Preamp pedal question

DesWalker · #1 09-16-2019, 04:53 AM

Hi All,

My first ever post on this forum. Mainly an electric player moving more into acoustic.

I own two acoustics, one with a Fishman UST and the other with a K&K pure mini. Both passive without any installed preamps so I’m wanting a preamp pedal to complement these two guitars.

My question regards input impedance. I have heard it said that piezo pickups benefit most from a preamp input impedance of 10M Ohm (found on some boxes) rather than 1M Ohm (found on some others). This begs the question why many boxes only have this lower figure including the one I am considering buying.

Can anyone comment on this 1M Ohm vs 10M Ohm comparison. Is it an issue that should dictate my choice of preamp ?

My other question regards the success (or otherwise) of plugging an acoustic preamp pedal into the FX loop Return of an electric guitar amp (bypassing the preamp stages but retaining use of amp reverb and master volume controls).

But my main question is the impedance matching one ....

Very many thanks for any replies.

Des

RockerDuck · #2 09-16-2019, 05:49 AM

I found UST's benefit from a LRBaggs Para DI and the Fishman aura. The K&K works with most any, but my preference is the Fishman Platinum Pro EQ.

guitarman68 · #3 09-16-2019, 07:25 AM

Hi Des,

and welcome to this forum.
Impedance mismatch can be an issue and discussions vary a lot about this subject.
One simple rule: With your preamp do not go to far under the impedance of the pickup. Better stay above or matched.
Another important thing to keep in kind when using passive pickups in your acoustics: do not use extra long cables. Mine are 10ft Mogamis.
An electric guitar amp will not be able to reproduced the complete sonic picture of an acoustic guitar. So I will recommend an acoustic amp or a flat response powered speaker.
Many great suggestions here on the AGF: Bose S1, JBL, Elite Acoustic ... I love my QSC K8.2. QSC CP8 is as wonderful at a lower price.
When it comes to chosing the right preamp, you can not go wrong with what already said: Baggs Para or Fishman Platinum Pro EQ. I add RedEye and Grace Alix.
If a 2-channel preamp is what you need when using both acoustics on stage, RedEye 2-channel or the pricey but wonderful Grace Felix is what I would recommend.
And ToneDexter is always worth taking a look at.

Peter Z · #4 09-16-2019, 07:59 AM

Quote:

Originally Posted by guitarman68

One simple rule: With your preamp do not go to far under the impedance of the pickup. Better stay above or matched.

Maybe I don't understand this comment correctly but I'd say the impedance of a preamp should be at least 10 times as high as the pickup imedance.
That is why preamps for piezo equipped adoustic guitars have input impedances between 1 and 10 MOhm. Otherwise you end up with a really thin sound. This happens when you plug a piezo into a line input of a mixer that usually has 10 kOhm.

guitarman68 · #5 09-16-2019, 09:32 AM

I would not say that the impedance of the preamp should be ten times higher than the impedance of the pickup. Perfect match would be perfect. That is what a company like Sunnaudio does when matching their preamps to the impedance of the Dazzo pickup: Both have exactly the same impedance. If you chose a preamp with an impedance a little under the impedance of the pickup , it normally will not result in a thin sound with all the body and bass missing. If you chose a preamp with an impedance above the impedance of your pickup, there should be no problem at all.
Some mixers have an additional high Z (Z for impedance) input for using with passive pickups.
Bose L1 or S1 have a switch to turn the line input into a high Z Input for the same purpose.
Maybe it makes it a bit clearer.
Best, Bob

DesWalker · #6 09-16-2019, 09:38 AM

Many thanks for the welcome and the replies. Please keep them coming. My budget is limited and I don’t like acquiring too much stuff so will give the acoustic preamp pedal into one of my electric power amps a go before looking elsewhere. I’m not an ultra tone junky anyway ...

I see there is already a slight disagreement. Maybe 1M Ohm input impedance is enough for a standard UST otherwise why are so many preamp pedals made this way ? But then I have seen other pedals selling the merits of 10M Ohm so is there really a difference ?

jonfields45 · #7 09-16-2019, 10:27 AM

Quote:

Originally Posted by DesWalker

Many thanks for the welcome and the replies. Please keep them coming. My budget is limited and I don’t like acquiring too much stuff so will give the acoustic preamp pedal into one of my electric power amps a go before looking elsewhere. I’m not an ultra tone junky anyway ...

