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Thread: Attenuators between a tube amp and the guitar speaker: some measurements and theory

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    Attenuators between a tube amp and the guitar speaker: some measurements and theory

    One important factor is the load the attenuator places on the amp: there is agreement that it should be like that of a guitar speaker, and we will discuss that more later, but first let's look at the impedance of a couple of speakers. First is one of my favorite speakers, the no longer produced Eminence Red Fang, an Alnico speaker. The most frequently shown plot is the magnitude of the impedance versus frequency. I have made some measurements with the speaker in a Line 6 open back cabinet, shown in the first attachment.


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    The measurement technique is very much like what I use for guitar pickups: A resistor its placed between one speaker terminal and ground, and the other terminal its driven with a voltage. This voltage to ground is measured as is the voltage across the resistor. The instrument is an inexpensive recording interface attached to a computer. The waveform used is a Golay complementary sequence. Cross spectral analysis is performed.

    The most noticeable feature is the low frequency resonance; the magnitude I measured is lower than the one published by Eminence. This is probably a function of damping introduced by the open back cabinet. The next most noticeable feature is the rise with frequency. One could look at this and might be confident that this is a result only of the inductance of gate speaker coil.

    However, there is more to it than that. The real (the apparent resistance as a function of frequency) and imaginary (the apparent inductance as a function of frequency) parts are shown in the next attachment.


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    Now we see that the rise in impedance is a result of both inductance and increasing resistance. The latter is probably a result of eddy current losses in the metal parts.

    The next question is what are all those little (maybe not so important) features? Are they the result of the measurements technique, including the cabinet, or are they a function of the driver? So here are the results for another speaker, a Celestion GP12-80. I wanted to measure a Celestial Blue, but it turns out that it did not survive Maria).

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    The overall impedance is very similar; some of the small bumps are similar, some are not. Thus we have a mixture of measurement and speaker effects.

    When I get to the next post, it will be about why we need to load the amp and what we need to do th get a scaled copy of the voltage across the load.

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    I use a speaker attenuator every time I gig. I use a Weber with the fancy speaker motor and stuff. I have the footswitch option so I can turn it off and on at will. Its great for bringing lead volumes up with the amp saturated. It has some treble cut/boost switches on it I rarely use. It does depend on the cab. AND Im not squashing the output.. just taking the edge off. Honestly though, as I mentioned in another post, for my last 20 watt build I simply added a $12 100 watt L-Pad from all parts on the back. I really dont hear a huge difference in tone when used in that manor. Everyone thinks its magic in the little studio where I use it.

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    Quote Originally Posted by olddawg View Post
    I use a speaker attenuator every time I gig. I use a Weber with the fancy speaker motor and stuff. I have the footswitch option so I can turn it off and on at will. It’s great for bringing lead volumes up with the amp saturated. It has some treble cut/boost switches on it I rarely use. It does depend on the cab. AND I’m not squashing the output.. just taking the “edge off”. Honestly though, as I mentioned in another post, for my last 20 watt build I simply added a $12 100 watt L-Pad from all parts on the back. I really don’t hear a huge difference in tone when used in that manor. Everyone thinks it’s magic in the little studio where I use it.
    Yes, I remember that post, and I think that it is important to find out how simple something can be and still work well, just as it is important to gain a better understanding of how things work by exploring more complicated designs.

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    A "thought experiment" that can be tested

    Suppose we haver a perfect device that can sample the voltage across a guitar speaker without affecting it and reproduce it exactly into any load at any power level. Connect an identical guitar speaker to the output. Would this second speaker sound just like the one one that is sampled? I think it must. Add a volume control to this device and you can scale the output while maintaining the sound as close as possible to the original given that the speaker is somewhat power sensitive as is human hearing.

    Now replace the speaker that is being sampled with a silent device that has the same impedance, and therefore, the same voltage across it. Now "the device" and the speaker connected to it are a very good attenuator or booster. Off course, a good ss amp approaches this ideal device very closely. Given that electric guitar is not exactly audiophile, the ss amp might not have to be the very best. A light, inexpensive switching amp might be good enough.

