DI boxes and impedence, what's it all mean?

[stevekane]

Hi, my first post for quite a while, the previous help I received from the forum was invaluable, many thanks.
Anyway our ukulele band is still plugging away and were running mics and ukes through a Studiomaster Club 16 mixer feeding RCF active speakers.
One of our chaps has a uke with a passive Piezo Pickup, we have no idea of make or model, and on occasions it can sound like an old bed twanging away, Im fairly sure its not the Quack that I have heard before, maybe lead length? So Ive been trying to get to grips with DI.boxes and I wonder if you could help me out? As I understand it, the Impedance of the Piezo pickup is very high,,does that mean that a piezo pickup only “produces” a very high Impedance signal, Im asking this because I think that there is no signal or power of any kind being fed to the pickup, its the pickup alone thats creating the signal? Our other Ukes have built in Pre-amps so don't have this problem except for one,,,and he has a Wireless connection instead of a lead,,I dont know what make it is but Im I right in thinking that his receiver plugged into the mixer will be producing a Line or Instrument level signal? So DI Boxes, I think I need to be looking for an Active box and one that “produces” up to
10Mohm?? So in the spec I need to look for the DI box Impedance?
BTW the mixer specs state 1.4 m ohm and 10 m ohm for the Mic and Line inputs so that seems to me to tie in? And lastly often the DI boxes will run off Phantom Power, however were using SM58s so couldn't switch on Phantom power,,this must often be the case or do some mixers have the option of Phantom P on individual strips? Bottom line,,is it possible to get a cheap DI Box with the required spec?
Steve.

[Drew Stephenson]

So, a DI box is for converting an unbalanced instrument level to a balanced microphone level*.
However your passive piezo mic is not kicking out a signal at normal instrument level, it needs some kind of pre-amp (similar to the ones built into the other ukes) to raise the signal up to normal instrument level. At which point you can plug it into an instrument input on the mixer or use a DI box to plug it into a mic input.
The pre-amp should have a very high input impedance option to handle the piezo signal. Something like this should do the job: https://www.gear4music.com/Guitar-and-B ... AyNTUwNTI5

You can run phantom power through an SM58 (and most other dynamic mics) with absolutely no problems, you don't need to worry about individually switched channels.

* They may also provide additional benefits like isolation but that's the main job.

[Tim Gillett]

...As I understand it, the Impedance of the Piezo pickup is very high,,does that mean that a piezo pickup only “produces” a very high Impedance signal...?

Yes. Not only that, high impedance signals shouldnt be run through long cables because the signal tends to get soaked up along the way. So not only is a DI or preamp is needed but the closer it is to the instrument, within reason, the better. A shortish lead from uke to DI or pre is best. This is why piezo pickups work well feeding a preamp built into the instrument. Only a very short internal cable is needed.

In the days of only valve amplifiers, high impedance guitar pickups were cheaper but shortish leads were necessary. I think it was Les Paul who got tired of the noise and interference when he played his guitars live. So he modded his guitars to low impedance balanced output which reduced the noise and interference at venues and allowed him to use much longer guitar cables when needed.

[Hugh Robjohns]

So Ive been trying to get to grips with DI.boxes and I wonder if you could help me out? As I understand it, the Impedance of the Piezo pickup is very high,,does that mean that a piezo pickup only “produces” a very high Impedance signal, Im asking this because I think that there is no signal or power of any kind being fed to the pickup, its the pickup alone thats creating the signal?

Signals don't have an impedance. It's the environment in which the signal exists that has source (output) and destination (input) impedances.

Piezo pickups generate a voltage when the crystal is deformed, in this case by sound waves, but there's very little current available (and negligible power), so it needs a very high impedance destination. Different piezo pickups need different impedance destinations, but most are in the 10 MegOhm range. Some are specifically designed to work with much lower impedance inputs, so it's important to know the details of the actual bug you need to connect.

Most standard active DI boxes offer 1 MegOhm at most, so you generally need a specialised preamp or DI box to interface a piezo pickup properly. Preamps that can be fitted to the instrument keep the high-impedance environment very small, which is a good idea for minimising noise and interference.

... but Im I right in thinking that his receiver plugged into the mixer will be producing a Line or Instrument level signal?

Usually... although some deliver mic level.

