silly transformer impedance question

jnovac1

Silver Supporting Member
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6,417
assuming that i am running a matched load, what would i expect to notice sound/feel wise between 4 and 16 ohms? the amp has an impedance selector, 4,8,16, i rewired my 2-12 cab so i can run either speaker or both, the way fred does the d/13 cabs. (it originally had a series/parallel switching scheme). so now i can run one speaker for 8 ohms, or both for 4 in parallel. have i sacrificed anything by not having a series option @16?
 

aynirar27

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With some amps, you have more or less negative feedback put back in to your signal depending on what you have the impedance set for
 
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HotBluePlates

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... what would i expect to notice sound/feel wise between 4 and 16 ohms? ... have i sacrificed anything by not having a series option @16?
Don't know; I never bothered trying both. If you're really interested, you can try both & see what you think.

With some amps, you have more or less negative feedback put back in to your signal depending on what you have the impedance set for
This is not the case for any amp I can think of (well, except those home-brew amps where the builder does it wrong :D).

Usually the negative feedback is tied to a single tap, such as the 8Ω tap in this 50w Marshall. All taps have voltage present when you play, so even if the speakers are on the 4Ω or 16Ω tap the 8Ω tap still has the same voltage present as it does when being used.

What this means is the same amount of feedback gets applied to the amp regardless of which speaker tap is used, which also means the amp has a consistent sound (and the Presence control has consistent performance across all the taps).
 

aynirar27

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Don't know; I never bothered trying both. If you're really interested, you can try both & see what you think.



This is not the case for any amp I can think of (well, except those home-brew amps where the builder does it wrong :D).

Usually the negative feedback is tied to a single tap, such as the 8Ω tap in this 50w Marshall. All taps have voltage present when you play, so even if the speakers are on the 4Ω or 16Ω tap the 8Ω tap still has the same voltage present as it does when being used.

What this means is the same amount of feedback gets applied to the amp regardless of which speaker tap is used, which also means the amp has a consistent sound (and the Presence control has consistent performance across all the taps).
you are correct, had a little brain fart there. I probably have built such an amp though!
 

jnovac1

Silver Supporting Member
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Don't know; I never bothered trying both. If you're really interested, you can try both & see what you think.
thanks, hbp. i can’t a/b it without re-rewiring the cab. really just curious about the diff (if any) between using all of the xformer windings or part of them.
ps, the rig sounds great at 4 ohms!
 

HotBluePlates

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10,291
... i can’t a/b it without re-rewiring the cab. really just curious about the diff (if any) ...
Understood.

In a way, I'm hoping you rewire to try it out. I suspect you'll get done with the re-wire & test only to say, "I'm not sure if I heard a difference or not."

I'd suggest if it's not a huge difference (and obvious to you after taking 10-20 minutes to re-wire the cab & try it), it the same as "no difference" (meaning any difference is small enough to not warrant choosing one over the other).

I had a similar situation when swapping the neck on my Tele. I made the change for playability, because my taste in what constituted a good neck shape changed over time. It was at least a 30-45 minute process changing necks & re-stringing. But there were some other differences about the new neck, and it was immediately obvious even after the time lag that the new neck sounded profoundly different than the old one (and fortunately better).

... just curious about the diff (if any) between using all of the xformer windings or part of them. ...
I'm very skeptical about the assertion that 16Ω uses "all of the windings" while 4Ω only uses "part of the windings." As I pointed out above, every tap on the OT is energized when you play regardless of whether a speaker load is attached. If anyone doubts this (and doesn't own a meter), we can take a power transformer and only hook up the tube heaters to the 6.3vac winding then see if they're will to grab hold of the unused high voltage winding! :D

However, I don't own an amp with 4/8/16Ω taps so I haven't tried this myself. I do have some old Vox amps with 8/15Ω taps, but use 2x12 cabs at either 8Ω or 16Ω total depending on the impedance of the individual speakers (and it would be a fair test to juggle between the 8Ω & 16Ω cabs because the speakers themselves are different-enough to be different-sounding).
 

jnovac1

Silver Supporting Member
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6,417
Understood.

In a way, I'm hoping you rewire to try it out. I suspect you'll get done with the re-wire & test only to say, "I'm not sure if I heard a difference or not."

I'd suggest if it's not a huge difference (and obvious to you after taking 10-20 minutes to re-wire the cab & try it), it the same as "no difference" (meaning any difference is small enough to not warrant choosing one over the other).

