Magic Box project. Dummy load + line out + reactance emulator + SS amp?

[nonost]

Hi guys. I'd like to get a line out of my amp in order to record it with the computer. It'd be great if it could also let me crank the amp at low volumes. I've come across a couple topics where the user induction posted some solutions to it, but I'm not sure I've understood everything ok. First I want to thank him for such an useful and interesting info.

That's the layout by induction posted here http://guitar-fx-layouts.42897.x6.nabble.com/DIY-Attenuator-td6223.html#a6239 I hope it doesn't bother him:



The layout it's quite easy but I have a few doubts since it's not a guitar pedal. Do the resistors need to be high power rating or do the 1/2 watt work? I mean the 22k and 47K.

The other doubt it's about the cabinet mode. I thought that in that position what you get is your normal amp + cabinet configuration but since at the bottom of the picture it is said that the line out is active in both modes I don't know what to think. What does the line out do in cabinet mode?

My idea is to plug that line out into my audio interface to record clips. Then I will need a cab simulator, either a stompbox or a plugin. So far I think I'm going to try the latter. If I've understood everything fine I wouldn't need anything else, just a cab simulator.

NOW, because it is a resistive dummy load and not at reactive  dummy load, it could be very useful to add a reactance emulator to the dummy load.

What is a reactance emulator? Till yesterday I had not f*cking idea, but the user induction do seems to have it. Here a quote where he talks about the Marshall Reactance Emulator
(http://freestompboxes.org/viewtopic.php?f=2&t=27638):

induction wrote

Remember, though, it only simulates the reactance effect, it's not a cabinet simulator. The reactance emulator is useful when your tube power amp is connected to a resistive dummy load instead of a speaker, or when you're using a solid state amplifier (or one with reasonably high NFB) and you want to recapture the sonic impact of the interaction between OT and a reactive load. In other words, it makes a solid state amp sound more like a tube amp. The cabinet simulator is for making a direct signal sound more like a miked cabinet. In general, the cabinet effect has a stronger impact on the frequency response of the system as a whole than the reactance effect, and they are used in different situations. If you re-amp a tube amp through a solid state power amp and run the result through a real cabinet, then you don't need a cabinet simulator, but you might still benefit from a reactance simulator. If you take a line out from the OT on a tube amp with a resistive dummy load, and feed it to a recording desk or PA, you may want both. If you take a line out with a reactive load, and feed it to a recording desk or PA, then you may want just the cabinet simulator. If you take a line out with a reactive load and feed it to a solid state power amp connected to a real guitar cabinet, then you might not want either.

Here the topic in this forum with the layout http://guitar-fx-layouts.42897.x6.nabble.com/Marshall-Speaker-Reactance-Emulator-td31283.html#a33469

My question here is: Would be useful/necessary to add a reactance emulator to the line out mode (pc recording, etc)?

Like I said at the top of the thread It would be a very versatile build (magic) if it would also work as a kind of attenuator. From the three topics mentioned above I know that it's possible with a few adds. If I've understood it well, you should add a SS amp to the line out od the dummy load and then the SS amp output to the cabinet. In that configuration a reactance emulator would be useful. So any recommendation for the SS amp? I play through an Orange AD30, which is 30 watts BTW.

In one of the topics mentioned induction said:

induction wrote

The trick is that you have to build the dummy load circuit specifically to suit the amp you want to use it with. The power resistor has to be able to handle twice the output wattage of the amplifier, and the lineout voltage divider has to give the appropriate range of output voltages.

I don't really understand the last part of the second sentence...Yes, I'm a noob and wouldn't like the idea of blowing up my amp.

I've been looking at the datasheets of some 16Ohm 100watts resistor and it's said that the should be mounted in a 995cm2 surface in order to full performance. I thought about using a 1590BB but it has  3 times less surface than recommended. Also I don't know if another kind of enclosure is more adequate for this purposes.

Well, this has been a bit longer than I'd thought. I know there are lots of questions but it would be great to make a box like this. So any experience/advice is very welcome!

[induction]

Hi nonost. I'm glad you're looking into this, as it's a great way to crank your amp quietly (like I do), or turn your tiny Fender Champ up to concert levels (like this guy does). I don't have much time to respond at the moment. But I'll try to answer your questions. If I miss anything, feel free to ask for clarification.

nonost wrote

First I want to thank him for such an useful and interesting info.

That's the layout by induction posted here ... I hope it doesn't bother him:

Not at all.

Do the resistors need to be high power rating or do the 1/2 watt work? I mean the 22k and 47K.

