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I guess the title of the thread says it all. I have a fully balanced tube preamplifier and I want to add a secondary unbalanced/single-ended output to drive a pair of subwoofers without compromising the balanced mode of the main outputs. I am guessing I need to interpose an isolation transformer. Yes? If so, what transformer(s) would you recommend? Since the sole purpose of the SE outputs is to drive very low frequencies, I would want an isolation transformer with good LF specs. Thanks.
Edits: 07/11/24Follow Ups:
Start with a set of Y adapters.
Or add a pair of balanced outputs to the preamp. Newer MP-1s have had dual outputs for years and they get added if an older one is updated.
Then get yourself a pair of Jensen subwoofer transformers. Although not shown on their website, they make balanced in to RCA out.
The Jensen transformer is optimized for subwoofer operation and so goes down to about 1 Hz unlike regular audio matching transformers.
This is the best way to do it unless a balanced input is added to your subs. This is the way I do it at home.
I contacted Jensen on this issue, prior to your posting, and the engineer who responded did not mention this particular subwoofer transformer as an option. I will get back to them. In the interim, I found out that the subwoofers I have in mind may in fact provide balanced inputs to the built in amplifier(s). That would greatly simplify the issue. I need to confirm this before purchase, but since I don't at the moment have $4000 burning a hole in my pocket (to pay for the pair of powered subwoofs), the project is on hold.
and works great. They make a mono version too; SUB-1XR.
On their website I find "SUB2RR", not XR. Is that the transformer you meant to recommend?
-
One other solution is the use of 1:1 trafo 10k+10k as
https://www.sowter.co.uk/specs/3575.php
taking the signal for pin 2 and 3 of tube output; secondary unbalanced connection with sub woofer that has a own filter inside, I suppose
Walter
The transformer output, as you outline, seems to be simplest with the only caveat that I need one with a very good low end response, because those are the frequencies I would feed to the subwoof, of course. Thanks for your response.
The 3575 is very good stuff
But look also Lundhal
Take care on Zout of preamp and it it has a Cap on output
Walter
Although the filter capacitance in a circlotron output stage, such as the one in my Atma-sphere MP1, can be viewed as being in parallel with the output, there is no series capacitance at the output. As I have mine configured, the output Z is about 100 ohms.
If Zout is 100 ohm the 3575 or similar do the job correctly
You can take the signal also from phase + and phase -
And the seondary in unbal
Walter
I think you can take the signal ffrom + of one phase and ground (pin 1)
I suppose that the Zin of the sub is high enough; in case you can add a 10kohm resistor in series; the sub must have a level regulation
Walter
The balance out of the pre if has a zero central it is connected to pin 1
Thi is balanced ground
And with the output line trafo is normal
Just to check
In a balance connection, pin one doesn't have anything to do with the signal. It is chassis ground for shielding purposes.
In a differential circuit, the center point between the two signal lines is grounded so between ground and either signal line you have identical but out of phase signal.
Those are two different things, related but different. People tend to conflate the two.
Balanced connection
Differential circuit
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
As wrote If the pre has a trafo there are two possibility.
And also if the circuit is w/o trafo ; you have the reference of the ground of circuit
In every case the use of 3575 Sowter, for example , is a fine solution
Walter
Pin 1 is not (signal) ground. Google "pin 1 problem" to find more than anyone wants to know! :^)
Does it have to be passive? I would probably use a couple opamps for this, one on each channel with their outputs summed resistively. This won't load the preamp output, and gain would be adjustable if necessary.
There are fewer devices that have higher transistor density than op amps. I seem to recall no shortage of posts on transistor unsuitability for audio TK... :)
Seems a transformer should be quite capable of doing this. Likely not an inexpensive one. Fortunately, signal level will be low... :)
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Thanks for the idea, but I think the implementation is over my head. Can you point to a schematic?
