Help with Variable Impedance Active Direct Box

[rp3703]

Not to steal from another product but everyone swears by the MW-1 for recording a clean guitar DI signal. For a transformer-less DI though, $800 seems a bit high. Yes it come with re-amping and a drive section but I don’t need that. I am interested in the adjustable input impedance though. Rod Elliott has a schematic for an active DI  here https://sound-au.com/project35.htm but how would you create an adjustable input impedance for it? I also read on this thread https://groupdiy.com/index.php?topic=3583.0, that adjustable input impedance is not necessary and just a gimmick but I think that pertains to transformer based DI’s. Thoughts?

 

[abbey road d enfer]

Rod Elliott has a schematic for an active DI

The actual input impedance of this DU is way too low, at about 23kohm, but it can be easily increased by replacing R5 with a higher value- typically 1Megohm - and disconnecting R1, VR1 and R3.
Others swear by higher inoput impedance like 10 Megohms. I don't. But it's quite easy to experiment.

how would you create an adjustable input impedance for it? You could replace R5 with a 10Megohm potentiometer, that may be quite hard to finf, or a multiposition switch that would select different resistors. he problem with that is that it would produce loud clicks when switching.
The best solution is to make R5 10 megohm and install the switch with the different resistors right across the input.

I also read on this thread https://groupdiy.com/index.php?topic=3583.0, that adjustable input impedance is not necessary and just a gimmick but I think that pertains to transformer based DI’s.
[/quote] It's a matter of taste, which can't be resolved for you by anyone else; you like it or not.
Me, I'm perfectly happy with 1 Megohm.
BTW the circuit would benefit using an OPA2134 instead of a TL072.

 

[rp3703]

Thanks for catching the input impedance on Rod’s design and the chip swap. I agree with you on the 1M. I got started down this rabbit hole when I tried using my JDI for guitar and noticed how bad it sounded. Once I realized the impedance was the problem, I tried the DI on my MPA2 and my guitar tone returned. My only issue with leaving it set at 1M is if I were to use the DI on a guitar with active pups. That’s where I think an adjustable input would come in handy. It would seem kind of hard to determine when you have it set right though.

 

[abbey road d enfer]

Thanks for catching the input impedance on Rod’s design and the chip swap. I agree with you on the 1M. I got started down this rabbit hole when I tried using my JDI for guitar and noticed how bad it sounded. Once I realized the impedance was the problem, I tried the DI on my MPA2 and my guitar tone returned. My only issue with leaving it set at 1M is if I were to use the DI on a guitar with active pups. That’s where I think an adjustable input would come in handy. It would seem kind of hard to determine when you have it set right though.

Active guitars already include a suitable load resistor.

 

[Bo Deadly]

I think variable input impedance sounds like a great feature. The tone of a guitar changes quite a bit depending on the input impedance. Not all guitar benefits from extracting every little high frequency detail. Sometimes you want something muted like for some rhythm strumming. To implement that, you could just use a rotary switch and pick some impedances like 2.7M with the switch open and then switch in resistors in parallel to get 1M, 100K, 68K, ... maybe down to something really low like even 1K to get a sort of weak guitar effect caused by the varied impedance of the guitar source with the low impedance of the load. Although I would at least experiment with using an A1M pot because then you can precisely adjust the tone by ear at lower Z settings which might be nice. You don't get the recall and adjustment at higher impedances isn't as precise but above a certain point it doesn't matter that much (thus the log pot).

Aside: I think muted guitar sounds are under valued. Everyone likes their instrument to be prominent but when played together you get a log of high frequency hash with no room for other stuff. So being able to restrict the detail of an e-guitar by directly manipulating the impedance it sees is desirable to me.

I would go a step further and make capacitance adjustable. Again a rotary switch with one position open and then caps of increasing size like 10p, 47p, 100p, etc. This will shift the high frequency cutoff down with each step which is different from the tone control on the guitar which just attenuates above a specific frequency.

This type of device makes a lot of sense to me because you're explicitly adjusting the DC and AC impedance of the guitar network respectively. You can effectively mimic the impedance of just about any amp input, high or low and long coiled cables and such. And noise performance will also be superior because you're loading the guitar directly.

For a cheaper, simpler solution, use ON-ON-ON toggles to give you 3 settings for DC impedance using shunt resistors and AC impadeance using caps.