Only the 16R has to have a high power rating. It's very low resistance and it's wired in parallel to the lineout circuit, so most of the current will go through the dummy or the speaker, whichever is selected. The lineout circuit is much higher impedance, so it reduces the current through those resistors quite a bit.

The other doubt it's about the cabinet mode. I thought that in that position what you get is your normal amp + cabinet configuration but since at the bottom of the picture it is said that the line out is active in both modes I don't know what to think. What does the line out do in cabinet mode?

The lineout and the cabinet mode are independent. It gives you a line level signal regardless of which mode you're in. For example, you can run your amp into your computer and monitor it through the amp (cabinet mode) or through the computer or audio interface (dummy mode).

My idea is to plug that line out into my audio interface to record clips. Then I will need a cab simulator, either a stompbox or a plugin. So far I think I'm going to try the latter. If I've understood everything fine I wouldn't need anything else, just a cab simulator.

...

My question here is: Would be useful/necessary to add a reactance emulator to the line out mode (pc recording, etc)?

It probably will help in dummy mode. Remember, a reactance emulator is just an eq, so it won't hurt anything. Your cab simulator probably doesn't have that eq section implemented, but I can't be sure. Try it out and see how it sounds. In my experience, a lineout from a tube amp with a dummy load sounds a little boxy, and the reactance emulator gets rid of the boxiness. In cabinet mode, the reactance emulator isn't needed because the amp can see the reactance of the speaker.

Like I said at the top of the thread It would be a very versatile build (magic) if it would also work as a kind of attenuator. From the three topics mentioned above I know that it's possible with a few adds. If I've understood it well, you should add a SS amp to the line out od the dummy load and then the SS amp output to the cabinet. In that configuration a reactance emulator would be useful. So any recommendation for the SS amp? I play through an Orange AD30, which is 30 watts BTW.

That's exactly how I use it. The SS amp you use depends on what your goal is (mine is to reduce power and volume, but it works just as well to crank it super loud with a big power amp), and how much color you want the ss amp to add. It can be a pure power amp, or a preamp+power amp, just beware of overloading the input in the latter case. I'd avoid a digital or modeling amp, as I've found it sounds terrible in that configuration. I assume you found the schematic and layout for my 1/2 watt Grace amp on the page you find the dummy load layout? Works great.

induction wrote

The trick is that you have to build the dummy load circuit specifically to suit the amp you want to use it with. The power resistor has to be able to handle twice the output wattage of the amplifier, and the lineout voltage divider has to give the appropriate range of output voltages.

I don't really understand the last part of the second sentence...Yes, I'm a noob and wouldn't like the idea of blowing up my amp.

The first part of the sentence means you'll want a 60W power resistor for use with your amp, which I assume you understood. The second part means that R1 and R2 form a voltage divider with the volume pot. The minimum lineout volume is 0, but the maximum depends on the value of R1 and R2. If you find your output is too loud (overdriving your ss input even at low settings), increase R1. If it's too quiet, even with the volume dimed, decrease R1. You won't blow up your ss amp if it's too loud, but in theory it could damage a component or two if you try to put a 20V signal into a preamp. Mostly it will just distort too much. The values you pick won't hurt your primary amp as long as you're in dummy mode or have a cabinet plugged in.  

I've been looking at the datasheets of some 16Ohm 100watts resistor and it's said that the should be mounted in a 995cm2 surface in order to full performance. I thought about using a 1590BB but it has  3 times less surface than recommended. Also I don't know if another kind of enclosure is more adequate for this purposes.

The trick is that the resistor shouldn't get too hot, but the solution depends on the output power of your amp. So just try it out. Turn your primary up as high as you'll ever need it, and play for 10 seconds in dummy mode. Then touch the dummy resistor and see how hot it is. If it got really hot in 10 seconds, you need to increase the size of your heatsink. If it doesn't get hot, play for a few more minutes and test again. Iterate until your satisfied it's not going to overheat.

Let me know how it goes and if you have more questions.

[nonost]

Thanks for your thoroughly answer, I really appreciate your help.

I think I see the whole picture now. Resistive dummy load it's just a resistor. A big resistor that doesn't talk to the OT of the amp in the same way a speaker does. So we emulate that behaviour with a circuit. That circuit could be built with small solid state components(like the Marshall Reactance Emulator) or with inductions and big stuff. Reactance dummy load implements the latter while resistive dummy load are just a resistor. Adding a reactance emulator to our resistive dummy load is an easy and cheap way of getting the higher quality sound of the reactance ones.

induction wrote

The lineout and the cabinet mode are independent. It gives you a line level signal regardless of which mode you're in. For example, you can run your amp into your computer and monitor it through the amp (cabinet mode) or through the computer or audio interface (dummy mode).

