Yesterday I read a thread in : Instruments and Amps ; Tube Amp Speaker Output To Line level for recording
One of the solution was to use an audio transformer for isolation & level matching, but audio transformer are expansive.
But a class2 transformer ( plug-in power transformer ) is cheap.
Nominal values are primary 120 volts / 60Hz secondary 12 volts / 330 mA .
Guitar signal is, I assume, between 80 Hz and 8 KHz, and 50 Watt in 8 ohms is 20 Volts rms.
If we scale according to frequency a transformer with a winding optimised for 120 Volts / 60 HZ can be used at 40 Volts / 20 Hz, without saturation.
So I made a frequency response test at a very low level ( signal generator output ) 3 Volts on the hi side I did get 0.35 on the low side almost flat between 20 Hz and 12 KHz. Of course magnetic level was very low, about 7.5 % of nominal value.
Then how should we make a more specific test to see if this cheap transformer may work in audio for this isolation & level matching situation.
One of the solution was to use an audio transformer for isolation & level matching, but audio transformer are expansive.
But a class2 transformer ( plug-in power transformer ) is cheap.
Nominal values are primary 120 volts / 60Hz secondary 12 volts / 330 mA .
Guitar signal is, I assume, between 80 Hz and 8 KHz, and 50 Watt in 8 ohms is 20 Volts rms.
If we scale according to frequency a transformer with a winding optimised for 120 Volts / 60 HZ can be used at 40 Volts / 20 Hz, without saturation.
So I made a frequency response test at a very low level ( signal generator output ) 3 Volts on the hi side I did get 0.35 on the low side almost flat between 20 Hz and 12 KHz. Of course magnetic level was very low, about 7.5 % of nominal value.
Then how should we make a more specific test to see if this cheap transformer may work in audio for this isolation & level matching situation.
I have used a toroidal power transformer as output transformer on a valve amplifier and it sounded OK.
As long as a mains transformer is not used in class A, I see no issues as the laminations (and ferrite core for a toroid) are the same, it is only the interwoven windings that differ.
You must be confusing line matching transformers (small and inexpensive) with output transformers (big and expensive).
Output transformers need to be able to handle the entire output power. Line matching transformers only transform voltage, without any meaningful current.
Output transformers need to be able to handle the entire output power. Line matching transformers only transform voltage, without any meaningful current.
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A 50 watt audio output may cost $75.00 and be 5 pounds .... interstage $15.00 and be 4 ounces .... but class 2 can be found at a dollar or so ( second hand ).
This is what I am looking for ....
This is what I am looking for ....
As a isolation transformer on the secondary side of the output transformer I don't see why it won't work. The class A comment was meant if you are using it as a single ened output transformer.
There are "audiophool" signal transformers at ridiculous prices, but you certainly don't need one of those. To create an isolated, balanced, line-out from a guitar amp, I've used this $7.55 (CAD) Triad TY-250P audio transformer, and it worked very well, no audible problems at all: TY-250P Triad Magnetics | Mouser Canada
That sounds really good for a power transformer. As you say, more than good enough for electric guitar!
Now it's a matter of getting other important details (such as signal level) right. Traditional line-out levels (there are at least three different standards) are all close to 1 volt output, which would require 10 volts input for your 10:1 transformer, and that's a lot of input voltage. (A 15-watt guitar amp only drives about 11 volts RMS right across the loudspeaker at the full 15-watt RMS output.)
You could go for an "instrument level" output signal, if that is sufficient for your recording equipment. This is somewhere in the ballpark of 100 mV RMS, and would require 1 volt across your transformer primary.
Let's say you wired 100 ohms across your transformer secondary. That would appear as 10k at the primary. Now if you wire a 1k resistor across the primary, to help flatten the frequency response, you basically have to feed it 1 mA RMS to get the voltage levels you want. All you have to do is choose the value of one additional resistor to get that right.
If you're using, say, a typical small 15 W valve guitar amp with an 8 ohm speaker, you can expect roughly 11 volts across the speaker at full output. This means you get 1 mA of current with a 10k resistor (close enough.)
So the recipe would be: 10k from speaker "hot" to one end of the signal transformer primary. Ground the other end of the TX primary. Wire 1k across that primary. Wire 100 ohms across the secondary. Run the secondary to a suitable XLR or stereo 1/4" jack for balanced output. Done!
The little Triad transformer is smaller, lighter, pretty inexpensive, and will also do the job just fine.
-Gnobuddy
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