Stereo amp parallel wired to mono - possible?

[hasselbaink]

Hello,

I have a Radford STA25 that I would like to drive a mono bass channel crossed at 100 Hz. My question is: Can I parallel (NOT bridge) the left and right output of the amp? I believe that the amps 8 ohm tap in that instance would act as a 4 ohm one.

Does anyone here have experience with a similar set up?

Sincerely
Kris

[Eli Duttman]

The schematic shows a common ground for both channels and GNFB. Bridging, with out of phase I/P signals, is the only 100% safe way to turn the unit into a monoblock. When you bridge, a 16 Ω speaker gets connected across the 8 Ω taps and an 8 Ω speaker gets connected across the 4 Ω taps. A 4 Ω speaker can't be used.

Very few amps work well, with simple paralleling. Unfortunately, heat and distortion are the typical result.

[barretter]

That would be a terrible waste of a Radford STA25!

[hasselbaink]

Originally Posted by Eli Duttman

The schematic shows a common ground for both channels and GNFB. Bridging, with out of phase I/P signals, is the only 100% safe way to turn the unit into a monoblock. When you bridge, a 16 Ω speaker gets connected across the 8 Ω taps and an 8 Ω speaker gets connected across the 4 Ω taps. A 4 Ω speaker can't be used.

Very few amps work well, with simple paralleling. Unfortunately, heat and distortion are the typical result.

Thank you Eli. How would you go about making a circuit to invert the signal to the one channel? I'm mostly a speaker builder so this amplifier business can be a bit intimidating at times: consider me a newbie. I'd like to avoid solid state anywhere in my signal path, thus it would be nice to be able to do it by other means.

Kris

[Eli Duttman]

Originally Posted by hasselbaink

Thank you Eli. How would you go about making a circuit to invert the signal to the one channel? I'm mostly a speaker builder so this amplifier business can be a bit intimidating at times: consider me a newbie. I'd like to avoid solid state anywhere in my signal path, thus it would be nice to be able to do it by other means.

Kris

What are you planning on using as a crossover? If you already have a low O/P impedance mono bass channel, the Sowter model 8920 will yield the requisite 180^o out of phase signals. Ground the violet and white leads of the paired secondaries. Take the signals from the pink and yellow leads.

[kevinkr]

Originally Posted by Eli Duttman

The schematic shows a common ground for both channels and GNFB. Bridging, with out of phase I/P signals, is the only 100% safe way to turn the unit into a monoblock. When you bridge, a 16 Ω speaker gets connected across the 8 Ω taps and an 8 Ω speaker gets connected across the 4 Ω taps. A 4 Ω speaker can't be used.

Very few amps work well, with simple paralleling. Unfortunately, heat and distortion are the typical result.

Eli, I am totally not following your comment above, normally the recommendation for using a stereo tube amp in mono is strap the inputs and outputs, and I can't see any reason why this wouldn't work fine on the Radford? For a 4 ohm load you would strap the 8 ohm taps and the grounds. The exception in my experience would be any amplifier that uses cathode feedback derived from the same secondary used to drive the speaker - in which case bridging is not a good idea either. (I've mono strapped ST-70, MKII, MKIII, Citation II, MC-30/60 etc without any problems.)

Can someone post the schematic for this specific version of the amplifier? (I looked at the standard schematic and saw no reason not to strap)

FWIW I don't think using a Radford STA-25 as a bass amp is its highest purpose, (I'm completely with barretter on this one) and I wouldn't expect it to do a great job either unless driving an insanely efficient horn.

[nigelwright7557]

Originally Posted by hasselbaink

Hello,

I have a Radford STA25 that I would like to drive a mono bass channel crossed at 100 Hz. My question is: Can I parallel (NOT bridge) the left and right output of the amp? I believe that the amps 8 ohm tap in that instance would act as a 4 ohm one.

Does anyone here have experience with a similar set up?

Sincerely
Kris

Short answer is no.

One option is to use two speakers, one off each amp.

[Eli Duttman]

Eli, I am totally not following your comment above, normally the recommendation for using a stereo tube amp in mono is strap the inputs and outputs, and I can't see any reason why this wouldn't work fine on the Radford? For a 4 ohm load you would strap the 8 ohm taps and the grounds. The exception in my experience would be any amplifier that uses cathode feedback derived from the same secondary used to drive the speaker - in which case bridging is not a good idea either. (I've mono strapped ST-70, MKII, MKIII, Citation II, MC-30/60 etc without any problems.)

Kevin,

Consider yourself fortunate. I've seen reports of heat and distortion. I KNOW bridging works. Bridged summing of the 2 voltage swings in the load is safe.

BTW, if the secondaries of the O/P trafos float, wiring the amp I/Ps in parallel and the O/P trafos in series is fine. As is the case for bridging, each section "sees" only 1/2 of the load. Anybody got a pair of 8Bs?

[tubelab.com]

Eli, I am totally not following your comment above, normally the recommendation for using a stereo tube amp in mono is strap the inputs and outputs.......Consider yourself fortunate. I've seen reports of heat and distortion.

I don't know anything about the Radford amp in question but I have indeed wired as many as 4 tube amps in parallel with nothing but ear splitting results. Recently I wired both channels of two Simple P-P amps in parallel to make a stereo amp using two boards. I then proceeded to crank up the B+ voltage until I was getting about 60 watts from each amp using JJ EL84's. I was planning to actually build my amp this way until I decided to try it with 2 OPT's instead of 4 OPT's. This resulted in some loss of power capability in the low bass region since I now have half as much iron per channel, but this is OK since my speakers have no response below 70 Hz and I have a sub.

[Printer2]

Schematic

http://www.drtube.com/schematics/radford/sta25ren.gif