Model 12 w/ guitar pedalboard?

[Hammarlund]

If I set the T12 input to "instrument", plug an electric guitar directly into the T12, and take an effect out using a half-inserted cable from the "insert" jack (so I can return a dirty signal to another channel) will the output from the T12 impedance and level work with the pedalboards?

In other words, does the "instrument" switch trigger any different impedance or levels in the insert jack?

 

[Mark Richards]

T12 input...does the "instrument" switch trigger any different impedance or levels in the insert jack?

Assume you're referring to Input 1 & Input 2, and not Track 12?

A typical electric guitar has an impedance of 40kOms. The instrument impedance switch converts the input to a very high 1M Ohms to adjust for the guitar's impedance.

OM Spec Page:
LINE (BAL)/INST input jacks (1-6)
Connectors: 6.3mm (1/4”) standard TRS jacks
(Tip: HOT, Ring: COLD, Sleeve: GND)
Maximum input level: +22 dBu (LINE)/19.8 dBV (INST)
Nominal input level: +4 dBu (LINE)/ 1.8 dBV (INST)
Gain adjustment range: -10 - +40 dB
Input impedance: 22 kΩ (LINE)/1 MΩ (INST)

L/MONO(BAL)/INST input jacks (7, 9)
Connectors: 6.3mm (1/4”) standard TRS jacks
(Tip: HOT, Ring: COLD, Sleeve: GND)
Maximum input level: +22 dBu (LINE)/19.8 dBV (INST)
Nominal input level: +4 dBu (LINE)/ 1.8 dBV (INST)
Gain adjustment range: -20 - +30 dB
Input impedance: 18 kΩ (LINE)/ 1 MΩ (INST)

R (BAL) input jacks (8, 10)
Connectors: 6.3mm (1/4”) standard TRS jacks
(Tip: HOT, Ring: COLD, Sleeve: GND)
Maximum input level: +22 dBu
Nominal input level: +4 dBu
Gain adjustment range: -20 - +30 dB
Input impedance: 18 kΩ

INSERT jacks (1-2)
Connectors: 6.3mm (1/4”) standard TRS jacks
(Tip: SEND, Ring: RETURN, Sleeve: GND)
RETURN (Ring)
Nominal input level: 0 dBu
Maximum input level: +18 dBu
Input impedance: 5 kΩ

SEND (Tip)
Nominal output level: 0 dBu
Maximum output level: +18 dBu
Output impedance: 100 Ω

 

[Hammarlund]

OK. So will I be able to use the "insert" (or a sub bus) to run effects through my guitar effect?

 

[Mark Richards]

Depends on the input specs and controls for your guitar pedal.
The Input 1 & 2 Insert circuits "Send" output a very strong 0dBu nominal / +18dBu max signal level.

For example,
My Guitar Pedal Digitech RP360 Input specs:
Analog Input Connections
Input Impedance: 1 MOhm
Maximum Input: +10 dBu

The Model 12 OM spec sheet - Sub Out:
SUB OUTPUT L/R jacks
Nominal output level: -2 dBu
Maximum output level: +16 dBu
Output impedance: 200 Ω

The impedance mismatch (200 Ohm going into 1M Ohm) means there's a possibility of lowering the signal, adding noise, or introducing some other type of signal degradation.

The Output mismatch (Max +16dBu into Max +10 dBu) means there's a potential to overdrive/introduce distortion into the signal.

That's why (although I can if I'm careful) I typically wouldn't use my RP360 as an external effect in the mixdown chain.

This may be moot re the Input 1 & 2 Insert jacks.

Looking at the Block diagram, it appears inserting the Tip ("Send" of the TRS plug) into the Input's Insert jack breaks out the signal on the Input circuit to send it to outboard gear (your pedal).

The outboard gear signal comes back into the same Input's Insert jack ("Return" via the Ring of the TRS plug), completing the Input circuit so the signal can go to the track.

So in the Block Diagram, it looks like the dry signal may not make it to the track once the path is interrupted by using the Insert jack; and returning the signal to the Input's Insert jack (via the TRS plug's Ring) may be necessary to complete the circuit and send the signal to the track.

A simple test of the Insert jack can confirm or refute this.​

 

[Logrinn]

Sure

 

[Hammarlund]

Thanks. Helpful re impedance and dB. Not being an electric player I do not always entirely grok impedance.

