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Need help! Preamp or amp circuit

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

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The input cutoff and output cutoff add and produce -3dB at about 150hz and -3dB at about 80Hz. If you want a -3dB total cutoff at 50Hz then both RC cutoffs must be 25Hz.

    the feedback capacitor is too big at 22uF so it has no effect on the total cutoff frequency. Even if it is 10uF then its cutoff frequency is so low (7.3Hz) that it also will not affect the total cutoff frequency.

    If you make the input capacitor, output capacitor and feedback capacitor all with a cutoff (-3dB) at 30Hz then the total cutoff will be -9dB at 30Hz and 30Hz will be at a fairly low level. If you make then all have a cutoff at 16Hz then the total cutoff will be -3dB at 50Hz that you want.

    But if the feedback capacitor is 10uF with a cutoff frequency of 7.3Hz then it won't affect the total cutoff so the input and output capacitors can have a cutoff frequency of 25Hz then the total cutoff is -3dB at 50Hz that you want.
     
  2. cyberquest

    cyberquest New Member

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    audioguru


    In post #78 you mentioned 50Hz. Thats why I mentioned the 50Hz in post #79. I did not mention that i wanted a 50Hz cut-off freqency, i assumed by you stating 50Hz in post #78 this was a cut-off freqency to use given by you becuase you mentioned that a voice cut-off freqency of about 50Hz would sound natural..

    I was thinking to use a lower cut-off freqency like 30Hz or 25Hz for better bass freqencies Would this lower freqency have a better all around sound for voice,radio, and bass freqencies or would 50Hz as you mentioned in post #78 be better to use cos; Im trying to find out from you what is the better freqency thats going to give good all around sound in my application.Please Advise!
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Some modern western music has frequencies down to 20Hz that you can also feel. But 20Hz needs very good pretty big speakers.
    My home stereo has speakers that have a cutoff at 50Hz but I have a bass-boost circuit that extends the bass down to 30Hz.

    My car radio has a 10" sub-woofer that also goes down to 30Hz and sounds and feels great.

    FM radio has a cutoff at 50Hz.
     
  4. dave

    Dave New Member

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  5. cyberquest

    cyberquest New Member

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    audioguru.

    Ok.. Radio has a cut-off freqency of 50Hz. Will 50Hz have good decent bass freqencies with that speaker that you posted in post#31.


    So. If i use a total of 50Hz for all RC circuits.

    Dose the 30Hz at the input of the Preamp stay at 30Hz.Or do i need to change it. YES or NO, if yes then at what cut-off freqency dose it need to be... Please Advise.

    For the feedback cut-off freqency dose the capacitor stay at 22uf or do i use the 10uf that you mentioned in Post #81 this will have a freqency of 7.23Hz. Correct!

    The output capacitor on the preamp will then feed the cut-off freqency for the input of the power amp and for the input for the headphone amp So I would just take the load for each RC input and calculate there cut-off freqency for the Preamps output capacitor. Is this correct!

    Also the Preamp output will also feed the input of the DECODER I would just use the Preamps output capacitor to calculate for the cut-off freqency for the decoder circuiit. Is this correct! Do I need to remove the 0.1uf capacitor on the input of the decoder. Advise>
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    No.
    Look at the datasheet of that small 3" speaker. Its cutoff frequency is 150Hz. It is too small to produce good bass. A few other 3" speakers produce some bass down to about 80Hz.

    I told you about the capacitors and I explained why but you don't understand so forget about it.

    Coupling capacitors are never two capacitors in series. Use only one coupling capacitor. Calculate its frequency so it passes touch-tones well.
    If you don't use a coupling capacitor from the output of the preamp to the input of the decoder then the decoder probably will not work because its input will have the DC voltage from the output of the preamp.
     
    Last edited: Feb 16, 2010
  7. cyberquest

    cyberquest New Member

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    audioguru.


    Why should i forget about what you explained to me in post #81 I only understand what i know which is not much with audio circuts and what you did explain to me, this audio stuff is new to me I don't have much experience with audio as you can see. This is the frist project for me that uses audio circuits. I been studying electronics for about 1year and everything I been learning is on my own. I do appreciate all the help that you been given me for my project and a part of learning dose come from those who kinow more about electronics, you have much more experience and many more years than i do in audio electronics.So give me some credit for wanting to know more and ask questions so i can better understand to know more and willing to learn more as i go.

    I been trying my best to understand audio electronics and i know your not here to teach me or anyone else on this forum but i did come here for Help and need your help with my project.

    P.s. Everybody needs to start out somewhere in learning the this stuff.
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You don't understand what happens when a coupling capacitor feeds a resistance. It has a cutoff frequency where the voltage at that frequency is reduced 0.707 times and the power output is half (-3dB). Lower frequencies are reduced more.
    Each RC coupling circuit reduces the output by -3dB at the cutoff frequency.

    Two RC coupling circuits add together and reduce the output voltage at their cutoff frequency to half which reduces the output power to 1/4. Frequencies 5 times higher than their cutoff frequency are also reduced but only a little.

    Three RC coupling circuits add together and reduce the output voltage at their cutoff frequency to nearly 1/3rd and reduce the output power to nearly 1/10th. Frequencies 10 times higher than their cutoff frequency are reduced a little.

    Therefore to reduce the effect of the RC coupling circuits adding together and eliminating all bass frequencies their cutoff frequency is reduced.
     
  9. cyberquest

    cyberquest New Member

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    In post #78.

    If the Preamp circuit stays (as is) and i use the 0.33uf coupling capacitor calculating with the 10k load on the Power amp input for its cut-off freqency I get
    48.23Hz.

    If i use the same 0.33uf coupling capacitor and calculate this with two 10k pots connected in series total would be 20K on the headphones amp input for its cut off freqency at 24.11Hz.

    Using the same 0.33uf coupling capacitor for the input of the decoder with two resistors in paralell is 50K gives me a cut-off freqency at 9.65Hz.

    If i change that coupling capacitor with a 0.56uf the freqencies are.

    With a 10k input load resistor is (Power Amp)
    28.42Hz
    With a 20k input load resistor is (Headphones)
    14.21Hz
    With a 50k input Load resistor is (Decoder)
    5.80Hz

    Would these freqencies work and have the cut-off freqencey
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You cannot connect the two volume controls in series. They must be connected in parallel. Then the cutoff frequency with a 0.33uF capacitor and two 10k pots is 97Hz.

    Using the same 0.33uf coupling capacitor for the input of the decoder with two resistors in paralell is 50K gives me a cut-off freqency at 9.65Hz.

    If i change that coupling capacitor with a 0.56uf the freqencies are.

    With a 10k input load resistor is (Power Amp)
    28.42Hz
    With a 20k input load resistor is (Headphones)
    14.21Hz
    With a 50k input Load resistor is (Decoder)
    5.80Hz

    Would these freqencies work and have the cut-off freqencey[/QUOTE]
    You will not find a capacitor that is 0.56uF. But 20k and 47k volume controls are common.
     
  11. cyberquest

    cyberquest New Member

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    audioguru.

    The 10k pot on the input of the headphone amp is a dual ganged audio taper pot. That pot is connected in parallel. I made a mistake. in parallel then that pot would subtacted in parallel which makes it a 5k which the cut-off freqency with the 0.33 coupling capacitor would be 96.46Hz... You're right!


    20,000 pot is common. 47,000 is not so common. I don't see many of them around.

    If i use a 50k audio pot in parallel then it will Make the headphone input a 25K pot and using the 0.33 will give a cut-off freqency of 19.29Hz.

    IF i use a 20K audio pot in parallel then it will make the headphone input a 10K pot and using o.33 will give a cut-off freqency of 48.23Hz.


    If i change that coupling capacitor with a 0.56uf the freqencies are.

    With a 10k input load resistor is (Power Amp)
    28.42Hz
    With a 20k or 50K input load resistor is (Headphones)
    14.21Hz, or 5.68
    With a 50k input Load resistor is (Decoder)
    5.80Hz

    Digikey has 0.56, 0.82, 1.0, 1.2 microfarad capacitors in metal film.


    will any of these cut-off freqencies work. and will they low enough. Out of these cap which one should work better or should i stay with the 0.33 in the Preamp circuit.
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Add up all the coupling capacitors and reduce the frequency of their cutoffs so that their total of cutoffs makes a final cutoff frequency that is about 50Hz for music and voices and about 300Hz for touch-tones.

    A dual 50k pot for the headphones amp with 0.33uF or 0.56uF coupling capacitors makes a good low cutoff frequency that is increased when added with other RC cutoffs.

    I think your small speaker will be limiting your low frequencies.
     
  13. Hero999

    Hero999 Banned

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    Disclaimer: I haven't read the whole thread.

    It depends on the bandwidth of the speaker, for example, there's no need to use a 1000µF capacitor in the output stage of an amplifier which is only driving a small 0.5W speaker, 470µF or maybe even 220µF will do for a cut-off of about 40Hz and 80Hz respectively. If it's a sub-woofer or a decent pair of headphones then the roll-off needs to be <20Hz.
     
  14. cyberquest

    cyberquest New Member

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    Im getting confused again here. You say add up all coupling capacitors together isn't the one coupling capacitor 0.33 on the output of the Preamp feeds the inputs for the Headphone amp and the Power amp for the Speaker.
    But on the headphone amp there two inputs Left and right for the headphones. how many coupling capacitors are there if both feed off the output of the Preamps output. Im not understanding this.

    Theres one input to the Decoder.
    Theres one input to the audio pot on the Power amp for the speaker.
    Theres the two inputs for left and right channels for the Headphone amp.

    How many coupling capacitors are going to feed all the inputs isn't it just the one thats on the Preamps output.
    You stated to use one coupling capacitor to feed a resistor on the inputs not two or more.. This I don't understand.
     
  15. cyberquest

    cyberquest New Member

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    Hereo999

    Thanks for your input.

    Yes i will being using A small 3'' 0.5W speaker but theres a bass freqency problem becuase of size, most will not reach a roll off freqency lower then 80Hz as audioguru stated. Unless your using a biger speaker like 8'' 0r 10''

    Most 2''- 3'' speaker that i seen are limited to 150 and 320Hz cut-off.

    If you have further suggestions Please comment.


    I need to use a small speaker for portability.

    As for the 1000uf capactior on the amp where do you see a 1000uf cap being use.
     
    Last edited: Feb 18, 2010
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    No.
    The input to the headphones amp has a coupling capacitor and their outputs have coupling capacitors. Then each channel has two coupling capacitors that must each have a cutoff frequency of 25Hz so the total is at 50Hz. A switch disconnects the speaker amplifier when hedphones are used.

    If you use the speaker then the headphones amplifiers are disconnected by the switch then the input capacitor of the amplifier must have a cutoff frequency of 50Hz or less. It is bridged so it does not have an output capacitor.

    Each headphones amp has an input capacitor and an output capacitor. Since most of your sound sources are mono then they are parallel.

    It is extremely simple.
     
  17. cyberquest

    cyberquest New Member

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    audioguru.

    Yes its extremely easy for you. Not so for me.

    Enclosed Amplifiers. Please edit the schematic so i can understand what capacitors to add and what capacitors need to be calculated on both amplifiers for a total of 50Hz or less.

    1.When the Preamps output is connected to the headphones left and right inputs on the amp dose the Preamps output (0.33uf 0r 0.56uf which ever one is used) coupling capacitor get connected to the headphones left and right inputs to. Or are the two 100uf capacitors on the headphones amp are those the input coupling caps for the headphones amp and the two 470uf caps are the headphones ouput coupling capacitors.If this is the case then the preamps output capacitor (0.33 0r the 0.56) dose it still need to be connect to the headphones left and right inputs . Yes or No. Advise?

    2. When the Preamp output is connected to the bridge amp with speaker and switch over with a on-on switch the preamps output coupling capacitor 0.33 or the 0.56 is connected to the bridge amps input because as you stated there is no input coupling capacitor for the bridge amps input. And are the 10uf or are 0.1 caps on the bridge amps output the coupling caps.. Yes or No. Advise?

    3. Now when I insert a stereo Plug into the output ear jack on a portable FM/AM radio then the other plug connects to a audio jack on my device then gets connect to the Headphone amplifier input when the Headphones amp switches over to the radio output will my audio jack need a coupling cap for DC voltage coming from the radio output or will the radios output suorce already have this coupling capacitor or the headphone amp on its input and output block for DC voltage. If this make any sense.
     

    Attached Files:

  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You should always have part numbers for ICs on your schematics. I don't know which decoder IC you have and I don't know the value of its input resistors.

    I also don't know the impedance of your headphones.

    The output coupling capacitor of the preamp connects to the decoder and to both volume controls of the headphones amplifiers. I don't know which decoder you are using so I don't know if it needs an additional input capacitor.

    No.
    They are not input capacitors. They are feedback capacitors.

    Yes but I don't know the impedance of your headphones so I don't know if the 470uf capacitors produce a 50Hz cutoff.

    Bad english.

    No. The 10uF capacitor is the feedback capacitor and the 0.1uF capacitors stop the IC from oscillating.
    Bridged amplifiers do not need output coupling capacitors.

    I don't know if your radio has an output coupling capacitor so you should add one or two. Is your radio mono or stereo?
     
    Last edited: Feb 19, 2010
  19. cyberquest

    cyberquest New Member

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    audioguru.

    Here's the information that was not included to edit amplifiers which was needed in post# 96.

    The decoder IC is the M8870 its in Post #6. I will not be using the 74LS148 for the display decoder. I have another IC in mind to control the display.

    Here the datasheet.
    http://www.datasheetcatalog.org/datasheet/clare/M-8870-01SMTR.pdf

    The impedance of my Headphones are 8ohms and 16ohms and i also have a 32ohm pair. But I use the 8 and 16 ohm headphones mostly for the portables.

    1.The coupling capacitor for the decoders input is 0.1uf, If I use that capacitor for the decoders input how would the preamps output capacitor affect the decoders input shouldn't the 0.1uf be remove on the decoders input so the preamps output capacitor is used. How would that affect the decoders function if the coupling capacitor 0.1uf was removed and the 0.33 or 0.56 was used instead of 0.1uf on the input of the decoder .

    3. My radio i listen to is a sony pocket mono FM/AM radio with a small 2'' speaker and it sounds pretty good for its size.
    I also have a sony walkman digital tuning stereo portable pocket FM/AM radio has no speakers comes with stereo headphones they are either 8 0r 16 ohms impedance.
     
    Last edited: Feb 20, 2010
  20. cyberquest

    cyberquest New Member

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  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The decoder has a 100k input resistor and a 0.1uF input capacitor so its cutoff frequency is 16Hz which is very low for touch-tones. When the 0.1uF capacitor is in series with the 0.33uF output capacitor of the preamp then the resulting 0.077uF will have a cutoff frequency of 21Hz will be fine.
    If your headphones have 8 ohms for each ear then a 470uF output capacitor will have a cutoff frequency of 43Hz which might be OK.
     

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