THAT transistor headphone amp (250ma discrete opamp) design sanity check.

[jcx]

oops, that wasn't so good - I know better than to largely design with the .AC loop gain in Spice, it can really give misleading results

I think this patches up most issues - I cut one output pair since I would use the 2.5 W rated sot223 pkg

[Russ White]

Wow, a lot to chew on JCX.

Very good info, thanks for the interest and help.

I am still thinking I would like to drop the second THAT device (at the diamond buffer) in favor of degenerated driver Qs.

Let me see if I can guess the purpose of Rs 5,6,23,24,31 and C15,C16.

I am guess that Rs form a voltage divider to help control the bias.

And that the C's help keep the impedance from the diamond buffer front end low at audio Fs. Correct?


Cheers!
Russ

[peranders]

About the diamond buffer: The relationship between R5 and R15 is too extreme. You will get big troubles when is comes to idle current due to temperature. I have tested this myself in my QRV-06 . You should have a quotient about 2-4 instead.

C10, C11, not a good idea.

[Russ White]

Originally posted by peranders
About the diamond buffer: The relationship between R5 and R15 is too extreme.

Thanks, I have actually changed that. I have a pretty extreme redesign I will be posting soon.

Mostly I am fretting over offset. I am think about adding another LTP to null it.

[Russ White]

Will be knocking out a prototype tonight.

I like this design better because it seems to have much better behaved offset. R12 and R13 are a multi turn pot with the wiper to VEE.

I have not decided yet to use JFETs CCSs for simple resistors to set the mirror currents. Using JFETs would allow more predictable circuit performance with varying supply voltages.

Also for now I will not use the THAT devices simply because I have none on hand for tonight's build.

Anyway here is the schematic and the LTSpice simulation should anyone want to play with it.

Cheers!
Russ

[Russ White]

And the simulation file.

[Nordic]

Sorry, What is THAT?

looks like this project fits my budget this month

[Russ White]

Originally posted by Nordic
Sorry, What is THAT?

THAT Corp makes some nice matched transistors.

[Nordic]

Cool, I'll look into it... my PCB is 99% done in eagle,

[Russ White]

I increased the size of C4 to 75pf and added 100ohms in series to give better phase margin. I also added the same to the input to act as a low pass.

I am still new to SPICE so I hope I used the probes correctly.

Now I get the following results:

Open loop gain: 70.8db

Phase margin at unity gain with 2.2K feedback R and 6pf comp is right around 36 degrees. The AOL falls of at about 28db per decade as it crosses 0db.

At 6db gain phase margin over 45 degrees and the slope is right about 20db.

Open loop gain reaches 0db at 10Mhz.


So I am guessing in this configuration (with the change in C4) the amp should be stable to unity and very stable above 6db.

Last night rigged a prototype on veroboard, and it worked just fine.

I have a couple of boxes of 2N5550 and 2N5401 I bought very cheap, they actually worked fine, though I am sure there are better Qs for the job.

I matched my Qs for gain, but not for anything else. Offset was <2 mv with the adjustment pot centered. It only took about half a turn to get it to zero.

Now I will refine it a bit and put together a proper headphone amp.

Cheers!
Russ