unbalanced output impedance matching?

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mus1k_freak

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Joined
Dec 13, 2009
Messages
156
Location
Philadelphia PA
Just wondering if someone could explain this to me quick, if i don't use a output transformer to balance my signal do i need to do anything besides the output cap (on a tube pre) before i go to the xlr connector for an unbalanced out? Thinking i can save some money on some budget/prototype builds bypassing an output transformer. Since its a poplar circuit here, use the NYD one bottle preamp for example.

I see that the original is unbalanced, i notice a 220k resistor on the output cap, would that need to be added to a tube circuit that just has a output cap? or would it have to be removed to add a transformer?
 
mus1k_freak said:
Just wondering if someone could explain this to me quick, if i don't use a output transformer to balance my signal do i need to do anything besides the output cap (on a tube pre) before i go to the xlr connector for an unbalanced out?
You don't need anything else, but it is good practice to add a resistor that leaks any DC residual. That avoids loud clicks when connecting the unit to the rest of the world.
Thinking i can save some money on some budget/prototype builds bypassing an output transformer.
Please note that the transformer generally steps down the signal level and the output impedance, making it compatible with other gear. You may have problems interfacing with the rest of your studio. The one bottle circuit has quite a large output level and impedance. Connecting it to a standard 10k line input may induce noise and headroom problems.
I see that the original is unbalanced, i notice a 220k resistor on the output cap, would that need to be added to a tube circuit that just has a output cap?
Yes, although not strictly necessary.
or would it have to be removed to add a transformer?
You would not have to remove it, but you could. Wouldn't change a thing practically.
 
if you skip the transformer, which is a felony in 7 states, then use a non electrolytic cap.

you could put a twin cap in the ground leg if you wanted, this would give you isolation without the trans,



 
ok cool! thats pretty much what i figured just from lots of trial and error. I tried a tube buffer i built tonight with a cinemag direct box transformer, and unbalanced out the sound was VERY similar, besides the different in gain of course. Both outputs had zero noticeable hum or noise, so far so good!
 
make sure you use a cap with a large safety margin on the voltage spec,

if the cap fails, you deliver HV into the next stage, which it might not like,
200 volts across a 600 ohm input trans, let's see, 10 ohms dcr, 200 v, = 20 amps primary current, this can not be good,

or, if you get good with jeff tanner and he invites you over to plug the box into a neve 128 track beatles console and you pop channel strip # 64, he will not be happy.
but i will, i can buy the blown strip on evilbay and own a piece of history,
 
haha ok im usuing sozo caps on the output i think they are rated for 400v? sometimes that extra transformer mojo is nice, sometimes saving the extra $$ is nice.
 
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