    Unlike many ideas, this one is double blind testable. You can can have someone else switch between the two, or not, and see what you can hear without knowing which is which ahead of time.

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    First of all, thanks for starting this thread. I actually planned something similar.

    Regarding speaker impedance, I think there are different effects to be considered.

    First there is the "print-through" of speaker impedance on the frequency response of the amp-speaker system. This depends mainly on the source impedance of the tube amp. A higher inner impedance will produce more print-through.
    The amp output impedance as seen by the speaker depends strongly on its NFB ratio, but also on tube type and operating conditions. Some measured values (@1kHz) relating to 8 Ohm outputs:

    JTM 45: 2 Ohm
    Super Reverb: 18 Ohm
    AC 30: 70 Ohm
    Tweed Deluxe: 80 Ohm
    Marshall 18W: 90 Ohm

    It seems that all the speaker needs to reproduce a corresponding frequency response would be a low impedance SS amp with a suitable resistor in series with the speaker. This also takes care of the looser damping of speaker ringing at tube amps.


    But the above only holds for linear operation of the tube amp's output stage. When overdriven output impedance changes and I assume that the frequency dependance of the loadline caused by the varying speaker impedance plays a major role regarding quality of sound/distortion.

    And this is why reactive dummy loads make sense.

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    And what of kickback voltages from an actual speaker that affect the reaction/response of the amp. Without an actual moving transducer that wants to recover to a fixed position this property is lost. How important it is could be brought into question. But it may be relative to the thread topic.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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    Quote Originally Posted by Mike Sulzer View Post
    ...
    Now we see that the rise in impedance is a result of both inductance and increasing resistance. The latter is probably a result of eddy current losses in the metal parts.
    ...
    Could the increasing resistance also be partly due to skin effect and proximity effect?

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    Quote Originally Posted by Mike Sulzer View Post
    Now replace the speaker that is being sampled with a silent device that has the same impedance, and therefore, the same voltage across it. Now "the device" and the speaker connected to it are a very good attenuator or booster. Off course, a good ss amp approaches this ideal device very closely. Given that electric guitar is not exactly audiophile, the ss amp might not have to be the very best. A light, inexpensive switching amp might be good enough.

    Unlike many ideas, this one is double blind testable. You can can have someone else switch between the two, or not, and see what you can hear without knowing which is which ahead of time.
    I think this is the basis for the Ho, or Ultimate attenuator. Except that design uses a simple load resistor rather than an active load. IIRC a 32 ohm resistor is used for all amps regardless of impedance setting. The premise being that although a speaker may be rated at a lower impedance, it's actual impedance across the spectrum is much higher when averaged.

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    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    And thank you for your reply. This is the next topic! I agree that the impedance with the overdriven output stage changes from linear operation, and since the major purpose of this type of attenuator is to allow the generation of output stage distortion at reduced sound levels, we should get the load right. But I would like to add that the transition between two tubes and one tube conducting, that is, when the A in AB moves to B, there also is an output impedance change.

    The output impedance is an instantaneous quantity that depends on output voltage and current; that is, energy storage matters.

    Quote Originally Posted by Helmholtz View Post
    First of all, thanks for starting this thread. I actually planned something similar.

    Regarding speaker impedance, I think there are different effects to be considered.

    First there is the "print-through" of speaker impedance on the frequency response of the amp-speaker system. This depends mainly on the source impedance of the tube amp. A higher inner impedance will produce more print-through.
    The amp output impedance as seen by the speaker depends strongly on its NFB ratio, but also on tube type and operating conditions. Some measured values (@1kHz) relating to 8 Ohm outputs:

    JTM 45: 2 Ohm
    Super Reverb: 18 Ohm
    AC 30: 70 Ohm
    Tweed Deluxe: 80 Ohm
    Marshall 18W: 90 Ohm

    It seems that all the speaker needs to reproduce a corresponding frequency response would be a low impedance SS amp with a suitable resistor in series with the speaker. This also takes care of the looser damping of speaker ringing at tube amps.


    But the above only holds for linear operation of the tube amp's output stage. When overdriven output impedance changes and I assume that the frequency dependance of the loadline caused by the varying speaker impedance plays a major role regarding quality of sound/distortion.

