Upgrading Sub Amplifier

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Brain Fried

Brain Fried

Member
Joined 2010
#1
OK i have a ASW -10-120 (aka as KLH) i keep killing the output transistors. I go through a set about once a month so i decided to look for better transistors and driver stage and i think i found what i am looking for i just want to make sure i am on the right track the stock output transistors are 2sa1516 and 1sc3907 and the drive stage is 2sd1563 and 2sb1086 i want to replace the outputs with 2sa1216 and 2sc2922 and the drive stage with 2sb649a and 2sd669a. So my main question is am i on the right track and do i need ot update anything else. Here is a picture of the schematic i will appreciate any help you guys can give.

An externally hosted image should be here but it was not working when we last tested it.
 
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#2
 
#3
What is the ±V ?

What is the voltage across R46, R47 ? Across R48, R49 ?

Consider MJL21193/94/95/96, should drop right in (no extra holes).

Consider MJE15032/33/34/35 drivers, they have reversed pin-out but should drop right in.
 
#4
If something is blowing up there must be a reason. I'm not sure just beefing up the outputs and drivers will put an end to it.
What Q devices are failing and what are the failure mechanisms. Things like accidental disconnecting of gear still powered up or shorting stuff??
This schematic is very basic! You might think about using a couple of freewheeling diodes on the output as well as HF loading zobels. Any service bulletins from the factory?
 
#5
Blowing devices

You need emitter resistors on the output devices, use 0.22 ohm or 0.27 ohm 5W types. Also add base stopper resistors on each output device right next to the base connection.

Diode D3 is shown backwards and the circuit cannot work with the diode shown in your schematic.

Check thr voltage across the bias spreader it should be less than 1.5v as the the output devices are not in the thermal loop.

Where are the bias spreader devices bolted to? They must be in contact with the drivers and NOT the output devices.

Of course upgrading to the larger MT200 Sankens makes the output stage more reliable in terms of SOA but without the above mods, the big Sankens will go just as quickly.

Steve Mantz
Zed Audio
Zed Audio Corporation
 
#6
infinia said:
If something is blowing up there must be a reason. I'm not sure just beefing up the outputs and drivers will put an end to it.
What Q devices are failing and what are the failure mechanisms. Things like accidental disconnecting of gear still powered up or shorting stuff??
This schematic is very basic! You might think about using a couple of freewheeling diodes on the output as well as HF loading zobels. Any service bulletins from the factory?
Click to expand...

The factory sub failed so i upgraded it to RFD2210 (aka HX2 Fosgate)Dual 4 ohms set at 8ohms this sub is rated at 1000watts RMS and i retuned the cabinet to 35Hz now i have sub that can take the heat and sounds great but now the amplifier output transistors are failing ( dead short ) and there no updates from the factory this design was discontinued eight years ago and it took a month to get this schematic.
An externally hosted image should be here but it was not working when we last tested it.


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#7
MOER said:
You need emitter resistors on the output devices, use 0.22 ohm or 0.27 ohm 5W types. Also add base stopper resistors on each output device right next to the base connection.

Diode D3 is shown backwards and the circuit cannot work with the diode shown in your schematic.

Check thr voltage across the bias spreader it should be less than 1.5v as the the output devices are not in the thermal loop.

Where are the bias spreader devices bolted to? They must be in contact with the drivers and NOT the output devices.

Of course upgrading to the larger MT200 Sankens makes the output stage more reliable in terms of SOA but without the above mods, the big Sankens will go just as quickly.

Steve Mantz
Zed Audio
Zed Audio Corporation
Click to expand...

There is emitter resistors on the output devices a .22/5w now for D1 it wrong on the schematic but right on the board here a picture of the top of the board

An externally hosted image should be here but it was not working when we last tested it.
 
#8
"You might think about using a couple of freewheeling diodes on the output "

If it had foldback current limiting I would agree with you, but it doesn't.

"HF loading zobels"

R52/C23 serve that function.

"Diode D3 is shown backwards and the circuit cannot work with the diode shown in your schematic."

