Hi all:
I'm a guitar player, and I built a resistive load box in order to operate my 100W tube amp head through my computer (a DI box - store bought) without speakers attached (without frying my output transformer)
My goal was to avoid paying for an expensive load box, since I happened to have an 8 ohm 100 W and a spare 1/4" input jack.
I do understand that pure resistive dummy loads aren't the best option for tone and feel, but the sounds I'm getting are definitely good enough for what I'm doing.
It's been a while since I've been able to play this loud amp through proper speakers (I live in an apartment), so I haven't heard it played proper but I'm experiencing very poor headroom on the amp's clean channel. I mostly play rock music so distortion isn't the worst thing, but I'd like to have the option for clean sounds! I've replaced the amp's tubes and it helped a little bit but did not solve the problem.
My question is this:
Could my dummy load be what's causing the distortion/breakup? Or are resistors pretty passive in that way?
I'm trying to trouble shoot and start with the simplest things first, but I don't know enough about this topic.
Cheers!
I'm a guitar player, and I built a resistive load box in order to operate my 100W tube amp head through my computer (a DI box - store bought) without speakers attached (without frying my output transformer)
My goal was to avoid paying for an expensive load box, since I happened to have an 8 ohm 100 W and a spare 1/4" input jack.
I do understand that pure resistive dummy loads aren't the best option for tone and feel, but the sounds I'm getting are definitely good enough for what I'm doing.
It's been a while since I've been able to play this loud amp through proper speakers (I live in an apartment), so I haven't heard it played proper but I'm experiencing very poor headroom on the amp's clean channel. I mostly play rock music so distortion isn't the worst thing, but I'd like to have the option for clean sounds! I've replaced the amp's tubes and it helped a little bit but did not solve the problem.
My question is this:
Could my dummy load be what's causing the distortion/breakup? Or are resistors pretty passive in that way?
I'm trying to trouble shoot and start with the simplest things first, but I don't know enough about this topic.
Cheers!
Resistors are as good as it gets for being an easy load, however if there is a problem with an amplifier that occurs under load (delivering real current into a load) then they will certainly show that up.
I'm not quite sure what you are doing but you can not drive the 8 ohm load and a speaker at the same time as that would be to low a loading.
Also be aware that an 8 ohm load is just that, 8 ohms. A real speaker has impedance that varies with frequency and that is often much higher than 8 ohms at higher frequencies and so would load the amplifier less.
I'm not quite sure what you are doing but you can not drive the 8 ohm load and a speaker at the same time as that would be to low a loading.
Also be aware that an 8 ohm load is just that, 8 ohms. A real speaker has impedance that varies with frequency and that is often much higher than 8 ohms at higher frequencies and so would load the amplifier less.
If the resistor is about the same impedance as your speaker,
the amp should work at least as well with the resistor as the speaker.
Do both amplifier channels behave the same with the resistor?
the amp should work at least as well with the resistor as the speaker.
Do both amplifier channels behave the same with the resistor?
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If you are really worried about the distortion inducing qualities of WW resistors ( in general ) then use High Precision WW resistors of 5ppm used in temperature sensors , it depends too on how WW resistors are wound and the material used.
Cheers Mooly, thx for the response.
I'm just driving the resistor. It serves to replace the speaker as a load so that I can operate my amplifier head at optimal power into my computer to record without shaking the windows of my neighbours. I have a DI box that intercepts the signal between the amp and speaker to send to a computer... but studios often replace the speaker with expensive 'reactive load boxes' that mimic the impedance curves of speakers for optimal "performance and tones".
Do you think those higher frequencies that experience less impedance would hurt the transformer?
I guess normally people use these simple resistive dummy loads in test scenarios... with a controlled/single frequency input to troubleshoot.
I wonder... speakers are inductive loads as I understand it- maybe it makes more sense to look into getting an inductor with 8 ohm impedance as my simple dummy load?
I don't think for a moment it will hurt anything but the resistors will be giving you a different sort of 'sound' in the voltage you tap off to record.
You could certainly try making something up to simulate a crossover... talking of which this simulation shows the effect really well. You can see how the output dips at some lower frequencies where the load pulls more current, while at higher frequencies the output voltage comes back.
If you were recording the output of that set up it would be totally different to a linear resistive load which would have a flat response.
Post #5980 here:
Amp Camp Amp - ACA
You could certainly try making something up to simulate a crossover... talking of which this simulation shows the effect really well. You can see how the output dips at some lower frequencies where the load pulls more current, while at higher frequencies the output voltage comes back.
If you were recording the output of that set up it would be totally different to a linear resistive load which would have a flat response.
Post #5980 here:
Amp Camp Amp - ACA
Could you be overloading the computer input and/or the DI box , what is the DI box you are using .