    And this is why reactive dummy loads make sense.

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    Quote Originally Posted by Chuck H View Post
    And what of kickback voltages from an actual speaker that affect the reaction/response of the amp. Without an actual moving transducer that wants to recover to a fixed position this property is lost. How important it is could be brought into question. But it may be relative to the thread topic.
    I think that is included in the impedance, in the imaginary or energy storage part, to the extent that the speaker behaves as a linear device.

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    Quote Originally Posted by Malcolm Irving View Post
    Could the increasing resistance also be partly due to skin effect and proximity effect?
    I had not thought of that; does anyone know the wire size used in a speaker coil?

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    Quote Originally Posted by Chuck H View Post
    I think this is the basis for the Ho, or Ultimate attenuator. Except that design uses a simple load resistor rather than an active load. IIRC a 32 ohm resistor is used for all amps regardless of impedance setting. The premise being that although a speaker may be rated at a lower impedance, it's actual impedance across the spectrum is much higher when averaged.
    http://www.ultimateattenuator.com

    This says it uses a reactive load, meaning, I think, an inductor. I have no idea why it does not need an impedance switch; that does not seem right.

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    Quote Originally Posted by Mike Sulzer View Post
    I had not thought of that; does anyone know the wire size used in a speaker coil?
    I guess JMF would know.
    I can do something else: I have a recone kit for a Celestion blue and can measure the series AC resistance at 1kHz and 10kHz of the voice coil alone.
    According to Zollner the increased loss resistance as well as the decreasing apparent inductance at higher frequencies are caused by eddy currents in the conductive ferromagnetic parts.

    Edit: Just found a value of 0.22mm diameter in the literature (Zollner).

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    Ah. That might be a new (ish) thing. The reactive load. I know I read somewhere that the Ultimate was a copy of the Ho and that they both used a plain ol' 32 ohm resistor. But I can't fact check it because I didn't document the info.

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    Quote Originally Posted by Helmholtz View Post
    I guess JMF would know.
    I can do something else: I have a recone kit for a Celestion blue and can measure the series AC resistance at 1kHz and 10kHz of the voice coil alone.
    According to Zollner the increased loss resistance as well as the decreasing apparent inductance at higher frequencies are caused by eddy currents in the conductive ferromagnetic parts.

    Edit: Just found a value of 0.22mm diameter in the literature (Zollner).
    The skin depth is about .45 mm for copper at 20 KHz. So it does not explain how we get from 6.5 ohms at 3 KHz to about 30 at 20KHz. Must be mostly eddy current losses.

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    Quote Originally Posted by Mike Sulzer View Post
    Suppose we haver a perfect device that can sample the voltage across a guitar speaker without affecting it and reproduce it exactly into any load at any power level. Connect an identical guitar speaker to the output. Would this second speaker sound just like the one one that is sampled? I think it must. Add a volume control to this device and you can scale the output while maintaining the sound as close as possible to the original given that the speaker is somewhat power sensitive as is human hearing.

    Now replace the speaker that is being sampled with a silent device that has the same impedance, and therefore, the same voltage across it. Now "the device" and the speaker connected to it are a very good attenuator or booster. Off course, a good ss amp approaches this ideal device very closely. Given that electric guitar is not exactly audiophile, the ss amp might not have to be the very best. A light, inexpensive switching amp might be good enough.

    Unlike many ideas, this one is double blind testable. You can can have someone else switch between the two, or not, and see what you can hear without knowing which is which ahead of time.
    This has similarities to the trans-impedance power amplifier used in some VOX's and the Fender Recording amp. The difference is the TI amp reflects the impedance of the actual speaker back the the driver amplifier's output terminals.

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    Quote Originally Posted by Helmholtz View Post
    ...
    According to Zollner ….
    ... in the literature (Zollner).
    Sorry to go slightly off-thread, but the book by Zollner does seem to be very useful and interesting. I found a rough English translation on the web, but is there any plan to publish a full English version?

