Apex AA14 Amp Class A 24V Supply

[printnik]

I don't like the bias current of the output transistors. Right now it's zero. Put in a simple VBE multiplier circuit.

 

[chermann]

sorry i dont catch you...?

 

[printnik]

Or do what printnik did and remove this biasing subsystem completely, and replace it with traditional Vbe multiplier (which normally is not needed in A class amps). The whole (and only?) interesting thing about AA14 was the novelty biasing circuit; without it - it's totally 'normal' amp, that might not be worth building, given all these troubles so far, unless someone puts some time to sim it properly.

replace it with traditional Vbe multiplier. 50ma bias.

 

[chermann]

oh...yes i know and simulated that your circuit with VBE multiplier is working.
What are the D1 and D7 are doing at the IPS ?

 

[chermann]

replace it with traditional Vbe multiplier. 50ma bias.

50mA bias? where?
if i short the input at your asc file and let run the sim i got Q17 und Q15 a bias current at the OPS transistors 120mA bias

 

[chermann]

With R16 333R i got with 22V rail a bias current about 1,45Amps.

enough for the day.

thanks
and good night

 

[chermann]

Hi

a played around and think to rebuild this amp and try it.
my final setup is, because i have 4R speakers.

R12_220R, R14, 91R
Rail is in my FW amps normally about 22V rail (or little less)

bias setting for class A:
R16 332 bias is about 1,45A
R16 bias setup is 1,67A with 316R ..my setup what i choose
R16 bias is about 1,9A....be carefully here..

Gain is according to mineks setup 21,5dB with R5 2k and R15 22k
with 1,4Vp input into 4R it is the max of about 35Watt
with 1,5Vp into 8R you get about 20WATT

actually i am not able to do more LT spice sim...have to learn...

chris

 

[chermann]

for learning for dummies....me

minek give us noobs a hint:
here:

minek created a simple 'Open Loop Gain and Phase simulation' for dummies step-by-step presentation.
Images and individual steps (1 asc file + image(s) for each step) attached in one zip file.

 

[printnik]

Hi. I added emitter resistors to the differential stage for better phase margin.

 

[printnik]

R28=200R

 

[chermann]

thanks you for your effort..
okay i check shortly

DC offset -50mV...not okay...for me..
Bias for a Class A amplifier is about Q17 73mA and Q15 -60mA

27 WATT @4R with 14,8Vp nearly symmetrical output voltage

 

[draganlalkovic]

Hi Minek

I check all resistor values - okay
diodes ok
all small BJT ok.
bigger BJT okay...

then i realized that the onsemi 1381 NPN is with hfe about 150 too much for the "auto biasing function" because if i change to the BD137 and BD136 which i had in the box- both amps are working and the ocp is not get on.
bias is starting with 1,44A @ 24 Volt and goes on one amp quicker then the other down to 1,250A

okay fine. but...

i had on the speakers measured the DC offset and i got at start up of the PSU 15Volt !! and then it gets more or less slowly down to 5V.
****.......this is bad!

so enough for today...

chris

"had on the speakers measured the DC offset and i got at start up of the PSU 15Volt !! and then it gets more or less slowly down to 5V."

I have the same problem with DC=18V. You can try to remove diode D1.
After removing, the amp started to work great and DC offset was 2mV.
Small signal transistors are BC550/BC560.

 

[chermann]

HEY..!
Cool that somebody tried something !
Thank you for that!
Do you drive the bias to Class A _Level ? ...700mA to 1,3A?


i will try this (Actually i am busy...)

kr
chris

 

[draganlalkovic]

Do you drive the bias to Class A _Level ? ...700mA to 1,3A?
Yes.

 

[chermann]

Fine !

I was a bit blocked because minek (well amp builder = expert) wrote that it is nothing special and its not worse a try. post 89)...so i slow down.

which transistor do you use for Vbe multiplier? is the TO92 as printnik supposed enough for class a biasing?

 

[draganlalkovic]

The transistor for the Vbe multiplier I'm using is BC 550 and is good enough for biasing.

If you are interested you can look at the D.Self Trimodal amp schematics for almost identical class a current controller.

 

[chermann]

ok..thx.

you just remove the D1 and keep the auto bias transistors? or do you remove the auto bias circuit (abc) completely. i am asking because minek told that it has with abc the amp has bad THD.

what is your bias actually?
do you have pics?

thanks

have fund

 

[draganlalkovic]

Just remove D1. The current is 0.69 A with 0.47 ohm resistors in emitors.

 

[chermann]

Okay i will try this

 

[chermann]

first quick test .

removed D1. supply is +/-15V rail because of small heat sink.
at starting the amp draw about 1,6 A and the auto bias is working after 4 min. to about 1,35A.
DC offset is about -3mV