Puzzling Issue but not a problem

[twotoedsloth]

Hello All, hope you are having a good weekend.

I'm mixing a live show today, and we just finished our sound check.

The reason I am writing is because the acoustic guitar player forgot to bring his preamp, and I don't have a preamp, or a DI box that supports 10megaOhms.

However, we plugged his guitar directly into a passive Radial DI, and amazingly it works quite well. What is going on here? As I said, not a problem, just weird.

Sorry if this is a bother,

Peter

[Hugh Robjohns]

Much depends on the specifics of which pickups are being used... but not all need to see 10M ohms, and most will still work into lower impedances... Some are even designed specifically to work with much lower impedances!

The tone usually changes significantly if a pickup expecting 10M sees something much less, but you can probably extract something perfectly workable with EQ.

An active DI would be better than a passive one... but it'll still give you something to work with.

[Wonks]

My passive K&K prefers 10k ohms to 10 Meg ohms. But I've had other pickups that do need the higher impedance.

You'd need to know what pickup he had. And he may just use a preamp for the tone controls on it, or possibly for IR impulses to give a better sound.

You are always going to get some sort of sound with a piezo, it just depends how much EQ it needs to make it sound like a decent acoustic.

[Sam Spoons]

Acoustic pickups range from 'passive' mag soundhole pickups which work much like an electric guitar pickup through passive piezo's to active systems with an onboard preamp (which may even have an IR loader built in) to onboard mics or SBTs or WHY and all possible combinations and variations of above. Some will sound rubbish without an external Hi-Z buffer some have one built in. A clue is if the guitar has a battery it usually doesn't need an external preamp or Hi-Z DI.

But, even quite sophisticated pickup systems may benefit from some kind of additional processing (with IR based being, usually, the most successful).

[AlecSp]

Back in my day, I learnt by experimentation. I would happily try an acoustic guitar through a passive DI, discovering that some behaved much better than others.

Sadly, learning through experimentation appears to be a lost art...

[twotoedsloth]

Hello,

Thanks for all of your responses, it is useful going forward. Unfortunately the guitarist didn't know what pickup he had, however it required very little EQ.

Perhaps this is because the DI box is feeding an Audient ASP 880, with the impedance set to High, and the highpass filter engaged. I'm not sure how high the impedance actually is, but it does affect the tonality of dynamic mics, but doesn't seem to do much to capacitor mics.

Anyways, the gig went well, everyone was happy- audience and performers.

Cheers gentlemen,

Peter

[Wonks]

So the low impedance XLR output on the DI box was fed into the TRS hi-z impedance input on the Audient?

A passive DI box 'reflects' the impedance of the input it's plugged into, and multiplies it by the square of the transformer turns ratio. This ratio is typically around 11.5:1, so has an impedance multiplication factor of about 133:1.

So with a 1.2k mic input you get around 159k. A 2k input you get around 266k With a 1 Meg input you get 133 Meg. Which could cause noise issues but will make some piezos very happy.

But if you were taking the output from the TS through on the DI box, you'll basically get the same impedance as the input you are plugging it into, if the DI box XLR output remains unconnected.

[ef37a]

"So with a 1.2k mic input you get around 159k. A 2k input you get around 266k With a 1 Meg input you get 133 Meg."

I think 133 M Ohms is stretching transformer theory a bit much Wonks? Maybe just simplifying? I would suggest inter-winding capacitance would shunt the input and drop the impedance well below that figure?

Perhaps worth mentioning that some of the various "DI*" boxes that exist in droves now are rather misnamed and actually contain low impedance "600 Ohm" transformers. They will sort of work in many situations but the frequency response would not stand close examination!

*To me a 'proper' passive DI box is of the sort being discussed. A step down transformer with a ratio of around 10 or 12:1.

Dave.

[Wonks]

I was quoting that from an article on the Whirlwind cables website, and they make DI boxes. But I'm sure it all gets complicated when you look deeper.

[Wonks]

But even so, if the transformer output was wired into a 1Meg input, then the guitar would certainly see a much greater input impedance than that.

[ef37a]

But even so, if the transformer output was wired into a 1Meg input, then the guitar would certainly see a much greater input impedance than that.

Hmm, LOVE to measure it mate!

Dave.

[Hugh Robjohns]

So the low impedance XLR output on the DI box was fed into the TRS hi-z impedance input on the Audient?

No! Sounds to me like the DI box was connected to the Audient's mic input... as is the intended usage. The preamp's impedance switch doesn't affect its own instrument input.

