Instrument signal muffled. Am I misunderstanding the chain?

Getting Stuff Done How do I .... ?
#31

I’m not as optimistic.

The pickup’s impedance is frequency dependent, but that relationship is rather complex and the guitar coil and low-impedance input form a 1st order resonant low-pass filter. That is definitely lossy. The signal is attenuated, low-pass filtered, and parts of it will vanish in the noise-floor. Also some sound characteristics change. For example the sustain is shortened when the coil pickups are not feeding a Hi-Z input.

If you heavily process the signal afterwards (loads of compression, distortion, chorus, etc.), you might be able to get away with undoing the effect of the low-pass. But keep in mind that this also depends on the pickup type and will be different for different guitars.

Meanwhile just inserting a 470kOhm resistor before the input, terminated with a 22kOhm to GND and ramping up the input gain by ~27dB will likely produce a better sound, compared to toying eith EQs.

#32

First off, it is crazy how much of a brainiac you are. :slight_smile:

Second. I will do this, but oh man, I am not an electronics guy. What’s the ELI5 version? Take a guitar cord, cut it, strip the wires, connect ? to ? and ? to ?. What component increases gain?

Third, do you think there is a pedal that does all this already? If so, I’m happy to buy it as a stopgap until my custom hifiberry is made.

#33

The phrase you want to look for is “buffer pedal.”
I did a quick search for guitar buffer pedal and found this one for about the price of a NYC hamburger:

I can’t vouch for it, I really mean just found it seconds ago, but that is the kind of thing to look for. For $17 and free shipping I would give it a try.

#34

If you’re willing to purchase off-the-shelf hardware, then any DI box will do (and do a better job than some hack using resistors only).

five? Don’t do this at home, kid! Ask your parents to help :slight_smile:

I was thinking do to a simple tentative hack with resistors that an ex EE student may have at hand, while you wait for proper hardware:

Guitar (source) o--+----R1----+---o Soundcard (sink)
                              |
                   +----R2----+
                   |
Ground   o---------+--------------o Ground

R1 = 470kOhm
R2 = 22kOhm

gain-loss = 20 * log_10 ((R1 + R2) / R2)

If you have some caps around you can do even better…

#35

Well I have a DI box, but it is USB and I’m attempting to create real-world conditions by using this hifiberry. So I’d rather put something in the signal chain from the guitar to the hifiberry instead of swapping input/output entirely.

Does the wattage rating on these resistors matter? I’m seeing 1/4 w, 1/2 w, etc on amazon.

#36

So in that case I would get a real DI box, this is standard on the stage. Anyone playing live and plugging direct into a sound system should have one, and some of the cheaper ones aren’t to expensive. Something like this:

Horizon IMP 2

Connected to the header marked ‘5’ in your image above, with the gain jumpers set to +32dB should be pretty close.

     Seablade

EDIT: I should clarify, a USB DI can still be a DI box, the term has become far more generic than the original devices that just took a high impedance unbalanced signal and output a low impedance balanced signal, but obviously it won’t help in your case unless you can run the USB DI into the Raspberry PI directly (I assume you have already tried this?)

#37

Much appreciated. How would I go about connecting this to the “5” from my image? Is that connector on the board some sort of standard connection? If so, what’s it called?

And why not just go from 1/4 to 1/8 into the mic jack? Is that problematic for some reason?

#38

What about this?

Seem ok?

#39

Yes, or even cheaper:

Note that you do need a XLR to RCA (or whatever the hifiberry has) adapter along with this.

#40

I would STRONGLY recommend not touching Behringer DIs with a well insulated pole.

While Behringer has released some usable products recently, their DIs were so notably bad in the past that I would not even use them for skeet shooting.

The DI I linked to is still considered one of the ‘cheaper’ DIs but is a workhorse you will find on most stages.

   Seablade
1 Like
#41

Not according to the spec sheet linked. You need to connect the XLR to the ‘5’ Header as I mentioned above, which is set up to accept balanced connections according to that spec sheet.

