january 2009

Using impulse response/convolution reverbs to help smooth out your sound.

Most of us would probably agree that the sound of a tasty reverb on a great guitar part is a thing of beauty. And you’d think that with all the latest generation of guitar plug-ins, this wouldn’t be that hard to achieve. Unfortunately, most of them are sorely lacking in this one all-important area. One way to get around this problem—aside from owning classic amps—is to use any of the widely available impulse response/convolution reverbs to help smooth out your sound.

As a quick reference, a convolution reverb is a process where a pulse or sine wave sweep is played back into a space or actual piece of gear such as an amp or reverb unit. Using a process called deconvolution, the sine sweep is then removed leaving a clean sample called an impulse response.

When needed, I tend to rely on a number of different plug-ins to get these great reverb sounds for guitar. Audio Ease Altiverb, Digidesign TL Space, Waves IR-1 and McDSP’s Revolver are used daily in my Pro Tools system, although there are several other excellent brands available as well. Specifics vary, but most will work across the popular platforms such as RTAS, TDM, VST and AU.

So, what is it about these IRs that make them better (typically) than a traditional reverb? While the quality of the actual sample is, of course, important, it’s the fact that you’re actually hearing the real thing. When you put your instrument “into” the reverb from an old Fender amp, the sound just comes alive. The depth and character of real reverb is an almost tactile issue, and each person will have their own opinions on their favorites.

One of my first go-to settings is Altiverb’s Fender Super Reverb. The samples done for Audio Ease by Joe Gore are simply selectable as Bright or Normal. The Reverb Time (RT) is fixed at 6.70 seconds, but you can shorten it if needed using the RT dial. Gore also has samples of a Magnatone 480, a Baldwin Amp and a Real Tube Reverb, all of which are also quite nice. Altiverb also lets you sample your own amps, which I and Vincent Miraglia of Analog Design group recently did when we recorded the tasty reverb of my 1964 Gibson Falcon amp; an amp I also turn to quite a bit when needed.

And don’t think about just using the sound of actual guitar amp reverbs. You can call up such goodies as EMT 140 and 250 plates, Echoplates, spring reverbs and echo chambers. By using a buss to send just a little bit to one of these IRs you can get dull-sounding electric guitar parts to stand out. For a really “wet” signal, enable the “Pre” button on your reverb send/fader. This will send the audio directly into the reverb, bypassing the fader level. You may have to pull your fader down to compensate for the dry/wet balance, but you’ll get a full wash of ’verb that way. Experiment with that setting for a lush sound.

Impulse response settings are not just for electric guitars or amp simulators. Amazing sounds can be had for acoustic guitars as well. Think about recording that beautiful acoustic you have in your bedroom, or in a small, tight space. Yes, it probably sounds good up close, but what if you wanted something more? This is the perfect situation to use an impulse response.

One of my favorites for acoustic instruments is The Stone Room at Masterfonics Studio in Nashville, Tenn. Available in the Waves IR-1, it adds an incredible sense of depth and brightness. Also, there are many other great sampled recording studios, scoring stages, churches, opera halls, arenas and even stairwells available. Not every studio/room works for the sounds you’re seeking, so its best to get a setting, and then on the plug-in itself run through different IRs. Sometimes the room is too big, too dark or too bright. That’s when it’s time to break out some more plug-ins.

The essence of dialing in one of these sounds is to get that perfect fit in a mix situation. To lift a track up or even set it back behind the rhythm section, you may want to try EQ-ing the reverb. Instead of just settling for the sound of that plate/amp/room, try placing an EQ after the reverb. While yes, some IRs have settings allowing you to EQ the actual output, they are usually limited in scope. I tend to use a good, flexible high-quality EQ such as the Sonnox Oxford EQ, Universal Audio Cambridge, URS A10 or the EMI TG1214. These allow me to really dig into which frequencies need to be boosted or cut. Of course, you can patch in a hardware reverb as well—whatever works best for you.

Placed directly after the reverb plug-in, you can then roll off the unwanted/unneeded low frequencies (usually below 120–150Hz) in the ‘verb itself to clean up the bottom of your mix. Then, by sweeping the EQ you can also boost and/or cut any frequencies to accentuate, like string noise on an acoustic or scratches on your electric part. Impulse response reverbs are simply great for adding character to a mix. For guitarists, it’s a surefire way to get that flat-sounding DI track or amp simulator to really shine. Poke around the Internet for yourself and see which version might work best for your studio setup. Your guitar will certainly thank you for it.

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A stereo live setup can make mix concerns a thing of the past

When performing regional and local gigs I run into situations where the stage mix is atrocious. Not hearing the mix correctly creates a struggle. Usually I’m sure the audience is hearing a mix that’s as bad as what I’m hearing—though often, thankfully, the front-of-house mix is much better than what I hear onstage. But concern about the mix is a huge distraction from performing well. Having my guitar set up in stereo has helped me find a sonic spot I can retreat to and regain the feeling and inspiration, even if the onstage mix is awful.

To morph your current amp and effects setup into a more dynamic stereo rig, you’ll need two combo amps or two amp heads and matching cabinets, cables and one or more stereo effect processors. The processors can be rackmounted units or stompboxes. An effects loop on your amp is desirable, but not required. A more advanced rig might include multiple stereo processors and a small mixer for blending them.

The Basics
In Diagram 1, we’re starting with a basic stereo rig. The stompbox processors consist of a distortion, a

stereo chorus, and a digital delay. This is a budget-conscious configuration that sounds great and is a breeze to set up. Connect your guitar into the distortion pedal, then route the distortion to the stereo chorus. Since the stereo chorus has a left and right output—it actually creates the first level of the stereo effect—we’ll send both outs into the digital delay, which can increase the stereo spread with the judicious addition of short or long echoes. Run the left/right signal out of the delay into two separate amps set for a clean sound. Position the amps a reasonable distance apart to enhance the stereo spaciousness.

