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Thread: valve amp output mods

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    Default valve amp output mods

    Hi there guys. Ive been given an interesting job. Im looking at a safe way to take the output of a valve amp and use the signal for an input to a mixer or PC. Is there such a beast? its use is for mixing but still want the valve amp sound.Any ideas???



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    You could just mike the output!

    Any hardwired connection will loose alot of the "distortion" that guitar amp nazi's so dearly love
    When I explained to the guy what avatar I wanted, that wasn't what I meant!

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    What is the amp? Perhaps try looking at interfacing a headphone circuit that could maintain the second harmonics that musos seem to love.
    "The saddest aspect of life right now is that science gathers knowledge faster than society gathers wisdom." - Issac Asimov

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    You could ask the question over here -->

    There are some really clever guys over there that will help with any problem. I've built a couple of amps from scratch, whenever I had a question they were very quick to answer.

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    Go to Jaycar or Altronics and a 600 to 600 ohm isolating transformer.
    Make an attenuator by using 100k resistor series with the the positive speaker lead and the positive of the transformer. Other leg of transformer to negative speaker lead.
    If level is too high place a low value resistor ( 1K ) across the two leads of the transformer.
    Other side of the transformer goes to the input of the mixer.

    Or, just use a DI Box that has a 40DB Attenuator.

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    A DI box splits the signal from the instrument and converts it to a low impedance symmetrical signal for a mixer and leaves the other signal line unchanged for connection to the amplifier input.
    They are usually not designed to handle the output signal from an amplifier.

    You could attenuate the output signal with a voltage divider and then use a DI box for a symmetrical mixer input and ground lift.

    However don’t just use a series resistor.
    As a rule of thumb the source impedance should be at least 10 times lower than the load.

    The amp output connected to 4k7 ohm in series with 47 ohm. The DI box instrument input (or the 600 ohm transformer mentioned above) is connected across the 47 ohm resistor.

    This will give you an accurate 1000:1 attenuation through the whole audio spectrum.

    The 4k7 should be at least 1/2W.

    If the amp is set very quiet you might need to use 470 ohm 5W instead if 4k7 ohm. Ideally connect a 560 ohm 5W across the 4k7 with a switch to select between 60dB and 40dB (approximately).

    From my experience a mike in front of the speaker has always been the way to go.

    EDIT:
    ... and never run a Valve amp without it's rated load !!!
    Always have the speaker connected or the correct substitute load which you would need to make up out of a lot of high power resistors.
    Last edited by Uncle Fester; 14-04-12 at 11:18 PM.
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    Quote Originally Posted by nomeat View Post
    A DI box splits the signal from the instrument and converts it to a low impedance symmetrical signal for a mixer and leaves the other signal line unchanged for connection to the amplifier input.
    They are usually not designed to handle the output signal from an amplifier.
    They can also be used on the speaker output as long as the signal level can be suitably attenuated. Hence the requirement for one with a 40DB Pad.
    Not the best option but, with the limited information provided, a possibility.
    The amplifier itself may have a line out jack for the DI.

    If this a guitar amp and the required sound is to replicate the sound they hear from the amplifiers speakers, there is no choice, it has to me miked.
    The way the actual speaker produces the sound is also part of the final product.

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    What you are looking for is called a "line out converter" or "Line output converter" they are very common just google either term and you will come up with heaps of ready made options.

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    Thanks guys i didnt even think of a mike , not sure if thats going to work for what we need with the mixing etc... will look for the Line out conveter before i bust out the tools.

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    I’ve played with different ways of recording guitar amps. I’ve tried recording without an amp with a DI box straight from FX pedals and it sounds really bad. The main reason for this is because of the dynamics that the amp, speakers and cabinet create. These are all important in creating a timbre or feeling of warmth to the sound. Valve amps are in my opinion sound the best. The way they distort (the good kind of distortion - Marshall or Mesa amp distortion) tends to squash rather than clip the waveforms that you get with cheaper amps. But push this all through crappy speakers, and you loose allot of the tones that most guitarists lust for.

    In summary, yes you can pick a signal from the output of an amp with a voltage divider, providing you load the output correctly (8ohms is pretty hard to get a decent attenuation ratio with off the shelf resistors. An 8 ohm resistor as the main load, with an additional two dividing resistors sitting across it in series as a divider in the order of a few Kohms would be good). Its also a good idea to get some galvanic isolation as mentioned previously with a matching transformer to stop earth loop links, and because valve amps have high voltages inside but you loose all the characteristics of the speaker and cabinet.

    My suggestion, if you are a hardcore purest, mike the speaker box!

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