just some curiosities about symasym4

[keantoken]

I have been studying the symasym4 design and am wondering a few things:
1: Why is there no bias on the input transistor?
2: Is the 100pF capacitor on the input to filter out radio signals?
3: My simulator shows that the bias on the QBD135 in the second LTP is around 350mV, and changing the POT doesn't do anything, why?
4: My simulations also show a strange wave in the Ib of the output transistors, it seems like a triangular wave subject to extreme crossover distortion. How does this recreate such a clean representation of the input from the emitters?
5: I heard there was supposed to be a miller cap or two somewhere, could someone tell me?


I really don't have a problem with this amp at all! I think it is awesome! I was just wanting to clear some things up before I started to inquire further.

Also, I was wanting a good PCB design so that I could make this amp and experience THE ultimate Hi-Fi home theater entertainment system that will BLOW YOU AWAY!

And yes, I want to use symasm4 because it seems that it is the most liked one (mybe I'm wrong).

My best regards to the engineer of THE ultimate Hi-Fi home theater entertainment system that will BLOW YOU AWAY!, MikeB!




edit: OOP! Forgot the schematic! Here it is:

[lineup]

hello.
You have any link to the schematic you talk about?
Much better chance to get some good response
if you make it easy for posters.

I guess MikeB has got this circuit for public at his website ..
He would be the best man to answer your questions in a good way.

SymaSym 4 is one of the earlier versions.
Right now SymaSym 5.3, as far as I remember from visiting his website.

[keantoken]

What is MikeB's website? I'm definitely interested!

[MikeB]

Keantoken, you find my website by clicking on my www-button.
There is also a schematic having devicenumbers, making it easier to talk about the circuit, also at the bottom is a biasing diagram, showing the currents.

To your questions:

1, There is a bias, it just works different than you are used to, but is standard for LTPs. Q7/8 form a constant current source, delivering 3ma into the diffamp. R14/29 "hold" the bases at ~0v, giving ~ -0.6v to their emitters. These 3ma are split into 2x 1.5ma through q1/2, creating ~1v across r5/6. This sets the bases of q4/12 to ~1v relative to upper voltage rail. Having the emitters at +0.6v to the bases (emitters up ! pnp devices) the voltage across R10 is ~0.4v, giving a current of ~5.8ma.
2, Yes, the 100pf forms together with r13 a lowpass filter
3, The voltage from base to emitter for T1 should read ~0.6v, if not, the circuit does not work. R23 + R22/24 form a voltagedivider, this multiplies up the vbe from T1. Assuming an open pot (0 ohm), the voltage divider would be 2000ohm + 500ohm. The voltage needed to have 0.6v across the 500ohm would be 3v over both. (0.6*2500/500) This way T1 would keep voltage at 3v. That's why it is called vbe-multiplier.
4, The Ib to the output devices can look very strange, but it's the voltage from base to emitter that count most.
5, Sym4 does not have any miller cap, sym5 does have some kind of one miller cap. (still not a real one)

Keantoken, once you understand LTPs, symasym is quite easy to understand, this might be a good training for you.

As Jan pointed out, the Vbe is not exactly 0.6v, more like 0.65v, somewhere between 0.6v and 0.7v.
And always remember, voltages are relative, they exist between 2 points. Often the 2nd point is simply ground (0v)

Keantoken, head up, as you are really willing to understand operation of an amp in detail you will become once a good designer !

Mike

[lineup]

Originally posted by keantoken
What is MikeB's website? I'm definitely interested!

More correctly I should have said SymaSym website by MikeB.

Anyway, besides as Mike said, clicking his WWW button, below all his posts,
here are some directlinks:

SymaSym 4, only schematic: http://www.lf-pro.net/mbittner/Sym5_...s/symasym4.GIF
SymAsym5_2 - Project page: http://www.lf-pro.net/mbittner/Sym5_.../symasym5.html
SymAsym5_3 - Project page
http://www.lf-pro.net/mbittner/Sym5_...ymasym5_3.html

Review by Pavel ( = PMA - An Amplifier man )
Symasym V5-2, amplifier designed by Michael Bittner, modified by Pavel Macura for Toshiba output devices
Pavel Macura, November 2005
http://web.telecom.cz/macura/review.htm

SymaSym topic, one of the longest in diyAudio history,
with latest news in development of SymaSym
Explendid amplifier designed by Michael Bittner, our MikeB

=============================================

This image from MikeB webpage, shows SymaSym 5.3
one channel completed with components and heatsink.

Note: This heatsink is too small for long time high power use.

[MikeB]

Lineup, thanks for doing the work !

Mike

[lineup]

Originally posted by MikeB
Lineup, thanks for doing the work !

that's what friends are for
next time you do something for me if I deserve it
maybe if they put me in house arrest again for minor offense

lineup



Nobody loves you when you're down and out
Nobody knows you when you're on cloud nine
Everybody's hustlin' for a buck and a dime
I'll scratch your back and you knife mine

I've been across the water now so many times
I've seen the one eyed witchdoctor leading the blind
And still you ask me do I love you, what you say, what you say
Everytime I put my finger on it, it slips away
Everytime I put my finger on it, it slips away

Well I get up in the morning and I'm looking in the mirror to see, ooo wee!
Then I'm lying in the darkness and I know I can't get to sleep, ooo wee!

Nobody loves you when you're old and grey
Nobody needs you when you're upside down
Everybody's hollerin' 'bout their own birthday
Everybody loves you when you're six foot in the ground


John Lennon, 1974, when he was still alive
Now he is six foot in the ground

[jaycee]

Keantoken, if you are using OnSemi's SPICE model of the BD135-139, be aware that it doesn't work. Use the model that Fairchild have on their website instead. Here it is:

* BD135/137/139 NPN EPITAXIAL SILICON TRANSISTOR
*------------------------------------------------------
* MEDIUM POWER LINEAR AND SWITCHING APPLICATIONS
* Based on BD135-16
* BVcbo: BD135 45V / BD137 60V / BD139 80V
* BVceo: BD135 45V / BD137 60V / BD139 80V
* BVebo: 5V
* Complement to BD136, BD138 and BD140 respectively
*------------------------------------------------------

.MODEL BD139 NPN (
+ IS =2.3985E-13
+ BF =244.9
+ NF =1.0
+ BR =78.11
+ NR =1.007
+ ISE =1.0471E-14
+ NE =1.2
+ ISC =1.9314E-11
+ NC =1.45
+ VAF =98.5
+ VAR =7.46
+ IKF =1.1863
+ IKR =0.1445
+ RB =2.14
+ RBM =0.001
+ IRB =0.031
+ RE =0.0832
+ RC =0.01
+ CJE =2.92702E-10
+ VJE =0.67412
+ MJE =0.3300
+ FC =0.5
+ CJC =4.8831E-11
+ VJC =0.5258
+ MJC =0.3928
+ XCJC =0.5287
+ XTB =1.1398
+ EG =1.2105
+ XTI =3.0 )

*-------------------------------------------------------
* FAIRCHILD CASE: TO-126 PID:BD135-16
* 2000-03-20 CREATION

[keantoken]

Thanks everyone! I always enjoy reading the replies of people who know what they're doing.



Anyone got a good PCB design? I am willing to take them for symasym4 and up.

[clem_o]

The PCB layout for the amp is downloadable from the symasym website. Have a look.

Cheers