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 16th March 2007, 04:11 PM #7321 kmj   diyAudio Member     Join Date: Feb 2004 Location: Falkenberg Oh, great. That took a load of me mind Now I'll just switch the busted transistors on the other channel, move everything back into the case and turn up the bias to 400mV or as much as the heatsink can tolerate. __________________ .....Where the music comes to die.
 16th March 2007, 04:15 PM #7322 AndrewT   diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Hi Kmj, 0r68 and 400mV gives Ire=0.4/0.68=0.588A. Ibias = 3*0.588=1.76A. The peak (ClassA) current into a load, for a push/pull amplifier, is 2*1.76=3.53Apk ClassA power = Ipk^2*Rload/2=3.53*3.53*8/2=50W ClassA Peak voltage at max ClassA power is Ipk*Rload=3.53*8=28.24Vpk=20Vac. This is worth saving and putting into a text file, or better, into a spread sheet. Personally I would bias for 1.76+0.1 amps to ensure the very curved knee in the transistor Ic vs Vbe at the near change to ClassB is avoided. I think running into this low Ic region causes an increase in open loop distortion and makes the NFB work very hard. I think that's why ClassAB produces more distortion when biased above optimum. I would also bias for the minimum impedance presented by the speaker rather than the nominal impedance. These extra levels of Iq will make the output stage run hotter and require larger heatsinks. __________________ regards Andrew T.
kmj
diyAudio Member

Join Date: Feb 2004
Location: Falkenberg
Quote:
 Hi Kmj, 0r68 and 400mV gives Ire=0.4/0.68=0.588A. Ibias = 3*0.588=1.76A. The peak (ClassA) current into a load, for a push/pull amplifier, is 2*1.76=3.53Apk ClassA power = Ipk^2*Rload/2=3.53*3.53*8/2=50W ClassA Peak voltage at max ClassA power is Ipk*Rload=3.53*8=28.24Vpk=20Vac. This is worth saving and putting into a text file, or better, into a spread sheet.
I have the calculation bookmarked but since it was a while ago I though that I rather ask one extra time and have it confirmed than make a misstake and have the amp unfinished another couple of months

Quote:
 Personally I would bias for 1.76+0.1 amps to ensure the very curved knee in the transistor Ic vs Vbe at the near change to ClassB is avoided.
Isn't that the same as 400mV across the resistor? I only meant to lower it if the sinks couldn't handle the needed dissipation.
__________________
.....Where the music comes to die.

AndrewT
diyAudio Member

Join Date: Jul 2004
Location: Scottish Borders
Quote:
 Originally posted by kmj Isn't that the same as 400mV across the resistor?
no.
Vre=400mV gives Iq=1.76A
Vre=423mV gives Iq=1.76+0.1A (1.86/1.76*400)

Adjusting the bias to achieve ClassA into a 6ohm load and reach the same peak voltage will require the bias to be increased to 2.45A and Vre=558mV
__________________
regards Andrew T.

 25th March 2007, 10:15 AM #7325 kmj   diyAudio Member     Join Date: Feb 2004 Location: Falkenberg I thought of a thing, the transistors on the driverboard are the MJE15032/33. I can't find them over here but the MJE15030/31 are available. If I would need to change these on one of the channels, are the compatible or do I need to change them on both channels? I heard that these are virtualy the same models. __________________ .....Where the music comes to die.
 25th March 2007, 10:17 AM #7326 jacco vermeulen   diyAudio Member     Join Date: Oct 2004 Location: At the sea front, Rotterdam or Curaçao Don't, send me an email, i'll mail you the 32/33. __________________ Why does it hurt when I Pee ? (Ay Ay Ay https://www.youtube.com/watch?v=w5fLGWrHouk)
 25th March 2007, 10:23 AM #7327 kmj   diyAudio Member     Join Date: Feb 2004 Location: Falkenberg email sent __________________ .....Where the music comes to die.
 25th March 2007, 11:00 AM #7328 pinkmouse   diyAudio Moderator     Join Date: Apr 2002 Location: Rotherham, England The 30/31s do work in the circuit, so if you want to get going, install them for the time being until you get the others. __________________ Rick: Oh Cliff / Sometimes it must be difficult not to feel as if / You really are a cliff / when fascists keep trying to push you over it! / Are they the lemmings / Or are you, Cliff? / Or are you Cliff?
kmj
diyAudio Member

Join Date: Feb 2004
Location: Falkenberg
Quote:
 The 30/31s do work in the circuit, so if you want to get going, install them for the time being until you get the others.
Thanks, but Jacco's on it!
__________________
.....Where the music comes to die.

 29th March 2007, 11:07 AM #7330 dr.strangelove3   diyAudio Member     Join Date: May 2004 Location: Leeuwarden I have skipped my KSA-50 project and moved on to the KSA-100 MKII. So now I have two very nice kits left. Troy did a hell of a job making all these kits! Al the electronic parts to stuff the Pinkmouse PCB's to make two channels. Except the output transistors. All quality parts like dale resistors. I'm asking what I payed for them. 75USD excl. shipment. So if someone is interested, send me a PM.

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