Help me change this DI input to 10Mega impedance

[Whoops]

If you want to make a di just for piezos (like for an upright bass), I have found charge amp designs giving the best sound, there are a lot of examples on the net, best explanation I found was
here.

In some bands that I do Live sound for I have been using for the Piezo pickups Radial PZ DI and the SB4, both are "optimised" for Piezo meaning one is able to provide 10M input impedance and the other 5M.
These are used in Double Bass and Cello piezo pickups without a preamplifier or buffer.
I tried using standard 1M input impedance DI's (BSS, Klark Teknik, LA Audio) and the sound is definitely much improved when using an higher input impedance DI, it's quite noticeable on the Double Bass but also on the Cello and even in acoustic guitar piezo pickups.

So if someone wants to start an argumentative loop about this it's their choice, but this situation is pretty well documented everywhere and personally to me it's proved since I did A/B tests, and the higher impedance DI's always won with piezos compared to the standard 1M.

Saying all this, I don't have the Radial DI's, they are from a colleague and as I'm more into building my own stuff than buying already made I would like to build my own DI's.
I'm finishing at the moment some Bo Hansen DI's which seem to have pretty good reviews around here and I'm implementing the 10 Mega inout impedance mod on those.

This Kevin circuit came to my attention, since it's small and simple, the pcb is the size of a standard card and also it's a transformerless circuit which brings the cost down.
Kevin has also a nice box for it.

If the circuit is useful and can be modified for an higher input impedance fine, if not I think I will be pretty happy with the modified Bo Hansen DI, even though the transformers brings the cost a bit higher.

Thank you all

 

[JohnRoberts]

IMO it's a good idea to make the input impedance switchable 1M /10M

Obviously, guitar/bass pickups are designed for guitar/bass amps whose input impedance is typically around 1M. So if you want to simulate this interaction with your DI, 1M is a good value.

However, if you use your DI in combination with a guitar/bass amplifier, your DI input is in parallel with the amp input. If both are 1M, your guitar pickup now faces 500k. Which does change the sound a bit - many players won't care, but some do. To minimize this sound change, your DI should have a much higher input impedance. 10 M would be a good value as it's the largest resistor value you can get cheap.

Good point, it is prudent to understand what termination the transducers are designed to interface with.

My old DI design included a unity gain buffered thru output jack so there was no reason to parallel inputs. The buffered unity gain feed could drive guitar amps or whatever.

JR

 

[MisterCMRR]

I think something that needs to be explicitly mentioned here is that the input impedance of the input forms a high-pass filter with the capacitance of the piezo (they're pretty much a totally capacitive source). So, for a piezo of 500 pF capacitance, a 10 MΩ input impedance makes the system -3 dB at about 30 Hz. But, if input impedance was only 1 MΩ, that 3 dB down point would be at 300 Hz. The piezo pickups I've seen (admittedly not a lot) are in that ballpark capacitance wise. And bootstrapping the input stage is a good way to raise the AC impedance without having to worry much about tiny bias currents in the input op-amp (you might even be able to use a good bi-polar input type.

 

[Whoops]

the input impedance of the input forms a high-pass filter with the capacitance of the piezo (they're pretty much a totally capacitive source). So, for a piezo of 500 pF capacitance, a 10 MΩ input impedance makes the system -3 dB at about 30 Hz. But, if input impedance was only 1 MΩ, that 3 dB down point would be at 300 Hz.

Yes,
Easy to see in this graph,
Although there’s an error in the legend the “typical active DI” is 1M and not 100K as the author stated, maybe he was thinking about passive DIs and not active, I don’t know.
But it’s easy to see the unnecessary mids, low mids, lows and sub loss.

 

[JohnRoberts]

Perhaps they were trying to make Leon Audio DIs look good...

JR

 

[Whoops]

Perhaps they were trying to make Leon Audio DIs look good...

JR

I checked the specs of my passive DI boxes, and the stated input impedance is 100k. So probably was a typo, it meant typical “passive” DI and not “Active”

 

[RuudNL]

There may be better designs, but this certainly works.
You can increase the value of R3 and R7, if you want a higher input impedance.

 

[JohnRoberts]

I checked the specs of my passive DI boxes, and the stated input impedance is 100k. So probably was a typo, it meant typical “passive” DI and not “Active”

A passive DI is basically a step down transformer.

The effective input impedance is the mic preamp input impedance stepped up by the transformer turns ratio squared. Working backwards from 100k looks like a transformer turns ratio that is the square root of 50 (around 7x).

JR

 

[Whoops]

There may be better designs, but this certainly works.
You can increase the value of R3 and R7, if you want a higher input impedance.

Thank you so much Ruud,
I can see there’s a battery connection.
Is it able to be powered by phantom power or is battery operated only?

 

[MisterCMRR]

A passive DI is basically a step down transformer.

The effective input impedance is the mic preamp input impedance stepped up by the transformer turns ratio squared. Working backwards from 100k looks like a transformer turns ratio that is the square root of 50 (around 7x).

JR

The Jensen JT-DB series "direct box" transformers are 12:1 and are used in many high-end boxes, including the Radial JEDI, of course. Their input impedance, which depends on the mic preamp's impedance of course, is usually well over 300 kΩ.

This might also be a good place to mention that, when circuit resistances get this high (10 MΩ or more), humidity can seriously shift the DC operating point of op-amps. It's a good idea to "guard" the high-resistance trace with ground traces all around it. A power rail next to it can, due to humidity-induced surface leakage, pull the input up toward the rail, reducing headroom and causing distortion.

 

[RuudNL]

Is it able to be powered by phantom power or is battery operated only?

Both.

 

[Bo Deadly]

There may be better designs, but this certainly works.

It should be said that this will not work driving an unbalanced load because driving cold to ground through 100R will cause distortion in the TL072. So it's really only for use with a balanced in like on a console. That is becoming less common. So unless you're trying to drive really long lines, I would just do an impedance balanced output. And it inverts incidentally. And 9V isn't going to give you a high level out.