Bass Guitar Headphone Amp?

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jman 31

jman 31

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Joined 2008
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#1
Hey, I'm new to these forums so please forgive me if this is not in the right place. I would like to build a headphone amp for my bass guitar and I have read tons of info on this subject. It has succeeded in making me extremely confused.

If anyone can point me to schematics and pictures for this project I would appreciate it greatly. I have looked over the cmoy options, but I don't quite understand how to convert it to work with the very low output of a bass pickup. Seems like every time I think I have found the best option, the next thirty posts go into all of the reasons why it's not.

Thanks in advance

Jeremy
 
#4
jman 31 said:
but I don't quite understand how to convert it to work with the very low output of a bass pickup.
Click to expand...

The only thing required is high input impedance (250k-1M) which can be easily achieved by adding a buffer opamp if the circuit doesn't allow for such a modification.

And no, the output of a bass pickup is not low, I've measured peaks of 3V on mine! But any passive pickup has considerable output impedance (usually about 10k) and no current output. Less so with active basses.

So your headphone amps only job is to act as a current amplifier, well maybe with a little bit of gain
 
#7
IMHO, do NOT just build the typical headphone amps some have suggested on this thread . For best performance, you want a high impedance input like wxn pointed out. I would do a search on guitar headphone amps schematics, there are so many out there. I myself only use JFET opamps in guitar amp circuits for this reason.
 
#9
leadbelly said:
IMHO, do NOT just build the typical headphone amps some have suggested on this thread . For best performance, you want a high impedance input like wxn pointed out. I would do a search on guitar headphone amps schematics, there are so many out there. I myself only use JFET opamps in guitar amp circuits for this reason.
Click to expand...

I guess that is really my problem. I haven't been able to find one that has some good instructions that I can follow step by step. I am perfectly capable of putting it together, but I don't have the technical knowledge to pick through all the posts and understand what everyone is saying and what each component is doing.

Can I use the same curcuit that I am looking at, but with a JFET op amp?
 
#10
jman 31 said:
Can I use the same curcuit that I am looking at, but with a JFET op amp?
Click to expand...

That'll work in general, but you want to make sure that the component values don't degrade the high imput impedance that the FET opamp affords you. If you want details you can post the schematic here and I'll mark it up. If you want to learn how they are designed you can look at Rod Elliot's site sound.westhost.com and see how he designs guitar circuits. He uses FET opamps in his designs.
 
#11
#13
try this one easy-to-assemble FET preamp:
http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp2.htm
i am beginner in electronics and have built one using 2N3819 FET. maybe it does not have enough gain to act as headblowing headphone amp, but it does a very nice job as a DI box. also i'm sure that it can be modified for use with potentiometer to adjust gain.
also note that bass/guitar produces mono output. but your headphones are stereo. and if you end up using single channel amp you have to connect output of the amp to the left and right channel leads on the stereo jack to produce pseudo stereo sound. otherwise you will hear the sound coming only from one of the cans.
 
#14
wxn said:
LM386 has naturally low input impedance - about 50k. You need at least 200k.

So looking at (http://headwize.com/projects/showfile.php?file=guitar_prj.htm), change R1a to something like 250k-1M (100k is NOT ok) and put a voltage follower [or a simple gain stage] between R1a and R2. Any decent opamp will do, but as pointed out, FET input is more suitable.

http://ourworld.compuserve.com/homepages/g_knott/elect254.htm
http://ourworld.compuserve.com/homepages/g_knott/elect253.htm
Click to expand...

I changed this curcuit to reflect the changes you suggested. What size would the resister in the purple box need to be if I use a 250K ohm pot? And did I do it right? I have some extra LM386 IC's that I would use for IC3.
 

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#16
Alright guys, I was digging through my stash and I came across a LF353N Dual Bifet Op amp. Will this work for the type of amp I want to build and what would be the best schematic to use if It will work. I am once again thoroughly confused as to which way to go, but I don't want to rush in and have to build it twice. Thank you so much for the suggestions so far!!

I also have a JRC4558 DX that I could scavenge out of a stereo receiver if that would be a better choice. Otherwise I'm back to ordering more parts.

Thanks
Jeremy
 
#17
Hi jman 31,

shematic you posted is not too smart for guitar. Pot in input for instrument is bad idea.

Better way - take simple Cmoy shematc with 10K volume pot, and before pot put input buffer ( take first opamp schematic from your posted schematic without pot, input directly to opamp's +in, R- 1Mohm).

you don't need any caps in signal path, guitar have not any DC on output.

if you need amp for guitar only, you don't need stereo, use one dual opamp - one half for bufer, other for Cmoy amp itself, connect output to both phones.

if you need "universal" amp, use dual opamp for Cmoy part and other one channel opamp for buffer.

Good luck!
Zigis.
 
#18
Zigis said:
Hi jman 31,

shematic you posted is not too smart for guitar. Pot in input for instrument is bad idea.

Better way - take simple Cmoy shematc with 10K volume pot, and before pot put input buffer ( take first opamp schematic from your posted schematic without pot, input directly to opamp's +in, R- 1Mohm).

you don't need any caps in signal path, guitar have not any DC on output.

if you need amp for guitar only, you don't need stereo, use one dual opamp - one half for bufer, other for Cmoy amp itself, connect output to both phones.

if you need "universal" amp, use dual opamp for Cmoy part and other one channel opamp for buffer.

Good luck!
Zigis.
Click to expand...

Thanks for the input, but you have got me more confused now. I am very visually oriented. I will sit down when I get a chance and work up a schematic for what I think it is you are saying.

Am I getting it right that you are saying to use the LF353N Dual Bifet Op amp that I have as the amp, and one of the LM386 op amps as the buffer that wxn was trying to explain?
 
#20
TheSeekerr said:
Other way around - you need high input impedance for the buffer, so use the Lf353 opamp there (BTW, why scavenge opamps? They're cheap....), and then use the 386 for amplification duties.
Click to expand...

I'm not scavenging the dual op amp, I have a brand new one. I just didn't want to pay shipping on a $2.00 op amp if what I have will suffice. If it won't then I will order another one. And I ordered 8 of the LM386's before I got an answer on my first post because I was in too much of a hurry. I guess I'm just being a cheap ***, but I'm trying to use up some of the surplus that I caused myself! :whazzat:

Thanks for the info. That will help when I draw the schematics.

Jeremy
 
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