The Next Step
In Diagram 2, we’re using rackmounted effects combined, using a mixer. Amp 1 is the main amp that your guitar is plugged into: it creates the preamp tone you’ll be using. Route the signal out of that amp’s effects loop send into the first effect. In this example, we’re using a TC Electronic 2290 because it has a direct signal pass-through that we can use to route a “dry,” unprocessed signal to another effects processor. Alternatively, you could use a line-level splitter, like the Whirlwind Splitter, to split the effects send output so it can feed the inputs of multiple processors simultaneously.
Route the left and right output signal from the 2290 to a stereo line mixer, a rackmountable Rane SM82S in this case. I’ve also routed the direct signal into a Lexicon MX400. Then come out of the MX400 left and right into the SM82S. The reason for the line mixer is so you can mix the stereo wet signal of each effect against the amp’s dry signal and maintain a clean signal path. The left and right outputs from the mixer are sent into the effects loop returns on the two amps.

There are, of course, many configurations you could use, depending on the gear you have and want to use. You can substitute combo amps or rackmounted preamps and power amps. You can use 4x10, 2x12, or 1x12 cabinets. Or, for maximum convenience, check out stereo cabinets like the Marshall 1936. Just pay attention to the ohms/watt ratings on the cabinets and the amp. You want to make sure they are matched.

A major benefit of this setup is you can use it with the “MIDI switching” setup we discussed in Premier Guitar’s November 2007 “Guitar Tracks” column. This will allow you the flexibility of using MIDI to change any of your effects patches. Enjoy, and let me know if you have any questions.
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A Fender Hot Rod Deluxe is on the fritz, and Jeff has five possible causes to check out.

Hi there,
I have a Fender Hot Rod Deluxe that is just six months old and well looked after. It has recently developed a popping, crackling sound when warmed up—even when there are no guitars or cables plugged in. What is wrong? Could the valves be on the way out? Is it something more serious?
Please help!
Ken Morton

Hi Ken,
Thanks for your question. The Fender Hot Rod series of amps is a very popular one, possibly their most popular. More than a few have crossed my bench over the years, some coming in for a “crackling” issue. There are many types of failures that cause this symptom. I’ll go through most of the typical ones here and tell you which ones I suspect might be the cause of the failure in your amp.

1. Input, footswitch and effects loop jack connections.
Jacks are probably the most physically used and abused parts in the amp. They are soldered directly to the circuit board, but are also attached to the front panel via the jack nut. This is usually not an issue unless the jack nut loosens due to vibration. Once the nut loosens slightly, the weight and motion of the guitar cable is supported solely by the circuit board solder connections. Over time, these connections can be compromised and cause crackling and an intermittent signal.

2. Broken solder joints on large components.
Another vibration-caused failure, the solder connections of larger-sized components, such as large 5-watt resistors, can eventually be compromised from the constant vibration of playing, or simply a rough ride in your trunk or the band truck.

3. Simple tube failure.
Although a tube may look good from the outside, there’s no way to tell for certain if it’s good or bad just by looking at it. Tubes can fail in many different ways. Output tubes will generally short internally and cause a fuse to blow, or they can become mechanically noisy and exhibit a “rumble” sound through the speaker when vibrated. Preamp tubes, on the other hand, will generally either become microphonic and make high pitched ringing or squealing noises, or will emit noises such as crackling and popping during idling. If the level of the noise is controllable in any way by the setting of the volume or tone controls, then there’s a good chance that the symptom is caused by a preamp tube and not an output tube.

4. Stressed component failure.
Some components become stressed over time when the typical operating parameters of the circuit are exceeded. This can easily happen with the failure of an associated component. Take for example an output tube. While a typical 2-watt resistor (used by many companies as an output tube screen grid resistor) will function fine under most normal operating conditions, that resistor may be pushed past its capabilities by constantly playing the amp at very high volumes or by an output tube shorting. This may cause excessive screen grid current to flow through the resistor, stressing it or causing complete failure. A stressed screen grid resistor can definitely cause crackling noises to occur. (As a side note, this is one reason to overbuild amps: to ensure the least possible chance of failures—using a 1-watt resistor instead of a half-watt resistor throughout the amp, for example, or a 5-watt in place of a 2-watt, etc.)

5. General component failure.
Any electrical component can fail at any time, causing anything from compromised operation to a complete shut down. I did a bit of research on this question to see if there are any known failures of this particular amp that I had not personally encountered, and it turns out that there may well be. According to the research, there may have been a bad batch of resistors used as plate resistors in the phase inverter circuit of the amplifier that were responsible for crackling noises in some amplifiers— a very strong possibility for the cause of your symptoms.

Having described the most likely candidates for the cause of your amp problem, I would say that based on your description of the amp needing to warm up first, and no guitar needing be connected, the most probable in your case is either 3 or 5. The easiest way for you to do some basic troubleshooting would be to first purchase one replacement 12AX7 tube. Then, one by one, replace each preamp tube and see if that alleviates the problem. If you still have the symptom, you should next replace the output tubes.

For the purposes of troubleshooting, you can install a new set without re-biasing the amp, but if they are indeed the cause of the noise then you should definitely take the amp in for biasing. If neither of the replacement tubes cure the problem, the amp will need to be taken in to your local experienced tech for further troubleshooting— but at least you’ll have yourself a couple of spare tubes for the amp, which no self-respecting tube-amp-playing guitarist should be without! ‘Til next time…

Jeff Bober
Co-Founder and Senior Design Engineer – Budda Amplification
©2008 Jeff Bober