[Avro_Arrow]

I should have mentioned that in the first reply...the error is so small that
the op amp feedback loop will correct it.
The end result would be that if the imbalance were bad, the amp would
clip sooner than if it were better balanced.
 
If you want to fine tune it some more, take out the op amp and measure
the output offset. Trim the bias current resistors to get the least offset.

 

[w62820616]

Quote:

I should have mentioned that in the first reply...the error is so small that
the op amp feedback loop will correct it.
The end result would be that if the imbalance were bad, the amp would
clip sooner than if it were better balanced.
 
If you want to fine tune it some more, take out the op amp and measure
the output offset. Trim the bias current resistors to get the least offset.

I bought some BC337 and 327 instead because they are cheap and have the same HFE.
 
http://www.fairchildsemi.com/pf/BC/BC327.html
 
I have a question about all the prefix on the transistor like TA,BU, do you know the meaning of these? Thank you!
 

 
Above is my most recent PCB layout. Please correct me if there is anything wrong or not good.
 
 

 

[Avro_Arrow]

For the suffixes you mentioned, BU means "bulk packaging" and TA means "Ammo Pack".
Ammo Pack is for automated placing machines.
 
It looks like you did a good job placing the components in a neat and organized manner.
The only problem is that your placement leads to an overly complex set of traces.
Routing is really an art form. Sometimes I redo a layout several times before I am satisfied.
 
Try and keep your power traces as short and thick as possible. Keep signal traces away
from power traces. Keep inputs and outputs away from each other. If signal/power traces
have to cross, do so at a right angle.

 

[Avro_Arrow]

Here is an example of a buffer I did:
 

 

[w62820616]

Quote:

Here is an example of I buffer I did:
 

Thanks for the reply! It is really helpful. Should I start layout the power line first?  Is your 3906 and 3904 have the same hfe? Thanks!
 
 

 

[Avro_Arrow]

I bought a batch of 25 each 3904/3906. The 3904 had an average hfe of 170. The 3906 had an average of 250.
By luck, I have five 3906 with an hfe of 170 from a different batch.
I will probably by another batch of 25 each and see if I get different values.
Or maybe I will just make due with the values I have...

 

[w62820616]

Quote:

I bought a batch of 25 each 3904/3906. The 3904 had an average hfe of 170. The 3906 had an average of 250.
By luck, I have five 3906 with an hfe of 170 from a different batch.
I will probably by another batch of 25 each and see if I get different values.
Or maybe I will just make due with the values I have...

Thanks for the reply! You should try bc327 and bc337. They are really cheap with hfe around 400. Another question, do you think R5 and R16 are necessary? Thanks!
 
 

 

[Avro_Arrow]

MPSA06 and MPSA56 would also be good substitutes.
 
R5/R16 are the resistors between the op amp and buffer?
It depends on the op amp used. It may or may not be
needed or have any effect. It doesn't hurt to have a small
value resistor. I often use a value of 49R9, sometime it
just makes a handy jumper...

 

[w62820616]

Quote:

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.
 

 
I'm using it with a dual rail power supply or a split battery pack
so my bypassing is to ground (0 volts).

 
Above is the revised design, I moved all the power thing to the top and shorten the signal channel.
 
 

 

[Avro_Arrow]

You should try and move all of your virtual ground components close to the output jack.
The output jack ground should be your star ground point. Power should also come in
near the BUF634 and transistors. They are the biggest power draw.

 

[w62820616]

Quote:

You should try and move all of your virtual ground components close to the output jack.
The output jack ground should be your star ground point. Power should also come in
near the BUF634 and transistors. They are the biggest power draw.

Here is the most recent one, I moved the buffer up so it is close to the power bus.

 

[w62820616]

Quote:

You should try and move all of your virtual ground components close to the output jack.
The output jack ground should be your star ground point. Power should also come in
near the BUF634 and transistors. They are the biggest power draw.

 

 
I added 2 1uf caps per channel for the transistor buffer (caps are near the output jack). I also added 1 uf cap for the buf627.

 

[Avro_Arrow]

I would still like to see the BUF634 closer to the out jack.
The trace between the BUF634 out and the out jack ground
should be short and thick.

 

[w62820616]

Quote:

I would still like to see the BUF634 closer to the out jack.
The trace between the BUF634 out and the out jack ground
should be short and thick.

As your command , I changed the ground error correction to soic8 so that I can free some space.
               

 

[Avro_Arrow]

Good use of radius instead of right angles.
Is there any way you can make the trace from the BUF634 to the headphone
jack thicker? Like maybe .07".
 
Time to get the board made and see if it works...