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Sound Reinforcement  Forums for Live Sound Professionals  Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Pro AV Forum => Topic started by: Jay Stephen on March 04, 2018, 07:10:37 am

I have an old but working RaulandBorg ("Webster") SS amp that I'd like to use as a subwoofer amp driving a 2x15 4 ohm cab. The amp has 70V and 25V output options. I believe the correct formula to arrive at output impedance is Output Voltage squared divided by nameplate output watts = speaker ohms.
So in my case, 25 squared divided by 125 = 5 ohms.
My cabinet is 4 ohms and I know I could open it up and put the 2 15's in series (16 ohms) but should I "cheat" and leave the cab at 4 ohms? Or, is there another workaround other than to go the series route?
Thanks.

I have an old but working RaulandBorg ("Webster") SS amp that I'd like to use as a subwoofer amp driving a 2x15 4 ohm cab. The amp has 70V and 25V output options. I believe the correct formula to arrive at output impedance is Output Voltage squared divided by nameplate output watts = speaker ohms.
So in my case, 25 squared divided by 125 = 5 ohms.
My cabinet is 4 ohms and I know I could open it up and put the 2 15's in series (16 ohms) but should I "cheat" and leave the cab at 4 ohms? Or, is there another workaround other than to go the series route?
Thanks.
I'm sure you will get some helpful advice once you use your real name as per forum rules.

Reposted. Apparently someone doesn't like me using the nickname I have had for many years. "I have an old but working RaulandBorg ("Webster") SS amp that I'd like to use as a subwoofer amp driving a 2x15 4 ohm cab. The amp has 70V and 25V output options. I believe the correct formula to arrive at output impedance is Output Voltage squared divided by nameplate output watts = speaker ohms.
So in my case, 25 squared divided by 125 = 5 ohms.
My cabinet is 4 ohms and I know I could open it up and put the 2 15's in series (16 ohms) but should I "cheat" and leave the cab at 4 ohms? Or, is there another workaround other than to go the series route?
Thanks."

Reposted. Apparently someone doesn't like me using the nickname I have had for many years. "I have an old but working RaulandBorg ("Webster") SS amp that I'd like to use as a subwoofer amp driving a 2x15 4 ohm cab. The amp has 70V and 25V output options. I believe the correct formula to arrive at output impedance is Output Voltage squared divided by nameplate output watts = speaker ohms.
So in my case, 25 squared divided by 125 = 5 ohms.
My cabinet is 4 ohms and I know I could open it up and put the 2 15's in series (16 ohms) but should I "cheat" and leave the cab at 4 ohms? Or, is there another workaround other than to go the series route?
Thanks."
A couple of points...
What is the actual power rating of the amp?
If there is a 70 and 25 volt output, there is probably a transformer on the output and that could be bypassed for a 4 ohm load.
Most of the 70 volt amps I have seen are relatively low powered and may not be suitable for sub applications.

Thanks. The power output appears to be 125 watts RMS. I used it on a cabinet with 2x12 16ohms and you are correct, it is pretty low power but it was sufficient for my purpose. Unfortunately I haven't been able to locate a schematic but I'd be interested in knowing how to bypass the output transformer and use it on the 4 ohm cabinet.

Thanks. The power output appears to be 125 watts RMS. I used it on a cabinet with 2x12 16ohms and you are correct, it is pretty low power but it was sufficient for my purpose. Unfortunately I haven't been able to locate a schematic but I'd be interested in knowing how to bypass the output transformer and use it on the 4 ohm cabinet.
NOTE: This is based on a general knowledge of 70 volt amps and may not apply to this specific amp!
Crack it open and have a look and see if you can locate the transformer.
There should be connections to the back panel for the speaker from one side of the transformer and on the other side of the transformer should be 2 connections coming from the amplifier cct board.
Disconnect those connections to the transformer and connect the speaker to the amplifier output.

NOTE: This is based on a general knowledge of 70 volt amps and may not apply to this specific amp!
Crack it open and have a look and see if you can locate the transformer.
There should be connections to the back panel for the speaker from one side of the transformer and on the other side of the transformer should be 2 connections coming from the amplifier cct board.
Disconnect those connections to the transformer and connect the speaker to the amplifier output.
Thanks. Attached the spec sheet and schematic for a RaulandBorg similar to mine. The output transformer configuration appears to be similar. Are you suggesting bypassing that transformer?

Thanks. Attached the spec sheet and schematic for a RaulandBorg similar to mine. The output transformer configuration appears to be similar. Are you suggesting bypassing that transformer?
Now that I see the schematic, perhaps not.
Try the speaker on the 25 volt output and see how it works.

Chances are running it on the 25v output will be fine. Optimal? No. But that amp was never going to be optimal anyway.
If you wanted more grunt, the Behringer iNuke series offer decent DSP and power for not a lot of cash.
Chris

I take it there is not a lot of "sub" demanded from this system as 125 Watts ain't a whole lot in the grand scheme of things.
Good luck with this and let us know how it worked out.

Seems like the transformer would saturate at the frequencies you'd want to use to drive a sub with...kinda rendering it less useful? Combine that with the low power output to begin with, and it doesn't sound like a winning combo...unless you only use it to add a little lowend support to your laptop speakers.