So DI Boxes, I think I need to be looking for an Active box and one that “produces” up to
10Mohm?? So in the spec I need to look for the DI box Impedance?

'Presents' rather than 'produces'... but yes, 10 MegOhms is a good safe generic figure. Ideally, though, find out what the specific piezo bug is and look up what it actually needs.

BTW the mixer specs state 1.4 m ohm and 10 m ohm for the Mic and Line inputs so that seems to me to tie in?

That doesn't sound right. Balanced mic inputs are typically 1.4 kiloOhms (k Ohms) and line inputs 10 kiloOhms. I think someone may have confused their prefixes....

...and lastly often the DI boxes will run off Phantom Power, however were using SM58s so couldn't switch on Phantom power...

Provided you're using balanced XLR cables for the mics, SM58s will tolerate phantom power quite happily. It's an inherent part of the design.

...this must often be the case or do some mixers have the option of Phantom P on individual strips?

Better mixers do have individual phantom switches per channel but, as said above, balanced dynamic mics simply aren't bothered by phantom power, so a global switch is not an issue.

Bottom line,,is it possible to get a cheap DI Box with the required spec?
Steve.

Cheap depends on your budget... but there certainly are DI boxes designed for piezo pickups... such as the Radial PZ-DI

https://www.radialeng.com/product/pz-di

Or the same company's Stagebug interfaces for attachmentvto the instrument

https://www.radialeng.com/product/stagebug-sb4

https://www.radialeng.com/product/stagebug-sb1

...other equivalent products are available from other suppliers.... but Radial gear is good and will last a long lifetime.

[shufflebeat]

Agree with all of the above (obvs) but would only add that my experience any decent active DI will work well with a raw piezo element. Orchid are always a favourite here, avoid the Behringer DI20 even if it’s free.

Some folk like the Behringer ADI21 for a super cheap option which gives you some EQ control and can run on batteries but if you have a respectable EQ on the desk just send it a good signal from a basic DI and do it there.

[Drew Stephenson]

Agree with all of the above (obvs) but would only add that my experience any decent active DI will work well with a raw piezo element.

I suggested a pre-amp above because the only instrument I have with a piezo but no built in pre-amp is my double bass and that doesn't always play nicely with DIs.

[Tomás Mulcahy]

Agree with all of the above (obvs) but would only add that my experience any decent active DI will work well with a raw piezo element. Orchid are always a favourite here, avoid the Behringer DI20 even if it’s free.

Some folk like the Behringer ADI21 for a super cheap option which gives you some EQ control and can run on batteries but if you have a respectable EQ on the desk just send it a good signal from a basic DI and do it there.

Yea it will work but you’ll often find it will twang like an old bed spring (as OP put it) compared to a pre amp with a more appropriate input impedance such as those Hugh recommended.

[AlecSp]

but Radial gear is good and will last a long lifetime.

Except in my experience with 2 brand new Radial DI Pros which experienced three failures between them in two years...

[Hugh Robjohns]

I explored my own ProD2s after you mentioned your woes. I found no signs of damage and thought the internal construction was pretty solid. The circuit board is well supported and the transformers seem secure — although they are pcb-mounted rather than bolted to the chassis as they are in some other designs I've seen.

Besides the ProD2s, I also have J48s, and a JPC, and all have been trouble free over many years of regular use.

[shufflebeat]

Agree with all of the above (obvs) but would only add that my experience any decent active DI will work well with a raw piezo element. Orchid are always a favourite here, avoid the Behringer DI20 even if it’s free.

Some folk like the Behringer ADI21 for a super cheap option which gives you some EQ control and can run on batteries but if you have a respectable EQ on the desk just send it a good signal from a basic DI and do it there.

Yea it will work but you’ll often find it will twang like an old bed spring (as OP put it) compared to a pre amp with a more appropriate input impedance such as those Hugh recommended.

You may be right but there’s so much variety in piezo pickups that we can’t know without more info.

As it stands, I’ve dealt with a lot of pickups in a live scenario and not come across one (that was properly fitted) that couldn’t be made listenable with a basic DI and decent channel EQ - no bed-spring issues.

[stevekane]

Well what a great response, many thanks everyone for taking the trouble to contribute and for firming up my thoughts about needing a DI. The technical explanations were really illuminating too. A member of the band is suggesting that we hold back and instead try to resolve the problem through the mixer, but Im sure this is a dead end and ignores the fact that we were unable to iron it out on our last gig. So Im on hold until we get a practice in and at least I have some basic understanding of what might be going on. I will report back in due course,,,,and many thanks to you all once again.
Steve.

