# Need help! Preamp or amp circuit

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by cyberquest, Jan 12, 2010.

1. ### cyberquestNew Member

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audioguru.

1. Is the Preamps input cut-off freqency 30Hz, if so. What is the Preamps output cut-off freqency .If the input and output cut-off freqency are the same do they add or subtact at the preamps total output cut-off freqency. What is the total output cut-off freqency at the preamps output.

Is the preamps 0.33 coupling capacitor there to calculate with the 50k load for the headphones input cut-off freqency. Then the load is in parallel then the 50k pot becomes a 25k load at 50k.

2. Will the Preamps total output cut-off freqency (add) to the headphones input cut-off freqency then adds to the outout cut-off freqency. if so. Then what will the total cut-off freqency be at the headphones output.

If my headphones is 8ohms and each capacitor is at 470uf then the output cut-off freqency will be 42.33Hz.
If you calculate for the headphones input cut-off freqency with a capacitor of 0.33 and a 50k load in parallel that
50k is subtract in parallel and becomes a 25k load giving you a output cut-off freqency of 9.65Hz.

Now. If you take the preamps input and output cut-off freqency and add them to the headphones input and output cut-off freqency you will get the total cut-off freqency at the headphones output. Is this correct! yes or no.

3. If the preamps output coupling capacitor of 0.33uf is in series with a 0.005uf and in series with the
0.1uf at the decoders input would i get a cut-off freqency of 305Hz at the input of the decoder for the touch-tones.. How will this affect the input of the decoder. What will be the cut-off freqency.

Wouldn't a higher cut-off freqency at the decoders input be better for touch tones. Then at 21hz as stated in post #100.

Last edited: Feb 21, 2010
2. ### cyberquestNew Member

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In question three.

I made a mistake in (post #101) it woud be a (0.0051uf) not a 0.005uf capacitor in series with the 0.1uf at the decoders input an in series with the preamps 0.33uf coupling capacitor.

Last edited: Feb 21, 2010
3. ### audioguruWell-Known MemberMost Helpful Member

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The mic is in parallel with R1 and they make a total of about 2.8k ohms.
R2 and R3 are in parallel and make 50k ohms. The total resistance is 52.8k ohms.
The capacitor C1 is 0.1uF so calculate the cutoff frequency. (30.3Hz).

It depends if the output capacitor is 0.33uF, 0.56uF or has a 0.1uF capacitor in series, and it depends on the resistance it drives.

They each cause the output level to be reduced 0.707 times at their cutoff frequency. So if they both have a cutoff frequency of 30Hz then the total cutoff frequency is 60Hz.

It depends on the value of the output capacitor and on the resistance it drives, and is affected by the input cutoff frequency.

Yes.

If all three cutoff frequencies are at 30Hz then the total cutoff frequency is 90Hz because they each reduce the level of low frequencies (a highpass filter).

0.33uF and 25k ohms produce a cutoff frequency of 19.4Hz, not 9Hz. 19Hz and 42Hz combine to produce a total cutoff frequency of about 61Hz.

The designer of the decoder IC used a coupling capacitor of 0.1uF for a cutoff frequency of 16Hz for important reasons. Maybe for a 350Hz dial tone to be passed without attenuation. Don't use the low value capacitor.

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5. ### cyberquestNew Member

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audioguru.

Ok. On the Preamps input capacitor is 0.1uf and the resistor network is 52.8k this will give a cut-off of 30.14hz.

1.If i change the coupling capacitor at the Preamps input to a 0.82uf and the resistor network stays at 52.8k this will give a cut-off freqency of 3.68Hz.

2. If i make the preamps output coupling capacitor of 0.56uf for the headphones input with a 100k load in parallel which makes this load a 50k at the next RC circuit this will give me a cut-off freqency of 5.68Hz.

3. On the output of the headphones the last RC circuit will be the impedance of a 8ohm headphones and the 470uf coupling capacitors for each channel.
If i use a 8ohm impedance Headpones with the 470uf capacitors this will give a cut-off frequency of 42.33hz.

Now if i combined the above 3 cut-off freqencies together this gives me a TOTAL of 51.66Hz at the headphones output.

But, WHAT If i remove the 470uf coupling capacitors and replace them with 680uf capacitors with a 8ohm headphones impedance this will give a cut-off freqency of 29.26Hz

Now combining the 3 new cut-off freqencies together this gives me a TOTAL of 40.32Hz at the headphones output....