I see there is already a slight disagreement. Maybe 1M Ohm input impedance is enough for a standard UST otherwise why are so many preamp pedals made this way ? But then I have seen other pedals selling the merits of 10M Ohm so is there really a difference ?

For maximum power transfer and maximum signal to noise ratio, matched impedance is best.

For pickups with large series parasitic capacitance (Piezo) or inductance (magnetic) the 10x rule is good (otherwise that series parasitic makes a bad sounding filter, high pass for a piezo and low pass for a magnetic).

For pickups with multiple heads that might in some rare cases be out of phase for some frequencies, then a dominant time constant is good (1M for a K&K).

A passive UST uses ceramic crystal piezo elements (most USTs are piezo polymers these days) and will probably handle the 1M recommended by K&K for their PUP just fine.

Another option is start with a 10M product and add a strapping 1M resistor to its input if you think something is amiss.

varmonter · #8 09-16-2019, 07:36 PM

First off welcome to AGF
Here you can find the answers to just about anything acoustic. Impedance matching is just that. The kk has a 1 mgohm output inpedance.
The reason kk makes their preamps
With a 1mgohm input impedance is just that.
It matches. Preamps like the grace design felix
Have switchable input impedance to help match
Several dif pickups. Not sure what your Fishman
Impedance is but if it's different than
1 mgohm than a pre with a switchable impedance might be the ticket..

T

S

Teleplucker · #9 09-16-2019, 07:58 PM

Impedance changes with frequency. That’s pretty fundamental in the study of electronics. I had a conversation with Darren at Fire-Eye (Red Eye) and he talked about how their DI was designed to be frequency reactive.
I’m sure his understanding of electronics is infinitely greater than mine, but the bottom line for me is his products work great.

vindibona1 · #10 09-16-2019, 07:59 PM

Quote:

Originally Posted by RockerDuck

I found UST's benefit from a LRBaggs Para DI and the Fishman aura. The K&K works with most any, but my preference is the Fishman Platinum Pro EQ.

I just recently went through this exercise, but was totally oblivious to the impedance issue. At home I auditioned the Baggs Para DI, the Fishman Platinum Stage and the Fishman Platinum Pro... I ended up keeping the Platinum Pro. After reading the question and initial replies I looked up the impedance listed at 1kOhm.

I liked the Para DI, especially for its build, but chose the Fishman Platinum Pro for some of its extra features: COMPRESSION was the big one for me and I didn't need a special cable for FX loop. It didn't hurt to have a built in tuner that can also act as a mute. It is also very compact and the outputs are all parallel with the unit laying flat. The vertical XLR of the Baggs DI was kind of annyoing. The Platinum STAGE, although it shares a similar name with the Platinum PRO is really similar in name only with a few features. The Stage was my least favorite of the three.

If I had the cash I'd probably have gone with the Alix Grace.

lschwart · #11 09-16-2019, 09:35 PM

Quote:

Originally Posted by vindibona1

I just recently went through this exercise, but was totally oblivious to the impedance issue. At home I auditioned the Baggs Para DI, the Fishman Platinum Stage and the Fishman Platinum Pro... I ended up keeping the Platinum Pro. After reading the question and initial replies I looked up the impedance listed at 1kOhm.

The input impedance of the Platinum Pro is 10 meg Ohms (same as the Para DI and the Stage). I think you got that mixed up with the output impedance of the 1/4" output.

Louis

DesWalker · #12 09-17-2019, 04:11 AM

Hi All,

Thanks so much for all the replies. After reading them and further googling on this forum I have discovered my issue is quite common. My conclusions so far ...

The K&K pure mini SBT in my Sigma S000M-18 has a recommended input impedance of 1 MOhm whereas the SRT passive UST on my Yamaha LL16 has a recommended input impedance of 10 MOhm.

