Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › Couple questions about my pocket amp design
New Posts  All Forums:Forum Nav:

Couple questions about my pocket amp design

post #1 of 50
2/13/12 at 3:14am
Thread Starter 

Hello Everyone,

 

                       I am trying to make a pocket amp and I am having troubles with Virtual ground circuit design. The Sch is shown below:

 

 

                                 psu.jpg

                                I have a tle2426 to get the middle voltage from 2 9 volts batteries, then it will send to a opamp for drift correction, the output of the opamp is connected to a buffer to increase the amount of currents that the virtual ground channel can handle. My question is, from the data sheet I noticed that I need couple capacitors from V+ and V- of the IC pins to ground(C9 C10 C7 C8). I don't know If I should connect those capacitors to the output of tle2426 or the output of the buf634.

            

                                 My second questions is about my amplifier circuitry, below is the design:

                          sch.jpg

                             It is a common OP+BUF design, I notice that in some of the designs, there will be a resistor connect the base of the transistor to the emitter, I don't know the why and what are the improvements. Also, I don't know If I should have R5 in between the output of the opamp and the push-pull buffer. The final question is, I am having distortion problem when I connect a inductive load (an earphone)to the output of the amp.

                               aawFile0.bmp    ddwFile0.bmp

                               Above, CH2 is the input and CH1 is the output (I have a lot of noises in my power supply because it is dying)

                               to the right is when I connect the amp with a earphone, you can clear see the distortion in the sinewave.

                 

 

                           

Many Thanks

LEE

 

Reply
post #2 of 50
2/13/12 at 5:28am

For your "virtual ground"

The bypass capacitors go from rail to rail as if were a single supply circuit.

 

For your "Amplifier circuit"

Why do you have "C3" when you have a virtual ground?

Try adding a small resistance in place of "C3" and see

if that clears up your distortion. 10 to 100 ohms.

Reply
post #3 of 50
2/13/12 at 6:08am

Consider this for your output buffer:

 

Buffer634.gif

Reply
post #4 of 50
2/13/12 at 9:43am

This one is almost exactly what you are doing:

 

Hybrid2.png


Edited by Avro_Arrow - 2/13/12 at 9:47am
Reply
post #5 of 50
2/13/12 at 10:47am

I would change those 15ohm resistors in the output stage. Making them smaller will increase idle current draw, but will probably help with what looks like wicked switching distortion. 

 

The 220 ohm resistor shown in the schematic immediately above is called a "bootstrapping resistor" and basically sets up a nifty little feedback loop to INCREASE the input impedance of the buffer.

 

I typed out how I think about it, but Wikipedia's explanation is a little more readable.

http://en.wikipedia.org/wiki/Bootstrapping_(electronics)

Reply
post #6 of 50
2/13/12 at 11:58am

I would drop R1/R2 down to 4R7.

Reply
post #7 of 50
2/13/12 at 12:04pm

Here is what I was mucking around with...

 

Buffer.JPG

 

The transistors all idle at 3.8mA

Reply
post #8 of 50
2/13/12 at 12:58pm
Thread Starter 
Quote:
Originally Posted by Avro_Arrow View Post

For your "virtual ground"

The bypass capacitors go from rail to rail as if were a single supply circuit.

 

For your "Amplifier circuit"

Why do you have "C3" when you have a virtual ground?

Try adding a small resistance in place of "C3" and see

if that clears up your distortion. 10 to 100 ohms.


Thanks for the reply, It this what you mean?

pwr v1.bmp

 

Reply
post #9 of 50
2/13/12 at 1:01pm
Thread Starter 
Quote:
Originally Posted by Avro_Arrow View Post

Consider this for your output buffer:

 

Buffer634.gif

Thank you!, is this a current mirror? or it just use the same type of transistor as a diode and we can heat couple it with the output transistor to increase the stability?
 

 

Reply
post #10 of 50
2/13/12 at 1:33pm
Thread Starter 
Quote:
Originally Posted by Avro_Arrow View Post

Consider this for your output buffer:

 

Buffer634.gif

 

Here is modified version

AMP2.bmp



 

Reply
post #11 of 50
2/13/12 at 1:52pm

Yes, this one is a current mirror.
Post #7 shows the transistor in place of the diode...

 

Quote:
Originally Posted by w62820616 View Post

Thank you!, is this a current mirror? or it just use the same type of transistor as a diode and we can heat couple it with the output transistor to increase the stability?
 

 



 


Edited by Avro_Arrow - 2/13/12 at 1:54pm
Reply
post #12 of 50
2/13/12 at 1:57pm


Don't use R7 with this design.

The bypass caps all go from rail to rail.

 

Op amps do not like large capacitor connected to their outputs ;)

 

Quote:
Originally Posted by w62820616 View Post



 



 

Reply
post #13 of 50
2/13/12 at 2:21pm
Thread Starter 

ould you explain why r7 is bad in this circuit? Thank you

Reply
post #14 of 50
2/13/12 at 2:33pm


As Nikongod pointed out, R7 is used to increase the apparent impedance at the input of the buffer.

It does this at the expense of stability. I don't think we need to make that trade-off as modern

op amps can easily drive the buffer.

 

Of course you are free to experiment with it...

 

Quote:
Originally Posted by w62820616 View Post

ould you explain why r7 is bad in this circuit? Thank you



 

Reply
post #15 of 50
2/13/12 at 3:04pm

This is what I was going to build to play around with.

I haven't designed a board for it yet but it doesn't really need one.

 

CMB Amp.png

 

I'm using it with a dual rail power supply or a split battery pack

so my bypassing is to ground (0 volts).

Reply
New Posts  All Forums:Forum Nav:
  Return Home
Head-Fi.org › Forums › Misc.-Category Forums › DIY (Do-It-Yourself) Discussions › Couple questions about my pocket amp design