I'm building a BA2018 preamp, and would like to have balanced outputs. There seem to be three options.
I've actually built two boards, though (as I say in the other thread) am having trouble with one of them, which might have to be ditched, hence am thinking ahead.
Other question. In the BA2018 thread, Wayne remarks (here) that a couple resistor changes (to R18 and R20) are needed for balanced inputs, as well as the big change of connecting -IN through R17. (The original schematic was for single-ended.) But these do not appear on the schematic linked from the store page (here). Are those changes needed or not?
- Build two boards, and make it symmetrical balanced.
- Use transformers to create the balanced output.
- Use a balanced line driver.
I've actually built two boards, though (as I say in the other thread) am having trouble with one of them, which might have to be ditched, hence am thinking ahead.
Other question. In the BA2018 thread, Wayne remarks (here) that a couple resistor changes (to R18 and R20) are needed for balanced inputs, as well as the big change of connecting -IN through R17. (The original schematic was for single-ended.) But these do not appear on the schematic linked from the store page (here). Are those changes needed or not?
"Are those changes (for balanced operation) needed or not?"
Yes, Wayne designed it and should know. With the changes,
the positive input gain is ( 27 / ( 27 + 10 )) x ( 1 + 27 / 10 ) so then
V+ gain is (27/37) x ( 1 + 27/10 )
V- gain is - ( 27/10 )
Subtracting the V+ and V- gains to get the balanced gain gives:
(27/37) x (1+27/10) + (27/10)
(27/37) x (37/10) + (27/10)
(27/10) + (27/10)
or a balanced gain of 54/10.
This is only about 3dB higher (rather than 6dB higher) than with the original single ended input network,
because that had no significant input attenuation (series 100R and shunt 100k).
Yes, Wayne designed it and should know. With the changes,
the positive input gain is ( 27 / ( 27 + 10 )) x ( 1 + 27 / 10 ) so then
V+ gain is (27/37) x ( 1 + 27/10 )
V- gain is - ( 27/10 )
Subtracting the V+ and V- gains to get the balanced gain gives:
(27/37) x (1+27/10) + (27/10)
(27/37) x (37/10) + (27/10)
(27/10) + (27/10)
or a balanced gain of 54/10.
This is only about 3dB higher (rather than 6dB higher) than with the original single ended input network,
because that had no significant input attenuation (series 100R and shunt 100k).
The answer is actually a qualified yes.
If you want balanced in and out... and, perhaps, single-ended in (and out) then you might want to consider the alternatives.
If you used two stereo boards -- one for each balanced channel -- then:
a) you don't need to change the resistors mentioned
b) you could arrange switching such that you could implement a circuit for a range of eventualities:
-- balanced in -- balanced out
-- single ended in -- single-ended out (just use half of each stereo board per channel)
-- balanced in -- single ended out
-- single ended in -- balanced out
I've been mulling over a variation that lets me build a balanced preamp for use in a prospective completely balanced system but hook it up to my current single-ended components in the meantime. It's a work in progress.
Here's a schematic that was floated earlier in the thread that might help.
If you want balanced in and out... and, perhaps, single-ended in (and out) then you might want to consider the alternatives.
If you used two stereo boards -- one for each balanced channel -- then:
a) you don't need to change the resistors mentioned
b) you could arrange switching such that you could implement a circuit for a range of eventualities:
-- balanced in -- balanced out
-- single ended in -- single-ended out (just use half of each stereo board per channel)
-- balanced in -- single ended out
-- single ended in -- balanced out
I've been mulling over a variation that lets me build a balanced preamp for use in a prospective completely balanced system but hook it up to my current single-ended components in the meantime. It's a work in progress.
Here's a schematic that was floated earlier in the thread that might help.
I've used Rod Elliott's Fully Differential Amplifier Item No P176, you can output the FDA to your two BA2018 PCB's.
One downside to balanced preamps is finding a volume control with four sections. I've used 24 step Vishay resistor four part pots for my builds along with Academy Audio balanced muse digital pots.
One downside to balanced preamps is finding a volume control with four sections. I've used 24 step Vishay resistor four part pots for my builds along with Academy Audio balanced muse digital pots.
Ya, the VC can be a problem.
This might be blasphemy not being a BA2018 suggestion.. but when people desire a balanced pre it seems like Bruno Putzey's "Purist Balanced Preamp" (aka BPPBP) from his article "The G Word, or How to Get Your Audio off the Ground", originally published in Linear Audio Vol.5... seems to get recommended often. I guess it must be "real decent". I don't have any experience with it personally. Author promises "staggeringly good sonics", "going to be the cheapest and best-sounding preamplifier you’ve ever built".
This might be blasphemy not being a BA2018 suggestion.. but when people desire a balanced pre it seems like Bruno Putzey's "Purist Balanced Preamp" (aka BPPBP) from his article "The G Word, or How to Get Your Audio off the Ground", originally published in Linear Audio Vol.5... seems to get recommended often. I guess it must be "real decent". I don't have any experience with it personally. Author promises "staggeringly good sonics", "going to be the cheapest and best-sounding preamplifier you’ve ever built".
Thanks, everyone. I've replaced the resistors.
Am I right that this is basically a matter of equalizing the gain between the two halves of the signal? Would running a single-ended input into the circuit with the resistors changed just lower the gain? Or is there some other effect?
Am I right that this is basically a matter of equalizing the gain between the two halves of the signal? Would running a single-ended input into the circuit with the resistors changed just lower the gain? Or is there some other effect?
There is a four-gang ALPS pot available---or a knock-off---which I've used in a few different projects. Khozmo makes great balanced pots, but they are expensive.