Hello..
I'm building a linestage and it will have 2 pairs of RCA outputs.
Should I just connect them in parallel?
Or is there something more I should do?
Is some active circuitry needed?
The linestage is itself a tube buffer..... (with reasonable output impedance)
The purpose/usage will be situations like driving a main amp along with a subwoofer amp, perhaps with only one powered some of the time...
and having a headphone amp connected as well as a main/speaker amp, but naturally only one powered...
I realize that the input impedance the linestage will "see" divides between the 2 amps.
Divide the sum of the 2 amps' input impedance by 2?
Did I say that right? I'm really tired right now.. sorry....
Thanks in advance!
Greg
I'm building a linestage and it will have 2 pairs of RCA outputs.
Should I just connect them in parallel?
Or is there something more I should do?
Is some active circuitry needed?
The linestage is itself a tube buffer..... (with reasonable output impedance)
The purpose/usage will be situations like driving a main amp along with a subwoofer amp, perhaps with only one powered some of the time...
and having a headphone amp connected as well as a main/speaker amp, but naturally only one powered...
I realize that the input impedance the linestage will "see" divides between the 2 amps.
Divide the sum of the 2 amps' input impedance by 2?
Did I say that right? I'm really tired right now.. sorry....
Thanks in advance!
Greg
This seems doomed to failure without more calculation. Details matter.
A generic "power amp" tends to have input impedance of <100 k ohms. Slightly less. The input impedance doesn't decrease for common emitter bjt inputs just because the power is off. Also 2 in parallel is <50k input, unless they are special with op amp inputs or something.
Tube outputs vary depending on type. A 12AX7 according to the old tung-sol datasheet allows 1.2 ma plate current at 250 v anode voltage, or 62k plate resistor. My 12AX7 output PAS2 definitely loses highs if the RCA cable is longer than 6'. That is what, 300 pf? That is on a ST120 amp input which is a bit higher than 100 k input by design. Two RCA cables in parallel is ~600 pf.
By contrast a 12BH7 from brinar allows 11.5 ma anode current @ 250 v anode. That is a bit more capable of driving capacitance, but requires a more capable power transformer to drive it.
You could increase the current drive available by using more of the voltage swing available from a vacuum tube to drive a transformer that would increase the current available. These transformers are strange & rare in class A service, probably expensive. Most of the ones on the market will be guitar amp models, that is to say, no highs are available by design.
You could use a 6l6 which allows 26 plate watts, or at 300 v anode voltage, maybe 86 ma (jedec datasheet doesn't have a current). The cheapest high fi transformer for that is a push-pull for two of those tubes, in which case you can use the 8 ohm tap and drive the speaker direct. That is a hifi tube power amp. You can drive a large number of parallel RCA cables with that, though that use is pretty silly.
The cheapest way to increase current is a transistor or op amp.
If you're driving a sub amp, usually it is assumed you will filter off the highs before they get to the amp. That involves inductors series the amp input, plus maybe a capacitor after the inductor to ground to make slope 12 db/octave instead of 6. That RLC network would have it's own impedance curve, likely higher current draw than one 6' rca cable. Of course the cheap way to filter off highs these days is with a op amp circuit. A 8 millihenry inductor these days is about $10.
So decide what you are doing, and calculate away. Since 6' rca cable won't get from my turntable location near my chair to my power amp at the end of the room, I put my line level PAS2 vacuum tube preamp in the attic. I can't get loudness control volume pots for it anyway. I'm using a disco mixer for the turntable with 33078 op amps, which took a year of modification to turn it from a hummy hissy beast (4558's) into something that sounds as good as the PAS2 when it was located inconveniently over there between the speakers & behind the organ.
You can use kirchoff's laws & network analysis to calculate your network current draw. Then make sure it doesn't exceed what the vacuum tube will put out. Or put the parts in spice, if it has models for 8 mH inductors and 12BH7. My op system won't run ltspice. Curses on windows, apple too, always requiring $$$ updates.