Ahhhh I see it now! That's perfect, you plug things into the box and then forget about it.

With things clearer now I'm going to build a box with the dummy load circuit + marshall reactance emulator. I will add a switch at the output of the circuit, just before the line out, that turns on/off the reactance emulator, just in case some cab sims would need it or not.

Then I will build a box with an SS amp, and since they are small and simple maybe a cab simulator as well. Both in the same box. My amp it's pretty loud, what I would like it's to reduce the volume. So the transparent and clean the SS amp is the better for me. I like both clean an overdrive sounds of my amp so I would like to keep it as much as possible. If yours is working good and doesn't add distortion I will build it. If no SS amp is used and I plug the line out to a cabinet, what would I get? Shitty sound? I don't see the point of the SS amp since I can use the volume of the dummy load circuit. I know it has a reason, huge reason...I just don't get it :)

induction wrote

The first part of the sentence means you'll want a 60W power resistor for use with your amp, which I assume you understood. The second part means that R1 and R2 form a voltage divider with the volume pot. The minimum lineout volume is 0, but the maximum depends on the value of R1 and R2. If you find your output is too loud (overdriving your ss input even at low settings), increase R1. If it's too quiet, even with the volume dimed, decrease R1. You won't blow up your ss amp if it's too loud, but in theory it could damage a component or two if you try to put a 20V signal into a preamp. Mostly it will just distort too much. The values you pick won't hurt your primary amp as long as you're in dummy mode or have a cabinet plugged in.  

Understood. So I probably will have to play with R1 in order to tweak it to my needs.

About the 16R resistor and heat dissipation I thought about using two resistors instead of just one. Intuitively it looks to me that it would dissipate everything better. Maybe I drill a few holes in a fancy pattern to let the thing breath. For some reason, getting 32R are easier to me than 4R or 8R. If I wire a couple of 32R rated at 50w in parallel(I'd get 16R), would I get 100w? Sorry if that's the silly question ever but I just want to be sure because it sounds too magical to me. That would be great though.

Again, thank you very much!

[induction]

nonost wrote

My amp it's pretty loud, what I would like it's to reduce the volume. So the transparent and clean the SS amp is the better for me. I like both clean an overdrive sounds of my amp so I would like to keep it as much as possible. If yours is working good and doesn't add distortion I will build it.

I'm very happy with mine, but the whole idea for me is to play quietly. If you don't push the 386 too hard, it's clean. But it will distort if you crank it up. So it just depends on how loud you want to play. It's super easy to breadboard, though. So you can always try it out and see whether you like it before you commit to building it.

 If no SS amp is used and I plug the line out to a cabinet, what would I get? Shitty sound? I don't see the point of the SS amp since I can use the volume of the dummy load circuit. I know it has a reason, huge reason...I just don't get it :)

The line-level output won't power a speaker. Think of R1, R2, and the top section of the volume pot as the top part of a voltage divider, with a 16R speaker as the bottom part of the voltage divider. The signal the speaker sees would be tiny. The practical effect is it makes no sound whatsoever. The purpose of a power amp is to provide a very low impedance, high current-capacity output with enough current to move the speaker and make sound. Otherwise you could just plug your pedals into a cabinet.

About the 16R resistor and heat dissipation I thought about using two resistors instead of just one. Intuitively it looks to me that it would dissipate everything better. Maybe I drill a few holes in a fancy pattern to let the thing breath. For some reason, getting 32R are easier to me than 4R or 8R. If I wire a couple of 32R rated at 50w in parallel(I'd get 16R), would I get 100w? Sorry if that's the silly question ever but I just want to be sure because it sounds too magical to me. That would be great though.

You got it right. As long as the two resistors are the same value, each resistor gets half the total current, which doubles the amount of power you can push through the dummy load.

[nonost]

 I'm going to build the dummy load + marshall reactance regulator first and then we will see. Right now I'm more interested in the silent pc recording mode so no hurry. But I think I will put Grace and a 2W clean amp that I've found somewhere in the same box.

induction wrote

The line-level output won't power a speaker. Think of R1, R2, and the top section of the volume pot as the top part of a voltage divider, with a 16R speaker as the bottom part of the voltage divider. The signal the speaker sees would be tiny. The practical effect is it makes no sound whatsoever. The purpose of a power amp is to provide a very low impedance, high current-capacity output with enough current to move the speaker and make sound. Otherwise you could just plug your pedals into a cabinet.

Got it! Hope you didn't go mad with that kind of amateur question.

Just one more quick question about the switch if you let me so, do I need a big switch for this circuit? The switches I use for stompboxes are 3A/125VAC or 3A/250VAC, but I don't know if they're ok for this purpose.