4. Now If i switch off the preamp and switch to the Power amp with speaker and use a 20k pot in parallel this would make the 20k to a10k and with a 0.56 coupling capacitor on its input will give me a cut-off freqency of 28.42hz at its output at the speaker, but I would not know the output cut-off freqency of the my portable radios output when the I switch to the radios output jack. Please Advise.

5. the 0.56 capacitor in series with the 0.1 capacitor on the input with a 100k resistor on the DECODER what will it cut-off freqency be. Advise.
Shouldn't i keep as close to its original input cut-off freqency of 16hz. But with the added 0.56 preamps coupling capacitor because this with be in series wih the decoders input.. Please Advise.

Last edited: Feb 22, 2010
6. ### audioguruWell-Known MemberMost Helpful Member

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The problem with your cutoff frequencies is that the 470uF output capacitor feeding your headphones is too small for good bass. Instead of making all the other cutoff frequencies much too low then simply use 1000uF output capacitors feeding your headphones.

FM radio stations have a cutoff frequency of 50Hz.

Adding a 0.56uF capacitor in series with the 0.1uF capacitor at the input of the decoder won't make much difference to its cutoff frequency.

7. ### cyberquestNew Member

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audioguru

The first think i throught of was to increase the 470uf caps to 1000uf but didn't think you can increase this capacitor twice the capacitance of the 470uf for the headphones RC circuit.This is why i lowered the first two RC circuits.
What would be a reasonable cut-off freqency for the first two cut-off freqencies. You might of stated this already. But Im just starting to understand this little a at a time.

Could I go higher then 1000uf if i need to for the headphones output caps, and what would be the capacitance limit that i could use for the headphones output capacitors.Or is there a limit of what can be used.

So. If FM radio freqencies cut off at fifty hertz then i wouldn't need to go lower or higher then 50hz for the cut-off freqences.. Correct!

What is the cut-off freqency of a 0.56uf cap in series with a 0.1uf with a load of a 100k resistor... How would i calculate for two capacitors in series.Can you give Example!

8. ### audioguruWell-Known MemberMost Helpful Member

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I have some small 1000uF/16V capacitors that would be perfect for your project.

If your cutoff frequency is 50Hz then the total of the radio station and yours would be a cutoff of 100Hz which is too high. But if your cutoff frequency is 20Hz then the total would be about 55Hz.

Two capacitors in series have a value using the same formula (product divided by the sum) as two resistors in parallel.
The two capacitors in series have a total of 0.085uF. The cutoff frequency with a 100k ohms load is 18.8Hz.

9. ### cyberquestNew Member

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If i make the total combined cut-freqencies at 20Hz and the radio stations has cut-off freqency is 50Hz dosen't this make a total cut-off freqency at 70Hz. Not 55Hz.

By adding 1000uf capacitor with a 8ohm impedance would have a cut-off freqency of 19.89Hz alone. Making the two other cut-freqencies at 0hz for a total combined cut-off freqency of 20Hz.

If you take the 3 RC circuits and dvide them for a total of 20Hz each RC stage would need to be at 6.6Hz. I would need a 3000uf cap for the headphones output caps to get 6.6Hz.

Is there a way to make each RC stage small enough and use 1000uf for the Headpones output capacitor and get a combined total cut-off freqency of 20Hz. I don't see how you can do this and use the 0.56 coupling capacitor for the first two RC stages.

10. ### audioguruWell-Known MemberMost Helpful Member

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This thread is now 8 pages!
Make the darn circuit and if the bass is not strong enough then make a bass boost circuit.

11. ### cyberquestNew Member

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Yes its 8 pages!

If you just told me what capacitor to use at each RC stage and pots to use for a cut-off freqency of 20Hz. Then trying to guess for myself, then seven pages ago the the circuit would have been build.

You really don't have to get upset about it. Its a forum where people come for help. And yes its my project.

Last edited: Feb 22, 2010
12. ### TecIENew Member

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Need help Audio Mic Preamp

Hi

I came across this preamp by doing a search on this site. I been trying to find a TL071
preamp circuit.

I have a direct upgrade replacement that has a lower operating supply voltage of 4.5V-16V
Will this amp circuit work as is with the TLE2071IP upgrade IC.

I like to use this preamp circuit.

I will be using 5.5V for my supply voltage.

Also is the 1k resistor directly connected to pin 7 or is it just connected to the positive supply
rail.Please explain. Will i need to add or change any parts for the circuit to be functional at
the supply voltage of 5.5V.

Datasheet.
http://www.electro-tech-online.com/custompdfs/2013/08/tle2071-q1.pdf

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13. ### audioguruWell-Known MemberMost Helpful Member

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Why do you have TWO posts about the same thing??

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