Question about Amp Distortion and Equalization

Discussion in 'Effects' started by Dorian, Aug 3, 2006.

  1. Dorian

    Dorian Active Member

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    My question seems like a general one, but it comes from working with some of the amp models on a VOX AD50VT modeling amp. The "problem" occurs when I set amp gains for a mild distortion for higher notes. This gives pleasant lead tones, so long as I stay away from chords and low notes. When I hit a low note simultaneously, the distortion of the high notes becomes quite severe, and my pleasant lead tones are seriously altered.  I want to minimize this effect.

    What I know is that the output from my pickups, the neck pickup in particular, is far greater for the low tones than the high tones. Even with a 500 K tone control set for maximum treble, the typical voltage output from my low string is about 10 times larger than from the top string.

    I have an idea for fixing the tone problem, but it requires two equalizers and seems sort of complicated. What I was thinking of doing is using the first equalizer with a contour to suppress the low frequencies going into the preamp. This way the distortion stage in the amp is not overwhelmed by the low notes. I would then need a second equalizer after the distortion stage to boost the bass to compensate. (This will require tapping into the VOX circuit before its clean solid-state power stage, since it has no effects loop. I haven't planned this in detail, but I think it should be manageable.)

    I am not quite sure what to expect, so before I buy a 2-channel equalizer I wonder if anyone has tried something like this? Maybe somebody has a simpler idea for keeping a mild distortion for high notes when playing chords, for example?
     
  2. Dorian

    Dorian Active Member

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    I post again in this inactive thread because I went ahead and did the modifications described in the previous post, and I have spent some time working with them. I bought a two-channel graphic equalizer for a paltry $80, a Behringer Ultragraph Pro FBQ1502, which is more than enough for a guitar amplifier, and modified my VOX AD50VT to accommodate it. Here is what I did.

    Basically, the amp modification was to add an external effects loop, which is missing in the stock AD50VT. There is a very natural place to put it in the amplifier electronics, but cheapness, I suppose, prevailed in product development, and these low-cost VOX amps do not have effects loops between their preamp and power amp stages. The higher-end Valvetronix amps do have effects loops, so this double equalizer modification should be possible with zero work. (Incidentally, the same modification is possible for the AD30VT and the 15 too. I actually used the schematic for the AD30VT for the modification because I did not have a schematic for my AD50VT. Amusingly, the component labels on the circuit board for the AD50VT were actually the same as those on the AD30VT schematic.)

    After a lot of thought, bad ideas, and a realistic assessement of the limit of my soldering skills, what I finally did was cut the ribbon cable from the digital board to the analog board of the VOX amp. This is where I put the effects loop. It turned out that the signal and its ground were the outer two wires of the ribbon cable, and were easy to cut and turn into an effects out and in. (The schematic is a block diagram, and you will need more details if you want to do the actual modification.)
    [​IMG]

    I now have the first equalizer channel between  my guitar and the input of the VOX amp to cut the bass, and a second equalizer channel in the effects loop to boost the bass from the preamp to the power amp. Today the lowest bass cut is -5 dB and the treble boost is up to +4 dB for the first equalizer. The second equalizer in the new effects loop does the mirror opposite, with +5 dB bass boost and -4 dB treble cut. The photo is out of date, but shows the general idea for the equalizer settings.
    [​IMG]

    I am so pleased with the result. Another blanket of mud has been removed from my tone. Chords are far improved when played through amp models that have distortion. Previously the low notes caused distortions of the higher notes, but now this effect is greatly diminished. Playing octaves or intervals across strings has a new clarity. I am really enthused. In the stock AD50VT, I could not use the Marshall amp models well. They sounded fine when I played on one string, such as a lead riff on the top two strings, but the tone turned to mud when I played chords or when my runs went down to the lower strings. I am now having a great time with the UK Modern amp setting (nominal Marshall DSL-2000), and this is influencing the type of music that I am playing.

    For the past couple of weeks I have left the equalizer settings fixed for all the amp models. The idea is that the equalizer will level the voltage output from my favorite pickup settings, so the distortion will be even across all strings. I first set the equalizer by ear based on the distortion, but then I checked it on an oscilloscope to see if I got it right. I was actually not too far off in my settings by ear, and I don't think the scope is necessary. (Incidentally, a volt meter on AC volts is probably good enough for calibration too.)

    One of the issues I am dealing now with comes from my coil tapping modification of the SG. The single-coil mode gives higher treble output, and this requires resetting the equalizer. I am still trying to decide on a compromise equalizer profile that works best for the various coil tap configurations that I use while playing. The same issue would be true for optimizing for more than one guitar, of course.

    An effects loop in the AD50VT is very handy. This is especially true for this VOX product line because their power amps are so clean. I put my BBE Sonic Stomp in the effects loop, and this seems the best place for it. I am thinking of a nice reverb in the effects loop too. Another thought is to put a compressor before the amp, and an expander in the effects loop. (Funny how this list keeps growing.)
     
  3. Bonfire

    Bonfire Active Member

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    well done! i'm glad it worked out for you mate!!
     