Using a 1 MOhm input preamp for both guitars will match the Sigma but will likely do little to stop the Yamaha sounding thin and quacky. Using a 10 MOhm input preamp for both guitars will match the Yamaha but may well (some people report this whereas others say it is a non-issue) make the Sigma sound boomy requiring heavy EQ either via the preamp EQ or a separate EQ pedal.

As has been said by one of you already, it seems so obvious that these preamps need building with switchable 1 MOhm to 10 MOhm impedances and yet very few and certainly none of the cheaper ones offer this. That has to be a gap in the market for somebody....

If I bought a 10 MOhm preamp to use directly with the Yamaha would putting my 1 MOhm Boss TU3 tuner in front of this for the Sigma tame any boominess or would the the low output impedance from the TU3 going into the very high input impedance of the 10 MOhm preamp pedal continue to cause boom issues ?

One thing is clear though. Namely a 1 MOhm preamp is a non-starter for the Yamaha so isn’t an option.

Rgds,

Des

jonfields45 · #13 09-17-2019, 05:32 AM

Quote:

Originally Posted by DesWalker

If I bought a 10 MOhm preamp to use directly with the Yamaha would putting my 1 MOhm Boss TU3 tuner in front of this for the Sigma tame any boominess or would the the low output impedance from the TU3 going into the very high input impedance of the 10 MOhm preamp pedal continue to cause boom issues ?

That is another good solution but very likely your K&K will sound fine into 10M.

vindibona1 · #14 09-17-2019, 08:16 AM

Quote:

Originally Posted by lschwart

The input impedance of the Platinum Pro is 10 meg Ohms (same as the Para DI and the Stage). I think you got that mixed up with the output impedance of the 1/4" output.

Louis

Yeah... You are correct. I know nothing about this (at least until now).
But the question of impedence nor did it come up in previous threads that I recall. I just went through this exercise and impedence wasn't even brought to my attention.

I looked at the specs again (for the Fishman Platinum Pro)...
Input impedance: 10M Ohms
Input trim gain range: -6dB to +14dB
Amp Output:
Level: -∞ to +6dBV
Output Impedance: 1k Ohms

I also checked out the specs on my other mini preamp, the Baggs GigPro...
Input Impedance: 10 Megohms
Output Impedance: 800 Ohms

It seems that the OP is quite focused on impedance. If it were, wouldn't the subject have come up in other threads?But is matching impedance to specific pickups important?

lschwart · #15 09-17-2019, 09:02 AM

Quote:

Originally Posted by vindibona1

Yeah... You are correct. I know nothing about this (at least until now).
But the question of impedence nor did it come up in previous threads that I recall. I just went through this exercise and impedence wasn't even brought to my attention.

I looked at the specs again (for the Fishman Platinum Pro)...
Input impedance: 10M Ohms
Input trim gain range: -6dB to +14dB
Amp Output:
Level: -∞ to +6dBV
Output Impedance: 1k Ohms

I also checked out the specs on my other mini preamp, the Baggs GigPro...
Input Impedance: 10 Megohms
Output Impedance: 800 Ohms

It seems that the OP is quite focused on impedance. If it were, wouldn't the subject have come up in other threads?But is matching impedance to specific pickups important?

An appropriate input impedance is important, but the fact is most units that are designed to take an input from a acoustic guitar pickup are designed to take pretty much anything you'd expect from the pickup systems that are out there.

Since there's no real "matching" at play here--the input impedance just needs to be significantly higher than the output impedance of the signal that's coming into it (the rule of thumb is 10x higher)--most units use a 10 meg Ohm input impedance, which will be fine for even the kinds of passive piezo pickups that need something very high. Even 1 meg Ohm will be fine for most pickups. And many will even be fine with the 220 k Ohm input impedance on some active DI's like the Radial J48. Lower than that and a lot of passive pickups will start to sound thin. People claim that with pickups like the K&K mini, there's a difference between 10 meg Ohm and 1 meg Ohm, but this difference has never been demonstrated beyond subjective reports. With that kind of pickup, it's just best for people to try units with 1 and with 10 and see if that (or other features) makes a difference to them. Most magnetic pickups and anything that's active, can pretty much go into anything, and the differences between different DI/Preamp/EQ units are really about features and aspects of design other than the input impedance.

Louis