Simple load box to attenuate speaker signal

[Jacew]

I have been tinkering with my Mustang recently. What I'm planning to do with it would be a sort effects box in front of tube amp. I already fitted it with jack to modify headphone out into line out. It doesn't work that well as it has some kind of speaker emulation (I think) and level is too low.

So I'm planning to build a simple load box (two 33ohm 50w resistors parallel + 5w resistor in front of output), so I could use speaker output.

How do I calculate the value of resistor for output? If anyone has a rough idea about that value, I would be interested to hear, what I need is just a starting point where I could start to iterate the value.

The schematics in web seem mostly to be for resistive loads or attenuators, which isn't what I need. It should be as transparent as possible, which seems exactly opposite of what general load boxes do (that try to emulate speaker.)

 

[Jacew]

Re: Simple load box to attenuate speaker signal

I'm answering my own post as I seem to have overlooked one thing earlier:

I need resistor either side of output: so I would have two 33 ohm resistors in series with output between. That would put 66 ohms to speaker out, shouldn't be aproblem for transistor amp. But what value resistor I need for line out? I don't think that 33 ohms is enough?

 

[DankStar]

Re: Simple load box to attenuate speaker signal

I'd use a DI box that could handle a speaker load

 

[Surgeon]

Re: Simple load box to attenuate speaker signal

Just buy an L-pad if you want attenuation. Basically a high-wattage pot. Plenty of implementation schematics floating around.

 

[Jacew]

Re: Simple load box to attenuate speaker signal

does that L-pad work between speaker output and amp in put?

DI-boxes I think are generally designed to push line level signal into recording devices etc. in other words, they emulate speaker, which is not what I'm after.

Anyway. I'm not going to buy something that I already have parts for and takes hour to build at max...

 

[Surgeon]

Re: Simple load box to attenuate speaker signal

does that L-pad work between speaker output and amp in put?

DI-boxes I think are generally designed to push line level signal into recording devices etc. in other words, they emulate speaker, which is not what I'm after.

Anyway. I'm not going to buy something that I already have parts for and takes hour to build at max...

Yes, seriously just research L-pad... I'm on my ipod otherwise I'd post a link in 12 seconds flat.

 

[Jacew]

Re: Simple load box to attenuate speaker signal

Yes, seriously just research L-pad... I'm on my ipod otherwise I'd post a link in 12 seconds flat.

Well, I already built it:

It's just two 33ohms 50w resistors parallel in load box input, with output jack parallel to this behind 22k 1w resistors. Works perfectly.

Output level of Mustangs speaker out through attenuator is about the same with guitar-straight-to-amp with master volume in Mustang at 3,5. Still low enough to keep digital hiss in check (It hisses way too much to my comfort about with master at 6 or so).

Sound quality is actually much better than I anticipated. Tube amp as final stage colorizing the models works brilliantly. Really sounds much better than Mustang on it's own. I thought it to be useful for clean and delay or modulation effects, but actually there's some really neat distortion sounds as well. Not something I would call "great sounding tone", but definitely useful.

Is there a way to add "safeguard" to this to prevent frying the tube amp if level gets too high through the pad? Like a fuse?

 

[crusty philtrum]

Re: Simple load box to attenuate speaker signal

Is there a way to add "safeguard" to this to prevent frying the tube amp if level gets too high through the pad? Like a fuse?

I'd say it's much more likely that you'll fry the Mustang before anything happens to the tube amp.

 

[Jacew]

Re: Simple load box to attenuate speaker signal

I'd say it's much more likely that you'll fry the Mustang before anything happens to the tube amp.

How? It has solid 16 ohms of load on output, Tube amp input behind 22k resistor is not going to affect the Mustangs power amp in any way, so only thing that I think could go wrong is the output signal overloading the input of tube amp.

 

[crusty philtrum]

Re: Simple load box to attenuate speaker signal

How? It has solid 16 ohms of load on output,

Solid ? It will only be 16 Ohms at 1,000 Hz .... 1K. Impedance is AC and frequency-dependant. Simply using resistors doesn't address the impedance factor that a speaker coil will have, or the fact that the impedance is changing as you play. If it works and sounds good, by all means go for it, but be aware that you may not be doing the Mustang's output any favours, and it may not endure the conditions over time. Maybe it will. Time will tell.

 

[Jacew]

Re: Simple load box to attenuate speaker signal

Solid ? It will only be 16 Ohms at 1,000 Hz .... 1K. Impedance is AC and frequency-dependant. Simply using resistors doesn't address the impedance factor that a speaker coil will have, or the fact that the impedance is changing as you play. If it works and sounds good, by all means go for it, but be aware that you may not be doing the Mustang's output any favours, and it may not endure the conditions over time. Maybe it will. Time will tell.

I thought standard resistor puts its resistance across the spectrum? How analog AC circuits can even be built stable if the impedance of resistors varies that much with frequency?

I'm pretty sure that Mustang is good for at least 8 ohms (maybe even 4 ohms...). And it's always safe to run higher impedance with transistor amp. They are very solid as far as you don't push them clipping or run output with too low impedance.

 

[crusty philtrum]

Re: Simple load box to attenuate speaker signal

I thought standard resistor puts its resistance across the spectrum?

The output of an amp wants to see impedance, not resistance. Impedance is measured in Ohms. A speaker with a rated impedance of, for example, 8 Ohms, will read somewhere in the high five/ low six Ohm range when measured with a multimeter because the meter will be reading DC resistance, not impedance.

But like i said, if it works and sounds good and you are happy, go right ahead. It will either survive or it won't. Personally i think padding a speaker output down to instrument level is too much and will have noticeably negative effects on the quality of the sound as well as putting the output stage at risk. It's not something I'd do without some kind of inductive load. But then again, it's not my amp to try to protect.

