Breaking in "guitar" electronics?

[Little Pigbacon]

Re: Breaking in "guitar" electronics?

Fair enough. "Inductive" here means that resistance is changing with frequency I assume, which is what a magnetic coil does. There is definitely a change in resistance here.

It is just that if the resistance changes with many Hz, I cannot think of a way to damage or otherwise change the output transformer. The reason why output transformer are damaged when run with no load is that they overheat and some of the winds melt. Changing the resistance pattern many times a second cannot build up heat.

Now, obviously you don't want to replace a 8 ohm impedance speaker with a D/C only resistor measuring 8 ohm D/C, because that will be a different resistance for A/C depending on frequency and the amp runs into the wrong resistance.

At the same time that illustrates why I am so skeptical about "artificial" loads damaging output transformers. The reason for me is that it is the speakers that constantly change resistance as you play. You play a full 1st fret F chord it has a different average resistance than if you play something high with the same volume. It is hard for me to imagine that the output transformer actually requires a changing resistance, and even if it does how do you ensure that your playing is the right pattern?

To me it looks far more likely that, if there is a difference at all beyond "no melting", that a constant resistance must be better.

I don't think the output transformer requires a changing impedance. (Changing with time, I mean.) Does it require an impedance that changes with frequency? Probably not, but I suspect it may require higher impedances at higher frequencies -- which is what you get with series inductance.

Here's a graph I found that maps out the impedance of a typical driver:

Source: TGP. http://www.thegearpage.net/board/showpost.php?p=12546970&postcount=15

The impedance changes in that peak area because of "back EMF", which means the magnetic field created by the current through the voicecoil is contributing to the impedance in that frequency range by inducing a current in the coil that opposes the primary current. But this is the overriding scenario only in the range of the resonant frequency. Much below the resonant range, the DC resistance of the wire takes over; much above, the simple series inductance takes over.

I think the key to a well-behaved, happy amplifier is to make sure that it doesn't see anything scary in terms of impedances at any frequency. Some amplifiers can overheat at higher frequencies due to oscillation, which was the case with a particular amplifier design I encountered where the amplifier was intended to have -3dB points of 0 Hz and 3 MHz. You had to be very careful with input signals and everything on the output side to make sure it didn't run away.

I try to treat guitar amps pretty much the same way, particularly tube amps, as their circuit topology usually implies current source, and you don't want an amp trying to shove specific amounts of current into the primary side of a transformer when the secondary is open. The impedance of the speaker load is reflected (and scaled) back to the power tubes through the output transformer, and without the speaker load, the primary is mainly just coil resistance (and series inductance). It's like when you plug in a power supply that's AC-to-AC into the mains, without the product connected to the power supply. You often blow up the transformer or, if you're lucky, just pop the protection fuse.

 

[uOpt]

Re: Breaking in "guitar" electronics?

But impedance is just the term for resistance that changes with frequency.

 

[Little Pigbacon]

Re: Breaking in "guitar" electronics?

lol

But what if I told you that impedance is a complex number with an imaginary component and a phase angle?

 

[uOpt]

Re: Breaking in "guitar" electronics?

lol

But what if I told you that impedance is a complex number with an imaginary component and a phase angle?

It is but that doesn't change how the changing resistance plays with the voltage and amps.

I just don't see how a changing resistance can be healthier for the output transformer than a constant one.

Look at your curve above. Whatever load the output transformer likes, let's say it is what is labeled at 400 (Hz presumably). Now you noodle a constant tone at the frequency where the speaker has the resonance peak. Resistance goes up and with constant power that means amps go down and voltage up compared to noodling the 400 Hz note.

I don't think it makes sense that this is somehow healthier to the output transformer than presenting the same resistance across the entire spectrum.

 

[Little Pigbacon]

Re: Breaking in "guitar" electronics?

I dunno, mang. I guess I'd like to hear from someone who has designed tube amps and analyzed their behavior extensively.

 

[koshikas]

Re: Breaking in "guitar" electronics?

guys i was recommended sort of hybrid solution by someone here, when i too was having my issues with a non-reactive load. The solution was to have a power resistors and a small speaker in parallel/series circuit. the speaker is not employed for sound, it is purely there for its reactive load from the voice coil. i could't sort out the impedance/resistance and wattage required for the speaker and power resistor and i abandoned the project. just thought of letting you guys know if you can figure it out.

 

[uOpt]

Re: Breaking in "guitar" electronics?

guys i was recommended sort of hybrid solution by someone here, when i too was having my issues with a non-reactive load. The solution was to have a power resistors and a small speaker in parallel/series circuit. the speaker is not employed for sound, it is purely there for its reactive load from the voice coil. i could't sort out the impedance/resistance and wattage required for the speaker and power resistor and i abandoned the project. just thought of letting you guys know if you can figure it out.

The way I understand it a large part of the varying (effective) resistance of the speaker coil is coming from the fact that it does move inside the magnetic field. A static coil will still have a capacitance and inductance which makes it react to A/C in interesting ways, but e.g. I don't think you will see that huge resonance peak in the chart posted earlier.

And even if all that is true, that still doesn't explain why varying frequency of any kind is supposed to be healthier in some way than just picking on constant resistance.

 

[Little Pigbacon]

Re: Breaking in "guitar" electronics?

