------
While the voltage off an electric guitar pickup can be a healthy 2.0 Volts or so peak-to-peak if you're playing hard,
------------
Guitar Output Impedance :
------------
In addition, guitars are very sensitive to the input impedance of an amp or DI box; the higher the input impedance, the better the frequency response. Typical guitar amps have an input impedance in the 1 megaohm (M•) range, which gives you a high-frequency response as high as 20 kHz with single-coil or humbucking pickups;
---------------
The electric guitar, due to the nature of guitar pickups, has a high output impedance.
------------
Most bass amplifier inputs usually aim for around 50,000 ohms, which is considered "high impedance" -- electric guitar/bass pickups and general use dynamic microphones' impedances are designed to be compatible with that input impedance.
---------




this is what i could find. by the way i am powering my lm4881 amp with 2 rechargeables = 2,5V if i can find a preamp circuit that would run with that voltage and can handle guitar signal without clipping that would be perfect.

Alper,

 

[PeterR]

Quote:

this is what i could find. by the way i am powering my lm4881 amp with 2 rechargeables = 2,5V if i can find a preamp circuit that would run with that voltage and can handle guitar signal without clipping that would be perfect. Alper,

2.5V is really low, can't you go to 4 cells for 5V?

What you need is something like this:
http://www.diyguitarist.com/PDF_File...itarPreamp.pdf
(make sure the opamp will work at the low voltage, for many 5 is already pushing it)

or this:
http://www.till.com/articles/GuitarPreamp/
(reduce R3 to ~3.3k for 5V, 2.5 should be a no go)

 

[aderici]

Hi peter,



This is my current setup, i want to add one stereo and one mono pot to input to adjust the guitar and mp3 levels individually.

i am a little bit confused and i dont know what value pot i should use and how should i wire it, by the way adding a pot to input would change the total input resistance and then the amplifier gain (feedback resistance / input resistance) would change??

i will really be happy if you can modify this schematic to add pots and send it back here.

thanks,

 

[aderici]

And by the way i will use this amp with an on board pre-amped guitar so i didnt add a preamp section

 

[PeterR]

Quote:

by the way adding a pot to input would change the total input resistance and then the amplifier gain (feedback resistance / input resistance) would change??

Yes, the input impedance as seen from the device connected to the inputs will change (depending on the drive capabilities of your sources you therefore might want to somewhat increase Ri). Gain of the amplifier itself (i.e. output voltage/voltage right before Ri - changing the overall gain is the point of adding the pots of course...), however won't.

Quote:

i will really be happy if you can modify this schematic to add pots and send it back here.

Use 10k log pots.

 

[aderici]

Peter thank you again for helping me,

i built this schematic and run it with 4 rechargeables (5 volts) it doesnt look a high-end device but it does its job i guess. Here is the picture of the completed unit.



By the way the gain pot creates some scratchy sounds while turning it. I dont thinks thats a problem with the pot because its brand new and i tried a few different ones, can this be an electronical problem? Got any ideas?

 

[PeterR]

Quote:

Originally Posted by aderici By the way the gain pot creates some scratchy sounds while turning it. I dont thinks thats a problem with the pot because its brand new and i tried a few different ones, can this be an electronical problem? Got any ideas?

Hey, that looks nice. If the pot has a metal shaft, try connecting the case of the pot to ground...

 

[aderici]

hi peter,

i sent my question to another thread and i wonder your opinion too,

i will build a switchbox that gets computer sound cards line out, and sends it to a speaker amp or a headphone amp,

what is the right way of doing this? Simply a 9 pin switch looks enough but since both amps will be always on, one amps input will be open on air, and while switching may there be dangerous pops for speakers?

should i ground the input of the amplifier that doesnt get signal? is it necessary to do something for pops?

i really would like to hear your opinions before building something,

Alper,