I have some questions about transformers.

honeyiscool

honeyiscool

Jan 28, 2011
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#1
If I understand transformers correctly...

1. Transformers are basically two coils wrapped around a piece of iron. One is input and the other is output, but only nominally, for they can be used backwards for reverse effect.
2. Depending on the number of windings on the input and the output, you either step up, step down, or maintain unity voltage.
3. As with all things in life, from input to output, transformers cause some energy loss.
4. Because transformers are passive devices, transformers cannot output more energy than they are given, but they should not lose too much, either.
5. Transformers are some of the most expensive parts in most of the devices they are found inside, and for whatever reason, they can't be made cheaply without sacrificing fidelity.

So...

1. Since transformers are passive devices, a step-up transformer that increases the voltage and impedance of the signal also decreases the current.
2. Meanwhile, a step-down transformer that decreases the voltage and impedance of the signal must increase the current.

Am I still doing OK? Here are my questions, then:

1. Why are transformers not common in solid state amps? Whatever they add to the sound, in theory couldn't we just have a 1:1 transformer? I understand that this would present some issues if someone runs it without a load and it would add a lot to the weight of an amplifier. But why isn't there a box that just has a big 1:1 transformer in it that can handle speaker signals that could "warm up" a solid state amplifier? And don't transformers also help prevent against certain types of damage (like accidentally connecting - to chassis ground in bridged amplifiers) and couldn't a step-up or step-down transformer help 8 ohm speakers look like 4 ohms or vice versa to amplifiers? With a simple ratio selector, it seems like they could be pretty useful. Even if such a box is many pounds and $200, I feel like it might be worth it to the right person.

2. In a 12:1 transformer used in a typical passive DI box, the voltage of the signal gets padded down to 1/12 of original, and the impedance of the mic input gets bumped up 144x to the bass pickup, loading the signal properly. Fine. Is the fact that there's also 12x the current in the output not an issue because there's just so little current in most audio signals to begin with?

3. I guess to expand on that, do speaker-level DIs have a transformer with a ridiculously large turns ratio or are they more of a more modest 12:1-ish transformer with a large resistor for padding? Does the padding happen at the input? Doesn't this generate heat? Does the fact that they're run parallel to an actual load mean that the transformer input itself doesn't actually see that much current?

4. I assume that a passive reamp box is just a step-down transformer of some ratio to go from line to instrument level? So what, this is basically a DI box but with the XLR in the input and TS on the output? Also, curious, just how different are line level and instrument level?

5. A step-up transformer seems to get used in many places (such as phono preamps and dynamic microphones) to passively step up input. What establishes the theoretical limit as to how much clean gain one can get this way?

6. Could one use two passive DIs w/ a special female-to-female XLR cable, with the receiving end wired backwards, to run a balanced signal between a bass and an amp?

Thanks for reading this far! I'm just really curious about audio signals and these magical things called transformers.
 
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#4
@OP:
Please take a run of some (not mentioned in your post) parameters of a transformer,such as:impedance,inductance,capacitance,core hysteresis,spike resonance,effiency,mass (weight),limiting physical dimensions,industrial costs.These should bring a more extensive answer to the "why not" question.
 
#5
Wow, great questions. I'll try to answer a few of these...

honeyiscool said:
If I understand transformers correctly...

1. Transformers are basically two coils wrapped around a piece of iron. One is input and the other is output, but only nominally, for they can be used backwards for reverse effect.
2. Depending on the number of windings on the input and the output, you either step up, step down, or maintain unity voltage.
3. As with all things in life, from input to output, transformers cause some energy loss.
4. Because transformers are passive devices, transformers cannot output more energy than they are given, but they should not lose too much, either.
5. Transformers are some of the most expensive parts in most of the devices they are found inside, and for whatever reason, they can't be made cheaply without sacrificing fidelity.

So...

1. Since transformers are passive devices, a step-up transformer that increases the voltage and impedance of the signal also decreases the current.
2. Meanwhile, a step-down transformer that decreases the voltage and impedance of the signal must increase the current.
Click to expand...
This is all basically correct.

Am I still doing OK? Here are my questions, then:

1. Why are transformers not common in solid state amps? Whatever they add to the sound, in theory couldn't we just have a 1:1 transformer? I understand that this would present some issues if someone runs it without a load and it would add a lot to the weight of an amplifier. But why isn't there a box that just has a big 1:1 transformer in it that can handle speaker signals that could "warm up" a solid state amplifier?
Click to expand...