 

[rp3703]

Again, the purpose of this device is to try and keep the signal being recorded and re-amped as close as possible to what is coming straight out of the guitar to begin with. While a wide variety of impedance options sounds like a good idea, if there is no way to confirm when the knob is in the “right” position, then the option is pointless.
From what I’ve read, most people agree that a 1M load sounds the best with humbuckers. Since most guitar amps have a HI input of 1M for humbuckers as well, would it make sense to also have the same 150K Lo impedance for single coils and active pickups? I wish there was a more scientific way to figure this all out.

 

[Bo Deadly]

If you're recording at the amp using a through connection, then the amp sets the impedance and so your DI should be very high input impedance. If you made it 1M, then the 1M impedance of the amp would be in parallel and result in a load on the guitar of only 500K. So you would want the DI to be much higher like 4.7M or maybe even higher. Just enough to keep the input from floating.

If you're recording dry without an amp, then you need to mimic what the amp is doing. High inputs are usually 1M. Low inputs are typically 68K. Capacitance can be unmodified (low) because the cable would be part of a dry setup but if you want to also mimic a longer cable you might measure one and add a small cap on a switch.

As for driving the amp for re-amping, you don't want to mimmic the source at all because you're just reproducing the voltage seen when recording so impedance should be very low. You might only have 33 ohms or so in series with the output for stability but you might leave it out altogether.

 

[abbey road d enfer]

Since most guitar amps have a HI input of 1M for humbuckers as well, would it make sense to also have the same 150K Lo impedance for single coils and active pickups?

Where does that figure of 150k comes from?
Leo Fender designed his single-coil guitars with 1Megohm inputs in view.
A Jaguar/Jazzmaster wants a high impedance input, due to its 1Meg pots.
For active p/u's, you don't care because load has no effect on the sound as long as it does not exceeds the drive capability. Active guitars are perfectly happy with anything from 20k to several Megs.

 

[rp3703]

I just read that the Lo input on most Fender and Marshall amps measured around 150K, never measured one myself. Now that I think about it, I think I had it backwards; Hi input was originally designed for single coils and Lo was meant for hum buckers. I guess since most everyone just plugs into the Hi then 1M would be the standard for all passive E-guitar pups.

What about passive E-bass?

 

[rp3703]

Here is where I got the 150K(Actually 136K)

"Lets take the High input. If you plug your guitar into that input (and nothing in the Low input), the impedance seen by the guitar is 1 megohm (see that resistor to ground marked 1M). The two 68K resistors in parallel (34K) are in series with the guitar and the grid. Since the impedance into the grid is infinite, no current flows through those resistors and the signal seen by the grid is exactly the same as the top end of the 1M resistor. That is, 100% of the guitar signal is applied to the grid. Summarising, this "High" input has an impedance of 1M and no attenuation of the guitar signal.

Now consider the "Low" input (with nothing plugged into the High input). Now you have two 68K resistors in series to ground so the input impedance seen by the guitar will be 136K. The connection to the grid occurs at the midpoint of these two resistors therefore the signal on the grid is one half of the signal generated by the guitar - in other words, -6dB (for voltage, attenuation of one half is a 6dB cut, for power an attenuation of one half is a 3dB cut). This input has one of the 68K resistors in series with the grid so it will have slightly different RF characteristics but this is of no consequence. Summarising, the "Low" input has an impedance of 136K and cuts the signal by 6dB. This "Low" input will also present a different sonic result because of the lower impedance. The higher the internal impedance of the guitar, the more noticeable this will be."

The Lo input attenuates signal so it's no longer a factor in my discussion but I wanted to relay the info on it.

 

[abbey road d enfer]

I just read that the Lo input on most Fender and Marshall amps measured around 150K,

  Correct. Two 68k resistors in series.

  Now that I think about it, I think I had it backwards; Hi input was originally designed for single coils and Lo was meant for hum buckers.

  No. Lo inputs were originally for allowing to turn down the volume and still have the volume post in a manageable position (not something between 0.5 and 0.75 on a 10-scale).
Almost nobody in their right mind use the Lo input.
Humbuckers want at least 500k and Tele/Strat single coils want to see min 250k.

  I guess since most everyone just plugs into the Hi then 1M would be the standard for all passive E-guitar pups.

  You finally come to the right conclusion.

What about passive E-bass?

Since the bass register requires less treble than the guitar, the loading effect is less questionable, but again the P-bass and J-bass have been designed in view of 1 Meg inputs.

 

[john12ax7]

  No. Lo inputs were originally for allowing to turn down the volume and still have the volume post in a manageable position (not something between 0.5 and 0.75 on a 10-scale).

