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2nd May 2017, 09:22 AM  #1 
diyAudio Member
Join Date: Jan 2009

apex b500 merging 4 channels into 2
Hi, its been a while since I last posted here, to make it as short as possible the idea is this, ive built 4 channels of the apex b500 years ago, then i built an smps power supply for them and safety circuit etc and put them in a box.
the thing is since I used the toshiba 2sa/2sc1943/5200 devices on them which dont have exactly the highest SOA I would like to simply take the 2 channels that use the toshibas and simply add the drivers to the other two channels. so that instead of 4 weaker channels I would have 2 stronger ones as they would have more output device pairs in parallel. My psu is quite capable and has output rails of about 85085 so this is one reason I would like to got for this the other one is that two of the 4 channels are built with less quality and they have HUM in them while the other two boards are built with more precision and detail so they are fine. my question is can I simply merge the outputs and take the signal to them from the active channel driver stage or would the drivers need to be made stronger due to the more output devices? I have attached the b500 schematic with 3 points labeled 1,2 and 3 of the possible places to connect the extra outputs from the other two channels, what do you think? 
2nd May 2017, 10:13 AM  #2 
diyAudio Member
Join Date: Jan 2009

I guess the question is can the driver transistor mje15032/33 drive each 8 output devices, as that is what will happen if I put additional 4 devices from the other channel on each side (npn/pnp) ?

2nd May 2017, 09:26 PM  #3 
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#3

3rd May 2017, 04:51 AM  #4 
diyAudio Member
Join Date: Apr 2003
Location: Százhalombatta, Hungary

The MJE15032/33 can drive 8 pairs, if You don't apply very low impedance load (2ohm, 1ohm), or higher rail voltage.
Sajti 
3rd May 2017, 06:24 AM  #5 
diyAudio Member
Join Date: Jan 2009

well normally one has such ridiculously low loads only in car audio subs etc normally in home or pro audio I see 4 and 8 ohms of impedance for a speaker set.
secondly I did some math , very basic but math and if I take the hFE of the output drivers and then take the worst case scenario current through the load and divide that I get something like this. my supply voltage is about 85085, which means that each side sees roughly 80v through the load, let's take 80v ad divide by 1.5 ohms we get about 53 amps current. then lets take 53amps and divide by 8 one side output transistors , we get about 6.6amps per transistor. according to the datasheet of 2sa1943 i get that for Ic 7A the hFE is around 60, so lets take our 53 amps divide by 60 is about 0.88 , heres where i get bit confused is this 0.88 or close to 1 amp the total base current for all 8 devices or the individual current per transistor base , I think its the total current for all 8 devices in one side? 
3rd May 2017, 06:41 AM  #6 
diyAudio Member
Join Date: Jan 2009

well basically I have a hard time understanding why many designs especially ones for DIY here which have 7 or 8 pairs of output devices use one of the output transistors in the final driver stage, because according to some simple math I just did here , at near maximum safe levels and very low impedance loads the base current doesn't exceed any significant numbers which a driver transistor like the MJE15032/33 couldn't handle?
Where could there be a situation where a 2sa1943 would need 500+mA of current in its base or say even 1A? because the MJE15032/33 is rated 8A max, so that gives about 1A max per output device if 8 are used. but I think at 1A of base current and a 2 or 1 ohm load the 2sa1943/2sc5200 would have long self destructed into flames from a 80v per side power supply that is strong enough to resist sag.? 
3rd May 2017, 06:47 AM  #7 
diyAudio Member
Join Date: Apr 2003
Location: Százhalombatta, Hungary

The driver has less work, if You just add some more output devices, but don't increase the output current.
But if You feel more comfortable with output devices as driver, let's do it! Anyway Fairchild makes FJP5200/FJP1943, which can be Your strong driver... Sajti Last edited by sajti; 3rd May 2017 at 06:50 AM. 
3rd May 2017, 07:43 AM  #8 
diyAudio Member
Join Date: Jan 2009

yeah but the question is , is there any necessity to use output transistors as the final base drivers if the device which is already there (mje15032/33) can do the job just fine?
the fairchild analogs to the toshiba transistors have a bit higher current ratings but I have toshiba original devices as output transistors. Well I will do some soldering later this week and then come back with the results basically 
4th May 2017, 02:37 AM  #9  
diyAudio Member

Quote:
Unfortunately, estimating load lines and peak power requirements for output transistors is not a simple, steady state DC calculation covered by Ohm's Law. You need to search load lines, DC and AC, for class AB audio power amplifiers to get a better understanding. As it is an engineering level topic, it may require a bit of heavy mathematics. I found this paper gave a fairly clear overview: http://highered.mheducation.com/site...alvinoch12.pdf 

4th May 2017, 06:30 AM  #10 
diyAudio Member
Join Date: Jan 2009

Ok I will read , I understand speakers are reactive , and so I was thinking that maybe using mje15032/33 could introduce some distortion upon heavy loading if at some point the driver exceeds it capability to power the bases of the output trannies.
although when I calculated using the hFE numbers mentioned for given Ic in the datasheets I used a load assumption which would go far lower thna I would ever play it, because normal home audio or pro speaker have 4 to 8 Ohms , I assume 1.5 ohms of load at 80v of voltage per side since my power supply is 85085 at these numbers I got a worst case output current of 53 amps through each sides 8 output devices. So I split the 53 amps upon my 8 devices and get about 6.6 amps per device which is very close to the toshiba datasheet where 7A is mentioned and a typical hFE is given to be around 60 at that collector current. So then I simply divide the 7 amps with the hFE and get about 111mA of base current but even assuming a much higher base current is still within reach of the MJE15032/33 Ic of 8A. Isn't this approach reasonable given that I used impedances much lower than would be actually used? The one thing I'm thinking about is whether I would need to up the bias current in the driver stage because additional output could mean that the bias could drop below a level where the transistors start to close upon certain conditions and introduce "zero crossing" distortion. 
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