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Old 7th July 2012, 01:26 PM   #1
emosms is offline emosms  Bulgaria
diyAudio Member
Join Date: Sep 2010
Default eq input and output buffer

I am building a guitar eqalizer and I want to add simple input and output buffers to get appropriate in and out impedances for the unit.
This is the EQ (the filter sections not shown):
Click the image to open in full size.
Here I plan to add simple as much as possible IN and OUT stages with 2SC1815 transistors
Click the image to open in full size.
Click the image to open in full size.
What about the noise of the 2SC1815 transistors? Is it going to be ok?
Could I have input impedance of 2M instead?
Would I benefit of a mosfet input buffer in terms of noise and keeping the high frequencies?
I know the input impedance of mosfet buffer could be 10M.
Here the mosfet input buffer:
Click the image to open in full size.
R2 - 10M (optional)
R3 - 10M
R4 - 22M
R5 - Jumper
R6 - 10k
R8 - 1M
R9 - 100 ohms
C1 - 0.1uF
C3 - 1uF
C4 - 100uF
Q1 - Mosfet transistor (2N7000, BS170 or similar)
I guess, the mosfet input would be overkill except i use the eqalizer as the first unit of the effects chain,
but I guess I would use it further on in the chain.
Best Regards

Last edited by emosms; 7th July 2012 at 01:34 PM.
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Old 7th July 2012, 06:25 PM   #2
emosms is offline emosms  Bulgaria
diyAudio Member
Join Date: Sep 2010
actually, dont understand how my eq works in the simulator, I have +9 on the opamp +VCC, +4,5 on the -VEE and the + input of the opamp is grounded..
so, if I use opamp in/out buffers like these:
Click the image to open in full size.
Click the image to open in full size.
- where is the virtual ground then?
- and how it is going to work at 4,5v voltage potential on the opamp +- supply..
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Old 7th July 2012, 07:14 PM   #3
emosms is offline emosms  Bulgaria
diyAudio Member
Join Date: Sep 2010
and I get 1T (teraOhm??) output impedance for the eqalizer without output buffer, which is insane itself???
Click the image to open in full size.
//that part, with the messed up supply and virtual ground. But the frequency responce and and gain is pretty ok with this weird unipolar supply...
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Old 8th July 2012, 02:02 AM   #4
diyAudio Member
Join Date: Jan 2008
If you want to run this from a single +9v supply you need to bias the op-amps to half the supply. So normally op-amps have +ve and -ve supply rails therefore the input is referenced to 0v. You will need to bias the input up to half the single supply voltage. There are numerous schematics available for guitar effects pedals that show single supply operation of op-amps in this type of configuration. Do some homework and if you still have a problem ask.

Both of your circuits shown above are wrong. The -vee should be connected to ground and the Vr connected to +4.5V

Last edited by sidsparks; 8th July 2012 at 02:08 AM.
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