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Old 14th April 2010, 01:49 AM   #1
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Default adding line out to guitar tube preamp

Dear all,

I have a preamp tube (behringer vt911) wich (after replacement off the tube) give me full satisfaction. But, I would like to add the only thing missing to my point of vue, which is a line out.

I have add, in attachment, the schematic of it with the additionnal line out (in red).

If I am right, adding a line out is like adding a D.I. (impedance change)? So, I guess I have to add a R.C. somewhere before the line out jack. Could you help me in this?

Thanks
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Old 14th April 2010, 02:36 AM   #2
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I don't know what you expect of the line output. What impedance do you require? You could connect it where you indicate but its impedance may not be appropriate to your use. What will you use it for? Of course it will be controlled by the volume control, and that may or may not be what you want.
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Old 14th April 2010, 12:22 PM   #3
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Hello Bob,

Thanks for your answer.

In fact I am willing to use the line-out to connect the preamp directly to a mixer (or recorder). Yes, I guess there 'll be a impedance mismatch and this the reason why I need some advises. How do I change the impedance to a line impedance?

The fact that the level of the line out 'll be controlled by the master volume is something that I want.

Cheers
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Old 14th April 2010, 09:05 PM   #4
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I have found a first answer using the schematic in attachment (impedance converter).

I would like to add a sort of amp simulation, any idea?
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Old 14th April 2010, 09:06 PM   #5
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Here is the schematic
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File Type: jpg FET impedance converter.jpg (33.4 KB, 258 views)
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Old 14th April 2010, 11:49 PM   #6
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The idea is correct. Take the high impedance signal and run it into an emitter follower (or source follower). Or, an opamp connected as a unity gain amplifier. All will do the job. It depends on what parts you have laying around, etc.

The trouble with that circuit is first, it requires a 9 Volt battery. Second, it's got an adjustment, another nuisance. And third, I don't know its output impedance but I don't think it can drive much. Emitter follower would, I think, be better. Use a 2N2222A for instance, set the bias at around a couple of mA and you are good to go.
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Old 15th April 2010, 09:34 AM   #7
Ian444 is offline Ian444  Australia
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bob, the unit already has a 9V power supply. The trimpot is no big deal, just set the 5VDC and thats it. The cct is a source follower. The thing I have a problem with is the Behringer cct is running the heaters at 4.5V and 9V as B+. I tried stuff like this 15 to 20 years ago and it never sounded good, the only good sound was what was coming out the guitar speaker. Even with 300V B+, still never achieved a line out sound quality near as good as what came out of the speaker, it's the combination of preamp/tone controls/amp/speaker that makes the good sound in my experience. hacaira you could try that buffer connected to the wiper of the tone control pot and see what happens.
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Old 15th April 2010, 01:35 PM   #8
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Hi Ian,

Thanks for the infos.

You are right, the sound coming out of the line out 'll never be as good as the sound coming out of a speaker but the purprose is just to be able to record ideas (even at night).

If I am right, the heaters of the double triode are connected in serie and then the 9V becomes 4,5 V in each heater?

What if I change the circuit to use 12V DC? Should it be better? What should be the best B+ voltage then?

Also, I have a NE5532AF (spare) that I could use to make the line out but I don't know how to connect it.

I am willing to upgrade this machine to have the best available sound. I'll change the tube for a Groove tube one and add a +9V output. If you have any suggestion to make it even better, don't hesitate.

Last edited by hacaira; 15th April 2010 at 02:03 PM.
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Old 15th April 2010, 03:51 PM   #9
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Voilą, I made modifications to the original schematic to include the line out.

But I feel like something is wrong. I thaught that there should be a shelf in the circuit in order to change the impedance from 1K Ohm (as reported by Berhinger) to 100 Ohm (line out impedance). Is it the opamp that does this job?
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File Type: png VT911schematic+lineout2.png (119.6 KB, 230 views)
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Old 15th April 2010, 05:36 PM   #10
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I think that should work. The place you call gain doesn't seem to be that. The opamp lowers the impedance but has a 1k resistor to protect it. That of course raises the output impedance by 1k.

You could bypass that resistor and have your line out but only when the circuit is operating. Else you will get the line out from your source follower in both modes.

You could modify the circuit to switch the input to the input of the opamp and do away with the source follower. Ib other words, you would disconnect the volume control wiper from the opamp input and connect the input instead.
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