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    Quote Originally Posted by Mike Sulzer View Post
    Suppose we haver a perfect device that can sample the voltage across a guitar speaker without affecting it and reproduce it exactly into any load at any power level. Connect an identical guitar speaker to the output. Would this second speaker sound just like the one one that is sampled? I think it must. Add a volume control to this device and you can scale the output while maintaining the sound as close as possible to the original given that the speaker is somewhat power sensitive as is human hearing.

    Now replace the speaker that is being sampled with a silent device that has the same impedance, and therefore, the same voltage across it. Now "the device" and the speaker connected to it are a very good attenuator or booster. Off course, a good ss amp approaches this ideal device very closely. Given that electric guitar is not exactly audiophile, the ss amp might not have to be the very best. A light, inexpensive switching amp might be good enough.

    Unlike many ideas, this one is double blind testable. You can can have someone else switch between the two, or not, and see what you can hear without knowing which is which ahead of time.
    I think this idea was tested pretty well around 1973-74:



    For the guitars & bass anyway, each column of speakers powered by pretty good hi fi amps for the times, McIntosh MC2300's. Driven from Twin Reverb Fenders. Yeah, I'd say "that works!" I'm sure the crowds agreed. Those who like it, like it a lot. Those who don't, well what can ya say...

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    Quote Originally Posted by nickb View Post
    This has similarities to the trans-impedance power amplifier used in some VOX's and the Fender Recording amp. The difference is the TI amp reflects the impedance of the actual speaker back the the driver amplifier's output terminals.
    What I am thinking of here is two simple pieces:

    1. A perfect voltage follower, or rather a follower with gain.

    2. A silent device that has the same impedance as a speaker.

    The first is what a good ss amp strives to be, The second has not yet been discussed here, but I think a passive device should do the job. Chuck H has sone posts with some pretty nice components, including a hum canceling inductor, and so I was thinking of building on that. For example, it appears that the inductor needs some eddy current losses. Could one use a short stack of really large steel washers as a poor's man toroid? I think I will try that and see how it works.

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    A very informative thread on Aiken's reactive dummy load:

    https://www.thegearpage.net/board/in...-load.1072793/

    Speaking of reactive attenuators one must also take into consideration that hooking up a speaker to a reactive load will boost the reactive effect (if not properly designed).

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    Quote Originally Posted by Leo_Gnardo View Post
    I think this idea was tested pretty well around 1973-74:



    For the guitars & bass anyway, each column of speakers powered by pretty good hi fi amps for the times, McIntosh MC2300's. Driven from Twin Reverb Fenders. Yeah, I'd say "that works!" I'm sure the crowds agreed. Those who like it, like it a lot. Those who don't, well what can ya say...
    That takes care of the boost! Not so sure about the attenuate.

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    Could one use a short stack of really large steel washers as a poor's man toroid? I think I will try that and see how it works.
    Why not? Problem: Undefined resistance between the washers. Eddy currents will want to travel around the cross section.
    Even the aluminum mounting/separating plate in Chuck's design helps somewhat.
    But why not use a solid steel rod as core in a 1mH choke?

    I use a modified Marshall PowerBrake PB 100 with 1mH//33 Ohm but plan to split the choke.
    I measured my 60s VOX alnicos and found average series inductance values of 0.7mH@1kHz and 0.4mH@10kHz.

    Zollner shows a design with 0.5mH//6R in series with 0.33mH//60R. The bass resonance is realized with 12mH//250.

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    Quote Originally Posted by Helmholtz View Post
    But why not use a solid steel rod as core for a 1mH choke?
    I figure that's a good idea. If you're trying to emulate a real speaker voice coil, they have steel magnet poles both inside & outside the coil. Which is bound to have some inductance magnifying effect over a voice coil type inductor without any iron/steel nearby.

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    Quote Originally Posted by Leo_Gnardo View Post
    I figure that's a good idea. If you're trying to emulate a real speaker voice coil, they have steel magnet poles both inside & outside the coil. Which is bound to have some inductance magnifying effect over a voice coil type inductor without any iron/steel nearby.
    Of course I meant winding it to around 1mH with core or start with lower value air core coil.

