Using headphone output on SS amp to drive a speaker cab?

MogwaiBoy

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My father and I are doing a few fix-ups on an old solid state combo amp. There isn't a single shred of info online about this amp ("P.18" with "Farfisa Amplifier" on the circuitboard, made by "Wellard Industries" in New Zealand. Presumably early 70s).

It has an unknown brand speaker with 8ohms stamped on it. One of the things my dad and I want to do is add an external speaker jack so it can drive an external 8ohms cabinet.

We were looking into it, then my dad says hang on a minute. The amp has a normal 1/4 inch "headphone" input on the front panel, and the wires are connected to the circuit and the speaker. My dad says that input will disconnect the internal speaker.

So my dad says we probably don't need to install an external speaker jack as the headphone input(output?) is probably exactly that already.

We know that impedance/loading won't be an issue because the amp is solid state - but we just want to know if running an external cab via a headphone output is totally safe and maybe even common practice?

This little amp has a phenomenal low volume cleantone by the way, and fuzzes up quite organically as you drive it. We have it in pieces on the kitchen bench at the moment... Any input would be appreciated. Cheers!
 

EightySix

Member
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1,782
The signal from the headphone output will be too low to push a speaker. You need to amplify the signal with a power amp, then to a speaker. But, most likely the signal will still be too low to feed the power amp properly... so you will probably need something like a microphone preamp between the guitar amp and the power amp to boost the signal enough.

Like this mic pre-
http://www.musiciansfriend.com/pro-audio/art-tube-mp-studio-mic-preamp

But by then there's so much background noise/hiss from amplifying the signal twice it's not very usable IME.
 

teemuk

Member
Messages
3,264
We were looking into it, then my dad says hang on a minute. The amp has a normal 1/4 inch "headphone" input on the front panel, and the wires are connected to the circuit and the speaker. My dad says that input will disconnect the internal speaker.
You're onto something but don't rush there yet.

Since it's a headphone output it's probably attenuated by a resistive divider since headphones need only few milliwatts of power and only few volts of voltage swing to achieve that.

If there is no such attenuation the amp will overpower and kill your headphones rather quickly.

If there is such attenuation there isn't going to be enough juice after it to drive a generic low-ohm loudspeaker.

We know that impedance/loading won't be an issue because the amp is solid state
That's not entirely correct.

Mismatch towards high-than-rated loads is probably ok. Mismatch towards lower-than-rated loads usually kills an old SS amp (which usually lack even the crudest protection circuits) in an eyeblink. If it's a very old design with transformer coupled output even both mismatches may kill it.
 




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