I had a similar situation when swapping the neck on my Tele. I made the change for playability, because my taste in what constituted a good neck shape changed over time. It was at least a 30-45 minute process changing necks & re-stringing. But there were some other differences about the new neck, and it was immediately obvious even after the time lag that the new neck sounded profoundly different than the old one (and fortunately better).



I'm very skeptical about the assertion that 16Ω uses "all of the windings" while 4Ω only uses "part of the windings." As I pointed out above, every tap on the OT is energized when you play regardless of whether a speaker load is attached. If anyone doubts this (and doesn't own a meter), we can take a power transformer and only hook up the tube heaters to the 6.3vac winding then see if they're will to grab hold of the unused high voltage winding! :D

However, I don't own an amp with 4/8/16Ω taps so I haven't tried this myself. I do have some old Vox amps with 8/15Ω taps, but use 2x12 cabs at either 8Ω or 16Ω total depending on the impedance of the individual speakers (and it would be a fair test to juggle between the 8Ω & 16Ω cabs because the speakers themselves are different-enough to be different-sounding).
much appreciate the comments. i am pleased with the results of my rewire, and will leave it in place. awaiting delivery of a suhr corso, this will allow me to run either the blue or the alltone and have a correct load. and the todd sharp sounds great @4 ohms parallel. thx again...
 

Glitch Magnet

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2,013
Understood.

In a way, I'm hoping you rewire to try it out. I suspect you'll get done with the re-wire & test only to say, "I'm not sure if I heard a difference or not."

I'd suggest if it's not a huge difference (and obvious to you after taking 10-20 minutes to re-wire the cab & try it), it the same as "no difference" (meaning any difference is small enough to not warrant choosing one over the other).

I had a similar situation when swapping the neck on my Tele. I made the change for playability, because my taste in what constituted a good neck shape changed over time. It was at least a 30-45 minute process changing necks & re-stringing. But there were some other differences about the new neck, and it was immediately obvious even after the time lag that the new neck sounded profoundly different than the old one (and fortunately better).



I'm very skeptical about the assertion that 16Ω uses "all of the windings" while 4Ω only uses "part of the windings." As I pointed out above, every tap on the OT is energized when you play regardless of whether a speaker load is attached. If anyone doubts this (and doesn't own a meter), we can take a power transformer and only hook up the tube heaters to the 6.3vac winding then see if they're will to grab hold of the unused high voltage winding! :D

However, I don't own an amp with 4/8/16Ω taps so I haven't tried this myself. I do have some old Vox amps with 8/15Ω taps, but use 2x12 cabs at either 8Ω or 16Ω total depending on the impedance of the individual speakers (and it would be a fair test to juggle between the 8Ω & 16Ω cabs because the speakers themselves are different-enough to be different-sounding).
I don't get the "use all of the windings" hypothesis either. If you have an OT that only has a 4 ohm tap, then you're using all of the windings. It's the same winding ratio as the multitap OT, so why would it be any different? Can the extra "unused" portion in any way hamper induction to the "used" portion just by having more wire "in the way"? I'm struggling to understand the implication.

The only argument I can offer for choosing 16 over 4 regards cable loss. Nothing to do with tranny taps. Let's say the speaker cable has a 0.1 ohm loop resistance. That results in a 2.4% loss with a 4 ohm load, but only 0.6% with 16. Not a big deal, but it's something. Mainly relevant with longer cables.
 
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gldtp99

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3,802
I agree with HBP on this.

I have one customer who claims that that amps sound better when running on 4 ohms (vs 8 or 16 ohms).

I don't know if they do or not---- I've never done a valid test with any of the 4/8/16 switchable amps I have.

I don't have speakers cabs that are switchable from 4/8/16 ohms in order to do such a test.

I had another customer come in to my shop and he was looking for a 4x12 cab to use with the Fender 5E5 based head he bought from me and had been playing live for a couple of years.

I had this Marshall Valvestate 4x12 that I'd loaded up with Jensen Mod 12-70's (8 ohm speakers ---- 8 ohm total load):



He brought in his head, guitar, pedals and I hooked it up with the head set to 8 ohms (had a rotary switch with a chicken head knob on the back for 4/8/16 ohm switching).

He proceeded to play at full volume (with/without pedals).

He held one feeding back note and reached behind the amp and switched between 4/8/16 ohm positions to see which position "sounded best".

He said he liked this "Tone Control" I'd put on the back of the amp and would often switch it around while playing to get different sounds.

I again tried to explain to him what Speaker Impedance Load was and why tube amps should run into a matched speaker load---- he didn't seem to care about that.