    Seablade
#42

And now replying to myself, their Spec Sheet isn’t completely clear honestly so I may have to eat my words:

5 Connector for balanced input (P6)

INPUT CONNECTOR P6

This allows you to connect an alternative input connector.

1 right +
2 –
3 GND
4 –
5 left +

So it says it accepts balanced connections, but their pinout is for unbalanced, so it isn’t completely clear.

EDIT: And reading their article on connecting dynamic microphones seems to indicate it does not support balanced connections:

Stage microphones usually come with a balanced XLR output. The DAC+ ADC does not support this out-of-the-box.

     Seablade
#43

If you want to go the full DIY route, a suggestion from drwhat on #ardour IRC: http://wiki.diyrecordingequipment.com/projects/categories/di/

1 Like
#44

This. x10. Behringer DIs are trash. All kinds of ground-loops and buzzing that no lift switch will ever fix. If anybody is looking for really nice DIs and is willing to spend money, I highly recommend the BSS AR-133 and the Countryman Type 85 FET. But the IMP-2 Seablade mentioned works just fine. It sits in the sweet spot where budget and quality cross each other, sort of like the SM-58, so it acts as a baseline that other DIs are compared to.

#45

Keep the context in mind. @ardour_fa3cv is building an inexpensive “play along” system for bedroom guitarists.

(and a couple more from https://github.com/cyrusadkisson?tab=repositories).

A Behringer device would likely not be the bottleneck, and it would still be expensive and over-sized compared to the rest of the components (RPi + hifiberry).

I hope the hifiberry developers deliver. This just needs a simple impedance-converter for the hifiberry. Realistically a hifiberry with Hi-Z will just need some additional resistor, capacitor, and matching the gain-stage. I don’t think you need a ground-lift or transformer at all.

#46

Spoke with hifiberry. The #5 connector is NOT balanced. The datasheet is erroneous.

Edit: But a DAC+ ADC Pro version is in the works (fall release?) which will have a balanced input.

#47

That is interesting, let me know what you turn up. Could be fun for projects such as the Zynthian for instance that Robin pointed me to in a different thread. Is this separate from the one with a High Impedance input for your guitar, or is it going to have both?

     Seablade
#48

zynthian mainly focuses on synths. MIDI in -> audio out. The first versions didn’t even have an audio-input.

#49

Ahh good point, I had assumed Balanced Output there as well, but you are correct, that was a poor assumption on my part.

    Seablade
#50

Meanwhile just inserting a 470kOhm resistor before the input, terminated with a 22kOhm to GND and ramping up the input gain by ~27dB will likely produce a better sound, compared to toying eith EQs…

You’re dealing with potentially very small signals already - so having to add another 27dB of gain just to overcome the passive attenuation and get back to the original level (actually this could be wildly off depending upon the input impedance of the soundcard, which we don’t know, but 10K was talked about) - may compromise the signal to noise significantly (unacceptably?).

Whatever the soundcard impedance (Rin) is, will effectively appear in parallel with your ‘R2’ of 22K. Something like:

1 / (1/R2 + 1/Rin))

There probably isn’t enough gain available in the ADC board to overcome the passive attenuation and also get to line level (and you might consider if the right place to be adding such significant gain - before any deliberate overdrive or additional gain effects - is a card supplied by power from a RPi).

These limitations also apply if you use ‘off the shelf’ passive DI boxes - the only difference being that now you don’t know what’s on the soundcard or in the passive box.

If you’re going to have to add external gain, why bother with the hack anyway - just do the design right with a suitable guitar friendly impedance to start with. Why not just wait for the custom DAC+ ADC board with the correct input impedance for guitar, rather than throwing away money on DI boxes or other hacks.

Does the wattage rating on these resistors matter? I’m seeing 1/4 w, 1/2 w, etc on amazon.

If you can burn out even a 1/4W resistor using a guitar pickup, you really need to change your playing style… :slight_smile: