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Solid state...transistor amp design problems


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Hey folks.  This is my first posting in the forum.  I have been working on making my own solid state amp from scratch...having some problems i guess understanding some basic concepts.  in short, this is what i have built ....  it doesn`t seem to put out any sound.  I`m hoping someone will have the time, knowledge and the grace to guide me in the right direction with it.  that would be super..  I`m thinking of this little thing as a pre-amp.  I`ve also been working on the secondary stage of the amp, but perhaps think it would be best to get the preamp up and running first.  Is there anything that sticks out as "could be changed and then it should work".  
I`m hoping whoever answers is a friendly sort.  I`m not shooting for the Nobel prize with this design, just something to plug my guitar into and get a bit of sound and tone out the other end.  Any help is greatly appreciated. Image shouold be attached of the hand drawn schematic.  Thanks!ampschematic.jpg

post-59998-14279144279283_thumb.jpg

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Hi there SILVERsound, and welcome to Electronics Lab  :)

How did you calculate the resistors? I belive they are totaly wrong, with maybe hte exeption of the colector resistors! ??? What are the amp used for? A guitar preamp? Have you measured the voltages on the transistors, I belive they are badly saturated.

Oops, i belive I am tired, i did not saw that it is a guitar preamp!

//Staigen

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You need to learn about how to bias a simple transistor.
You still have the transistors saturated (the collector voltage is as low as it can go) so they are not an amplifier.

I corrected the biasing so that the collectors of the transistors can swing up and down and swapped the two transistors so that the low input impedance of the second transistor is driven from the low output impedance of the first transistor for low signal loss.

The circuit is completely wrong for a guitar pickup that needs a preamp with a very high input impedance and low gain. This circuit has a low input impedance for a dynamic microphone and even has too much gain for a low level microphone.

Here is a simple guitar pickup preamp circuit:

post-1706-14279144279722_thumb.png

post-1706-1427914428_thumb.png

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I still have some old BC109 transistors from when I worked at Philips 44 years ago.
It is replaced by the BC549 which is also low noise.

Now there is another forum with this simple guitar preamp circuit.
On the other forum somebody thought it is a power amplifier to drive a speaker!
The person didn't know how to bias the transistors so continued to use the adjustable pots.

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ampinprogress8.jpg
still trying to work with the help from the forum.  another day of no progress...the amp still isn`t producing any sound.  any ideas what i can do from here?  i have put in the resistors on this drawing as audioguru suggested, measured the voltages and noted the readings....  scratching my head today.  um....  what`s wrong?

hopefully it isn`t frowned upon to post the same topic at more than one forum online.  having read audioguru`s posting on the other forum where i posted this topic...it seems perhaps i shouldn`t be hearing a sound coming from the output side of this preamp.  is that correct?  someone on the other forum mentioned that i should set the bias of the transistors to get a voltage of substantially over .7 volts from the base to emitter.  i`m really a newbie to building my own transistor sound amplifier....i`m not sure if this is accurate about the VBE.  is it?  is perhaps my preamp as posted, with the voltages i measured in blue, working?  boy am i confused.

not sure if it matters...but i should also note that both transistors are now bc109

thought perhaps the other transistor was bad.....but not really sure how to tell if a transistor is good....i get the diode readings from the base in both directions to determine that the flow is unidirectional, but should the readings vary somewhat...?  and if so, by how much?  how do you determine from the readings if the transistor is "healthy"?
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the amp still isn`t producing any sound.  any ideas what i can do from here?  i have put in the resistors on this drawing as audioguru suggested, measured the voltages and noted the readings....  scratching my head today.  um....  what`s wrong?

What is the input source? A dynamic microphone? A dynamic microphone has a coil and a magnet. This preamp will not work from a common electret microphone unless the microphone is powered from a 10k resistor connected to the filtered positive supply.
The input impedance is too low to be driven from a guitar pickup but it might produce some weak bass sounds.

What is at the output of this preamp? it will not drive a speaker, it drives the input of a power amplifier that drives a speaker.

someone on the other forum mentioned that i should set the bias of the transistors to get a voltage of substantially over .7 volts from the base to emitter.

That is completely wrong.

not sure if it matters...but i should also note that both transistors are now bc109

If they are connected properly (not with the pins mixed up) then the preamp should work perfectly.

not really sure how to tell if a transistor is good....i get the diode readings from the base in both directions to determine that the flow is unidirectional, but should the readings vary somewhat...?  and if so, by how much?  how do you determine from the readings if the transistor is "healthy"?

Most multimeters measure the current gain of transistors. The two "diodes" in a transistor are supposed to conduct when forward-biased and not conduct when reverse-biased, exactly like real diodes. Also measure from collector to emitter that should not conduct.
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i shouldn`t be hearing a sound coming from the output side of this preamp.  is that correct?  

Yes no amplifier will make any sound without a speaker and in this case a power amplifier is required to drive a speaker.