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    Quote Originally Posted by Mike Sulzer View Post
    What I am thinking of here is two simple pieces:

    1. A perfect voltage follower, or rather a follower with gain.

    2. A silent device that has the same impedance as a speaker.
    Exactly that^^ would have been one of my proposals too. BTW, when I use the term "reactive dummy load", I always mean "A silent device that has the same impedance as a speaker". The HF choke doesn't necessarily need to have Eddy current losses. The upper frequency response of a speaker can be modelled/fitted very well with 2 chokes and parallel damping resistors. The bass resonance requires an additional parallel resonant circuit wired in series with the DCR and the HF choke(s).

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    Quote Originally Posted by Mike Sulzer View Post
    Suppose we haver a perfect device that can sample the voltage across a guitar speaker without affecting it and reproduce it exactly into any load at any power level. Connect an identical guitar speaker to the output. Would this second speaker sound just like the one one that is sampled? I think it must. Add a volume control to this device and you can scale the output while maintaining the sound as close as possible to the original given that the speaker is somewhat power sensitive as is human hearing.

    Now replace the speaker that is being sampled with a silent device that has the same impedance, and therefore, the same voltage across it.
    In addition to having an impedance, the real speaker generates a back e.m.f.
    Is it necessary for the 'silent device' to also generate this back e.m.f.?

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    Quote Originally Posted by Malcolm Irving View Post
    Sorry to go slightly off-thread, but the book by Zollner does seem to be very useful and interesting. I found a rough English translation on the web, but is there any plan to publish a full English version?
    Yes, Prof. Zollner's book is highly valuable (to me) and the most comprehensive opus on electric guitars and equipment I know. The german version comprises more than 1200 pages in 2 volumes (I payed ca. 150€ for both some years ago). I think they call it POTEG= Physics of The Electric Guitar in English. Meanwhile he has published a third volume covering electroacoustics for stage and studio (about 600 pages).
    Zollner is the mastermind of GITEC. AFAIK they started English translation activities about 2 years ago. You may find out about the progress contacting them via their webpage:
    https://gitec-forum.de/wp/en/gitec-c...tars-and-gear/

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    Quote Originally Posted by Malcolm Irving View Post
    In addition to having an impedance, the real speaker generates a back e.m.f.
    Is it necessary for the 'silent device' to also generate this back e.m.f.?
    Yes, but this reflects in the complex impedance as this determines the interaction and relation between current and voltage. If the reactive dummy load shows the same impedance, it should be fine. The real speaker stores and releases motional energy and re-converts it into electrical energy while the reactive load directly stores and releases electrical energy by its reactive components.

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    Last edited by Helmholtz; 11-14-2018 at 10:58 PM.
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    Quote Originally Posted by Malcolm Irving View Post
    In addition to having an impedance, the real speaker generates a back e.m.f.
    Is it necessary for the 'silent device' to also generate this back e.m.f.?
    As Helmholtz said.... But let's also look at it from a bit different viewpoint. Do the following for a frequency, and then generalize it for a function of frequency in the audio range:

    1. Apply a sinusoidal voltage. It has an amplitude and the locations of its zero crossing can serve a phase reference.

    2. Measure the resulting current, both amplitude and phase relationship to the driving voltage. (We assume that a single sine wave is produced; this is a result of linearity.)

    3. Then ask this question: is there any other property that the response could have?

    The answer is no. The ratio of amplitudes and difference of the phases is all the information there is because a sinusoid is completely determined by its amplitude and phase.

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    Quote Originally Posted by Helmholtz View Post
    Why not? Problem: Undefined resistance between the washers. Eddy currents will want to travel around the cross section.
    Even the aluminum mounting/separating plate in Chuck's design helps somewhat.
    But why not use a solid steel rod as core in a 1mH choke?

    I use a modified Marshall PowerBrake PB 100 with 1mH//33 Ohm but plan to split the choke.
    I measured my 60s VOX alnicos and found average series inductance values of 0.7mH@1kHz and 0.4mH@10kHz.

    Zollner shows a design with 0.5mH//6R in series with 0.33mH//60R. The bass resonance is realized with 12mH//250.
    One can spray the washers with lacquer to insulate them, choosing washers with the right thickness. All I found in the garage was a nut that fits on a 3/8" rod. The inductance drops a factor of three between 120 and 1 000 Hz. That is too much.