The mic inputs of the ASP880 can be switched between three input impedance options: low (220 Ohms), medium (1k2 Ohms) or high (2k8 Ohms).

So all you wrote about reflected impedances across transformers, while broadly true, is not relevant to the OP's application.

[Mike Monte]

I for one have never found the need to use an active DI box when doing live sound for band work in my area.
I have had no-ill-issues running acoustic guitars through one of my Horizon Stereoline passive DI's.

10+ years ago I purchased an LR Baggs active DI - and I have yet to use it!

This thread is a reminder to me to check the battery (which I store outside the unit) as I'm sure that it is out-of-date by now.

[twotoedsloth]

Hello,

Hugh is correct, I was running the guitar (unknown), to the passive DI box (Radial ProD2), to the mic preamp (Audient ASP880, with highpass filter and set to high impedance).

As mentioned, I have no idea which pickup the guitarist was using (and neither did he), but as I could not see a magnetic pickup in the soundhole, I am assuming it was a piezo of some sort.

I just remembered that I could have used my Royer dBooster2 as an active DI, don't know why I didn't think of that yesterday. The passive box did the job just fine, so no harm, no foul.

Happy Monday everyone.

Peter

[James Perrett]

As mentioned, I have no idea which pickup the guitarist was using (and neither did he), but as I could not see a magnetic pickup in the soundhole, I am assuming it was a piezo of some sort.

Are you sure that there wasn't also a preamp on the guitar?

[Wonks]

Hugh is correct, I was running the guitar (unknown), to the passive DI box (Radial ProD2), to the mic preamp (Audient ASP880, with highpass filter and set to high impedance).

The 1 Meg Hi-Z impedance only works on the DI input on channels 1 and 2. It;s not switchable.

The impedance selected on the selector switch on the front panel for each channel is primarily for the mic input, and Hi is 2.8k, Med is 1.2k and Lo is 220 ohms. It's in series with the signal from the line input, so will also have an effect on the line input impedance as well (though as line input feeds are normally from electronically driven outputs it has very little effect on these apart from maybe minor changes to signal levels).

Switching to Hi on the acoustic guitar channel will have helped the raise impedance of the DI box seen by the guitar pickup. But at the moment we still don't know whether this was a directly connected piezo or whether it came from a preamped output in the guitar.

Can you remember what the make of guitar was?

[twotoedsloth]

Hello James,

I didn't notice a preamp on the guitar, the cable connected to the strap button on the end of his instrument. He could have stashed a battery inside the guitar somewhere, but I don't think he had a control panel where he could adjust settings. He just plugged his guitar cable into our DI box, he didn't seems to make any adjustments.

I'm always amazed at how guitarists will spend years getting the tone they want from their instrument, and then when they show up for a gig, they just completely rely on the sound tech, no matter how godawful it sounds.

[Hugh Robjohns]

The impedance selected on the selector switch on the front panel for each channel is primarily for the mic input, and Hi is 2.8k, Med is 1.2k and Lo is 220 ohms. It's in series with the signal from the line input, so will also have an effect on the line input impedance as well ...

The line inputs pass through a hefty attenuator pad before heading into the mic preamp, and that pad is the dominant element in defining the line input impedance (which ranged from around 6k to 8k5 Ohms with different impedance settings when I measured it).

The impedance selector no effect on the instrument input as that passes through a separate buffer and is routed to the preamp after the impedance selector circuitry.

[Wonks]

Some guitars have the preamp located around the upper section of the soundhole so it's pretty much invisible unless you are looking for it.

On Taylors, you've got three small flat knobs on the upper front bout facing towards the headstock, so again, quite discrete.

Sometimes the preamp is built into the output jack.

It's quite rare for a factory fitted guitar to come with a pure piezo system with no preamp. The after-market ones like K&K can be fitted without a preamp to avoid modifying an old acoustic guitar, but the piezos that K&K use don't need a very high impedance input like some types of piezo do.

But even with a built-in preamp, you may want to use an external preamp simply for its local tone-shaping qualities rather than for it's actual pre-amp section.

We'll probably never know, unless you come across the guitarist again!

[twotoedsloth]

Slightly off topic,

But wouldn't a preamp mounted onto the guitar affect the acoustic sound? I'm a bass player, and I use a Fishman pickup that clamps onto my bridge. I take it off whenever I'm not using it, as it (slightly) mutes the sound, maybe it only bothers me, but I'm convinced I can hear it. I use a Radial J48 DI box, which has a lowish impedance, 220k ohms, but it works just fine for me. That being said I have been borrowing a PZDI from work, also from Radial, and I think it sounds better, especially with the low pass filter engaged. I'm going to have to buy one when the university starts up again in September.

