ok thanks. I'm still trying to understand the particulars of biasing this output stage. If I have enough heatsink then I'm assuming I could take my bias a little higher. Maybe pushing 1.2-1.3 amp? But with the diodes bypassing source resistors I need to make sure my bias voltage across the resistors stays below the turnon point of the diode, correct? Does it matter how far below? For example if I were to replace my source resistor with a single 10 watt .22 ohm and then set bias voltage to .22volt for an amp, does that set the bias to low for the bypass diode?
BTW if anyone else is reading this a caution. I almost burned up my frontend board by not being careful. I'm using an older B&K signal generator and was trying to figure gain of the amp. The transistors on the cascade got very hot before I could kill power. The problem turned out to be an appreciable amount of dc offset coming from the generator. This was with it set for ac couple. Make sure your equipment is working correctly before using it.
BTW if anyone else is reading this a caution. I almost burned up my frontend board by not being careful. I'm using an older B&K signal generator and was trying to figure gain of the amp. The transistors on the cascade got very hot before I could kill power. The problem turned out to be an appreciable amount of dc offset coming from the generator. This was with it set for ac couple. Make sure your equipment is working correctly before using it.
OK thanks Audiosan. I just now realized that the schematic I've been working from does show 4 pair outputs. Mine has only 2 pair. So what can I realistically get as far as class A watts from this? Running @.66amp per transistor after 2 hours my heatsink is only at 118 deg F. The transistor case is at 135deg F. And with +/-45vdc rails what can I get in AB?
at this moment you have about 25W class A @ 8ohm. and about 110W class A/B. if your sinks is at 47c you don't realy have to much to go on. build one more chassis and run mono blocks with 4 pairs each. or get a transformer with lower voltage and use it as a v2 if you want more class A power.
I tried what Andrew said which was add 1 more 1ohm resistor. I could not reach .333mv across source resistor w/P1-P2. I ended up at .266mv across.333ohm. So that comes out to 3.12 total for 2 pair. Ambient temp is 70deg F and heatsink is holding steady at 113deg F. I intended to use with older Maggies that are not efficient. Don't I need higher voltage to drive the speakers. I have a multitap xfmr and could go lower ac volts. How do I get more class a power with lower voltages? How come I can't get the bias any higher also?
its not 3.12A. its 1.6A. its the total current of the N ch batch. not N+P.
no need for that ekstra 1ohm resistor. stay with the 2 originals.
you can crank up the bias a littlebit. 800mV over 0.5ohm resistors gives you a total of 1.6A bias. and 36W dissipation pr device.
With lower voltage you can crank up the current without the dissipation being too high.
but be carefull so that the diodes don't start conducting.
and also think of the transformer VA rating. With half the output voltage, the VA rating is halfed too. current capability always stay the same regardless of voltage.
no need for that ekstra 1ohm resistor. stay with the 2 originals.
you can crank up the bias a littlebit. 800mV over 0.5ohm resistors gives you a total of 1.6A bias. and 36W dissipation pr device.
With lower voltage you can crank up the current without the dissipation being too high.
but be carefull so that the diodes don't start conducting.
and also think of the transformer VA rating. With half the output voltage, the VA rating is halfed too. current capability always stay the same regardless of voltage.
How?
With 800mV across the Rs and the parallel bypassing diode, that diode is already turned on HARD.
It starts to turn on ~ 400mVf and is fully on @ ~ 600mVf and will be passing 100s of amps @ 800mVf.
The PSU and the output devices will have given up long before you can get to 800mVf.
Andrew thanks for clarifying above about the 400mv threshold for the diodes. The 800mv was throwing me off also. I did try the extra 1ohm resistor but then the highest bias setting I could dial in was 275mv over the .333 ohm resistance. Do you know why I can't dial it higher? Is the heatsink temps i'm seeing on the high side? I considered on the low side. I can put my hand on the sinks and hold it there for as long as I want. What temperature in degrees would be considered too hot? Anyway I have more heatsinks or option for forced air if needed.
Audioson, not sure I'm agreeing with you on VA rating on xfmr. The VA rating stays the same across different Voltages and currents. If I am using a multitap xfmr rated at 1000VA and I have 100V supplied from secondary then it can supply 10amps. If I tap for 50V on secondary then it can supply 20amps. I'm ignoring higher voltage drop in secondary because of higher current.
Audioson, not sure I'm agreeing with you on VA rating on xfmr. The VA rating stays the same across different Voltages and currents. If I am using a multitap xfmr rated at 1000VA and I have 100V supplied from secondary then it can supply 10amps. If I tap for 50V on secondary then it can supply 20amps. I'm ignoring higher voltage drop in secondary because of higher current.
no you can not. a 1000VA 25-0-25-0-25-0-25V transformer can handle 10A pr sec winding. if you use it as 25-0-25V or as a 50-0-50V do not mather. you are limited to 10A unless you can parallell 2 and 2 windings.
the reason you can not crank the bias any higher is the value of R5/R6 on the schematic from the F5turbo article.
the reason you can not crank the bias any higher is the value of R5/R6 on the schematic from the F5turbo article.
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OK so I did some further reading and I agree with you on the kVA rating of the xfmr. So help me understand my xfmr specs. I have a 2kVA xfmr. It has a single multitap primary rated at 415vac. It has 2seconday windings that can be wired in series for 240vac or in parallel for 120vac. So what is the current rating for the secondary? It also states that it can handle peaks to 6kVA.
Thanks for the info
Thanks for the info
Yes the same transformer but operated at lower input voltages on pri and therefore lower output on sec. So I was thinking that the lower voltage would allow higher sec. current by 2000/lower volt=higher current. What I now hear is that if the secondary is in series you can only get the 8.3 amp output no matter what the secodary voltage is. Correct??