    I kind of like the "toroid" because the flux is confined and the complete magnetic path gives more H. But the rod could be more practical.

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    Quote Originally Posted by Helmholtz View Post
    Yes, Prof. Zollner's book is highly valuable (to me) and the most comprehensive opus on electric guitars and equipment I know. The german version comprises more than 1200 pages in 2 volumes (I payed ca. 150€ for both some years ago). I think they call it POTEG= Physics of The Electric Guitar in English. Meanwhile he has published a third volume covering electroacoustics for stage and studio (about 600 pages).
    Zollner is the mastermind of GITEC. AFAIK they started English translation activities about 2 years ago. You may find out about the progress contacting them via their webpage:
    https://gitec-forum.de/wp/en/gitec-c...tars-and-gear/
    Thanks for the link! Looks like a really great site

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    I kind of like the "toroid" because the flux is confined and the complete magnetic path gives more H.
    And a toroidal core has minimal leakage and avoids the PU interference issue as reported by Chuck. But as the choke will have to handle several amp(ere)s, saturation may be an issue without an airgap.

    On the other hand I think a steel box for the load should effectively shield against radiated mag. fields interfering with the PUs even with a rod core coil.

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    Quote Originally Posted by Mike Sulzer View Post
    Suppose we haver a perfect device that can sample the voltage across a guitar speaker without affecting it and reproduce it exactly into any load at any power level. Connect an identical guitar speaker to the output. Would this second speaker sound just like the one one that is sampled? I think it must. Add a volume control to this device and you can scale the output while maintaining the sound as close as possible to the original given that the speaker is somewhat power sensitive as is human hearing.

    Now replace the speaker that is being sampled with a silent device that has the same impedance, and therefore, the same voltage across it. Now "the device" and the speaker connected to it are a very good attenuator or booster. Off course, a good ss amp approaches this ideal device very closely. Given that electric guitar is not exactly audiophile, the ss amp might not have to be the very best. A light, inexpensive switching amp might be good enough.

    Unlike many ideas, this one is double blind testable. You can can have someone else switch between the two, or not, and see what you can hear without knowing which is which ahead of time.
    The bad cat unleashed was sort of like that iirc.

    In more recent news Boss have just announced something similar with a lot of bells and whistled to boot:
    https://www.boss.info/us/products/wa..._amp_expander/

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    Quote Originally Posted by Zozobra View Post
    The bad cat unleashed was sort of like that iirc.

    In more recent news Boss have just announced something similar with a lot of bells and whistled to boot:
    https://www.boss.info/us/products/wa..._amp_expander/

    bad cat:

    HERE'S HOW IT WORKS

    The Unleash V2 converts the high voltage speaker output of any guitar amplifier up to 100 watts into a line level signal, sends it to our proprietary reactive load circuit that responds like a speaker and then into an ultra-transparent class D amplifier which allows any wattage amplifier to be adjusted between 1-100 watts without any change in dynamics or tone.


    It looks as though they are attenuating the amp output, and then feeding that through a network that simulates a guitar speaker. IMO that misses the point completely; the idea is to load the amp properly so it responds as it would into a guitar speaker, and then attenuate, or reamplify, and then feed a guitar speaker. Perhaps it is just a very bad explanation.

    Boss:

    The boss might be loading the amp properly, but what it does after that is not clear to me, except there is an ss amp for reamplifying.

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    Quote Originally Posted by Helmholtz View Post
    And a toroidal core has minimal leakage and avoids the PU interference issue as reported by Chuck. But as the choke will have to handle several amp(ere)s, saturation may be an issue without an airgap.

    On the other hand I think a steel box for the load should effectively shield against radiated mag. fields interfering with the PUs even with a rod core coil.
    Yes, you would need enough steel so that it would not saturate. On the other hand, the rod would have an effective permeability of under 10, and so I think you would need a lot more turns. I am not sure which weighs less: the steel shielding box or the steel toroid.

    I have some 1 mH (I think) coils intended for crossovers, air core. Maybe it would be easiest to make a "Chuck H" sandwich, and then experiment with adding more metal until the eddy current loss is about right.

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