(Assuming we're talking about speaker transformers and not power supplies) Expense and weight of the transformer, and the added expense of housing it in terms of a stouter chassis, extra shipping weight, ect... And I don't believe transformers are the missing mojo that would make a SS amp sound like a tube amp. And of course due to the losses, you have less power delivered to the speakers.

And don't transformers also help prevent against certain types of damage (like accidentally connecting - to chassis ground in bridged amplifiers) and couldn't a step-up or step-down transformer help 8 ohm speakers look like 4 ohms or vice versa to amplifiers? With a simple ratio selector, it seems like they could be pretty useful. Even if such a box is many pounds and $200, I feel like it might be worth it to the right person.
Click to expand...
That's what the taps on an output transformer are for, but yes, such a gizmo could exist. It would cost a lot, be heavy, probably have to be sold with a lot of snake oil magic, and then people wouldn't understand how to use it, blow up their amps, and blame their impedance matching transformer.

3. I guess to expand on that, do speaker-level DIs have a transformer with a ridiculously large turns ratio or are they more of a more modest 12:1-ish transformer with a large resistor for padding? Does the padding happen at the input? Doesn't this generate heat? Does the fact that they're run parallel to an actual load mean that the transformer input itself doesn't actually see that much current?
Click to expand...
JUST SPECULATING HERE but it would make sense to have a plain "12:1 ish" transformer with a series resistor(s) to the speaker lines, and maybe a capacitor to block any 'stray' DC currents. Yes the resistance of the transformer primary and series resistor is in parallel with the speakers, but is so high (greater than 1k Ohm I'd guess) that it's not significant. Since there's not much current flowing, the resistor shouldn't get hot.


5. A step-up transformer seems to get used in many places (such as phono preamps and dynamic microphones) to passively step up input. What establishes the theoretical limit as to how much clean gain one can get this way?
Click to expand...
The input impedance of the first stage of the amplifier. Too low of an input impedance will load down the feeble voltage and there won't be enough to amplify. A high-impedance first stage doesn't load the signal down, even if there isn't much voltage gain, the output of the first stage can output more current which can then be amplified more easily. Condenser mics, for example have either an FET or tube buffer amp in the mic body because impedance of the condenser element is so high it's not practical to send it down a cable, it would pick up too much noise.

6. Could one use two passive DIs w/ a special female-to-female XLR cable, with the receiving end wired backwards, to run a balanced signal between a bass and an amp?
Click to expand...
I think there would be too much loss for this to be practical. Perhaps an active DI feeding a passive DI would work. There's been a few high end basses made with XLR outputs, that would be ideal (but bass and guitar electronics are still 'unbalanced').

A couple of other comments about transformers: as the frequency goes down, they become less efficient, and of course can't pass DC current at all. As frequency goes up, the become more efficient, but as frequency increases the inductance of the transformer coils starts to affect things. So I'm sure spec'ing a transformer for an amp means juggling a lot of contradictory factors...

A.H. may come along and shoot holes in all this, that's OK.
 
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#13
Crater said:
That's what the taps on an output transformer are for, but yes, such a gizmo could exist. It would cost a lot, be heavy, probably have to be sold with a lot of snake oil magic, and then people wouldn't understand how to use it, blow up their amps, and blame their impedance matching transformer.
Click to expand...

Weber sells one (I'm sure a statement like yours has been paired with one like mine many times, by the way).

Z-Matcher-100 100w Impedance Matcher

Here's the transformer itself WZC-100 Transformer
 
#15
The simple answer is that transformers are used to match impedances as well as matching voltages. That's why passive DI boxes use transformers, to convert the high impedance output of a guitar to the low impedance input of a mixer.

Tube output stages do not like driving the low impedances of speakers so they have an output transformer. It's huge because of the large amount of current passing through it. Modern solid state amps have no problem with low impedances, but there are early SS amps that did require output transformers. I used to have a Univox amp with one.
 
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#17
You ask a lot of questions. I'll answer a few:

1) There have been transistor amps with output transformers - McIntosh made some. They were high quality amps, but...even without the transformers, McIntosh probably would have made very good amps. It's been quite a while since those days, and having transformers in their amps didn't prove to be a huge selling point. As they involved quite a bit of expense, the market spoke, and we don't see such things nowadays.

2) Yes - you are correct.

