USB Audio Interface -> 1M Ohm Impedance -vs- 470kOhm Impedance .... Differences ?

BenIfin

BenIfin

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Been offered an Original Blue RME Baby Face in mint condition really cheap - maybe for a small PC rig .

It is still fully supported by RME and has very good 48 Sample / 48k RTL of around ~4.7ms - RTL better than most 2025 USB Interfaces at 32 Samples / 48K

My query though is that the "Guitar Input" is 470k Ohm and not the "standard" 1M Ohm ?

What differences am I likely to hear and feel if running at 470k Ohm as opposed to 1M Ohm in another similar Interface ?

Guitar is a 6k non-Active PAF Style Bridge Humbucker in my partscaster

-> more / less high end ?
-> more / less mids and lows ?
-> more/less gain hitting the AD circuitry ?
-> sharper / duller attack ?
-> more / less noise ?

Any tech'ys here can chime in ?
 
I don’t think you’ll notice any difference; it would need to be lower still to mess with a guitar input signal. Although 1 Meg is the most common, 470k is still a high input impedance but I’m led to believe that some physical amps are lower (eg look up the input impedance that a Vox AC30 apparently has - it’s a fair bit lower still).

The loss of higher frequency happens with crazy low input impedance - a traditional fuzz face has an input impedance of 10k or so IIRC. This does make things sound more dull…. Which is one of the reasons why a fuzz face sounds different if you put a buffered pedal first in the chain and why it won’t do the volume thing properly.

If you’re going into an fx pedal first (Kemper, helix, whatever) and sending that to the interface, they’ll solve any issues with impedance. If you’re going straight in I think 470 will still be more than enough to not notice any difference at all so either way I don’t think it’s an issue
 
RME stuff has the best reputation, I haven’t tried one though. I don’t think there’s much difference 470k to 1m either. 220k there’s a little high end roll off.
 
Compared with 1 MOhm a 470 kOhm input impedance tames the guitar signal very slightly (less than 0.5 dB). But it also tames the resonance peak of the pickup in the upper mids by about 2 dB (single coil measurement). So it will appear as slightly duller attack / less character / less harsh.

Examples for devices with around 500 kOhm input impedance are a traditional Tube Screamer or the jumpered input of e.g. a Marshall Plexi.
 
Hmmmm ..... a little less harsh high end and a little bit less attack sounds pretty good to me.

According to "AI"

=> the Vox aC 30 has a high-impedance Normal and Top Boost channel around 250k and a lower-impedance Normal channel around 68k

=> the The Fender Deluxe Reverb has 1 Meg ohm for the "bright" or "hi" input, and 136k ohm for the "normal" or "lo" input on each channe

I have zero idea if this info in right or not.

However, when I do use Vox or Dlx Rev Amp Sims, not knowing any of this, I have always prefered and used the Vox Normal Channel and the Dlx Rev Normal Channel :)

Go figure !
 
BenIfin said:
Hmmmm ..... a little less harsh high end and a little bit less attack sounds pretty good to me.

According to "AI"

=> the Vox aC 30 has a high-impedance Normal and Top Boost channel around 250k and a lower-impedance Normal channel around 68k

=> the The Fender Deluxe Reverb has 1 Meg ohm for the "bright" or "hi" input, and 136k ohm for the "normal" or "lo" input on each channe

I have zero idea if this info in right or not.

However, when I do use Vox or Dlx Rev Amp Sims, not knowing any of this, I have always prefered and used the Vox Normal Channel and the Dlx Rev Normal Channel :)

Go figure !
Click to expand...
The AC30's input impedance is 1 MOhm or higher (High inputs) and 68 kOhm (Low inputs). This means all channels have both impedance options. I've never come across 250 kOhm inputs on AC30s.

I like the Bright channel on the Low input with my Tele.
 
DieSchmalle said:
The AC30's input impedance is 1 MOhm or higher (High inputs) and 68 kOhm (Low inputs). This means all channels have both impedance options. I've never come across 250 kOhm inputs on AC30s.

