hvyj
2609 posts
Dec 11, 2014
4:39 PM
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Why do harp amps have a 1/4" line out instead of an XLR line out? I sometimes play through an SWR Baby Baby Blue which is an acoustic bass amp. It has an XLR line out which has been useful in situations where I would otherwise have to mic the amp. But the harp amps I've seen with a line out all have a 1/4" line out which requires a separate DI if you have a long run. Doesn't seem to make sense. So, why is this?
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timeistight
1665 posts
Dec 11, 2014
6:47 PM
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Money.
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Greg Heumann
2908 posts
Dec 11, 2014
7:50 PM
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The common amp line out circuit is high impedance and is derived the final amp output (i.e. the output transformer secondary - the same signal that is going to the speakers.) A low impedance/XLR line out typically signals a pre-amp out signal, which would be cleaner. Doesn't HAVE to be that way, but that's the way it is.
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/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
Bluestate on iTunes
Last Edited by Greg Heumann on Dec 11, 2014 7:50 PM
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hvyj
2610 posts
Dec 11, 2014
7:56 PM
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On my SWR the XLR out has its own volume control. FWIW.
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5F6H
1845 posts
Dec 12, 2014
4:36 AM
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Because most chassis either have a 1/4" jack/9mm hole going spare, or are more easily converted to take that over an XLR socket.
Speaker driven line outs are more typically low impedance, sometimes more towards medium, & aiming for 0.5-1.5VAC out. You could tweak a speaker driven voltage divider to produce whatever voltage & impedance the next device expects to see (this is a stumbling block in many cases as older PAs often had 1/4" hi-Z inputs, more lately lo-z TRS jacks are more common, but they sometimes have a 1Meg input for instruments too). You could wire a 1/4" to XLR (pin 2 hot) to connect to a typical mic input, but your signal will be unbalanced...still "work" though.
A line out volume is nice (especially if set up time is limited) & easy to install (usually the load is on/in series with a pot), but if your values are ball park you should be able to get what you need by balancing amp & PA volume.
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Last Edited by 5F6H on Dec 12, 2014 4:37 AM
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J_Bark
33 posts
Dec 12, 2014
4:44 AM
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Ted Weber sells a balanced line out kit for a very modest cost ~$40 as I recall.
I plan to add one to my little Kalamazoo Model II
Jerry
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Greg Heumann
2911 posts
Dec 12, 2014
8:28 PM
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Thanks, 5F6H - I misspoke above. Most line outs are indeed low impedance. However they are typically wired unbalanced. A DI box or an adapter is still required to convert from unbalanced to balanced to connect to a PA. The transformer is probably NOT necessary.
Balanced vs unbalanced is about wiring only, not impedance. In unbalanced wiring, chassis ground is connected to cable shield which is connected to connector ground at the other end. A single center conductor carries the "hot" side of the signal, and the cable shield is the path for the negative side of the signal.
In balanced wiring, chassis ground is connected to cable shield and then to the chassis ground of the device at the other end. There are TWO wires inside the shield which carry the "+" and "-" of the signal, so it is completely independent of the shield.
Balanced wiring is typically associated with low impedance and unbalanced with high, but it doesn't have to be that way. Balanced wiring has better noise immunity.
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***************************************************
/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
Bluestate on iTunes
Last Edited by Greg Heumann on Dec 12, 2014 8:30 PM
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