It is but that doesn't change how the changing resistance plays with the voltage and amps.

Actually it does, and here's why:

It might be tempting to assume that, once your amplifier is operating at a certain frequency, seeing a certain impedance across the speaker terminals, that the speaker load behaves just like a resistor of whatever magnitude impedance we get from the speaker at that frequency. However, we still have to model the speaker load as a complex impedance -- a resistance in series with an inductance, at the very least.

With a simple resistance, the phase relationship between the current through the resistance and the voltage drop across it is as simple as it gets -- it's zero phase angle. This means the current and voltage are always lock-step in phase with one another, and that current through the resistor always means voltage drop across it (and vice verse), and power is always dissipated by the resistance. It never stores energy. (Again, we're talking about lumped-parameter approximation, ideal circuit elements, etc.) The instantaneous current through and the instantaneous voltage drop across are "happening" together.

An inductor stores energy, though. So does a capacitor. Again simplifying and assuming ideal circuit elements, an inductance or a capacitance in a circuit does not dissipate energy. Instead, they store and release energy as "stuff happens" in the circuit, each in their own way.

A capacitor resists instantaneous changes in the voltage across its terminals (plates, etc.). It stores energy in the electric field that results from the separation of electrical charge on one plate from the charge on the opposite plate. The voltage across the capacitor changes as charge moves from one plate to another, which is current. Forcing a current through the capacitor moves electrical charge and changes the voltage across it. But, again, it doesn't happen instantaneously -- at least, not usually. When we're dealing with sinusoidal signals, the voltage change across the capacitor lags behind the current through it. How much? 90 degrees -- 1/4 of a cycle at a given frequency.

An inductor does pretty much the same thing, except that it's the inverse of a capacitor. The inductor resists instantaneous changes in the current through it. If you're an amplifier, you put a voltage across it and let the current change as it will. And what the current will do is lag behind the voltage by 90 degrees -- 1/4 of a cycle at a given frequency.

When you throw in a resistance, in a simple series combination with an inductor (or capacitor), the same concepts apply -- except that the phase angle isn't -90 or +90 degrees anymore -- it's somewhere between zero and one of these endpoints. The equations start with simple trigonometry and lead to phasor math.

Here's some cool stuff to look at:

https://www.google.com/search?q=voltage+vs+current+inductor&source=lnms&tbm=isch&sa=X&ei=YxGiVPnOBcSLyATkuoCgAw&ved=0CAgQ_AUoAQ&biw=1280&bih=870#facrc=_&imgdii=_&imgrc=TBKI-WNBpCUCjM%253A%3BhcZgmuSgHj8ewM%3Bhttp%253A%252F%252Fmacao.communications.museum%252Fimages%252Fexhibits%252F2_4_4_5_eng.png%3Bhttp%253A%252F%252Fmacao.communications.museum%252Feng%252Fexhibition%252Fsecondfloor%252Fmoreinfo%252F2_4_4_PhaseShift.html%3B724%3B306

The consequences of this, for the purposes of our discussion? Inductances and capacitances aren't done behaving like inductances and capacitances after we arrive at a frequency-dependent magnitude impedance number along a curve. An audio amplifier -- guitar, bass, hi-fi, PA -- without a speaker load is an incomplete circuit. We don't use them in isolation, and I have a feeling they are usually not designed in isolation. I think someone designing an amp is probably thinking about the speaker, too, and about what's going on between the two, across a wide range of frequencies.

I'd love to hear from a successful designer of tube amps. What is she expecting to see across the speaker terminals, in order for her amplifier to be well behaved, and to what extent do those expectations inform the design?

 

[uOpt]

Re: Breaking in "guitar" electronics?

That still doesn't explain why a variable behavior for different frequencies should somehow be better for the output transformer.

I just don't see how you damage the output transformer unless with insufficient load.

 

[ImmortalSix]

Re: Breaking in "guitar" electronics?

All of us guitar guys are a tad, what us in the holler call, tetched..

Haven't heard that in a while! My dad says it - he's from South TN / North GA :laughing:

 

[koshikas]

Re: Breaking in "guitar" electronics?

guys I think you ought to take this discussion to a newer thread. I think original topic has nothing got to do with the importance of what you guys are discussing here.

I know this might sound so stupid, probably as much as 'breaking-in guitar electronics'. for purely a DI output, can a amplifier be engineered to skip output transformer altogether without losing the output tubes. on most DI outs built into most amps, the signal is taken right after the preamp skipping the power section. and the output transformer while appreciably has lots to do with the final tone, was employed primarily to convert the voltage based signal of the output tubes into current required for the speakers. so minus the speakers, we should be able to take out the output transformer too, and maybe substitute it with an inductor only. this way we will be able to retain power stage characteristics while eliminating the complexities of output load of transformers and how it should be (maybe not if the power tubes themselves are as fussy for their load)

I think there is enough organic circuitry in a tube amp even without a output transformer.

Sent from my XT1033 using Tapatalk

 

[Mr. B]

Re: Breaking in "guitar" electronics?

Maybe we need to be breaking in cases as well. You know; throw them down a few flights of stairs, spill some beer on them, scuff them on some rough pavement. If the guitar spends a lot of time in the case, the new case may be keeping the guitar from developing character by association.