I recall the original intent of the 2nd input was to allow 2 players to plug into the same amp.

 

[john12ax7]

If the goal is clean DI then just go with 1M, it's an expected standard. A good fet opamp,  like an OPA134, as a unity gain buffer and 1M resistor will get you there.

 

[abbey road d enfer]

I recall the original intent of the 2nd input was to allow 2 players to plug into the same amp.

It has never been advertized as such. Having input 1 and input 2 is what's made to cater for two players.
Actually one was often used for guitar and the other for a vocal mic.
Plugging two guitars in one input works somewhat, but is an abuse.

 

[dbelousov]

The only reason to make impedance higher than 1M is the unbuffered parallel connection with other devices.

Active pickups/electronics don't have to face lower impedance, so the variable impedance is redundant in this case.

The only reasonable purpose of variable impedance is to "mimic" impedances of very particular amps.

This MW1 device has lots of futures, hence the price. A simple high fidelity buffer can be made for a little fraction of it without any compromises in sound quality.

 

[rp3703]

Thanks dbelousov. So you're saying that if this DI were to have a through connection, it would be preferable to design that with a Hi-Fi buffer rather than passively because a passive thru will screw up the input impedance of the DI?
I would like to add a thru for running to a tuner and a monitor matrix. Any designs you would suggest?

 

[dbelousov]

I suggested this in some other thread. It was implemented with different component values, but it's not that crucial. The "real" isolation is impossible to achieve without trafos, though, but it should work fine in most cases.

 

[Whoops]

Others swear by higher input impedance like 10 Megohms. I don't. But it's quite easy to experiment.

We had that conversation some days ago in another thread.
What people found and also myself is that for Piezo pickups (normally used on acoustic guitar, Doublebass, Cello, ectc) and piezo only, a 10Mega input impedance on a DI provides a better load. But only for Piezos

For passive guitar and bass pickups (electric guitar and bass) the 1 Mega input impedance that is standard in most active DI's is adequate and it's whats used also on most guitar amplifiers inputs.

 

[Whoops]

I also read on this thread https://groupdiy.com/index.php?topic=3583.0, that adjustable input impedance is not necessary and just a gimmick but I think that pertains to transformer based DI’s. Thoughts?

No it's not a gimmick on DI boxes and you should try it yourself, it's quite noticeable.
I think that thread was discussing it on Mic Preamps and not on DI boxes input, so I don't think is that much relevant to an Active Direct Box

I am interested in the adjustable input impedance though.

I don't know the box you referred to, so I can't comment on how they have the adjustable input impedance.
But for a DI box input I find a 3 position switch with 3 different input impedance settings like they did it on the PZ Radial DI works great and gives me all the range and settings I need.

You have 1Mega for Electric Guitar/Bass magnetic pickups, 10Mega for Piezo pickups, and 220k.
I love the 220K setting on bass, it's on the low side for magnetic pickups so you loose high end, nice for an old school 60s/70s bass sound, similar to a passive DI input.
You can also use any setting in any instrument or pickup you like and use it has an EQ, and No it's not a gimmick these 3 settings are so far apart that it's quite noticeable.
Also after you try a 10Mega input DI on a Piezo (like this radial or the Country Man) you will never go back.

On a DIY DI project like the Bo Hansen DI, it's probably possible to adapt a switch to change the input impedance for different values. I will probably do the same concept of the Radial PZ-DI on my Bo Hansen DI boxes

 

[Whoops]

The only reason to make impedance higher than 1M is the unbuffered parallel connection with other devices.

The biggest reason and the only one I see for Active DIs with higher than 1M input impedance is to provide a proper load for Piezo pickups. A lot of piezo pickups have around 1Mega output impedance.

Radial has 2 options , the "SB4 Piezo" with 5mega and "PZ-DI" with 10mega input.
From the Radial website:

"the SB-4 features a very high 5 meg ohm impedance that smooths out troublesome peaks that give piezo transducers a bad name. This higher impedance also broadens the frequency response for a more natural and pleasing sound."

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

"The third setting is an ultra-high 10 meg ohm impedance that is specifically designed for piezo transducers, helping compensate for the harsh or squawky tone that can occur when a piezo is connected to a lower impedance input."

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

The trustworthy Countryman Type 85 also has 10mega input impedance, and it's quite well regarded for many years now

https://countryman.com/product/type-85-direct-box/