[Philbo King]

Hi, my first post for quite a while, the previous help I received from the forum was invaluable, many thanks.
Anyway our ukulele band is still plugging away and were running mics and ukes through a Studiomaster Club 16 mixer feeding RCF active speakers.
One of our chaps has a uke with a passive Piezo Pickup, we have no idea of make or model, and on occasions it can sound like an old bed twanging away, Im fairly sure its not the Quack that I have heard before, maybe lead length? So Ive been trying to get to grips with DI.boxes and I wonder if you could help me out? As I understand it, the Impedance of the Piezo pickup is very high,,does that mean that a piezo pickup only “produces” a very high Impedance signal, Im asking this because I think that there is no signal or power of any kind being fed to the pickup, its the pickup alone thats creating the signal? Our other Ukes have built in Pre-amps so don't have this problem except for one,,,and he has a Wireless connection instead of a lead,,I dont know what make it is but Im I right in thinking that his receiver plugged into the mixer will be producing a Line or Instrument level signal? So DI Boxes, I think I need to be looking for an Active box and one that “produces” up to
10Mohm?? So in the spec I need to look for the DI box Impedance?
BTW the mixer specs state 1.4 m ohm and 10 m ohm for the Mic and Line inputs so that seems to me to tie in? And lastly often the DI boxes will run off Phantom Power, however were using SM58s so couldn't switch on Phantom power,,this must often be the case or do some mixers have the option of Phantom P on individual strips? Bottom line,,is it possible to get a cheap DI Box with the required spec?
Steve.

It is difficult to make a passive piezo pickup to sound good with a PA. They have incredibly high impedance, usually have a very weak signal, and have a very peaky frequency response in the highs.
At least that was my experience trying to add a piezo to my dobro.

If possible, convince them to get an active pickup (which contains circuitry that addresses all the above problems).
Or just mic it.

[shufflebeat]

It is difficult to make a passive piezo pickup to sound good with a PA unless you provide an appropriate interface for it to give it’s best.

FTFY

At least that was my experience trying to add a piezo to my dobro.

I hesitate to argue with direct experience but what pickup did you use and how did you deal with it to get such poor results?

If possible, convince them to get an active pickup (which contains circuitry that addresses all the above problems).

An appropriate active DI is effectively a preamp with no controls which, for some players and lots of sound-techs, is the best kind.

Or just mic it.

More worms, pass the tin-opener, please.

[ef37a]

For those with a bit of a DIY bent and a solder iron, modding an existing active DI is pretty simple if it is of the old 'thru hole' component construction.

You just need to identify the op amp bias resistor, almost always 1 meg Ohm (Brown Black Green) and replace it with a 10 meg resistor (Brown Black Blue)
In such an older pedal the op amp will likely be a TL072 or one of its 'family' and will not care a jot. The input device might be a discrete junction FET but again, the gate 'tie down' resistor* can simply be upped in value.

Do not be concerned about dire warnings about static damage, such kit is not that easily damaged in my experience. Maybe don't wear you nylon frillies"!

Yes, cables make a difference but not in the same way as magnetic pickups. Piezo pups are essentially a capacitor so the cable acts as a simple attenuator, not just a 'top chop' as for magnetics. Best way would build the preamp into a small tin along with a 9V battery and have a sub 1m cable and the box on your belt.

*Much the same as the "grid leak" for a valve but that has a rather different function.

Dave.

[stevekane]

Well Dave,,,I dont quite know what to say,,apart from I dont know what to say…
But many thanks indeed for putting it all together for me. Whilst I felt I understood the first stage in swopping a 10meg for the 1meg resister, after that I was all at sea and I would be straying into areas well beyond my ken,,
Changing the pickups for ones with onboard preamps is a solution, but probably a bit too drastic, what I will do though is to tell the band not to buy instruments with straight Piezo “bugs”.
Steve.

[Tomás Mulcahy]

As it stands, I’ve dealt with a lot of pickups in a live scenario and not come across one (that was properly fitted) that couldn’t be made listenable with a basic DI and decent channel EQ - no bed-spring issues.

