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Old 12th November 2008, 05:22 PM   #11
wxn is offline wxn  Lithuania
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LM386 has naturally low input impedance - about 50k. You need at least 200k.

So looking at (http://headwize.com/projects/showfil...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/homep...t/elect254.htm
http://ourworld.compuserve.com/homep...t/elect253.htm
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Old 13th November 2008, 01:50 AM   #12
jman 31 is offline jman 31  United States
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Thanks guys, I'll look that stuff over and try to decide the best route. I will also post up the schematic as soon as I get a chance for the one on headwize that I was looking at.
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Old 13th November 2008, 05:48 AM   #13
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try this one easy-to-assemble FET preamp:
http://www.hawestv.com/amp_projects/...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.
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Old 13th November 2008, 11:57 AM   #14
jman 31 is offline jman 31  United States
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Quote:
Originally posted by wxn
LM386 has naturally low input impedance - about 50k. You need at least 200k.

So looking at (http://headwize.com/projects/showfil...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/homep...t/elect254.htm
http://ourworld.compuserve.com/homep...t/elect253.htm
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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Old 13th November 2008, 01:47 PM   #15
wxn is offline wxn  Lithuania
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You did it right. And you can omit the resistor, it is not necessary in the case. And you can't use another LM386 for the buffer - that would defeat the purpose. You need something like TL072 there.
Oh and I forgot to mention that you'll need virtual ground for an opamp to function correctly. Like in this circuit:
http://ta2020.huuryuu.com/ChuMoy_e.html
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Old 13th November 2008, 07:48 PM   #16
jman 31 is offline jman 31  United States
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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
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Old 16th November 2008, 12:29 AM   #17
Zigis is offline Zigis  Latvia
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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.
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Old 17th November 2008, 11:57 PM   #18
jman 31 is offline jman 31  United States
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Quote:
Originally posted by Zigis
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.
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?
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Old 18th November 2008, 04:51 AM   #19
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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.
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Old 18th November 2008, 11:43 AM   #20
jman 31 is offline jman 31  United States
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Quote:
Originally posted by TheSeekerr
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.
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!

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

Jeremy
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