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Old 28th October 2004, 12:17 AM   #1
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Default Building A Direct Output Line For An Amp

A friends site has a low level line out circuit for use on the speaker output of an amp for reducing the signal to a suitable level for feeding mixing consoles, computers or even a house stereo.

My concern is that the high wattage will fry your run of the mill resistors.
I am assuming if there is a speaker load on the speaker output, you may be safe but if there is not I know for certain the resistors will fry.

I will generally have a load on the speaker line out but if I don't......

Will I need high wattage sand stone resistors or what?

I've worked really hard on perfecting this amp and really want to use it's sound for recording without having to mic the speaker cab.
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Old 28th October 2004, 02:19 AM   #2
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Try this.

Get yourself a 10K pot at RatShack. One end to speaker hot and the other to speaker ground. The center tap and ground goes to the console. Adjust as needed.
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Old 28th October 2004, 04:11 AM   #3
Enzo is offline Enzo  United States
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Use Ohms Law. Across 8 ohms, 50 watts means 20 volts of signal. Even in the absence of a speaker load, the voltage will not rise in a solid state amp. If it is a tube amp running unloaded, then the line out is the least of your worries.

Remember that amps do not "push out" power, they just make it available and the load DRAWS the power from the amp. So the amp will not be pushing its power into the poor little line out resistor. Wattage per se is not the concern.

So start with the 20v - or whatever voltage your amp might produce. And if you want to use for example a 1000 ohm resistor, you can solve for current and then power. In the case of the 1k resistor you get a whopping 20 ma - .02 amp. That equates to 400mw or .4 watt dissipated in the resistor. A 1 watt resistor would be fine.

But since the line out is high Z, a larger resistor would be fine. The 10k pot suggested above would use even les power. 20v across it yields only 2 ma. That means the 10k pot dissipates only 40mw - .04 watt.

It won't be burning up soon.
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Old 28th October 2004, 12:42 PM   #4
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If this is a guitar amp you are talking about, you may be dissapointed with the sound of the "direct out". The sound of the speakers used has a significant bearing on the overall sound of the amp, and a line out will miss that. Also, as most guitar speakers do not use tweeters, the amp usually has a lot of top end boost to compensate, giving a very toppy sound when taking a line out.

Quite often recording studios have a small soundproof room specifically for micing a guitar amp cabinet - the amp can be cranked as far as needed to get the right sound without deafening everyone.

Cheers
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Old 28th October 2004, 01:59 PM   #5
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Default ...I dunno man, I've fried some resistors already

I've fried some resistors already though......

Centauri, thank you for assuming I'm stupid and don't know any of that common information you shared with me.
If I simply wanted to mic my speakers, I wouldn't be building a line out circuit.......

Yes the amplifier section of a guitar amp has a broad frequency spectrum, the tone stack section of the preamp generally has a treble control set typically somewhere around 7 khz or even as high as 10 khz.

Mine is set at 10 Khz, right where all those sweet 2nd order harmonics primarily are.

In other words, even with a direct out line, you're not going to be getting an abundance of highs that ranges over your amps treble control frequency point.
Even if this is an issue, a treble bleed cap is easily added and can even be wired on a pot to properly cut the treble. Just like your guitars tone control.

I plan to try this circuit with 1/2 watt resistors and use whatever caps necessary to tailor the sound. I'm also going to add a pot to adjust the level of output.

Hopefully the resistors don't fry because in my experiences with high wattage sources, they burn up if the amp is at mid to high volume settings......
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Old 28th October 2004, 02:43 PM   #6
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MrCheeks,

Settle down, I am not assuming you are stupid - just sharing my observations taking a speaker line out in live work.

And yes, caps can be added to tailor the output sound. For what its worth, the line out used of Fender "60 Series" amps (tube amp) consists of 560ohm 1/2W resistor from speaker line to hot of line out socket, 100ohm 1/2W resistor from hot to cold of line out socket, and 22ohm 1/2W resistor from cold of line out socket to earth. Even with these relatively low values, they don't seem to have any problem with ratings.

Cheers
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Old 29th October 2004, 07:11 PM   #7
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Default The 10 K pot.....

Well, the 10k pot worked great. I already knew it would but guess what.

I cranked the volume a bit and it fried EXACTLY like I said. I mean come on, you're talking about 120 watts running through a 1/2 watt pot.

For what it's worth, I added a 10 uf cap to be switched in and out for giving a cab simulating type effect for when I'd be running straight into a PA or mixing board. It worked beautifully.

So was frying that pot a fluke or what?

I have serious doubts but maybe....
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Old 29th October 2004, 07:28 PM   #8
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Default Re: The 10 K pot.....

Quote:
Originally posted by MrCheeks
Well, the 10k pot worked great. I already knew it would but guess what.

I cranked the volume a bit and it fried EXACTLY like I said. I mean come on, you're talking about 120 watts running through a 1/2 watt pot.


So was frying that pot a fluke or what?
If you have 20volts across the 10K pot then the current through it will be .002 amps. That means the power disappated in the pot will be .04 watts.

Perhaps you connected it up wrong?
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Old 29th October 2004, 09:25 PM   #9
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Default Musta been the electrolytic caps....

I was toying with some different electrolytic cap values so it's likely in doing so I increased the signal draw and smoked it.

Anyway I'm off to rat shack in a bit here to pickup a new 10 k pot being I have no more lying around and don't want to wait to order one.

I am thinking though, can I also use a pot to vary the amount of treble bleed caused by the electrolytic caps?

A 10 uf doesn't quite cut enough and a 47 uf cuts too much unless I crank the 10 k wide open.

....So I imagine I could run a lead from the hot to the input of another 10 k pot then it's output through the 47 uf cap to ground. This is post the 10 k pot for my line out of course.

Wutcha think?
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Old 29th October 2004, 10:32 PM   #10
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Default Re: Musta been the electrolytic caps....

Quote:
Originally posted by MrCheeks
I was toying with some different electrolytic cap values so it's likely in doing so I increased the signal draw and smoked it.
No, that isn't likely. Before you turn another pot into toast, show us how you connected it.
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