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I have a dc coupled 6sn7 preamp with 2 set of outputs (in parallel)
similar to dc version below:
I am driving a tube amp with 470k input z and a behringer dcx2496 crossover on the other outputs with 20k input z.
When I unplug the behringer the soundstage expands and everything sounds more dynamic and better.
Any ideas of how to isolate the behringer are welcome.
The crossover is only handling music below 80hz going to a adcom 555
and subs.
\
thanks
Follow Ups:
I'm sure you'll find that the other posts answer your actual question quite a bit more than mine, but I'd like to respectfully suggest that you are wasting your time trying to fit a piece of Behringer gear into your rig. Behringer stuff has absolutely horrible build quality, and just about every piece of their equipment I've seen except for one notable exception has failed in less than 5 years. Switches are horrible, caps are as cheap as possible and go bad very quickly, the list of issues goes on and on.
I would suggest finding a better crossover with a higher input impedance and seeing if you still have this issue. I could never bring myself to modify a piece of gear just so it could deal with a Behringer device.
That's my personal opinion and two cents.
I measured mine (the cheapest which I use for my subs and it works fine and has for 8 years) and it has the promised 25K input impedance. It might be that the input impedance on yours is very low and you are stressing your pre.
Add another cathode follower stage to drive the Behringer separately.
If you don't have space for that, consider a FET follower instead?
Can I have one gain stage dc coupled to 2 cathode followers?
Or do I need to convert the preamp to 4 channel
Yeah, you can do that.
You can try paralleling tube halves and halving the cathode resistor, or set up four separate cathode followers. (Separate cathode resistors, coupling caps, etc)
Based on your description, I would opt for four independent cathode followers.
So you are passing full range to your main speakers, why don't you try a voltage divider on your main amplifier output to feed line level voltage to the crossover, it may actually sound better all around.
Buffer the I/Ps of the Behringer crossover, with FETs or tubes. Doing so will make certain that the line stage is not loaded down.
You've found out the hard way that cathode followers can't work miracles.
Don't assume the Behringer specified 20 K I/P impedance is correct. When both devices are connected in parallel to the line stage, the combined load will follow the textbook reciprocal formula of:
1/Z net = 1/Z 1 + 1/Z 2 + 1/Z 3 ...
Eli D.
Something is wrong with the the schematic values. Looking at the lower version, the second 6SN7 triode can't possible be carrying 7.15 mA of quiescent current. That would produce 326V across the 47K cathode resistor, a physical impossibility considering B+ is only 250V. Maybe the "7.15 mA" notation is intended to represent combined current into both triodes.
Anyway, I SPICEd the lower circuit just for grins. With a 20K load (in addition to the 100K shown), the output clips at better than 30V pp. You might be right about the Behringer, Eli.
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Buy Chinese. Bury freedom.
heres a picture of the behringer
I just remebered that I am using an rca to xlr adapter into the behringer inputs
maybe thats part of the cause?
Edits: 09/16/14 09/16/14
"I am using an rca to xlr adapter into the behringer inputs"
Do you have specs for the in/out impedance of the adapter?
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Buy Chinese. Bury freedom.
You're thinking of the good transformer based adapters. But most sold are nothing more than wire jumpers.
There are many discussions on this subject of cheap unbalanced to balanced conversion, and legitimate different approaches for input versus output as well as transformer based versus OPAMP input and output stages.
I suggest people do their own research on this 50 year old plus subject. Way too much to post here.
its just a plug that has one signal and 2 ground pins like bottom picture
That's opamp based circuitry. It's quite possible that the I/P impedance is different when the unit is powered on and when it's powered off.
More than 1 SS recording device has required buffering, when used in combination with tubed phono sections. Remember, BJTs are inherently low impedance devices. BJT circuits elevate the I/P impedance by the application of NFB. Take the power away and you have the low impedance of the base/emitter junction.
Constructing a JFET buffer is not difficult. Assemble a regulated bipolar PSU from a "Rat Shack" 25.2 VCT trafo, 4X Schottky diodes, and 7812/7912 3 terminal regulator ICs. The only cap. in the signal path is between the FET's source electrode and the load. The minimum coupling capacitance is 3.3 μF. That will work (barely) into the IHF "standard" load of 10 K.
Eli D.
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