Back on topic, I think that the guitarist in question was playing a Taylor, but if he had knobs on the top, they must have been very discreet.

My mother plays a Larrivee, it takes a battery so it must be active, but there are no knobs to adjust, she does this on an LR Baggs ParaDI, which works well enough for her. I guess this means the preamp is built into the output jack. Is it common for piezo pickups to clip or do they saturate like a transformer? I've never noticed it in the past, but with no controls to adjust it seems rather brave.

[Wonks]

Piezos don’t clip or saturate. They are voltage generators, so they just produce more output.

Given enough pressure, they can generate enough voltage to create a spark across an air gap (i.e. several hundred volts) - if you’ve used a handheld gas lighter that gives put a spark, then that’s almost certainly a piezo crystal.

But an under-saddle piezo strip doesn’t get 'squashed' much at all by the vibrations, so the output is pretty low. An under-saddle strip is actually not best positioned as there is very little up and down movement from the string vibrations - most of the string energy is used in moving the saddle backwards and forwards as the string tension increases and decreases with the string vibration. So placing it against the front or rear of the saddle is a lot better. but isn't as easy to engineer a solution that works without making the bridge/saddle very complicated with adjusting screws to clamp the saddle to the piezo. Sticking it under the saddle is a lot easier and the string pressure will keep the saddle and piezo element in contact (providing the base of the saddle slot and the base of the saddle is flat).

It's part of the reason why piezo USTs have that quack.

Sticking the piezo elements inside the guitar under the base of the bridge gives a subjectively more natural sound, though maybe without some of the note definition that a UST strip gives. Because the elements in these aren't being squeezed between something solid and something vibrating, the signal generating effect is smaller, so you need larger elements to compensate. And because they are larger, you pick up vibrations that may be quite out of phase with each other at high frequencies and partially cancel out.

The standard Taylor volume/treble/bass controls are very near the neck join, not on the top of the guitar.



But there are some Taylors that have a 3rd party system installed.

This 814e has an LR Baggs Element VTC system, with the volume and tone controls hidden inside the soundhole, so you wouldn't know it had any system fitted at all if there wasn't a lead plugged into the endpin. The battery is normally in a clip or a pouch attached to the rear of the neck block. so doesn't affect the vibrations of the top or sides.



You can only just see the controls from this angle.



Obviously the tone control on this is almost certainly a basic treble roll off, so this is where an external preamp with tone controls or IR loading capability comes in handy.

[twotoedsloth]

Hello and thanks for the detailed response, Wonks.

Does the high end roll off obviate the need for 96kHz?

As mentioned earlier, I use the lowpass filter on my double bass, but doesn't the guitar need the brightness that would be eliminated with a high end roll off?

Many thanks,

Peter

[Wonks]

Like everything, there can be 'too bright'. You can fit a wide range of strings on acoustics of different materials, and some are much brighter sounding than others (especially if brand new). Also, some acoustic amps (if you aren't using a PA) can be quite bright in themselves. And on some songs you might want a slightly less bright sound if you are mainly singing over the top of the guitar, as opposed to a pure instrumental where you might want all the detail you can get.

And just because it's there doesn't mean you have to use it. I'd bet most of those single tone controls rarely get touched.

[Sam Spoons]

I do roll of the treble on my Emerald (with B-Band UST and preamp) but my other acoustics don't have a tone control

Does the high end roll off obviate the need for 96kHz?

You don't need 96kHz to reproduce the highest frequencies you can hear, never mind the highest reproduced by most guitars. The CD standard of 44.1kHz at 16 bit is more than good enough for anything most of us are capable of hearing in the real world.

As mentioned earlier, I use the lowpass filter on my double bass, but doesn't the guitar need the brightness that would be eliminated with a high end roll off?

Many thanks,

Peter

Depends on what sound you are trying to achieve, and it depends on how your pickup system reproduces the sound of your guitar. My Emerald sound more real with the treble rolled off, my gypsy jazz guitar through the Tone Dexter sounds so close to the miked sound that I'm happy to record with it.

UST's often sound harsh and 'quacky' without some intervention, I believe this is because the transducer is being squeezed between the saddle and the bridge as the 'big tone' bridge on my Rob Aylward Selmer style does not quack despite using exactly the same transducer. I believe this is because the transducer is epoxied into a slot routed into the underside of the floating bridge so is not under any pressure. The same applies to K&K type SBT's which are bonded to the bridge plate and do sound more natural.