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Audio stage needed for simple radio
http://diyaudioprojects.com/Forum/viewtopic.php?f=10&t=6338
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Author:  Yungman [ 12 Sep 2018, 18:56 ]
Post subject:  Re: Audio stage needed for simple radio

OK, this I pull out from my butt!!! No guaranty it will work, but at least you can try:

Attachment:
Output stage.jpg


To gain more voltage swing, I use common collector on both top and bottom.

1) I roughly calculate the resistor value so I get 0.4mA current on the input chain that drive the base of Q1 and Q2. ( R1, R2, R3 and R4).
2) This will give about 0.8V across R1 and R4.
3) 0.8V across R1 and R4 will make Q1 and Q2 conduct about 5mA. But it's likely not balanced and the output voltage will swing to one rail or the other.
4) R5, R6, C2 and C3 form a negative feedback. If Q1 conduct more current than Q2, then the output will swing up. But then it will pull the input junction up also and decrease the drive of Q1 and increase the drive of Q2. This is negative feedback to keep the output roughly about 1.5V ( half way between the two rails).

R5 and C2 and C3 create a dominant pole for the closed loop DC feedback of pole frequency fp= 1/( 2pi X 5.6K X ( 220uF/2)) = 0.26Hz.

But this is just guessing, too much work to do detail calculation and deal with stability. Think about this circuit to make sure it works. I am just going through the thinking with you. If this works it will definitely give you the maximum swing with such low voltage. If you can use MOSFET, that would be much easier.

This stage do have gain. the gain is about (2/7.6) X -10 = -2.63.

Author:  Friend [ 12 Sep 2018, 19:38 ]
Post subject:  Re: Audio stage needed for simple radio

Quote:
He mentioned the teach want him to look into sliding bias.
NOT HE, IM A GIRL! :blush: I just use my dad's e-mail address! and yes the teach did suggest we research the sliding bias circuits.

I'll get a suitable mosfet & give Yungmans modifications of the sliding bias circuit a college try, and if they can't be made viable the it will be proven to teach.

Also in Yungmans post he did say he didn't like the circuit as the audio would clip on transition, I understand that to mean it has to be overdriven in the first place (clip) before the current increase to handle the new larger signal?

Earlier today I found a circuit, (attachment), on a Russian site that claims to be a Gray amplifier, I'm told that these are called "feed forward adaptive bias" circuits, It seem it uses the rectified input signal to increase bias on loud passage. It looked simple so I breadboard it and its aweful! Even if you run it at 12 volt it distorted badly, if you increase the idle bias so it doesn't distort badly, the thing passes more current than is good for the transistors (hot).

I'm told that there is a circuit that works on 3volts designed by someone who used to write for r&ec magazine. Course, I can't find it online.

I'm beginning to like the idea of a simple class A with big battery.

I'll check out the butt circuit :eek: in the morning. Its late here now, Time for bed!

Sweet dreams, Ami. :sleep:

Author:  Gio [ 12 Sep 2018, 22:03 ]
Post subject:  Re: Audio stage needed for simple radio

Hi Ami,

Take a look at this simple amplifier circuit: https://www.instructables.com/id/Rechar ... amplifier/

It looks like it meets most of your requirements. A 3V power supply and the amplifier section uses BC327 and BC548. A BC549 should work as a substitute for the 548.

Cheers

Author:  Yungman [ 12 Sep 2018, 23:16 ]
Post subject:  Re: Audio stage needed for simple radio

Friend wrote:
Quote:
He mentioned the teach want him to look into sliding bias.
NOT HE, IM A GIRL! :blush: I just use my dad's e-mail address! and yes the teach did suggest we research the sliding bias circuits.



You don't have to get offended, How do I know from your user name. I have been trying very hard to help you. I am not into political correctness and call YO.

Author:  Friend [ 13 Sep 2018, 04:52 ]
Post subject:  Re: Audio stage needed for simple radio

Yungman wrote:
Friend wrote:
Quote:
He mentioned the teach want him to look into sliding bias.
NOT HE, IM A GIRL! :blush: I just use my dad's e-mail address! and yes the teach did suggest we research the sliding bias circuits.



