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
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
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.
Very few amps work well, with simple paralleling. Unfortunately, heat and distortion are the typical result.
re. bridging
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
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 180o out of phase signals. Ground the violet and white leads of the paired secondaries. Take the signals from the pink and yellow leads.
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.
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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? 😉
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.
Have a look at this thread
http://www.diyaudio.com/forums/soli...ridge-parallel-them-use-them-mono-blocks.html
Though concerned with solid state amplifiers it would work fine with valve amps.
Stuart
http://www.diyaudio.com/forums/soli...ridge-parallel-them-use-them-mono-blocks.html
Though concerned with solid state amplifiers it would work fine with valve amps.
Stuart
Radford
Well... while the Radford STA25 III is a desirable amp, in the e-bay sense of the word, it is hardly a superior design. Actually I have had it triode wired with 6L6 tubes in stead of the EL34 and the phase splitter has been redesigned.
All else being equal, I still think the original is a descent amp.
Kris
Well... while the Radford STA25 III is a desirable amp, in the e-bay sense of the word, it is hardly a superior design. Actually I have had it triode wired with 6L6 tubes in stead of the EL34 and the phase splitter has been redesigned.
All else being equal, I still think the original is a descent amp.
Kris
Hi Eli,
I will be using a passive line level XO - a gentle roll off 1. order RC filter 100 or 150 Hz. That is why I want to use the Radford because it is going to be audible well into the midrange. The rest of the audio band will somewhere in the near future be handled by a single ended 2A3 amp.
The mono bass channel is a 100 liter sealed box with two ATC 15" studio woofers wired in series making a 16 ohm load. The woofers are 91 db sensitive. Should be well suited for a bridged amp.
Best
Kris
I did that back when I had 2 Citation II amps (60W/Ch) paralleling the 16 ohm taps to get 120W/Ch into 8 ohm speakers. You are correct that you would need a 4 ohm speaker to double the power if you tied the 8 ohm taps together. If you ran with the 8 ohm taps tied together and an 8 ohm speaker you would have no increase in power but your distortion would be almost cut in half.
( I still have one Citation II and a Citation I preamp. I had to sell the other Citation II when I moved to Florida because the air conditioning could not keep up. )
( I still have one Citation II and a Citation I preamp. I had to sell the other Citation II when I moved to Florida because the air conditioning could not keep up. )
Unless you have experienced it directly I think that information is largely anecdotal. While I haven't strapped that many tube amps I have never encountered an issue doing it, and don't know anyone who has. Almost every manufacturer whose literature even discussed this issue recommended strapping. Ever wonder why the Dyna ST-70 came with a stereo/mono switch and a very clear set of instructions for strapping in the manual? A couple of roadies I have known in the past indicated that strapping big MAC amps was not an uncommon practice. Funny that you mention the 8B - I used to know someone who had a pair of them mono-strapped, he found that the 9 sounded a bit better than the strapped 8B for roughly the same power into a pair of Quads. (77s I think)
I don't really think much luck is involved, this was a common practice in the past..
Until the advent of solid state amplifiers I doubt anyone "bridged" amplifiers for additional power output, and having the additional complexity of a phase splitter in front the amplifier seems like unwarranted complexity in most cases - particularly if you want to maintain a pure tube signal path.
There is a good reason why solid state amplifiers aren't generally strapped (some monolithic based are internally strapped but use small series resistors on the outputs to guarantee load sharing) and that is because they tend to behave a lot more like an ideal voltage source than a tube amplifier and if they are not absolutely identical across the entire frequency range the other amplifier will look like a short to the amplifier with the higher source impedance.
Tube amps for the most part have a high enough source impedance that current sharing into a load is reasonably good if not ideal. Pentode amplifiers in general should not have much of a problem with strapping. Not to say it isn't a very good idea to make sure that the gain and frequency response of the two channels to be strapped is well matched to reduce degradation of amplifier performance which IMO is the good reason for not strapping - and I don't think bridging is the answer either. Use bigger amplifier if possible IMHO..
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