I think a differential opamp is probably the easiest approach if you're into DIY. Take a look at figures 1,2, and 3 in the data sheet below. Digikey has these in the DIP8 package.
I like the idea of using two high CMMR op-amps to pick out differential left and right and convert to mono.
If on a tight budget, discrete 0.1% 25ppm/C resistors could be used with a less expensive op-amp.
I'd also consider breaking the ground between the sub and the pre-amp with a 100 ohm resistor. i.e. RCA shell to Sub grounded to pre-amp ground through ~100 ohm 1W.
- Bypass the 100 ohm with a 0.1uF > 24V NPO ceramic to control RF interference.
- Reference the INA's power and MONO ground to RCA Shell.
- You may be able to get away with using two phone chargers for a +/- 5V supply if the main power amp delivers full power to the speakers at 1-2V rms.
- The 100 ohm could go up to 1K and the cap to a 0.01u if you run the circuit from +/- 15V.
You could possibly even add a 3rd amp to make a "Differential send" for higher rejection of the ground noise between the pre-amp and sub. The input signal reference to the INA goes to the mono-signal ground and the output reference sense resistor goes to "RCA shell." I've done something similar to this 3 decades ago and it worked well.
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.. . .Oh!. . .Remember: Modifying things voids their warranty.
I'm very much into DIY but not too comfortable with solid state circuitry, so thanks for the reference.
Well, there are a lot of one-off test equipment accessories and convenience circuits (like timers) that can be built with SS. These things are generally much easier to build this way than with tubes. Just last week I threw together a pulse generator with a couple NE555 chips for burst-testing amplifiers. The worst thing is that most components like caps and resistors have different ratings than for tubes, and I don't have as many of those types of parts on hand. I use the gold-plated Perma-Proto breadboards from Adafruit for many projects because they make the job go a lot faster. What you need for the balanced-unbalanced conversion is a perfect candidate for this. Just my 2¢.
Just put the same load on the negative as the plus. 20K from the negative to ground and a 20k pot to positive will not disrupt your balanced output.Alternatively, the way I do it, is to use a 5K pot off of the amplifier output to drive a subwoofer amp.
Edits: 07/12/24
Thank you very much for the response. It reminds me that I once talked to the late Bill Thalmann about this problem, and he suggested 20K resistors, too. But why a 20K pot vs a resistor on the positive leg?
In any case, the circuit if I tap off the linestage would be...
20K between the negative phase of the balanced output and ground.
20K potentiometer or resistor between the positive phase of the balanced output and positive of the SE output.
Is that correct? I guess the 20K pot on the positive leg would permit control of gain, but the subwoofers I plan on using have built in attenuators.
If it is 100K, then you just need to take the signal off the positive and put a 100K resistor from the negative leg to ground. One resistor
I haven't purchased the subwoofs yet, but I can certainly find out the input impedance of the built-in amplifiers. I hate to reveal my ignorance or at least my insecurity, but I assume you mean the negative phase of the balanced output, when you refer to the "negative leg". Then connect the ground lug on the RCA SE output jack to ground per usual.
That couldn't be simpler.
Not all balanced outputs are referenced to ground. This should be confirmed by reviewing the specs or the schematic for the preamp before proceeding.
Exactly.
In a pure balanced line, there is no ground. There is only a shield, which is NOT the signal ground of components at either end. This is best implemented with a transformer. In that case, you can ground one end of the secondary to get a single-ended connection.
Many (most?) modern balanced components are more like differential lines, where there are two signal paths in opposite phase, which share a common signal ground. This is a decent approximation as long as the impedance to the source's common signal ground is much, much smaller than the receiver's impedance to ground. That means you can't just connect one side to ground.
Many problems are caused by the confusion between signal and shield grounds ... :^)
Yes. Ground is floated at the balanced outputs. Which is why I originally thought I needed to use a transformer, in the way that you mention which is to ground one side of the secondary while taking the SE signal off the other side of the secondary.
Edits: 07/13/24 07/13/24
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