If you reverse D3 it will have no bias, I know it will work with D3 removed, I'm not sure what the function of D3 is as shown.
 
#11
Or more accurately what's the bias current thru R50 and R51. Its key to recheck the bias here, esp with going to new device types. The feedback will go nuts if the bias is being starved. If the op-amp is in a socket it might be prudent to remove it and ground its output and check bias. There is one bias pull down resistor near D3 whose value isn't listed, that maybe key for sliding bias as op-amp levels change with feedback.



Outputs maybe OK to switch to ON from the Sanken upgrade but the Ft don't match by a factor of 8-10, might be worth it to try as the cost of Sankens if you can find them,

TO-126 as drivers maybe more work switching to TO-220 ie fitting new reversed Heat Sinks and drilling PCB holes. Might be worth going all out with ON if the lower gains and Ft's work out for stability.

These TO-126 Fairchilds are slightly better than Hitachi at Mouser 0.38 each
NPN 2SD669 with Fairchild KSC2690A as modern subs
PNP 2SB649 with " " KSA1220A "
 
#12
Hi there.

Check the resistance accross the speaker binding terminals at the speaker itself and check to see if the resistance has not fell below it's rating. For example, a friend of mine had the same problem and we found that the speaker was being driven too much. When this happens the voice coil heats up too much and this can lead to the insulation on the voice coil melting and the voice coil wires start to short out.

I measured the resistances of two 15" bass subs rated at 4 Ohms each and they had dropped to 2.3 and 1.7! - this would totally blow your output transistors if there is no speaker/amp protection circuit on your amp.

Hope this helps.
 
#14
My WAG is that at clipping something is "hanging up" causing cross conduction, Thus shorting out across the rails. If you can't test for this then set things up to keep from clipping in operation. Note the gain of this amp is pretty hot probably for stability using the op-amps dominate pole. The transistors Ft probably need to be beyond the op-amps OL unity gain frequency since there is some gain on the OP stage.
 
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#15
infinia said:
My WAG is that at clipping something is "hanging up" causing cross conduction, Thus shorting out across the rails....
Click to expand...
Yes. During clipping the output devices will saturate heavily and "stick".

Perhaps that could be fixed by adding a few diodes as shown below.

The bias arrangement looks a bit dodgy too, unless Q3 and Q8 are tracking the driver's temperature?
 

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#16
godfrey said:
Yes. During clipping the output devices will saturate heavily and "stick".

Perhaps that could be fixed by adding a few diodes as shown below.

The bias arrangement looks a bit dodgy too, unless Q3 and Q8 are tracking the driver's temperature?
Click to expand...
Hi
How do we know this is sticking? any references ? Baker clamp?

Or keep the amp from clipping in the first place by careful setting of the preceding levels.

Another possibility is the thing can break into oscillations getting hot until meltdown. Try a 0.02 uF cap at the op-amps output to ground, this can stabilize things w/o affecting the response. R52 would be showing signs of excess heat for symptoms of this.

I guess the tempco tracking is by air, If the OP device gains are high enough.
 
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#17
Pro amp with the same topology here It looks weird because they take the speaker at the opposite polarity. This is done to eliminate insulators on the OP devices, ie increased reliability. Heatsink and OP device cases are at ground. Very clever those Californians.
Note protection diodes D1 and D2 tied to the op-amp supply, this is to help output from latching up?
Resistors marked FP stands for flame proof. LOL

Note C4 is an AC op-amp load. they're using ON drivers with slower TOS OP devices. There's hope for djk's suggestions I think.
 
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#20
" I'd do it anyway. It's cheap and easy."

It's your time and money, notice QSC doesn't use them either.

"Note protection diodes D1 and D2 tied to the op-amp supply, this is to help output from latching up?"

D1, D2 are power supply rectifiers. As you near clipping the supply rails approach zero volts, and the speaker voltage is rectified and fed to the opamp.

I would try a 2K2 resistor across C23, and adjust the bias to 340mV on the bases of the outputs.
 
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