 

[Jacew]

Re: Simple load box to attenuate speaker signal

The output of an amp wants to see impedance, not resistance. Impedance is measured in Ohms. A speaker with a rated impedance of, for example, 8 Ohms, will read somewhere in the high five/ low six Ohm range when measured with a multimeter because the meter will be reading DC resistance, not impedance.

I know and that was what I meant, just mispelled impedance as resistance. If the resistor impedanced varies, and so does the speaker impedance, how do they differ in practice? Of course the physical resistance of air has effect on speaker impedance, but the actual difference can't be anything meaningful. By logic I wouldn't think that resistors impedance could vary much with frequency. Otherwise almost every AC resistor circuit would be on a constant risk of shorting out wouldn't it?

I'm not arguing, just interested about it, and would like to understand what you mean...

But like i said, if it works and sounds good and you are happy, go right ahead. It will either survive or it won't. Personally i think padding a speaker output down to instrument level is too much and will have noticeably negative effects on the quality of the sound as well as putting the output stage at risk. It's not something I'd do without some kind of inductive load. But then again, it's not my amp to try to protect.

Well, the "negative" effect of sound is that you don't get the amp voicing through. Sounds hideous for recording or PA. But it's perfect if you want to use it just for preamp effects like I do with this. It doesn't color the tone almost at all.

I'm not worried about frying that Mustang, They have absolutely no value at all, and it isn't good, or loud enough, that I would have other use for it anyway.

 

[crusty philtrum]

Re: Simple load box to attenuate speaker signal

I know and that was what I meant, just mispelled impedance as resistance. If the resistor impedanced varies,

Resistors don't have impedance, they have resistance. The two are different things, although both are measured in Ohms. A speaker has a coil of wire and thus inductance comes into play. Resistance is not frequency dependant, impedance is.

Don't worry about it, just play guitar. The Mustang will survive or it will die. Just play the guitar.

 

[Jacew]

Re: Simple load box to attenuate speaker signal

Resistors don't have impedance, they have resistance. The two are different things, although both are measured in Ohms. A speaker has a coil of wire and thus inductance comes into play. Resistance is not frequency dependant, impedance is

?? Are saying resistors doesn't have effect on AC? They are constantly used in AC circuits to direct and attenuate signals, so that seems just blatantly wrong. And I don't think inductance has anything to do with this: As long as power amp has proper load, it should work to my knowledge.

Care to explain?

 

[crusty philtrum]

Re: Simple load box to attenuate speaker signal

And I don't think inductance has anything to do with this: As long as power amp has proper load, it should work to my knowledge.

Well then you go with what you think you know and ignore me, that's fine. I'll be happy just to know you are playing your guitar.

 

[Jacew]

Re: Simple load box to attenuate speaker signal

Well then you go with what you think you know and ignore me, that's fine. I'll be happy just to know you are playing your guitar.

Yes I do because you don't provide any proof for any of your claims. You just avert the questions and act condescendingly, like you're trying to scare people away from experimenting. Or you just don't know and try to act smart.

If other things than impedance matters in anyway for an amp, why is impedance all we need to know to safely connect speaker to power amp? It affects the way it sounds of course, but it won't break down as long as it has a proper load. If this is not true, prove it.

I'm confronting you here simply because someone, who might be reading this in the future, sees your "credentials" and decides, "oh, that guy must know", and shys away from trying something like this. Despite the fact it's cheap, simple, and it works very well.

 

[crusty philtrum]

Re: Simple load box to attenuate speaker signal

Yes I do because you don't provide any proof for any of your claims. You just avert the questions and act condescendingly, like you're trying to scare people away from experimenting. Or you just don't know and try to act smart.

If other things than impedance matters in anyway for an amp, why is impedance all we need to know to safely connect speaker to power amp? It affects the way it sounds of course, but it won't break down as long as it has a proper load. If this is not true, prove it.

I'm confronting you here simply because someone, who might be reading this in the future, sees your "credentials" and decides, "oh, that guy must know", and shys away from trying something like this. Despite the fact it's cheap, simple, and it works very well.

I have been playing and messing with electronics for 45 years. I started building tube amps almost 25 years ago. And i'm no part-timer, these things are ALL I've done across all those years. There are threads here about builds and restorations I've done. There are forum members who own stuff I've built or have worked on for them, stuff that they love that and has never faltered.

Your arrangement is working, you are happy and not too worried even if the Mustang pooches out, so you're all good, just play. The output stage of the Mustang may or may not hold up over time, that remains to be seen.

 

[Jacew]

Re: Simple load box to attenuate speaker signal

I have been playing and messing with electronics for 45 years. I started building tube amps almost 25 years ago. And i'm no part-timer, these things are ALL I've done across all those years. There are threads here about builds and restorations I've done. There are forum members who own stuff I've built or have worked on for them, stuff that they love that and has never faltered.

Then why it's so hard for you to answer a simple question?

1) If your claim that resistors ohm rating can't be used to calculate their values for AC circuit is true, how correct value of resistors should be calculated?

2) If there's more in speakers than just impedance that affects the stability of power amp, why it's safe to run whatever speakers with whatever amp as long as impedance matches?

I have provided both, theoretical and practical evidence here that resistor in place of a speakers works as a safe and simple attenuator. If you can't answer to those two questions, it's same to just ignore your claims as a personal opinion or purposeful deception.

(Don't worry, I'm not trying to take your job )

 

[crusty philtrum]

Re: Simple load box to attenuate speaker signal

(Don't worry, I'm not trying to take your job )

Hehe, I don't worry, you wouldn't have a hope. As you seem keen to make evident.

1) I never claimed that

2)Inductance, for one thing.