Reducing the speaker output of a tube amp to line level

[zonta]

Is this possible?

If the amp has no line out, can we reduce the external cab out to line level in order to be able to send the signal to the souncards line input (but without blowing the card or even the pc out)?

If possible, I would like to install the circuit (or even a single resistor...i don't know...you tell me) in a separate box without modifying the amp.

Actually I do not see any reason why it would not be possible to reduce the speaker level output signal to line level but I may be wrong. Does the signal lack quality this way?

I know that best deal to record an amp is micing it, but that is usually pain in the neck and yet I know people who achieve good results by direct injection method. Perhaps a di box would help a lot. But of course first of all I have to find a way to reduce the speaker out level to line out level.

Please help.  :wink:

Thanks.

[Peter Snowberg]

It is not only possible, it's really easy.

Two things to remember:

(1) Much of the sound of an amp comes from what the speaker does to the signal (including reflecting some of the energy back through the transformer in reverse and to the output tubes) so if you want any kind of realistic sound, IMO, you need to keep the speaker in the circuit.

(2) Tube amps **REQUIRE** a proper load on the output transformer, without that you will watch your output tubes work as fireworks.  

Adding a direct output is really simple if you don't want to emulate the speaker-microphone response. All you need to do is to wire a 10K resistor from the speaker hot lead to the wiper of a 10K pot. Ground the CCW end of this pot, and take your output signal from the ground and the wiper contact.

That's one way.... another which gives you more constant output impedance is to take the speaker hot, connect that to a 4.7K resistor, connect the other resistor lead to the CW end of a 10K pot, connect the wiper to a 3.3K resistor and connect the other side of that resistor to ground. Take your signal from across the 3.3K resistor.

Either way will work fine, just always use a speaker or a good attenuator too. You can get away with using a big power resistor instead of the speaker, I've never had trouble, but some feel that the amp may get damaged with this approach. An 8 ohm speaker will have a minimum impedance of 8 ohms, but depending on the frequency and speaker, it may actually be as high as a hundred ohms. It's a complex equation which is frequency dependant. Randall Aiken has some great info about that and building a good speaker simulator.

http://www.aikenamps.com/spkrload.html
http://www.aikenamps.com/

Take care,
-Peter

[zonta]

Tube amps **REQUIRE** a proper load on the output transformer, without that you will watch your output tubes work as fireworks.

lol i know this.

peter, is it possible for you to draw me a schematic for one of those suggestions? well, schematics for both would be even better but i don't wanna bother you much.

highly appreciated mate. thanks a lot.  

cheers.

[aron]

Take a look at the cabinet voiced line out of my Carvin amp:

[Peter Snowberg]

No worries.

http://www.freewebs.com/qrp/snippits.html

I like the filtering in the schematic Aron posted. If you look at the first link I posted to Aiken's site, you can see how the impedance rises with frequency. The capacitor across the output in the Carvin will tame some of the highs that are accentuated by the higher impedance at higher frequencies.

The resistor values I posted are really general... You could vary them over a huge range as they're just a divider that work off the ratio between the two. Once you start adding a filtering cap, things get more complex. I would build the Carvin circuit, but instead of the 470K, I would make that a 220-270K in series with a 500K pot. I like knobs.

Take care,
-Peter