It does seem like the impedance for the inserts (5k ohm) is substantially different from the SUB outputs, so one of them may play more nicely than the others. I'll give it a shot, since VOX doesn't publish the input range #s for the stompbox, either on their site or in their manual. I suspect my (cheap!) modeling pedal won't play nicely but it sounds like I'll be unlikely to fry it merely by testing.

FYI, the input jacks work like most others I've seen: The top/shallow part of the jack is the send and the bottom/deep part of the jack is the return. You can half-plug a cable into top half only, and take a "copy" of the signal (post-preamp of course) without affecting the underlying signal passing through. Only when you have plugged it all the way in (so there is a return signal) does it interrupt the channel.

 

[Max Relic]

Thanks Mark Richards and Hammarlund for the interesting discussion. In general, a signal going from low impedance output to high impedance input will translate well with minimum loss; the opposite (high Z output to low Z input) will degrade the signal and should be avoided when possible.

 

[Tahiche]

I’m such a noob when it comes to impedance and levels. I’m trying, though. Had to take a look at an Ohmms calculator to try and understand the figures and also the impedance section on the manual.
Here are some quotes I found:

the impedance relation between outputs and inputs needs to be considered, but only in the following way:
Always make sure that a device’s input impedance is higher than the output source impedance of the device that drives it.

and…

Nowadays, nearly all devices are connected bridging -- low-Z out
to high-Z in -- because we want the most voltage transferred
between components.
If you connect a low-Z source to a high-Z load, there is no
distortion or frequency-response change caused by this
connection. But if you connect a high-Z source to a low-Z load,
you might get distortion or altered response.

Which is precisely what @Max Relic Is saying.
According to the Model 12 manual
insert jack send Output impedance: 100 Ω
Sub output jack Output impedance: 200 Ω
Aux output jack Output impedance: 200 Ω

A guitar pedal expects high impedance input (see a lot of different figures 300K, 500K, 900K and about) so in theory both insert jack send (jack halfway in tip) or sub out should be ok. Right?.
For anyone considering this stuff is “obvious”, let me tell you it’s not! . For example I just read a guitar output is apparently between 10KΩ and 40KΩ. That’s many, many times more than the output impedance of the mixer. Yet it’s ok?.
According to this rule the output impedance of the insert, sub or aux are a LOT lower than the input of a pedal, so it should be fine. Obviously the levels have to be taken into account and are surely critical but I guess you can lower the output volume/knob (except on the insert jack, which might make a difference).
This is hard to grasp…

 

[Tahiche]

Hi again.
Seems like I found a “gotcha”: fuzz pedals.
I have a Willy Mammoth clone and a 90’s BigMuff that I use a lot. Apparently fuzz pedals are very demanding and expect a high impedance, such as a guitar’s output, to work and respond well. That’s why it’s recommended that they’re first in chain and connected directly to the guitar.
Given the aux/sub/insert sends on the Model 12 (and any mixer I guess) are very low impedance, the fuzzes would probably not work well here. Right?.

 

[Max Relic]

[...] I just read a guitar output is apparently between 10KΩ and 40KΩ. That’s many, many times more than the output impedance of the mixer [...]

That's the reason why you find one or more "Hi-Z" or "Instrument" selectors on most audio interfaces, mixers and Model Series siblings that can accept direct input of guitars or bass guitars

[..] the levels have to be taken into account and are surely critical but I guess you can lower the output volume/knob (except on the insert jack, which might make a difference).
This is hard to grasp… [...]

Again gainstaging is the key, adapt levels in order to obtain a good signal to noise ratio (easily done on the Model 12 thanks to its wide dynamic range and low noise floor) and not too high to avoid distortion. You judge by ears and looking at level meter, train yourself by practicing.

 

[Mark Richards]

I’m such a noob when it comes to impedance and levels. I’m trying, though.

"Impedance Matching" (From the 2488/DP-24/32/SD forum's "Equipment Related Tips" sticky thread) may be helpful.

While focused on the portastudios, other information (listed in the Index to the "Equipment Related Tips" sticky thread) may be helpful also.

 

[Max Relic]

That’s why it’s recommended that they’re first in chain and connected directly to the guitar. [...]

Distortion is substantially -but not limited to- an excess of preamplification. Until your guitar has active electronics, just plug the cable into your preamp/distortion pedal that will raise the signal from the pick-up, then pass it to whatever effect you like. If you are going the simplest way and don't want to do particularly elaborated recordings, you don't need to plug the guitar into the Model 12 preamp and then send it to the Fuzz. If you intend to record both clean and effected guitars on separated tracks, then you already have the picture.