Answering all of your questions:
1) resistors do not create distortion, it all comes from amp/preamp
2) your amp is properly loaded ... assuming it´s happy with an 8 ohm load.
3) tell-us-what-your-amp-is.
Brand and model.
I´m amazed by the continuous flow of questions by people asking about something and not offering basic information.
4) Surprise!!! Tube distortion is harsh.
And even "clean" channel already has some distortion if it´s a Rock amp.
Not usually annoying because what you hear is HEAVILY filtered out by a steep lowpass filter, killing everything above 2500 to 3500Hz at steepn 24dB/octave or so.
Usually known as "guitar speaker"
Check it yourself, here´s a popular and generic Eminence speaker:
Notice between 2200Hz and 10kHz you have dropped a whopping 36dB or so.
**attached below**
5) also Guitar amp output transformers don´t need to be extra wide band as in Hi Fi,often contribute some loss too.
6)
You are missing all the HEAVY EQ I mentioned above.
You need to build and add a "speaker simulation" EQ between the attenuated amp signal (you must pad it down to 1V RMS or lower to avoid destroying your DI box ... unless it already has a "Speaker level" option, with the attenuator built-in.)
Attenuate anyway, since the Speaker Emulator only handles 1V or less signals.
For an inspiration, have a hard look at popular "Red Box" DI or equivalent boxes do, or what Guitar Amps use as "compensated line out"
Notice STEEP filtering.
Those who master simulators might simulate it and post the frequency response.
EDIT: and set your clean channel volume to a lower level.
You don´t hear it because you are using a resistor, but I bet you are playing at eardrum busting levels 😉
EDIT2: no crossovers in Guitar cabinets.
1) resistors do not create distortion, it all comes from amp/preamp
2) your amp is properly loaded ... assuming it´s happy with an 8 ohm load.
3) tell-us-what-your-amp-is.
Brand and model.
I´m amazed by the continuous flow of questions by people asking about something and not offering basic information.
4) Surprise!!! Tube distortion is harsh.
And even "clean" channel already has some distortion if it´s a Rock amp.
Not usually annoying because what you hear is HEAVILY filtered out by a steep lowpass filter, killing everything above 2500 to 3500Hz at steepn 24dB/octave or so.
Usually known as "guitar speaker"
Check it yourself, here´s a popular and generic Eminence speaker:
Notice between 2200Hz and 10kHz you have dropped a whopping 36dB or so.
**attached below**
5) also Guitar amp output transformers don´t need to be extra wide band as in Hi Fi,often contribute some loss too.
6)
^^^^^ THAT.
You are missing all the HEAVY EQ I mentioned above.
You need to build and add a "speaker simulation" EQ between the attenuated amp signal (you must pad it down to 1V RMS or lower to avoid destroying your DI box ... unless it already has a "Speaker level" option, with the attenuator built-in.)
Attenuate anyway, since the Speaker Emulator only handles 1V or less signals.
For an inspiration, have a hard look at popular "Red Box" DI or equivalent boxes do, or what Guitar Amps use as "compensated line out"
Notice STEEP filtering.
Those who master simulators might simulate it and post the frequency response.
EDIT: and set your clean channel volume to a lower level.
You don´t hear it because you are using a resistor, but I bet you are playing at eardrum busting levels 😉
EDIT2: no crossovers in Guitar cabinets.
Attachments
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Reactive loads (mimicking loudspeaker impedance) are used (and often preferred as well) because their impedance characteristics alter frequency response of power amps with low damping factor. (These would be most amps aimed for musical instrument amplification). Basically a loudspeaker introduces an impedance peak at low frequency known as the resonant frequency and rising impedance at high frequencies. An amp with low damping factor will introduce higher voltage gain at these frequencies so basically its response will feature boost at lower and higher frequencies.
This is not apparent in all amps (i.e. amps with lots of NFB) and basically amps will feature such characteristic in varying degrees. Some not at all, some plenty.
Whether load is reactive or resistive, one still needs some kind of "cabinet emulator" to mimick the frequency response introduced by the speaker system (the distinct lo-pass filtering being the most prominent characteristic of it). Without that the signal will contain all high order harmonics of distortion -unattenuated- and the tone will be much harsher and fizzier than filtered tone (which we are more associated to hear in case of overdriven tones).
A loudspeaker is not only an inductive load. The motor also has electroacoustic properties that introduce capacitive reactance, namely at the aforementioned resonant frequency. Inductive characteristics start to matter more towards higher frequencies.
Also, reactive load is much harder for the amp than a purely resistive load! Inductance will aid harmful "flyback" voltage transients so potential for arcing increases, capacitive reactance increases potential for oscillation, and often the impedance may also dip below rated nominal impedance.
This is not apparent in all amps (i.e. amps with lots of NFB) and basically amps will feature such characteristic in varying degrees. Some not at all, some plenty.