Or buy or build a transistor or opamp RIAA preamp or mixer. Much cheaper. Not as fashionable as vacuum tubes. My mixer allows 3 inputs on at the same time, so I don't have to walk around & turn the selector knob on the PAS2 preamp and change the volume knob for different output levels from turntable, CD player, radio. All volume pots are preset on a mixer.
Happy calculating.
A generic "power amp" tends to have input impedance of <100 k ohms. Slightly less. The input impedance doesn't decrease for common emitter bjt inputs just because the power is off. Also 2 in parallel is <50k input, unless they are special with op amp inputs or something.
Tube outputs vary depending on type. A 12AX7 according to the old tung-sol datasheet allows 1.2 ma plate current at 250 v anode voltage, or 62k plate resistor. My 12AX7 output PAS2 definitely loses highs if the RCA cable is longer than 6'. That is what, 300 pf? That is on a ST120 amp input which is a bit higher than 100 k input by design. Two RCA cables in parallel is ~600 pf.
By contrast a 12BH7 from brinar allows 11.5 ma anode current @ 250 v anode. That is a bit more capable of driving capacitance, but requires a more capable power transformer to drive it.
You could increase the current drive available by using more of the voltage swing available from a vacuum tube to drive a transformer that would increase the current available. These transformers are strange & rare in class A service, probably expensive. Most of the ones on the market will be guitar amp models, that is to say, no highs are available by design.
You could use a 6l6 which allows 26 plate watts, or at 300 v anode voltage, maybe 86 ma (jedec datasheet doesn't have a current). The cheapest high fi transformer for that is a push-pull for two of those tubes, in which case you can use the 8 ohm tap and drive the speaker direct. That is a hifi tube power amp. You can drive a large number of parallel RCA cables with that, though that use is pretty silly.
The cheapest way to increase current is a transistor or op amp.
If you're driving a sub amp, usually it is assumed you will filter off the highs before they get to the amp. That involves inductors series the amp input, plus maybe a capacitor after the inductor to ground to make slope 12 db/octave instead of 6. That RLC network would have it's own impedance curve, likely higher current draw than one 6' rca cable. Of course the cheap way to filter off highs these days is with a op amp circuit. A 8 millihenry inductor these days is about $10.
So decide what you are doing, and calculate away. Since 6' rca cable won't get from my turntable location near my chair to my power amp at the end of the room, I put my line level PAS2 vacuum tube preamp in the attic. I can't get loudness control volume pots for it anyway. I'm using a disco mixer for the turntable with 33078 op amps, which took a year of modification to turn it from a hummy hissy beast (4558's) into something that sounds as good as the PAS2 when it was located inconveniently over there between the speakers & behind the organ.
You can use kirchoff's laws & network analysis to calculate your network current draw. Then make sure it doesn't exceed what the vacuum tube will put out. Or put the parts in spice, if it has models for 8 mH inductors and 12BH7. My op system won't run ltspice. Curses on windows, apple too, always requiring $$$ updates.
Or buy or build a transistor or opamp RIAA preamp or mixer. Much cheaper. Not as fashionable as vacuum tubes. My mixer allows 3 inputs on at the same time, so I don't have to walk around & turn the selector knob on the PAS2 preamp and change the volume knob for different output levels from turntable, CD player, radio. All volume pots are preset on a mixer.
Happy calculating.
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The advantage of individual active outputs, one amp won't influence the level or quality sent to the other amp. Some input stages present nasty loads when power is off.
What are the input impedances of the components that you will connect to the tube buffer?
Can you post a schematic of the tube buffer?
Can you post a schematic of the tube buffer?
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Umm...well...it depends on what the user wants to tolerate. If the output is as high impedance as suspected, then the different loads will interact with each other, as will interconnect cable C. Maybe that's desirable in someone's world, but not in mine. In fact, high output Z isn't in my world either. But some people think of design errors as "color", so who knows.
They could be fairly high, or not. The point is, in a good design you shouldn't have to even give them a thought. Shouldn't have to think about cable C either, but in this case it could be a factor.