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In Reply to: RE: How to devise a single-ended output from a balanced preamplfier posted by Lew on July 11, 2024 at 12:36:20
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.
Follow Ups:
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¢.
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