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Old 16th March 2007, 03:11 PM   #7321
kmj is offline kmj  Sweden
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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.
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Old 16th March 2007, 03:15 PM   #7322
AndrewT is offline AndrewT  Scotland
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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.
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Old 16th March 2007, 03:29 PM   #7323
kmj is offline kmj  Sweden
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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.
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Old 16th March 2007, 03:42 PM   #7324
AndrewT is offline AndrewT  Scotland
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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
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Old 25th March 2007, 09:15 AM   #7325
kmj is offline kmj  Sweden
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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.
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Old 25th March 2007, 09:17 AM   #7326
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Don't,
send me an email, i'll mail you the 32/33.
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Old 25th March 2007, 09:23 AM   #7327
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email sent
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Old 25th March 2007, 10:00 AM   #7328
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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.
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Old 25th March 2007, 10:03 AM   #7329
kmj is offline kmj  Sweden
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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!
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Old 29th March 2007, 10:07 AM   #7330
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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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