Speaker Cable Vs Screened Cable

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Speaker Cable Vs Screened Cable

Postby calvinheuer on Wed Sep 01, 2010 10:35 am

Hi Guys,

I'm new to the forum, so forgive me if this has been answered in detail already...Allow me to explain:

I own a VoxAC151TV Heritage amp as well as a VoxAC30CC2X. I also now have a small (fairly basic) powered Yamaha Mixer system with 2 x 8ohm passive speakers for home mixing use.

The VoxAC30 has two outputs at the rear: One Extension and one External Speaker output. These are both 1/4inch unbalanced connections with a variable impedance that can be adjusted from 16ohms to 8ohms by the flick of a switch. The only difference between these two outputs is that the External Output mutes the internal speaker.

The first question is:

Is it possible to send the signal from the AC30 (from the Extension Speaker Output at 16ohms) to the PA unbalanced 1/4inch input?

Question 2:

What type of cable do I use to send the signal from the back of my AC30 to the PA. Is it a non-screened speaker cable or a screened guitar instrument cable?

Question 3:

If this is not a good idea or possible, what other options do I have: Is Miking the amp / or using a DI box the only other alternatives?

I recently played in a gig where this was done (I think it was using a guitar screened cable...the guy who made the connection did not speak English so it was difficult to determine what was right or wrong), but could not help wondering if the sound was suffering unnecessarily due to impedance mismatching or better yet...damaging my amp. Nonetheless, the sound produced through the PA system was not bad.

The VoxAC30 manual says you should only use a speaker cable (non-screened) for Extension and External Outputs leading to an Extension Speaker cabinet. But it does not say if you can connect this directly to a PA / Mixer input?

Any thoughts here?
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Re: Speaker Cable Vs Screened Cable

Postby calvinheuer on Mon Sep 06, 2010 12:58 am

So after some research (The Guitar Amp Handbook by Dave Hunter):

Connecting the Ext/Extension Seaker Out on a VoxAC30 to a PA (or anything for that matter) using a screened cable is never a bright thing to do...although the sound may appear to work...the extra impedance produced from an instrument cable places extra stress on the OT and can potentially fry the Output valves and OT and output stage circuitry.

SO....that's at least one problem solved (ALWAYS USE A SPEAKER CABLE)....but it still does not answer my question of connecting the Speaker Out of the Vox directly to a PA? I would presume this is a NO, as all the literature seems to say you need a direct speaker / driver load of 16 ohm connected to this output. Am I correct in saying this?

I also found an interesting DI box made by Behringer (Ultra-G GI100): Active box. By using the 2 x -20dB PAD switches this box boasts the ability to pick up a signal from a loudspeaker output of an amplifier and connect directly to a console, provided the amplifier output does not exceed 3,000 Watts (4 ohms). SO I presume, based on what I have read, that the VoxAC30 Main Output... and for that matter (Ext/External Speaker Output)....all put out a 16 ohm load. So using this DI box is also not possible...without overloading the DI box. Am I correct in saying this?

This brings me to another question to mind: If the DI box above boasts the ability to connect directly to a console via an Amp Output.....then what cable is used......a speaker cable or a instrument cable...??? Based on the above...I would say a Speaker cable...But all the DI box literature I have read says that a DI box Input receives a screened instrument cable? So now I am confused as to if this option of connecting an AMP Output to a DI box is possible?

It would be great to get some professional advise here. Anyone ever used a DI box like this?

Kind regards
Calvin
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Re: Speaker Cable Vs Screened Cable

Postby calvinheuer on Mon Sep 06, 2010 1:09 am

Also,
Since I have previously connected my amp using a screened instrument cable from the Extension Output to a console input, how can I now determine if the output stage of my amp has been damaged? Is it necessary to take it into an amp tech to test the OT?

I have ordered a complete JJ Tube set for the Vox from Eurotubes. I intend to re-tube the amp next week anyway...so I'm not too stressed about the current output tubes being a little fried.