Last I heard that amp was still going strong and when he played at House of Blues in Dallas he told me that the headliner's Marshall died during his set and he borrowed my customer's head to finish the show---- he said the headliner liked the amp very much.

I thought the OT in the amp would have failed from hot switching over and over but it never did.

The OP should ask this guy about 4 vs 8 vs 16 ohm tone----- he's the expert. But he's very big and naturally built like a powerlifter and has a reputation for violence.

I'm glad he liked the amp I built--- it was one of the first scratch built 5E5 based heads I did @15 years ago.
 

VICOwner

Gold Supporting Member
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1,847
I’ve had the opportunity to toggle between the 4 and 16 ohm settings on several of my amps through my Marshall 4 12 1960AV and I hear no difference. I thought that using the whole winding had to be better but it doesn’t matter. It could somewhere but not at my shop.
 
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I think the differences are heard by some not because of unused windings at the o/t end of thing, but rather, the speaker.
4 vs 16 ohms, it is said, sounds different due to more/less voice coil resistance causing variations , and I've seen it said that any thicker wire used in the same voice coil gap reduces that gap, thus increasing the efficiency of the speaker.
Are these things true?
I've never tried to test it.

just copied this from another forum

" A higher impedance/resistance drops more voltage. Higher voltage equates to higher amplitude which in turn equates to more travel. The speaker cone follows the frequency’s amplitude.

On the other hand lower resistance has higher current, less travel and more control. As I have mentioned before coils and magnets love current."
 

teefus

Member
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8,947
there was a well-known builder that used to claim that most amps sounded better at 16 ohms because you were using all of the output transformer windings. i, personally, have not noticed much if any difference in using the different taps with the correct load.
 

Tone_Terrific

Silver Supporting Member
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32,712
A higher impedance/resistance drops more voltage. Higher voltage equates to higher amplitude which in turn equates to more travel. The speaker cone follows the frequency’s amplitude.
But the voltage across the speaker coils remains the same, otherwise power would change.
That's what the switch matches.:huh
 

HotBluePlates

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10,291
But the voltage across the speaker coils remains the same, otherwise power would change.
That's what the switch matches.:huh
Remember, the speaker impedance is different, so voltage must also be different to maintain same-power. If you double the tap-impedance, voltage increases by √2.

20w = (8.94v)^2 / 4Ω
20w = (8.94v * √2)^2 / 8Ω
20w = (8.94v * 2)^2 / 16Ω
 

Tone_Terrific

Silver Supporting Member
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32,712
Remember, the speaker impedance is different, so voltage must also be different to maintain same-power. If you double the tap-impedance, voltage increases by √2.

20w = (8.94v)^2 / 4Ω
20w = (8.94v * √2)^2 / 8Ω
20w = (8.94v * 2)^2 / 16Ω
But the speaker only responds to power since speaker load must be increased if you need to change the impedance on the amp.

This just does not read right.
" A higher impedance/resistance drops more voltage. Higher voltage equates to higher amplitude which in turn equates to more travel. The speaker cone follows the frequency’s amplitude.

On the other hand lower resistance has higher current, less travel and more control. As I have mentioned before coils and magnets love current."
You don't get more cone movement without more power.
You can't get more with a mismatch.
 

HotBluePlates

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10,291
... since speaker load must be increased if you need to change the impedance on the amp. ...
I'm talking about fundamental properties of transformers, and a single output transformer with multiple secondary taps (4/8/16Ω). If you play through such an amp/OT, at any single moment the 4Ω tap has the least voltage present, the 8Ω tap has √2 more voltage present, and the 16Ω tap has 2x the voltage present as on the 4Ω tap.

All of my examples assume we are developing a single amount of power on the primary side, so that we're understanding the secondary taps in a like-for-like scenario.

This just does not read right.
Now be aware my assertions here do not imply support for Eugene's assertion that you also quoted. I don't know anything about "Speaker cone travel." Nothing I've said should be "controversial."

Instead, what I'm saying is as simple as Impedance Ratio and Turns Ratio. If the secondary (intended) impedance is higher, the impedance & turns ratio from primary-to-secondary is smaller, which means the voltage step-down is smaller.

Impedance Ratio
4,000Ω : 16Ω = 250:1
4,000Ω : 8Ω = 500:1
4,000Ω : 4Ω = 1000:1
The Turns Ratio between primary and secondary is the square-root of the Impedance Ratio (do we need a math proof to agree on that?).

