Beginner question on transformers

[Nyman]

Please note that this is a beginner question. I am basically trying to understand the way transformers work in an audio circuit, so I'm specifically not interested in amplification. General discussion on transformers is however welcome   Altered frequency response is a non-issue, reasonabe amount of noise and distortion is also a non-issue.

My example project consists of a very rudimentary schottky diode clipper. One diode goes from signal to ground and another one the other way around.  The circuit is preceded by a booster and followed by other pedals. Let's say I'd like to up the voltage a bit to meet the forward voltage of the diodes sooner, thus lowering the clipping treshold. What I'm thinking now is using a step-up transformer to achieve this without amplifying the signal. Please tell me if that reasoning is faulty in some sense, I'm sort of fascinated about the idea.

I have tried looking up basics of transformers on the net. The usage examples however mostly involve a configuration where AC goes from input to ground, and the secondary coil operates a load such as a loudspeaker, a motor or something similar in sorts of a loop. How am I supposed to wire up the transformer in my case? Even if the ground is isolated on the input side, it would be connected on the output side because I daisy-chain my pedals. Adding an isolation or step-down transformer after the circuit solves this, but wouldn't I still have to use something for a ground?

This is probably a piece of cake for you folks. I'd appreciate a passive approach if possible. Also remember that I already have a buffer before the circuit.

[brett]

Hi

Please tell me if that reasoning is faulty in some sense, I'm sort of fascinated about the idea.

Your reasoning is completely good. Usually we'd use transistors to increase the voltage, but there's no reason why a transformer isn't a good idea. In fact you'll often see a transformer used in "octave" (frequency doubling) effects and similar circuits (ring modulators), in order to reduce the effects of the forward voltage of the diodes. It is also worthwhile knowing that Si diodes have a Vf of 0.7 V whereas Ge diodes have a Vf of 0.3 V. So a Ge diode like a 1N60 is handy in these circuits.

A handy rule of thumb when choosing a transformer is that the voltage gain is proportional to the square root of the impedance ratio. An 8:1k transformer has an impedance ratio of 1:125, and the sqare root of 125 is about 11, so the voltage gain is about 11.

Any more questions? Just ask.
cheers

[armdnrdy]

My thought is why don't you design your build to run off of a higher voltage? 12 volt, 15 volt.

[Kesh]

you mean like this?

[Nyman]

It is also worthwhile knowing that Si diodes have a Vf of 0.7 V whereas Ge diodes have a Vf of 0.3 V. So a Ge diode like a 1N60 is handy in these circuits.

I'll take a look at the germanium diodes available at my supplier. I think the forward voltage of my schottkys is around 4.5. It would be great to get even lower than that. I've also heard germanium diodes clip softer. I wonder though if that effect could be replicated just by adding a simple low pass filter after the diode clipper.

A handy rule of thumb when choosing a transformer is that the voltage gain is proportional to the square root of the impedance ratio. An 8:1k transformer has an impedance ratio of 1:125, and the sqare root of 125 is about 11, so the voltage gain is about 11.

I didn't know that. Ultimately that means the 600/10k transformer I have lying around is just about right for my purpose. Thanks!

My thought is why don't you design your build to run off of a higher voltage? 12 volt, 15 volt.

It's because in this case it will be a completely passive almost matchbox-sized device, which will be placed after my booster that's practically always on. That's a good point though if I were to have some sort of amplification in the circuit.

you mean like this?

Yes, that is it! I somehow thought the signal was supposed to be isolated from the output or something. Many thanks.

[brett]

Hi again

I think the forward voltage of my schottkys is around 4.5.

Schottky diodes are simply half of a normal silicon diode and have half the voltage drop (0.7 / 2 = 0.35V) which is very handy for this application. (normal diodes have 4 parts - wire, N semiconductor, P semiconductor, wire. Schottkys have 3 - wire, N or P semiconductor, wire).

It's a good thing that your circuit will be 'driven' by a booster. The pickups themselves might struggle to 'energise' the transformer (transformers 'lose' some energy, which might be a large proportion of what your pickup is putting out).
cheers

[dmc777]

I've been researching when I can on transformers as well. I have the origianal V1 MXR M118 Analog Delay with the original transformer but at some point in time the original owner must have damaged the transformers cable because the cable, or transformer cable with 2 prongs, is only about about 6 inches in length so the pedal has to be bascially right by the outlet. So....If anyone cares to answer, is it possible to add a new transformer WITH a 3rd ground plug? That way it will be plenty of length to power the pedal and the unit will be properly grounded. Hope that makes since. What what I need to do and would the original transformer work but just add a new pigtail?

Also, may I ask what you are building exactly and what you're trying to achieve by adding this in front of your booster? Thanks to everyone in advance.

[PRR]

> Yes, that is it!

This plan is incomplete. It does not show any resistance in series with the diodes. If a high power source were connected through an ideal transformer, the diodes would not be able to clip the high power available.

In most real-audio situations there is always significant resistance in source or transformer. While it may not be an actual resistor, you need to think about it (and usually show it on an equivalent circuit).

Transformers do not increase Power.

A voltage step-up means a lower input impedance.

1:2 step-up is a 4:1 reduction in input impedance.

We rarely have a ton of impedance mis-match to throw-away with transformer loading.

Also the typical 50K-500K guitar-chain impedances are much higher than audio transformer impedances. A transformer winding is a tight lump of conductor and insulator. This is a capacitance. A long guitar cord is also a capacitance. Long cords suck treble. Transformers in guitar lines suck treble. Step-up transformers suck more treble.

Incidentally it is not trivial to adjust transformer step-up ratio. You can only do so much with taps. You can use a pot but that increases suckage.

Amplifiers ARE the answer. They can increase audio Power. They can lightly load the source (guitar), apply arbitrary voltage gain, arbitrary current-gain to hit the diodes, and can cover the power loss in a gain control pot.

That's why essentally ALL diode-clip pedals have amplifiers.

The prime disadvantage is the need for a power supply. Passive clipping would avoid that, IF we had ample excess audio-signal power to spare. But we usually don't. And I see other power supplies on your pedalboard.