SOA Old Vintage Xtors Vs new MJ21195/6

[jstang]

I upgraded my vintage Marantz Model 500 amps some time back to MJ21195/6 outputs. The Safe operating area of MJ21195/6 is about twice the Current at the Same voltage as the stock 1973 SJ2500 series....

Any harm in adjusting the amps SOA to be broader to match the MJ21195/6 SOA? Or just increasing the Current SOA by 40 or 50 %?

SOA is adjustable on this amp. The amps power supply, wiring and cooling is very robust. At the the current limits the outputs rarely go over 110*F as it is fan cooled. And that is with some very demanding 4 ohm loads ( Ohm Fs ). I feel there may be some benefit for low freq control of the very heavy Ohm F cones.

Thoughts?


johnk

[jstang]

Shameless bump... Any one?

johnk

[taj]

I'm only a beginner, but here's my take.

The SOA, as I'm pretty sure you are aware, is a graph of a given device's ability to handle current@voltage for a specific amount of time, and I assume at a given temperature before it burns out.

The only thing you can [typically] adjust in an amplifier is its quiescent bias current through the output transistors. That lowers a device's SOA figures somewhat by raising the temperature of the output transistors substantially. Your new transistors are very robust though. So steady-state heat is your concern (unrelated to amount of signal being amplified), and that comes down to how much cooling you have in place. So bear that in mind while adjusting the bias.

The other [main] things that will affect your device SOA are the rail voltages (I doubt that's adjustable, but could be I suppose), and the output current drawn by low impedance loads (lower than 4 ohm speakers). That's obviously adjustable by simply paralleling more speakers.

I guess it's just the concept of 'adjusting the SOA' that had me scratching my head. You did that (raised the SOA) by replacing the original output devices with MJ21195/6's.

I'm hoping any EEs here will correct me on any of that. And I have no advice to offer on adjusting the bias, that's beyond my abilities.

..Todd

[jstang]

Well the amp is an old Marantz Model 500 from 1974. Its current limiting feature (SOA) has 2 pots for each channel to Adjust the SOA current limit. One pot for the negative feed (PNPs) and one for the positive feed (NPNs).







The graph above from the service manual shows the current limit line. The idea is to place a capacitive load on the output and that simulates the affect of the speakers reactive load.

The current limit line in the case of the Model 500 ( many old Marantz ) can be adjusted up or down.... by a turn of the pot... Also has an adjustment for bias and balance of the two differential pairs for each channel....

johnk

[MJL21193]

Originally posted by jstang
Well the amp is an old Marantz Model 500 from 1974. Its current limiting feature (SOA) has 2 pots for each channel to Adjust the SOA current limit.

As taj said, idle current or bias current. SOA = Safe Operating Area

You want the amp to run a higher idle (quiescent) current, that's what those pots are for.

Quiescent Current
The standing current that flows in a circuit when the signal is not applied. The quiescent current is usually very low or lower than when processing a signal.

Sorry, guilty of not reading your entire post. Disregard the above obvious stuff.

[taj]

Johnk,

Ahh.. I see. Like some sort of adjustable crowbar protection circuit. You lost me with the capacitive load part. Is that service manual online somewhere, sounds interesting.

But if it's a current limit scheme, and you're not pushing the limit, does it have an effect on the low freq. you mentioned. Or are you wanting to push the amp harder, thus causing the low frequencies to hit the current limit too soon for the new transistors?

..Todd

[jstang]

Originally posted by MJL21193



As taj said, idle current or bias current. SOA = Safe Operating Area

You want the amp to run a higher idle (quiescent) current, that's what those pots are for.



Sorry, guilty of not reading your entire post. Disregard the above obvious stuff.

Originally posted by MJL21193



As taj said, idle current or bias current. SOA = Safe Operating Area

You want the amp to run a higher idle (quiescent) current, that's what those pots are for.

Not correct in the case of the Model 500. Bias, Differential balance and +/- SOA are all separate adjustments.

In the attached schematic the Bias adjust is on the Left side - Select R202.

The SOA adjusts are post R209 & R213 in the middle of the schematic.


johnk

[taj]

It does look like it could be an adjustable crowbar circuit, sensing from the output collectors. It's a bit too low resolution for me to see it comfortably though. And I'm not sure I would analyze it correctly if I could see it clearly anyway.

..Todd

[jstang]

Originally posted by taj
Johnk,

Ahh.. I see. Like some sort of adjustable crowbar protection circuit. You lost me with the capacitive load part. Is that service manual online somewhere, sounds interesting.

But if it's a current limit scheme, and you're not pushing the limit, does it have an effect on the low freq. you mentioned. Or are you wanting to push the amp harder, thus causing the low frequencies to hit the current limit too soon for the new transistors?

..Todd

Yes an adjustable SOA Current Limit protection circuit.

The circuit prevents Transistor over loads at high or low output Voltages.

Low output voltages cause the outputs to exceed Current VS Vce limits.

Hence current limits at high output Voltage is different than lower output voltages... This is whole Transistor SOA issue....

The Marantz SOA protection circuit can be adjusted to match the output transistors.....

Speakers are re-active loads due to the weight of the cone, the coil and magnet... What happens is at Resonance Freq, the speaker is generating voltage... That is where the control issue occurs...and the Current limiting because an issue....

The speakers affective impedance drops at the Resonance Freq and places extreme loads on the amp....

The Ohm F is a 3.8 ohm speaker.... But drops very low at Resonance Freq....and that is where I think I am bouncing into the current limit.....


johnk

[jstang]

Originally posted by taj
It does look like it could be an adjustable crowbar circuit, sensing from the output collectors. It's a bit too low resolution for me to see it comfortably though. And I'm not sure I would analyze it correctly if I could see it clearly anyway.

..Todd

I will find a place to post higher res.....


jk