Thank you very much, I have almost all ready to do the order! :)

[induction]

nonost wrote

Got it! Hope you didn't go mad with that kind of amateur question.

Not at all. Glad to help.

Just one more quick question about the switch if you let me so, do I need a big switch for this circuit? The switches I use for stompboxes are 3A/125VAC or 3A/250VAC, but I don't know if they're ok for this purpose.

Thank you very much, I have almost all ready to do the order! :)

Power is Voltage * Current. 3A at 125VAC is 325W. Your amp puts out 30W. Should be fine.

[nonost]

Ok. I've already ordered the parts. I'm going to add a Simple CabSim + switch after the Marshall Reactance emulator in case there's room enough for it.

I will post about the progress next week.

Thank you, I'm learning a lot!

[nonost]

Today I feel like drilling. Well, not actually...but I had to do it! I had populated all the boards weeks ago and they were bored. From left to right: charge pump, Marshall Reactance Emulator, Simple CabSim and Line Out.

[nonost]

I've been doing some tests with the dummy load and the line out of it directly to the audio interface, no pedals, just two cables. Sounded horrible. Cab sims are needed. I tried with some impulses and it was day and night. The cleans are very decent but in the very moment I turn up the gain knob of the amp, the quality starts to degrade. Tomorrow I will try adding the Marshall Reactance Emulator.

pd: I've had to increase R1 a lot. Between 512k and 1M seems to be the point. I'm playing through a very very loud 30w amp.

[nonost]

Should I be dealing with ground in a special manner? I'm not having problems but in some way it worries me, I don't want to blow up the amp. I'm doing as if it was a guitar pedal.

The tone is letting pass through a lot of treble content. Need to try different values tomorrow.

[induction]

nonost wrote

Should I be dealing with ground in a special manner? I'm not having problems but in some way it worries me, I don't want to blow up the amp. I'm doing as if it was a guitar pedal.

I treat mine like a pedal. Nothing in your build has an actual connection to ground until you connect the output from your amp to the input of your Magic Box. So that means there's only one ground connection in the entire chain and everything is fine. This assumes your SS amp is powered by an adapter or a battery. If you plug into a slave amp (or anything else) that has a direct ground connection, you may want to put an isolation transformer between them.

The tone is letting pass through a lot of treble content. Need to try different values tomorrow.

I included the tone control in my lineout box because I built it long before I built the reactance emulator. I was essentially trying to use the tone control to simulate the reactance effect. If I built it again, I'd skip the tone control altogether. I set it as flat as possible these days. A tone control could be useful for shaping, but as you've discovered, you'll want one that's a bit more subtle than what I used.

[nonost]

I'm having problems with the tone control. Fully counterclockwise it gets a bit muddy, but a good sound. You move the knob clockwise a little bit and the sound is ruined. It's hard to explain, it's metallic, like with a tiny bit of distortion (like poor digital distortion). It's there, you can feel it. I've tried different resistors and capacitors with no success. I'm going to remove the tone control and see what happens.

I'm testing with a small ss practice amp and an audio interface. So I guess I would need an isolation transformer.

[induction]

I wouldn't worry about the transformer for testing, unless you're getting unacceptable hum.

Are you in dummy mode when you're testing? If not, and you don't have a cabinet hooked up, your amp may try to push too much current through the lineout, which can cause distortion. Double-check your power resistor joints as well.

It's also possible that the distortion is coming from overdriving your SS amp input or audio interface. Line level is much higher than a guitar output.

[nonost]

So far no hum at all.

I'm in dummy load. The resistor are working fine, I've checked them as I turn up the volume/gain and they get hotter.

I'm going to remove temporarily the tone control and see how it goes. Like I said, the tone control spoils the sound very badly.

[nonost]

I'm back. I got rid of the trebly/harsh/metallic sound by adding a 10nf cap across the line out jack. I can't believe how well this works. I've removed the tone control so right now I only have the voltage divider (100k pot with 5.1k series resistor) along with the dummy resistors. I will add the tone control later on.

So far the sound it's pretty decent. I've done some tests adding some digital cabsims and I'm sure I can get great results.

Today I've added the Reactance Emulator without success. There's something wrong with it, it distorts very unpleasantly and it's way too bright. I've checked opamp voltages and they look ok... Anyway, I'm adding the Simple CabSim next and see how it goes.

[Blackboarcult]

I laid out the reactance emulator a couple of days ago, if it can be of any help, here it goes.
Should be verbatim from schematic.
I plan to verify it tomorrow IF I get the amazon delivery for my new breadboard & parts.

Cheers