As have I. I learned a long time ago that it is far better to try a very high impedance pre-amp if the instrument does not have a pre amp. IME it is quicker and more productive than mucking around with eq on the desk which I find is a bubbles in wallpaper pursuit . I have a Barcus Berry and a pair of Triton Big Amp (used to have others). On some piezo's the Big Amp make it worse, and the Barcus Berry is just right. They each have a different input impedance. Very rarely has the 1M input of a DI been better than either of them if the instrument does not already have a pre-amp.

The Triton are really great, bought a pair to use with a selection of generic piezo discs for contact mic applications in the field.

[shufflebeat]

As it stands, I’ve dealt with a lot of pickups in a live scenario and not come across one (that was properly fitted) that couldn’t be made listenable with a basic DI and decent channel EQ - no bed-spring issues.

As have I. I learned a long time ago that it is far better to try a very high impedance pre-amp if the instrument does not have a pre amp. IME it is quicker and more productive than mucking around with eq on the desk which I find is a bubbles in wallpaper pursuit . I have a Barcus Berry and a pair of Triton Big Amp (used to have others). On some piezo's the Big Amp make it worse, and the Barcus Berry is just right. They each have a different input impedance. Very rarely has the 1M input of a DI been better than either of them if the instrument does not already have a pre-amp.

The Triton are really great, bought a pair to use with a selection of generic piezo discs for contact mic applications in the field.

That’s perfectly in keeping with my experience. There are two ways to plug into my Tonedexter, a regular TS cable (1 M-Ohm) or a TRS (10 M-Ohm). They provide very different environments for high impedance sources to work into, so far we agree.

When hi-Z pedals weren’t on most people’s radar we made clones of the Barcus Berry Matchbox for mates who were amazed by the improvements appropriate impedance matching could make. I have no issue with the concept or anything you’ve said. However, as previously mentioned, I’ve still not come across a piezo source that couldn’t be rendered acceptable (bed-spring free) using a good active DI and a decent desk EQ and if I had to make a semi-informed recommendation, which is what this is, that would be it.

I’ve not encountered the Triton Big Amp before and will be exploring further, thanks for the revelation.

[adrian_k]

I’m just wondering, given that (from the OP),

and he has a Wireless connection instead of a lead

how that affects things?

He’ll be plugging into a transmitter, what sort of impedance does that present? Put a preamp before the transmitter?

I sorted out my mandolin piezo with a Triton Bigamp, very impressed with it, but gets a bit more tricky with wireless because of phantom power..

[shufflebeat]

I chose to replace a passive K&K setup in my mandola for an active Baggs Element because I wanted to use a wireless system and those I had access to (£) had a fairly low input impedance. Complete transplant was the only reliable solution.

[Drew Stephenson]

I’m just wondering, given that (from the OP),

and he has a Wireless connection instead of a lead

how that affects things?

Two different band members I think.

[adrian_k]

Ah. Too many words for my small brain!

[shufflebeat]

Yes, the only other uke player with a similar problem is using a wireless connection. Assuming this is at the cheaper end of the market it may be the same issue I faced with the passive K&K. His/her solution will be either sack the wireless and use an active DI or other appropriate high impedance (Hi-Z) input pedal or keep the wireless but fit a preamp to the instrument feeding it.

[stevekane]

Sorry, I may have caused a bit of confusion, two of the ukes have straight Piezo pickups with no preamps, but only one is causing me problems, Im now fairly sure that the reason for this is that one is connected via a wireless transmitter, and that or its receiver is converting it to a Line Level signal. Bearing in mind whats been said, I think it might be that even that instrument could sound better if its Impedance was properly matched?
Steve.

[Hugh Robjohns]

.... two of the ukes have straight Piezo pickups with no preamps, but only one is causing me problems, Im now fairly sure that the reason for this is that one is connected via a wireless transmitter, and that or its receiver is converting it to a Line Level signal.

There are several possibilities (aren't there always?)... but the most likely is that the transmitter on the uke is not presenting a sufficiently high impedance for the piezo pickup to work properly.

If you could tell us the make & model of both the pickup and wireless transmitter we would be better placed to confirm and advise.

[stevekane]

Hugh, just for clarity and your last post may still stand,,but the uke with the wireless transmitter (what are they called?) is working just fine,,maybe it could work better but its quite usable as it is.
Steve.