  4. skidshark

    skidshark Active Member

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    the effects loop sounds like a very worthwhile mod to the adxxvt line....i don'r suppose you took any pics while elbow deep doing it did you?
     
  5. Dorian

    Dorian Active Member

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    Skidshark -- Sorry, I wasn't confident that the modification would work without noise or buzz, so I didn't take any pictures. I was also a bit annoyed that I could not figure out how to add the return after the vacuum tube stage (the vacuum tube circuit is very, very weird), so I thought I would try a different connection later. It turned out to work quite well electrically. I am happy with it and I will keep it. On the other hand, it is not my best work mechanically. I just spliced some coaxial leads into a cut ribbon cable, and ran the wires directly out the vent slots in the back of the amp. Very temporary, I thought, but it has been sitting this way for two weeks now. I do intend to get back to it and put in proper chassis-mount connectors, probably standard phone plugs with insulation from the chassis, but I haven't had the time. When I do, I'll take photos. This effects loop modification is really pretty easy, and it gives a lot of new capability to these inexpensive Valvetronix amps.
     
  6. vic108

    vic108 Well-Known Member

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    WOW! Very cool! Glad it works for you.
     
  7. skidshark

    skidshark Active Member

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    i'd like to add that to my 30....
     
  8. *ess*

    *ess* Member

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    cut a slot in the front and mount the EQ in there!

    cool mod.
     
  9. Dorian

    Dorian Active Member

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    Skids (and Vic) -- Pictures. Yes. And more explanation.

    Sorry to be slow, but the effects loop modification was working so well that I did not get around to opening the cabinet again until today so I could make it permanent.

    Here is a picture of what I did a few weeks ago. It was quick and easy, and a way to test if the effects loop was a good idea. There is a ribbon cable in the VOX AD50VT, circled in red, that connects the upper digital board to the lower analog board in the amp. The outer two leads on the ribbon cable, conveniently accessible, are the analog input and the analog ground. I peeled these two leads from the others, and cut them in the middle. The black is the analog input signal, and you can see that I soldered the center leads of two gray coaxial cables to the two ends of the cut lead. One of them was the output and the other the input for the effects loop. The analog ground was soldered to the coax shield, but I was careful to keep it away from the case of the amp. The VOX schematic shows some isolation between the analog and digital grounds. The case is the digital ground. This was quite an easy modification, and it worked very well.

    [​IMG]

    I wanted to do the modification properly, with chassis jacks and no dangling wires. Another improvement was that I wanted the effects loop modification to be invisible when nothing was plugged in, so the amp would act as it did when it came from the factory. I bought a couple of chassis-mount phone jacks with switches, and selected the circuit below. Notice that when a jack is plugged into either the input or output of the effects loop, the analog and digital boards are disconnected. This seemed safest, especially if I remember to plug in the left Effects Out jack first, but I hope the switches don't get dirty. I am going through two of them in series.

    [​IMG]

    I decided to respect the VOX separation between analog and digital grounds, so I isolated the phone jacks from the case using insulating washers. This would keep the analog ground separate from the digital one. I was lucky today to find these washers when I wandered through our local electronics surplus store, along with other mounting hardware, two 10 K pots for another project, and enough phone plugs and coax to make two 3 foot cables. (All for $ 2.45.)

    [​IMG]

    The hardest part of this project for me was drilling the holes in the sheet metal of the VOX chassis for the phone jacks. They have to be pretty big holes to accommodate the insulating washers.

    I soldered the coax to the jacks before mounting them. It looked too difficult to get a soldering iron up to the jacks if they were mounted first. 

    [​IMG]

    Here are a couple of shots of the inside and outside of the jacks for the effects loop. 

    [​IMG]

    [​IMG]

    All went smoothly without problems. The only stupid thing I did was to close up the case before I connected the loudspeaker. Fortunately I noticed quickly that the amp was unnaturally quiet, so the revalation came immediately. So far I have done all the tests that seem appropriate, and all work fine. The amp sounds like new when the effects loop cables are unplugged, or when the in and out are jumpered. With the two channel equalizer it is, I think, much better than new.

    The last little thing I need is proper labels.  I am a bit worried that some fool, possibly me, will plug an 8 ohm loudspeaker into the output from the digital board. The back panel labeling could be misleading. I suppose I should paint over the VOX labels and paint on new ones. Is there an elegant and easy way to do this?


    Ess -- I liked your idea of cutting the slot in the front of the amp and mounting the equalizer in it. Sadly, there is not quite enough space to do this. I did check.
     
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  10. davewolf

    davewolf New Member

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    Do you still have and use that Vox? I love my Vox AD50VT so yeah I stumbled across this post in a search to add an effects loop after years of using it as it was out of the box. The only difference between yours and mine is your ribbon cable is dominantly white wires with a black where mine is opposite black with a white. More interesting is you could totally bypass the built in preamp and feed just about whatever preamp you want even a pedal board using the effect return only accessing the power amp part only. Using a little switching you can switch between the built in and whatever else you choose even an acoustic preamp. I more or less modified for the purpose of being a Swiss Army knife to accept whatever guitars or preamps and setup I want to plow through it.
     
    Last edited: Feb 22, 2020

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