what transistor should i use, ive got some bc109c would they be ok? and what ic should i use for the high pass and low pass op amps?

thanks

freddy

 

The opamps will not work with the input DC voltage at ground. It must be half the supply voltage.
Make a voltage divider from the supply with two equal-value resistors (3.3k) in series and a 10uF capacitor to ground as a filter.

The lowpass filter needs one capacitor to be twice the value of the other.
Nobody makes a 3.5nF capacitor so use a 3.3nF capacitor for the first one, use two 3.3nf capacitors in parallel for the second one and change the 10k resistors to 12k.

All electronic circuits need a supply bypass capacitor, especially when powered from a battery that has an internal resistance that rises as it runs down.

 

The LF351 is not powerful enough to drive low impedance headphones.

If the gain is too high by the LF351 or by the transistor then the distortion will mess up the bandpass filter.

The highpass filter must be driven from a very low impedance like the output of the LF351, not by the high output impedance of a transistor.

 

hi audioguru

after reading the last couple of posts a few times i now understand about the values of the capacitors and resistors for the lowpass filter are incorrect and more importantly understand why...

re the lf351, whilse it cant 'power' the headphones could you think it would be ok for the filters and the led part of the circuit?

thanks for your help

 

Hello,

If Audioguru says leave the transistor out, so be it.
Leave also the headphone connection out.
Leave than also the 220 μF elco out and connect the filter directly to the 351.

Gretings,
Bertus

 

so even if i cant hear it in the headphones and have to further amplify it the signal is still there i presume..

and i hope this is enough to drive the other part of the circuit???

i hope, i really, really do....


thanks guys, will order some bits tomorrow

what op amp will i use for my high pass and low pass filters

 

hi

audioguru recently posted a circuit diagram, it noted 'connect to half the voltage supply and a filter capacitor' on a note to a high pass filter....

how do i supply half the supply voltage (9v) and what would i use as a filter capacitor?

 

I like the TL072 low noise high bandwidth dual opamp ICs for audio.
The OPA2134 is better but costs more and is not needed for your "telephone quality audio".
The LF351 opamp can drive the highpass filter plus drive a capacitor and volume control into an LM386 little power amplifier to drive your headphones.

I explained how to make "half the supply voltage" with two series 3.3k resistors from the supply to ground and a filter capacitor at their output.

 

thanks audioguru,

 

hi

ive built the additional circuit (x2 filters highpass then lowpass and the transistor part). I just need to supply half the supply voltage to the 10k resistor going to the + input on the first filter. I haven't got any 3.3k resistors, ive tried a couple of things and looked up about a potential divider but not really getting much luck.

Can i use two 1k's in seris, like this

0v --------[1k]---*---[1k]--------9v


* to 10K resistor
HERE where marked *

where would i put the 10uf capactitor would it be between the poiint marked * and 0v?

hope you can help me guys,

thanks!

 

Hello Fred,

Yes, from the * to 0 should be the capacitor for decoupling.

Greetings,
Bertus

 

Hello Fred,

Yes, from the * to 0 should be the capacitor for decoupling.

Greetings,
Bertus

and the two 1k resistors in seris is that ok?

 

Hello,

Here I have drawn Audioguru's changes and your 1k resistors in the drawing.

Greetings,
Bertus

 

Based on Thevenin's Theorem, it should be possible to achieve the Vcc/2 reference and at the same time provide a 10K equivalent resistance by using a 20K resistor pullup and a 20K resistor pulldown on the positive input to the first opamp. The two 1K resistors and the 10uFd cap would then not be needed.

hgmjr

 

hi guys

i decided to start building the circuit shown above first as it seemed the most simple design. It's on breadboard, what can i do for testing, what kind of readings should i be gettingon the outputs of the filters?

 

HELP!!

I have started building the circuit using a lm386 for the first filter, when connected it makes some very strange sounds, distorted like a fog horn!! have looked up lm386 datasheet and the connections i have

could i be my chosen capacitor http://uk.farnell.com/panasonic/ecqb1h223jf/capacitor-0-022uf-50v/dp/1198538?_requestid=435907

or is it something else

can anyone help me please!!

 

Have you put the large valued capacitor across the power supply as was strongly recommended?

hgmjr

 

yes its there but the 1st filter just dont seem to filter much at all!

ive checked cchecked and double checked but at the moment i cant find anything wrong. I am just getting sounds like pulses about 20 every five seconds through my speaker, very hard to discribe! Will do some more checking but everyting seems to be in the right place, i though i would build the circuit 1 filter at a time and then added on the transistor driver bit once the filters are sorted.

thanks for getting back to me

 

An LM386 is a little power amp, not an opamp. It has built-in biasing so that its inputs work at 0VDC. Its minumum allowed gain is 9 but the filter needs an opamp with a gain of only 1.
An opamp needs the two 1k resistors to bias its inputs at half the supply voltage.
An LM386 power amp will not work if its inputs are biased at half the supply voltage.
Why didn't you use an opamp?

 

sorry, what should i of used for the two filters?

thanks