WOPL running 4 Ohm speakers

[Gepetto]

I am not exactly clear. Is it one or both of the diodes that we must remove?

If you only want to unclip the top half of the signal, remove D13L/R
If you only want to unclip the bottom half of the signal, remove D14L/R

If you want to unclip both top and bottom signal halves, remove both D13L/R and D14L/R

You have been warned and are on your own after that...

 

[plex]

I understand now. Thanks for your help.

 

[Gepetto]

Remember, you are on your own...there is reasons there are so many blown up PL amps out there. This is one of them.

 

[WOPL Sniffer]

Remember, you are on your own...there is reasons there are so many blown up PL amps out there. This is one of them.

Yeah, not good just to run an extra set of 901's. I can buy a set of 901's for $3-$400, or I can blow up a $2000 amp just to do it.

 

[WOPL Sniffer]

Yeah, not good just to run an extra set of 901's. I can buy a set of 901's for $3-$400, or I can blow up a $2000 amp just to do it.

That is one of the first things in my "New WOPL" handout all my customers get, no twisting extra sets of speaker wires together. There is a reason for that. Heat is our enemy and doing that will cause premature failure sooner rather than later and added stress on other components aint good either.

 

[Gepetto]

When driving 4 ohm speakers (which are quite likely not 4 ohms but something less than 4 ohms) The VI limiter is set to deliver 10.6A at 42.5V peak. This means that each output is providing ~3.6A. The 90V, 50msec (20 Hz) SOA rating of the MJ21195/96 pairs is 4A.

The SOA of the original PL909 transistors is far less than that of the MJ21195/96 transistors so you can guarantee that they were violating the rated SOA often.

Yes the VI limiter is set conservatively to protect the amp from failure.

 

[MusicSteve]

The VI limiter works as designed. I snub the 4 ohm condition at about the same watts out as 8 ohms for thermal reasons (PL let you blow your amp up if you wanted, I am trying to avoid that). The PL400/700 are not thermally designed to handle CW output at 4 ohms. You are aware of that.

The VI limiter will work anywhere along the linear spectrum when you are driving a load. When you hit around 42.5V peak into a 4 ohm load, you will begin to activate the VI limiter and the signal will flat top limiting the current into the load. That is exactly where you were at 30.05Vrms into your 4 ohm load (10.6A!!). A VI limiter responds to both the voltage and current that is going into the load. The simple spice model I sent you should help explain that, it is truly linear in nature and will protect anywhere along the signal spectrum that it needs to.

Joe
Good to know, So When when this happens what happens?, to the output to the speaker and/or the amp? any weird sounds in the speaker? how do you know it happens?

 

[Gepetto]

Joe
Good to know, So When when this happens what happens?, to the output to the speaker and/or the amp? any weird sounds in the speaker? how do you know it happens?

The output to the speaker flat tops at the VI limit...

 

[MusicSteve]

The output to the speaker flat tops at the VI limit...

So it like stop at that limit and keeps playing?

 

[plex]

Remember, you are on your own...there is reasons there are so many blown up PL amps out there. This is one of them.

Understood, will post results when alterations are made.

 

[Gepetto]

This explanation will show what happens at SOA protection onset. SPICE models show the behavior at the 8 ohm and 4 ohm condition.

PS: it will do the same thing at 8 ohms when the amp encounters the limit imposed by the power supply voltages. That will not be the VI limiter engaging, it will be the limitation imposed by having either 80V rails (PL400) or 100V (PL700).

 

[Gepetto]

So it like stop at that limit and keeps playing?

Correct, does not shut down, just protects the amp and moves on. It behaves on a cycle by cycle basis, limiting only peaks that exceed the SOA limitation.

 

[MusicSteve]

Correct, does not shut down, just protects the amp and moves on. It behaves on a cycle by cycle basis, limiting only peaks that exceed the SOA limitation.

Thanks Joe
Sounds like a good thing to have hooked up

 

[MusicSteve]

Understood, will post results when alterations are made.

You might want to run it into 8 ohms to verify everything works ,,, before doing this modification, You still could have another problem IMO

 

[WOPL Sniffer]

Yup, this opens the flood gate to dumb shit happening. Bad move. Might as well put a "Stupid Switch" on the back of the amp so everybody can do it.

 

[WOPL Sniffer]

Why have the SOA circuitry then. Do the American thing and save 50 cents on each assembly and remove it..

 

[FredR]

Joe
Good to know, So When when this happens what happens?, to the output to the speaker and/or the amp? any weird sounds in the speaker? how do you know it happens?

Sounds like distortion. When you hear someone say an amp sounds like shit into low impedance loads but fine otherwise, that is a possibility. Actually, I usually hard wire in my stupid switch.

 

[MusicSteve]

This explanation will show what happens at SOA protection onset. SPICE models show the behavior at the 8 ohm and 4 ohm condition.

PS: it will do the same thing at 8 ohms when the amp encounters the limit imposed by the power supply voltages. That will not be the VI limiter engaging, it will be the limitation imposed by having either 80V rails (PL400) or 100V (PL700).

Thanks Joe
I love Troubleshooting!

 

[MusicSteve]

Sounds like distortion. When you hear someone say an amp sounds like shit into low impedance loads but fine otherwise, that is a possibility. Actually, I usually hard wire in my stupid switch.

Thanks
The more information the better. I been playing around w this too Lol

 

[George S.]

Why defeat the improved circuitry? I'd personally run one set of Bose 901 until I could build a second amp. Even if it looked like it might be a year or two. Looks like reliability is the trade off. Any issues are going to be time intensive and expensive to fix.