I've merged your posts, in future use the modify button to add something to an existing post, it looks like this .
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so, if i`m to understand correctly -- this pre-amp could quite possibly be functional at this point?  
hmmm...that would be great actually.  that means i can start working on the speaker driving portion.
am i right in my understanding....that these voltages i posted as "measured" from the made version of audioguru`s schematic are in the "ok range"?  should i proceed to the next step?

ahhhmmmmm....and i`m not seeing a modify button here....i`m simply replying to my post...should i be looking somewhere else to keep a dialogue going?   not simply click on reply?

audioguru, to answer your question -- til now it has been simply a 1/4 inch jack.  i`ve been touching the end of the cable to see if i can get a hum/pop/or rattle of any kind to show up on the other side.  i have, at one point, plugged my 50`s silvertone guitar into it....but it produced no sound through the amp.  i was thinking i would hear something through the amp before putting a microphone into it as you suggested.   do you think with those voltage readings i posted on your schematic that it is worth my while at this point to "hook it up" and do a microphone test?  

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A high current push-pull amplifier at the output will produce sound from a high impedance speaker, but the amplitude will be low. A higher voltage supply will allow for greater amplitude, and adding transistor amplifier stages will produce greater amplitude. The input input impedance to the amplifier can be increased with higher input impedance transistors.

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so, if i`m to understand correctly -- this pre-amp could quite possibly be functional at this point?

Yes, if the capacitors have correct values, have the correct polarity and the amplifier has an input impedance of  47k or more.

am i right in my understanding....that these voltages i posted as "measured" from the made version of audioguru`s schematic are in the "ok range"?

The voltages show that the mic preamp will work well.

should i proceed to the next step?

Yes.

i`m not seeing a modify button here....

It is at the top of each of your recent posts (before you press REPLY beside DELETE and another button.

with i`m simply replying to my post...should i be looking somewhere else to keep a dialogue going?   not simply click on reply?

Use MODIFY to change a previous post. Use REPLY to continue in the thread.

i`ve been touching the end of the cable to see if i can get a hum/pop/or rattle of any kind to show up on the other side.

That is an extremely poor way to test an amplifier. Use a proper signal instead.

I have, at one point, plugged my 50`s silvertone guitar into it....but it produced no sound through the amp.

If it has a guitar pickup then it should have produced some sound so maybe a capacitor value is wrong.

do you think with those voltage readings i posted on your schematic that it is worth my while at this point to "hook it up" and do a microphone test?

Yes. Try a dynamic mic or an electret mic with a 10k resistor to power it.
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ahmmmm.....is that what you`re looking for????


Thanks for trying but that's not what I meant you should do.

What I mean is don't keep adding successive posts to a thread. If someone's replied since your last reply, use the reply button to respond. If no one has responded and you're just adding more information, use the modify button rather than adding a reply.

I'll sort the thread out when I get the time.
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audioguru asked "Yes, if the capacitors have correct values, have the correct polarity and the amplifier has an input impedance of  47k or more."

well audioguru, here are the capacitors i`m using at present.  numbers are based on the schematic i supplied (found above)...sorry, i don`t have the spice skills.  or spice for that matter.  easier for me to scribble by hand i reckon.
so, here they are.
C1 - mica B152
C2 - electrolytic 16v 1000uF
C3 - electrolytic 16v 100uF
C4 - electrolytic 16v 100uF
C5 - propylene(?)(blue)  B 472K  2KV
C6 - electrolytic 16v 100uF
C7 - electrolytic 16v 47uF

i wasn`t able to hook up an active microphone today, but will get a chance soon.  what do you think audioguru?  are these suitable caps?


What I mean is don't keep adding successive posts to a thread. If someone's replied since your last reply, use the reply button to respond. If no one has responded and you're just adding more information, use the modify button rather than adding a reply.

ok, i think i get the hang of it now.   posting on the forum i mean.  electronics are still illusive to me i must admit.  thank you all for your help so far.

-ooops....how does one truncate a posting for a specific quote?  seems like i have failed to do so.....
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Here are the capacitors I`m using at present.  Numbers are based on the schematic I supplied.

In addition to not knowing how to simply correctly bias a transistor, you don't know how to simply calculate vales for capacitors. Your input capacitor is so small that it does not pass audio frequencies.
I commented on the capacitor values for my mic preamp.

sorry, i don`t have the spice skills.  or spice for that matter.  easier for me to scribble by hand i reckon.

I didn't use Spice to make my schematic nor to modify it. I used Microsoft Paint program.

post-1706-14279144281225_thumb.png

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Thread sorted, all double/triple posts are merged and the old posts restored. It's weird how theSILVERsound's post count is now when he has five; me deleting and undeleting all the posts has confused the forum software.

The formula for the lower frequency cut-off is: C = 1/(2pi*FR)

Where C is the capacitance in Farads, R is the input/output resistance in Ohms and F the frequency in Hz which should be 20 maximum for audio but it's good to go a little lower for good bass response.

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