Whether load is reactive or resistive, one still needs some kind of "cabinet emulator" to mimick the frequency response introduced by the speaker system (the distinct lo-pass filtering being the most prominent characteristic of it). Without that the signal will contain all high order harmonics of distortion -unattenuated- and the tone will be much harsher and fizzier than filtered tone (which we are more associated to hear in case of overdriven tones).
A loudspeaker is not only an inductive load. The motor also has electroacoustic properties that introduce capacitive reactance, namely at the aforementioned resonant frequency. Inductive characteristics start to matter more towards higher frequencies.
Also, reactive load is much harder for the amp than a purely resistive load! Inductance will aid harmful "flyback" voltage transients so potential for arcing increases, capacitive reactance increases potential for oscillation, and often the impedance may also dip below rated nominal impedance.
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I seem to recall some speaker simulators that are software based that you can run your signal through. I did a quick look, have not read anything here, just think it could be a starting point to search for something.
Top 7 Free Amp Sims: The best freeware virtual guitar amp plug-ins - gearnews.com
Top 7 Free Amp Sims: The best freeware virtual guitar amp plug-ins - gearnews.com
Teemuk thanks for all the info!
Can you explain why the amp experiences more voltage gain despite the increase in impedance? is it due to phase shifting/voltage lagging? I learned some basics about AC circuits and phasors in engineering school - but I'm overall pretty 'green' as we say here (new to circuit design/build). I'm surprised to hear about this voltage gain.
JMFahey thanks a lot!
my responses to yours below 🙂
1) Thanks a lot. This was the main purpose of this thread so I appreciate that. People's different responses though are opening so many different doors of info hahaha definitely in the rabbit hole
2) Yup, 8 ohms 🙂
3) Ceriatone OTS HRM 100. Its a dumble HRS clone
4) great graphic, thanks!
5) never thought before about a transformer's frequency response and affect on tone. very cool - makes sense.
6) I'm actually using the Redbox 5 - that's what I meant by DI, hahaha. Maybe DI isn't the correct term for what it is. Wasn't expecting to meet guitar people, so figured a more accurate description of it wasn't necessary.
my responses to yours below 🙂
1) Thanks a lot. This was the main purpose of this thread so I appreciate that. People's different responses though are opening so many different doors of info hahaha definitely in the rabbit hole
2) Yup, 8 ohms 🙂
3) Ceriatone OTS HRM 100. Its a dumble HRS clone
4) great graphic, thanks!
5) never thought before about a transformer's frequency response and affect on tone. very cool - makes sense.
6) I'm actually using the Redbox 5 - that's what I meant by DI, hahaha. Maybe DI isn't the correct term for what it is. Wasn't expecting to meet guitar people, so figured a more accurate description of it wasn't necessary.
The easiest way to see it is compare normal 8 ohm operation which produces a flat frequency response to an open circuit secondary. Say the tubes pump 8 Volt-Amps (VA) into the transformer primary. The secondary produces 8V at 1 A across the 8 ohm load. But what happens when the secondary goes open? The tubes already dumped 8 VA into the magnetic circuit as magnetic flux. Since the secondary infinite resistance no current flows but the same magnetic flux is causing a voltage to be developed on the output. The transformer tries to produce the same VA's on the output, since no current flows it jacks up the voltage.
It is similar to a standard transmission on a car, in gear and running you hit a patch of ice and the engine revs higher and the wheel spins faster. Now back to the 8 ohm load and 8 VA. If the speaker has a resonance that goes up to 16 ohms then the voltage goes up to 16 V and the amperage goes down to 0.5 A 32 ohms and the voltage goes up to 32 V and the amps down to 0.25 A. (I like easy numbers in examples 🙂 )
Now your amp has NFB and it reduces the apparent output impedance of the amplifier. Some amps do not have NFB and the effect is more pronounced. This is part of the reason some speakers sound good with some amps and not others where they would sound bright and boomy. Or on the flip sid sound dull. Then there is the case of an amp that has NFB and reduces the effect of the speaker impedance but is driven into clipping which kills the NFB. All of the sudden the speaker sounds brighter and flubbier. Or you have the amp voiced to sound good in overdrive but it sounds lifeless when not with the speaker.
A whole lot of fun.
The secondary can be viewed as the load being driven by an ideal voltage source and a source resistance in series driving a load. With the load resistance low (8 ohms) there is some of the voltage source's voltage dropped across its internal resistance. With the impedance peak there is less voltage dropped across the voltage source's resistor and more developed across the load. We se the same thing with a loaded and unloaded power transformer. But the power transformer is being driven by a low impedance source, which is not what a pentode is. Because it is high impedance the impedance of the secondary is higher, the same amp run with the output tubes run in triode has a lower impedance as triode operation has a lower plate impedance. Another reason amp's run in triode may sound different into speakers that have the rising impedance.
Am I beating this to death?
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