DO I need to stress about the OT? I only played the amp like this once off for about 1-1,5 hours.

Kind regards
Calvin
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Re: Speaker Cable Vs Screened Cable

Postby Wayne Alexander on Sat Sep 11, 2010 6:40 pm

Never connect any cable to the speaker out jacks on the back of am amp except a SPEAKER cable, and a proper load. A SPEAKER CABLEW has two fat conductors. If you use an INSTRUMENT cable with one skinny conductor and a shield, there's a chance the signal will melt the skinny conductor, leaving your amp with no load, which will burn up the output transformer or output tubes and related components over a very short period. Your output transformer and power tubes also require a proper LOAD connected to the speaker-out jack at all times. A connection to a power amp or PA system input via any sort of direct cabling is not a proper load. Especially with Voxes, the load must match the impedance required by the transformer - a 16 ohm load for the 16 ohm tap on the output transformer, an 8 ohm load for the 8 ohm tap on the output transformer.

If you want to take the output from the speaker out jacks on the back of your amp into a PA system or power amp, you must have a SPEAKER CABLE connecting the amp to a load, some sort of transformer to correct the impedance, most likely a pot to allow adjustment of the gain, and then a shielded cable to connect from there to the power amp or PA system.

You can have the amp modified (go to a tech, you questions indicate you don't personally have the skill level to do this) to have a LINE OUT connection, in which case you connect that via a shielded cable to the power amp or PA system, or you can use a load box or attenuator that has a LINE OUT wired into it, the THD Hotplate has this, but you cannot wire the speaker out connection directly to a PA system or power amp without great danger to the amp.

The other classic, and safe, way to connect the output of your amp to a power amp or PA system is to put a microphone in front of the amp, and connect that to your PA system via a microphone cable.

I can't tell you whether you've hurt your amp by what you've done so far, but if it's working, I wouldn't obsess about it , but do not do it again.
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Re: Speaker Cable Vs Screened Cable

Postby calvinheuer on Sun Sep 12, 2010 7:49 am

HI Wayne,
Many thanks for your response. Much appreciated and very informative. I will be purchasing a Weber Attenuator and another as you suggest that has a proper DI option to do this in future.

I have replaced all the tubes (everything except the rectifier tube), and I can say that the amp is sounding much better now. But I still cant tell the sonic difference between a good sounding OT and a bad sounding damaged one....So I may have to get in touch with vox and ask them directly what a damaged Vox OT is supposed to sound like.

All in all, my amp is sounding better now....but then again, I changed over to JJ tubes....so that has obviously made a huge sonic difference to the standard Sovteks that the amp comes standard with.

I have also purchased a ShureSM57 last week and will just do it through a mic in future...it sounds better anyway as you get the Celestion (Blue) sound dynamics through the PA...much better than the somewhat sterile DI sound from an amp into a PA.

Thanks again
Kind regards
Calvin
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Re: Speaker Cable Vs Screened Cable

Postby jaywalker on Mon Dec 13, 2010 5:18 am

There are DI boxes specifically designed to go between the "speaker out" jack and the speaker. You still use speaker cable from the "out jack" to the DI box and from the box to the speaker, then you can use a balanced xlr cable from the DI box to the board. You still have the speaker sounding out and you can do whatever you want with the signal that goes into the board. H&K "Red Box" is one example, another is a cheapie by Behringer the GI 100 which also has optional cab simulation.
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Re: Speaker Cable Vs Screened Cable

Postby calvinheuer on Mon Dec 13, 2010 8:00 am

Thanks,

I will check this out as an option.

I have since purchased a whirlwind DI with a Jensen Transformer...very nice passive DI box and silent too! But I must admit I still prefer the idea of miking an amp into a PA...it sounds so much better with the speaker characteristics thrown into the equation.

Regards
Calvin
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