3 and 4) I could answer from a theoretical view, but there's likely someone knowledgeable on specifics of what's out there, etc, so I'll leave it to them.

5) The theoretical limit to voltage gain (it's not free gain really, as you don't gain power). This isn't some simple Physics equation - any given input topology that you might feed the output of an input transformer to has an optimal input impedance for best signal to noise ratio. Wind a transformer with more turns, the input impedance is higher,and the input stage's noise current starts to rise and dominate the noise of the system. Too low of an input impedance, and your'e giving up voltage - the input stage's noise voltage now dominates the noise of the system. As with many things there's a "Goldilocks" point where things, while not perfect, are optimized. Where that point is is something that you hope the circuit designer optimized. Curiously, many effects pedals do better with a lower input impedance than what we feed them - a bass wired with parallel coils in a humbucker will actually be quieter, even when the gain is turned up to compensate for less level, than one with the coils wired in series.

6: Yes - Radial makes a system to do just what you describe - take an unbalanced signal, balance it, and run it a long distance through a balanced cable (at lowered impedance) without degradation due to cable capacity or hum issues. Their solution even solves the gender issue on the xlr's - you don't need a special cable.
 
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#18
honeyiscool said:
1. Why are transformers not common in solid state amps? Whatever they add to the sound, in theory couldn't we just have a 1:1 transformer? I understand that this would present some issues if someone runs it without a load and it would add a lot to the weight of an amplifier. But why isn't there a box that just has a big 1:1 transformer in it that can handle speaker signals that could "warm up" a solid state amplifier? And don't transformers also help prevent against certain types of damage (like accidentally connecting - to chassis ground in bridged amplifiers) and couldn't a step-up or step-down transformer help 8 ohm speakers look like 4 ohms or vice versa to amplifiers? With a simple ratio selector, it seems like they could be pretty useful. Even if such a box is many pounds and $200, I feel like it might be worth it to the right person.
Click to expand...

Peavey and Glockenklang already sell standalone matching trafos, and have for years -- so it might be worth asking about how many of us have ever seen even a single one in the field? IIRC you're looking at something that weighs at least triple what the amp itself will these days, at least at those power levels. Here's a commercial amp that gives a good idea of the form factor tradeoffs: en/Warwick---Products--Amplification--Warwick-Hellborg--Hellborg-Power-Amps--MP-500--Features.html

Be sure to really look at the frequency response curves of any candidate pieces, especially the less expensive options. In general, you may find that the drawbacks often outweigh any potential benefits, IME and IMO.

honeyiscool said:
3. I guess to expand on that, do speaker-level DIs have a transformer with a ridiculously large turns ratio or are they more of a more modest 12:1-ish transformer with a large resistor for padding? Does the padding happen at the input? Doesn't this generate heat? Does the fact that they're run parallel to an actual load mean that the transformer input itself doesn't actually see that much current?
Click to expand...

For a basic approach, look at how 80s era Fender tube amps did this. A very simple resistive pad (just one pot and one resistor taken at the feedback return tap in some cases) is more or less all you need, although as always, there are many nuances and safety considerations that may not be immediately obvious. Since the pot or pad is typically at least a few thousand times the impedance of the load, heat doesn't have to be a big issue at all. They didn't even bother with a transformer for what they called a line or recording out, but the end user could/can plug in a standard issue DI or even just an isolation (1:1) trafo in some cases, as needed. Many people find some extra signal conditioning desirable though. Things get trickier with SS amps, but there's far less motivation to use speaker level DI feeds with those in most cases.

honeyiscool said:
Thanks for reading this far! I'm just really curious about audio signals and these magical things called transformers.
Click to expand...

It's well worth reading through the Jensen white papers on their website, especially those written by Bill Whitlock. If you don't feel like registering you can find some of them on other websites as well.
 
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#20
SS rigs can be designed to operate at a low output impedance that already matches a speaker's impedance.
Tubes inherently operate at high impedance, thousands of ohms.
The transformer matches the tube's high impedance to the speaker's low impedance.

Transformers won't make an SS amps sound like a tube rig.
A good transformer will make an SS amp sound like an SS amp.
A crap transformer will make a good amp sound like a crap amp.

It's like taking your mother-in-law on vacation.
Takes up a lot of room, doesn't make the vacation any better, adds to the expense, and makes noise.:woot:
Sorry all you ma's-in-law.:bag:
 
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