Turns Ratio
√(4,000Ω : 16Ω) = √(250:1) = 15.81 : 1
√(4,000Ω : 8Ω) = √(500:1) = 22.36 : 1
√(4,000Ω : 4Ω) = √(1000:1) = 31.62 : 1​

Are you seeing how that "√2" term is starting to sneak in? The Turns Ratio is equal to the Voltage Ratio between windings, since Faraday's Law says essentially that 1 volt on a primary turn results in 1 volt on a secondary turn.
Let's put 400v across our 4kΩ primary to see what happens.

Voltage Step-down
4kΩ : 16Ω -> 400v / 15.81 = 25.3v on 16Ω secondary
4kΩ : 8Ω -> 400v / 22.36 = 17.89v on 8Ω secondary
4kΩ : 4Ω -> 400v / 31.62 = 12.65v on 4Ω secondary​

Note the voltage on the 8Ω tap is √2 more than the voltage on the 4Ω tap. The voltage on the 16Ω tap is √2 more than the voltage on the 8Ω tap. √2 * √2 = 2, so the voltage on the 16Ω tap is twice that on the 4Ω tap.

What does that mean for power?

Power
4kΩ : 16Ω -> 25.3v^2 / 16Ω = 40 watts on 16Ω secondary
4kΩ : 8Ω -> 17.89v^2 / 8Ω = 40 watts on 8Ω secondary
4kΩ : 4Ω -> 12.65v^2 / 4Ω = 40 watts on 4Ω secondary
We could have found the exact same voltages by using Ohm's Law & the Equation for Power to find what voltage we should expect on a given speaker load for a set amount of power output. Try 40 watts on 4Ω for example:

Power = Voltage * Current
Current = Voltage / Impedance
Power = Voltage * (Voltage / Impedance) = Voltage^2 / Impedance

Voltage^2 = Power * Impedance
Voltage = √(Power * Impedance)

40w into 4Ω
Voltage = √(40 watts * 4Ω) = √160 = 12.65 volts​
 
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HotBluePlates

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10,291
But the speaker only responds to power since ...
The Navy taught me Ohm's Law in a very specific way. Our homework one night was to write the following 100 times on a sheet of paper:

"The Current in a circuit is directional proportional to the Voltage across the circuit, and inversely proportional to the Resistance of the circuit."

We came in the next day and the instructor showed us the sentence above correlated with

Current = Voltage / Resistance
The instructor then explained the reason for understanding Ohm's Law in this format is we can never directly change the current in a circuit. We can change the voltage across the circuit (with wall voltage, batteries, power supplies, etc), and we can change the resistance of a circuit (add more in series, or reduce resistance by adding parallel paths).
_________________________________________________

Similarly, we can't directly change Power into a speaker. We can use the OT secondary taps to change the voltage applied to the speaker, so to get the Same-Power into different speaker loads the transformer designer just applies More-Voltage into Higher-Impedance.

It's handy that at the same time, when you put the rated-impedance onto those various secondary taps that they reflect the same impedance onto the primary, so the primary-circuit behaves consistently and develops the same power.
 

Tone_Terrific

Silver Supporting Member
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32,712
^Quite right.
I was trying to point out that the statement I quoted defies Ohm's Law.
Sometimes such quotes are misread and misquoted or out of context but that one should not be circulated, imo.
 

HotBluePlates

Member
Messages
10,291
I think the differences are heard by some not because of unused windings at the o/t end of thing, but rather, the speaker.
4 vs 16 ohms, it is said, sounds different due to more/less voice coil resistance causing variations , and I've seen it said that any thicker wire used in the same voice coil gap reduces that gap, thus increasing the efficiency of the speaker.
Are these things true?
...
For myself, I haven't delved into the depths of what details of a speaker's construction makes do whatever, because I'm don't expect I'll ever scratch-build a speaker or have any real way to alter how the speaker does what it does. I dig into how the parts of the amplifier do what they do, because those are what I do build, modify, etc.

...
just copied this from another forum

" A higher impedance/resistance drops more voltage. Higher voltage equates to higher amplitude which in turn equates to more travel. The speaker cone follows the frequency’s amplitude.

On the other hand lower resistance has higher current, less travel and more control. As I have mentioned before coils and magnets love current."
This sounds like B.S. to me.

The reason I keep defaulting to fundamental principles is it's very easy to use logic to come up with flat-wrong ideas if we don't work from proven laws, etc.
 

HotBluePlates

Member
Messages
10,291
... I was trying to point out that the statement I quoted defies Ohm's Law. Sometimes such quotes are misread and misquoted or out of context but that one should not be circulated, imo.
Gotcha! I thought, "this does not read right," referred to what I posted, rather than what you quoted Eugene quoting.
 




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