I like the Bright channel on the Low input with my Tele.
Click to expand...

So much for Google AI :)
 
Doubling impedance at low values like 20k up to 40k is going to be night and day like rolling the tone knob from all the way down to 5. When you’re up to 500k to 1m it’s very subtle. Like turning the presence knob from 5 to 5.5.
 
From my own experience, you will likely run into the most noticeable differences when dialing down your guitar's volume pot. Personally, on the HX Stomp/Floor, I often prefered lower input impedances (even in cases when it wasn't a necessity such as when using fuzzes), and they even only start at 220k.
And well, you could still slap whatever buffer in front and call it a day.
 
Thanks all.

Am a bit dumb on this stuff :(

=> so if I grab a basic 1 Meg Ohm Buffer Pedal and run my guitar into that first and then into the 470k ohm RME input, that "makes" the RME Input equivalent to a 1M Ohm input ?

=> I presume all Buffer pedals are active and not passive ?

Sorry for the beginner questions.
 
BenIfin said:
=> so if I grab a basic 1 Meg Ohm Buffer Pedal and run my guitar into that first and then into the 470k ohm RME input, that "makes" the RME Input equivalent to a 1M Ohm input ?
Click to expand...

Kinda. It's rather that the RME input is sort of overridden, hence simply not relevant anymore.

BenIfin said:
=> I presume all Buffer pedals are active and not passive ?
Click to expand...

A passive DI box might work, too. There's dedicated ones for electric guitars. You might need to raise the input gain on the RME, though, no idea how much it'd affect the overall noise floor.
 
Sascha Franck said:
Kinda. It's rather that the RME input is sort of overridden, hence simply not relevant anymore.
Click to expand...
Thanks. I gather the RME 470k Input Impedance becomes irrelevant because the Buffer Pedal Output Impedance will be well under 470k ohm ?
 
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BenIfin said:
=> I presume all Buffer pedals are active and not passive ?
Click to expand...
A buffer is an amplifier, so yeah... all buffers are active. They take the passive signal of the pickups (if they´re passive pisckups, of course) and "convert" them to active low impedance signals. They preserve frequencies and prevent them to get lost because of cables capacitance and whatnot. With a 1M buffer, your guitar goes to an 1M input, no matter what comes after... because you now have a low impedance active signal that will work just fine with anything.

Well, that was a simplified explanation (my limited knowledge doesn´t let me go much further either... ha!).
 
Sedaxel said:
A buffer is an amplifier, so yeah... all buffers are active. They take the passive signal of the pickups (if they´re passive pisckups, of course) and "convert" them to active low impedance signals. They preserve frequencies and prevent them to get lost because of cables capacitance and whatnot. With a 1M buffer, your guitar goes to an 1M input, no matter what comes after... because you now have a low impedance active signal that will work just fine with anything.
Click to expand...

^^ So ^^ it really does make the 470k Input Impedance on the RME "irrelevant" -or- as if it were actually a 1M Ohm Input Impedance ?
 
Sascha Franck said:
Obviously correct (my previous reply might've been misleading).
Yet, a passive DI box might serve the purpose, too. You obviously don't get a buffered output signal, but a low impedance one.
Click to expand...
That´s right.

A passive Di box is tricky. You´ll not only need a good impedance ratio, but also you have to take into account the impedance of the audio interface. If it´s the Hi´Z input, then you´ll have no problem (except the format of the input... you know, jacks and XLRs...).

If you´re connecting to a line or mic input (not a Hi-Z one), then you could get short in the impedance conversion. That´s because passive DIs, based on transformers, don´t have a "fixed" impedance. They have a ratio instead. Their input impedance will be the result of the input impedance of the preamp, multiplied by the impedance ratio of the transformer.

Example: if your console has 2kOhm preamps, and you use a (say) Radial JDI (which has a 144:1 impedance ratio), your guitar will see 2000x144=288kOhm.

But if you have a Midas MR18, with 10kOhm line/mic preamps, your guitar would see 10000x144=1.44MOhm.
 
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