Bo Hansen DI design question

[Gus]

When I looked at the schematic(I like the low parts count well thought out circuit) I wonder why I have not seen one with a bootstrapped buffer stage for a little more input impedance?
I like it uses BJTs. I avoid JFETs when I can.
Remove the filter cap at the bias network and install a cap from the 2nd transistors emitter to the input 470ks and 1meg node. I would then reduce the 470 bias resister values.

There is nothing wrong with the stock circuit this idea is if you want more input impedance if the device driving the DI works better into a higher impedance. You need to be careful with the cap value picked.

 

[Newmarket]

When I looked at the schematic(I like the low parts count well thought out circuit) I wonder why I have not seen one with a bootstrapped buffer stage for a little more input impedance?
I like it uses BJTs. I avoid JFETs when I can.
Remove the filter cap at the bias network and install a cap from the 2nd transistors emitter to the input 470ks and 1meg node. I would then reduce the 470 bias resister values.

There is nothing wrong with the stock circuit this idea is if you want more input impedance if the device driving the DI works better into a higher impedance. You need to be careful with the cap value picked.

What problem do you have with jfet devices in general ?
What source impedances are you thinking about ? Piezo transducers ?

 

[Ricardus]

Somewhere in the Bo Hansen thread there is a JFET mod that was tested, and worked. I know you said you don't like to use them, but I'm just throwing it out there.

 

[Gus]

BJT transistors are more predictable and often cost less.
In simple circuits JFETs often need to be selected. There are ways to make circuits to "drop in" non selected JFETs but they are more complicated.

I just posting if one wanted a higher than 1 meg in parallel with the transistor input resistance (beta, Re and emitter resistor set this) so the input resistance is less than 1meg
One can even make the circuit change switchable by moving one end of the cap to different nodes.

Also what bootstrap cap value you pick can change the low frequency interaction with something like a guitar this can be fun to adjust.
I posted a circuit at diystompboxes called the NPN boost ver 2 that showed the interaction(this was years ago so the links might be broken to the graphs)

For piezos I would most likely use a JFET or tube I have done experiment in the past with different input HPF -3 settings and loading of guitars into different types of effects some work better at 220K others at 470K 1 meg or higher. It is about the interaction with the cable, input loading and RLC output of the guitar at different settings.

Also something a little different to try.

 

[CJ]

Gus, what do you think of the Countryman?

i thought nothing special,

maybe i built it wrong

 

[Newmarket]

Also what bootstrap cap value you pick can change the low frequency interaction with something like a guitar this can be fun to adjust.
I posted a circuit at diystompboxes called the NPN boost ver 2 that showed the interaction(this was years ago so the links might be broken to the graphs)

Yeah. The lf response is definitely a thing with this type of "bootstrapping". The Pete Cornish buffer is worth a look in this regard.

 

[Gus]

CJ
A little off topic
You might like this

NPN Boost Version 2 - Vero

 

[CJ]

I gotta try that!

 

[Chandra]

Here is a simple transistor preamp, tested to 9 or 18 volt supply.
Article explains some details.
Alternative transistors can be substituted.
You might find it useful.
Regards.

 

[Whoops]

Easy to have 10 Meg input impedance on the Bo Hansen DI just with a swap of 4 components:

https://groupdiy.com/threads/bo-hansen-di-layout.36569/page-45

 

[Gus]

Whoops

That is not the point.

Yes it simple to add a JFET no big deal.

Why assume it is about piezos?

Think about what I posted.
I often post hints and hope that people might do some testing/reading maybe even try it out and learn something.
I stopped posting complete answers why make it easy for the people who just copy.

Gus

 

[ChuckHanning]

Hi all,
Like everyone else I got one of the recent circuit boards and am currently putting together the DI. I had a couple questions about increasing the capacitors.

1. For the 10 uF that bootstraps the 1Meg input, I also have some 82 uF caps. Would increasing them in crease the low end input?
2. Increasing the 100uF that bypasses the phantom input to 220 or 470 uF would increase the current for transients?
3. Lastly increasing the output coupling capacitor to 100 uF instead of 10 would increase the low end linearity for mic preamps that typically have an input impedance of 1-2 k?

Thanks everyone in advance for opinions!
Chuck