You don't have to get offended, How do I know from your user name.
I'm not offended, sweetie, it doesn't matter, or shouldn't matter, I think its funny, perhaps I need a more feminine name, and avatar picture! BTW I've signed Ami, my real name is almost universally accepted as a girls name? It means FRIEND
Yungman wrote:
I have been trying very hard to help you
Yes, you have, and I appreciate it. On some forums no one even bothers answering. Actions speak louder than words, and I don't care for politics myself either, I will see if your ideas can be used today when I see Greg. I'm not snubbing you, its just that if I don't present classwork that is as required, my marks will be spanked :blush:

Gio wrote:
Take a look at this simple amplifier circuit:
Thanks, I will play with that too, its a shame we can't use another transistor, it would be easier to make a complementary stage like Yungman last offered with driver, there are loads of examples on the net that would run on a three volt battery.

Luv, Ami!

Author:  Friend [ 14 Sep 2018, 10:54 ]
Post subject:  Re: Audio stage needed for simple radio

OK Diy audio project peeps! This is where I'm at. This is harder than I thought.

First I tried the instructable circuit that Gio linked. Its not a sliding bias circuit but does use parts from the play list, and keeps the total transistor count to three. Unfortunately, it burned out the little speaker I was using because the transistors turned hard-on, (snigger) :lildevil: maybe because the biasing is scat.

Attachment:
instructable.jpg


The second is the Yungmans slider circuit. With a little bit of modification to make the control transistor bias adjust, it did work well. (Thanks). But.... no voltage gain would bring the transistor count to four, and the nine volt supply as well.

Attachment:
Alan slider.jpg


The follower with gain circuit just didn't work for me unfortunately.

Attachment:
Alan gain.jpg


I need to do more research.

Cheers, Ami.

Author:  ILoveHiFi [ 14 Sep 2018, 15:08 ]
Post subject:  Re: Audio stage needed for simple radio

The schematic will work with about 1.5v dc accross the speaker, R2 sets the bias current for transistors and biasing point is dependant on gain.
Increase R2 to reduce the dc voltage at output to about 1.5v.

Cold start of 1-1.2v would be perfect then it reaches 1.5v as it warms up

Author:  ILoveHiFi [ 15 Sep 2018, 15:15 ]
Post subject:  Re: Audio stage needed for simple radio

https://www.instructables.com/id/Rechar ... amplifier/
I was talking about this schematic

Author:  Friend [ 15 Sep 2018, 20:04 ]
Post subject:  Re: Audio stage needed for simple radio

ILoveHiFi wrote:
https://www.instructables.com/id/Rechargable-pocket-sized-amplifier/
I was talking about this schematic

Thanks I got that.

Arranging the bias to suit a particular transistor beta is bad practice 101. It would make a good thermometer tho. Take it into a warm room and it stops working!

Edit. I've added a more recent pic as an avatar to save further embarrassment.
Luv, Ami

Author:  Yungman [ 16 Sep 2018, 00:00 ]
Post subject:  Re: Audio stage needed for simple radio

Yungman wrote:
OK, this I pull out from my butt!!! No guaranty it will work, but at least you can try:

Attachment:
Output stage.jpg


To gain more voltage swing, I use common collector on both top and bottom.

1) I roughly calculate the resistor value so I get 0.4mA current on the input chain that drive the base of Q1 and Q2. ( R1, R2, R3 and R4).
2) This will give about 0.8V across R1 and R4.
3) 0.8V across R1 and R4 will make Q1 and Q2 conduct about 5mA. But it's likely not balanced and the output voltage will swing to one rail or the other.
4) R5, R6, C2 and C3 form a negative feedback. If Q1 conduct more current than Q2, then the output will swing up. But then it will pull the input junction up also and decrease the drive of Q1 and increase the drive of Q2. This is negative feedback to keep the output roughly about 1.5V ( half way between the two rails).

R5 and C2 and C3 create a dominant pole for the closed loop DC feedback of pole frequency fp= 1/( 2pi X 5.6K X ( 220uF/2)) = 0.26Hz.

But this is just guessing, too much work to do detail calculation and deal with stability. Think about this circuit to make sure it works. I am just going through the thinking with you. If this works it will definitely give you the maximum swing with such low voltage. If you can use MOSFET, that would be much easier.

This stage do have gain. the gain is about (2/7.6) X -10 = -2.63.


You said it's not working, label some voltages on the circuit to troubleshoot. What is the voltage at the output? Is it around 1.5V or is it rail to one side or the other.

I just put in the rough value, the value of the resistors needs to be adjusted. When you get down to this low voltage and you have to use bi-polar transistors, nothing is that accurate. This is a homework assignment, not a product. Just get it working and show to the teacher.

I didn't realize your old avatar was your picture, I thought someone just put a cute little girl as avatar. Pretty girl.

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