Input Transformers as a passive buffer

[Bill Mountain]

I have a love/hate relationship with single transistor boosts and distortions.

As a bass player who likes to either plug straight into a pedal for a quick jam or who uses tuners and other buffered pedals live and at formal practices, I prefer my pedals to have either a high input Z or an input buffer.

I also love simple single transistor designs.  I mean I reeeeaaallllyyy love them!

Now you can see my troubles.

I prefer my pedals to sound the same no matter what is plugged in front of it.  This has led me on a search for either high input impedance circuits or passive buffering techniques.

I have been on the lookout for an input transformer but the specs are a little confusing.  I understand the turn ratio but they also list input impedances as well.  Can I ignore these and just be aware of the actual turn ratio or should I be looking for a specialized input transformer for HI Z instruments?  What do mixing boards and preamps with input transformers use for impedance matching?

Am I barking up the wrong tree?

Is there another way???

[Bill Mountain]

Or I could just stick with FETs and tube designs and then I can have my cake and eat it too.

I wonder if they make any single triodes???

Off to google!!!

EDIT:  Don't look at this post as a reason not to offer ideas related to my first post.  I was just trying to show that I have given this some thought and I just wanted to say that I prefer transistors over tubes for simplicity and I prefer NPN's over JFETs for availability and consistency.

[amptramp]

There are high impedance transformers, but these are wound with tens of thoudsands of turns of exceptionally fine wire and are usually rated for only a few milliamps of signal with no DC signal through them.  If you can find them, they cost a fortune and they do not amplify the signal or reduce its impedance - you need active gain devices to do that.

There are some phenominal single triodes that were made for television RF amplifier service.  I have some 6HA5's squirreled away somewhere for this kind of thing.  These are "frame grid" tubes that use a crossed screen material like a window screen rather than loops of wire for the grid.  This allows the grid to be less microphonic and gives the tube a high transconductance and low noise level.

FET's are easier to use, bipolar transistors are easier than FET's to bias and op amps are dead simple to use.  Imstrumentation amplifiers offer high impedance and voltage gain selected by one resistor.

You can have a buffer feed a low-impedance transformer if you feel the need for signal isolation.

[Bill Mountain]

There are high impedance transformers, but these are wound with tens of thoudsands of turns of exceptionally fine wire and are usually rated for only a few milliamps of signal with no DC signal through them.  If you can find them, they cost a fortune and they do not amplify the signal or reduce its impedance - you need active gain devices to do that.

There are some phenominal single triodes that were made for television RF amplifier service.  I have some 6HA5's squirreled away somewhere for this kind of thing.  These are "frame grid" tubes that use a crossed screen material like a window screen rather than loops of wire for the grid.  This allows the grid to be less microphonic and gives the tube a high transconductance and low noise level.

FET's are easier to use, bipolar transistors are easier than FET's to bias and op amps are dead simple to use.  Imstrumentation amplifiers offer high impedance and voltage gain selected by one resistor.

You can have a buffer feed a low-impedance transformer if you feel the need for signal isolation.

But what kind of transformers do passive DI boxes use?  Maybe I could buy a cheap used one and pull out the tranny for some experiments.

[R.G.]

This has led me on a search for either high input impedance circuits or passive buffering techniques.

There are no passive buffering techniques. Signal buffers are active, because they turn a high impedance signal into a low impedance one at the same or higher voltage. This is a power gain, and the only way to do that is with an active circuit.

I have been on the lookout for an input transformer but the specs are a little confusing.  I understand the turn ratio but they also list input impedances as well.  Can I ignore these and just be aware of the actual turn ratio or should I be looking for a specialized input transformer for HI Z instruments? 

Transformers are complicated. Transformers (mostly) don't have impedances, they have ratios. The ratios change voltage levels by the turns ratio, current levels by the inverse of the turns ratio, and *external* impedances by the square of the turns ratio. They also have a built-in set of parasitic imperfections. These imperfects amount to internal impedances; you need to use the transformer with external impedances that make the internal impedances acceptable for your purposes.

Transformers are complicated.

What do mixing boards and preamps with input transformers use for impedance matching?

They mostly don't use transformers the way you're thinking, with just a transformer at the input that the guitar plugs into. Some do, I'm sure; but the good ones have carefully thought out the consequences of using a transformer, and the bad ones... well, they haven't carefully thought it out.

A guitar signal is fairly small. Making it even smaller is not usually a good idea from a frequency response and noise standpoint. You generally want whatever it drives to be at least ten times higher in impedance than the guitar pickup itself.  This is something like a 10K resistor in series with a 2H inductor. So you would need the transformer plus the reflected loading to be bigger than 100K resistance and 20H of primary inductance. This gets unwieldy really quickly. A 20H inductor is getting into the size of a small tube amp output transformer. Making one is probably possible. But it's expensive and difficult. Which leads to:

But what kind of transformers do passive DI boxes use?  Maybe I could buy a cheap used one and pull out the tranny for some experiments.

It's a mixed bag. If they go to the nominally 600 ohm input on a board, they have to be a step down in voltage; the 600 ohms at the board and the (ideally) 100K or larger means the voltage ratio is the square root of 600/100k = 0.077, which is a bit more than 1/16th. It's a signal *loss* of about 14 times, which then has to be made up at the preamp, adding in the preamp noise to do it. It can be done, but it's not what you'd ideally prefer to do. And that's for a marginal 100K loading. You'd really like to have that be 250K to 1M, which would tilt the signal loss even more.

Cheap ones will have dreadful - er, excuse me, - unfortunate signal coloration and probably treble loss. The cheap ones aren't good, the good ones aren't cheap, and not for mojo advertising reasons.

Probably the best thing to do is to put a buffer in your guitar or as close to it as you can get it.

[Bill Mountain]

Probably the best thing to do is to put a buffer in your guitar or as close to it as you can get it.

Thanks for the lengthy explanation as always.  It's not what I wanted to hear but it makes sense.  What I think I need to do is to look at ways to raise the input impedance of single NPN circuits (or just stop caring and play my damn bass).  Or I could do some tests to see how noticeable the treble loss really is.  One idea I had last night was to put in some sort of bright switch to cut some highs when going buffered.