convert Stereomour to a preamp.

[Paul Birkeland]

Like this.

[drewh1]

Ha, I just looked up voltage divider and get it now!  Thanks again Paul, you are a very patient man.

drew.

[Paul Birkeland]

Now is a good time for cheap experiments. 

[drewh1]

Got the parts and took about 10 minutes to put in. I put them across the binding posts and ran a lead to the plus on the 3d set of RCA's ( I only use one input anyway)  so I assume I can use it either way without modifying anything, speakers or padded outputs.

It works great!  I am running at about half way on the volume pot. 

Thanks so much, now I can spend hours comparing it to the Nelson Pass Buffered Passive Preamp I just built!  thanks goodness for hobbies. Hope you are all well at Bottlehead.

drew.

[Paul Birkeland]

Awesome, I'm glad that worked out for you.  If you need more or less gain, you can mess with the resistor ratio to fine tune things. 

[drewh1]

Just smoked one of my RAAL Tweeters.  I was going to change out the volume control on the Stereomour.  Inputs and outputs were connected. I turned the power off the Stereomour but not the power amp. When I pulled the power cord from the Stereomour to disconnect it, got a loud hum and one of my tweeters went up in smoke. Fried the Ribbon. 

Thankfully everything else seems fine including the tweeter on the other channel.

All of my power is from a Panamax 5300 power unit. Supposed to have all sorts of safety stuff.

Things had all been working fine and sounding great prior to this disaster.  Any ideas what might have happened?

thanks - Drew.

[Jamier]

The turn on/ turn off procedure is: power amps; turn on last, turn off first. With the power amp left on, you probably gave the tweeter the full power capability of that amp instantaneously.

Jamie

[drewh1]

Well thanks - but the preamp was already powered off. Nothing happened till I pulled the plug. Don't see why that would have caused a problem unless there is some kind of ground short.

[Paul Joppa]

Grounds are involved. The inputs and outputs are of course grounded to the chassis, which in turn is grounded to the safety ground through the power cord. You might look into whether the sources and/or power amp are safety grounded. It could get complicated to figure out exactly what happened, but it may well have to do with disconnecting the safety ground connection.

Ribbons are fragile! Normally a second or higher order passive crossover is needed to protect them. We've learned this lesson the hard way some years ago.

[Doc B.]

Looking over the specs, the power manager you mentioned appears to have a built in power on/off sequencer that does what we are talking about regarding preamp on first/amp on, amp off first/preamp off.

[drewh1]

Thanks for feedback -

Paul - Alex at RAAL helped me with the final circuit and values for the tweeters, so I am guessing he would have caught a problem in the xover. and it definitely has to be some problem with grounds. No other damage but the tweeter. From now on, that power amp is definitely getting shutoff and disconnected before I do anything else but i would like to figure out if there is something else that needs to be fixed.

If you have any other ideas let me know.

[Jamier]

Drew,

     No disrespect intended. I have learned the hard way, myself, about turn off procedures. I have F’d up tweeters too, so I got religious about the “turn off “. Sorry about your RAALs. I have never used them but I hear they’re very nice.

Jamie

[drewh1]

No worries - I appreciate your help.

drew.

[drewh1]

I think I found the problem -  I reused two of the input RCA connectors for the Voltage divider output.  The problem is although they had ground wires connected, they only had ground when the selector switch is in the correct position.  Which of course it wasn't, it was on the input RCAs.

Therefore, there was no ground to the RCA connectors going to the power amp. So I imagine that when I unplugged the safety ground, the capacitors discharged directly to the only available path - through the RCA's to the power amp (which was still turned on)

Does that seem like a possibility?   At any rate, these need to be grounded directly to the chassis, so that is happening now.

[Jamier]

Drew, to be clear, I would not disconnect any component until everything is powered down. Also, it wouldn’t hurt to wait a few minutes after power down to begin disconnecting stuff.

Jamie