Using Randall RG-80 as a recording preamp - speaker load?

[daffe]

Hi,

Newbie here, newly converted to the beauty of ss guitar amps.

I would like to use my RG-80 combo as a recording preamp.  The goal is of course to plug-in directly into a di box into my recording interface while disconnecting the speaker to record silently.

My question is : How should I go about doing that.

1- Should I use the speaker out jack or the effect send from the amp to the di box?

2- What about the speaker load?  Do I need some sort of load compensation for the power amp?

Maybe I'm missing something so feel free to add any other consideration to the discussion.

Many thanks.

[teemuk]

Well, what's the input sensitivity of the DI box? That will define a lot of things because if you use the speaker output as signal source you might have to attenuate the signal considerably. That signal will have very high amplitude and in worst case the signal can toast the input stages of your recording equipment. But, doesn't RG-80 have a post-power amp stage "signal output" jack, which is already attenuated? The schematic I'm looking at shows one but there might have been plenty of revisions.

Most solid-state amplifiers cope without a speaker load but this is not true in every case. So, if this one does do not assume that it's a general rule. ok? In some SS amps the speaker load is as essential as in tube amps and some amps will oscillate without one. Anyway, RG-80 is a pretty basic SS amp and it should work without a speaker and it should not be damaged if you omit it. You should consult the manual, though. If it says no, then don't do it. If there's no remark about the issue then the amp likely runs fine without a speaker. If the output signal has "whistle" or the amp seems to behave strangely then it is likely oscillating and needs the speaker to remain stable. In some cases omitting the speaker upsets the output's DC offset.

You can experiment with all the "output jacks" quite safely as long as you keep the master volume level down. Just make sure you don't short the output. Plugging cords in and out of the speaker jacks can do that momentarily. Turn the amp off always before that.

Then, do note that in this amplifier the reactive load of the speaker forms a filter that alters the power amp stage's frequency response. If you omit the speaker this filter will be removed and the power amp will sound different. It will also sound different with different speakers (e.g. headphones, dummy loads etc.) due to that filter. You can expect that most of solid-state amps have this kind of interaction.

Whatever of the "output jacks" you decide to use as signal source, you still likely need to compensate the loss of speaker cabinet somehow – in other words, you need a cabinet simulator that emulates the cabinet's frequency response. This is different issue than the filter formed by the power amp's interaction with the reactive load. That was an electric filter, this is an acoustic one. In roughest form the cabinet emulator is a steep low-pass filter with a cut off frequency at about 5 kHz. Omitting this can make the amp sound too "harsh" with distorted signals since you let all those high-frequencies pass unattenuated. With distorted tones the outcome will be screeching, obnoxious high end - but if you record clean, "acoustic" stuff the HF content may actually be beneficial.

Furthermore, speakers are very non-linear devices and very often the speaker and its cabinet are major contributors to the amp's tone. Thus the signal you get from the "output jacks" alone may prove to be a huge disappointment. Too "sterile", too "screechy", "missing something" and all that. If it were up to me, I would just use the conventional recording technique with a microphone in front of the speaker cab but since you need to record silently it obviously isn't an option.

Anyway, if absolute silence is not required then I would try recording with mic and low volume. It often works better than blasting the amplifier at "10" anyway.

In conclusion, let's make it clear that there really aren't right or wrong techniques. That is of course excluding the techniques that damage the equipment somehow. In recording the only thing that matters is the outcome. You should experiment on which configuration gives the best tone. Hopefully this post gave some insight on different variables affecting each option.

[daffe]

teemuk,

Thanks for your very detailed reply.

I got a lot more info to get where I want now.

The DI box I use is one of those Behringer DI100 active DI's.  It can take a very high wattage and has 2x -20db attenuation pads available.

Your right about the signal output jack on the RG-80.  I forgot about that.  I should try this first.

The service manual doesn't have any warnings about using the amp without a speaker load but says "less than 8 ohms load impedance not recommended".  Impedance load has always got me confused so I don't know what to make of this.

As for the speaker acting as an electric and acoustic filter, I know I'll have to use some sort of cabinet simulation.  Some DI's have some internal cab sim circuits or I can just use some eq or even plugin in my DAW.  The electric filter part has me wondering if getting one of those MASS devices from Weber VST would be useful.  It is an attenuator with an actual speaker motor in order to replicate the non-linear impedance load of a real speaker (or something like that...).  These are intended for use with tube amps so I don't know if it's relevant or maybe even harmful to a ss amplifier.

So I just need to make sure unplugging the speaker is safe to try this out.

[teemuk]

A "dummy load" is just like any other load so it works with solid-state amp equally as it works with a tube amp. The only difference is that SS amps do not necessarily need one, whereas running a tube amp without a load will toast it. The harm that these dummy loads may cause for amplifiers comes from the fact that people can "turn their amp to 10" without the loudness being an issue and in such condition the amplifier works under a greater stress.

The electric filter.... Well, if you have a graphic EQ you can easily fake the power amp's response to a speaker load. The filter typically introduces a gain boost at speaker's resonant frequency (which is around 60 – 100 Hz) and boosts the gain at the high frequencies. The response difference is shown in figure #1 of this old study: http://milbert.com/Files/articles/TvsT/tvtiega.pdf

The figure shows why the speaker's impedance "must" be faked by the dummy load in case of tube amps. Well, that issue applies to many SS amps as well if they are designed to interact similarly with the speaker. It's not just a tube thing.

Speaker impedance and things related to that is a bit too extensive topic to be explained briefly but it's something that is definitely worthwhile of learning.

In short, impedance is a measure that indicates that a specific circuit element has a "resistance" that varies according to signal frequency. Even the unit of impedance is the same as that of resistance, an Ohm. The difference between these measures is that resistance stays constant throughout all signal frequencies while impedance does not. But basically these measures express the same thing.

Speakers have impedance because they are not purely resistive devices and their load (in ohms) will therefore vary according to frequency. That's why we also state the "nominal impedance", which is just that: A nominal figure that represents the most common load (in ohms) the speaker introduces. The 8-ohms stated by the manual is simply the lowest load impedance the amplifier can safely handle.