Thank you Where and Kiwi

[EveningSky]

Examples would be a speaker simulation out of a guitar amp intended for recording purposes.

I find the signal too weak to feed directly into my AD converter.

I want to boost the signal by running it through a preamp at a low amplification setting.

I notice that some preamps have line input, mic input and instrument input.

Some only have mic and instrument input.

 

Is there a problem with feeding a line signal through a hiz or lowz input?

 

Kiwi, help me understand what a -10 db versus +4 line signal means?

Does it always mean that the -10 is unbalanced while the +4 is balanced?

Is the +4 stronger than the -10db? Perhaps a dumb question, but I need to ask.

 

Yours, ES

[where02190]

+4/-10 has nothing to do with balancing.

 

+4dbu is approx 11.2db hotter than -10dbV.

 

Don't plug a line level signal into a mic input unless it has at least a -20db pad engaged.

[Kiwiburger]

Depends on your mic preamp.

 

If your Mic preamp gain can be set to zero, that means you get zero gain. If the output trim is also set to zero, you should get out what you put in.

 

I'm not 100% clear how to compare dbU with dbV. Something to do with the different voltages used. For practical purposes I don't think it matters. A lot of modern gear lets you use inputs and outputs as either balanced or unbalanced, using either TS or TRS cables.

 

With balanced cables, you are running two identical signals on two seperate wires - except the polarity of one is reversed. At the receiving end, the polarity of the reversed one is reversed again, creating two identical signals that sum together.

 

Whenever you sum two identical signals together, you get a 6dB increase in level. So unless things are electronically compensated for, there is a 6dB increase if you run a balanced TRS cable compared to an unbalanced TS cable.

 

The idea of balanced cables is to reject hum. Whatever enters the cable is cancelled out to zero at the point where the final polarity reversal takes place. You lose that benefit if you choose to use unbalanced - as well as losing that 6dB boost.

 

Hi-Z or Low-Z is a different issue. It's about how much current the input stage will try to suck. Inputs that are design for low impedance are designed for higher current. If your source can't supply a high enough current, the sound suffers. Or the circuit overheats.

 

Hi impedance stuff doesn't try to suck as much current. Impedance is related to resistance - the higher the resistance, the less current can flow.

 

E.g. a hi-z guitar pickup connected to a low-z mic preamp can sound strangled because the mic preamp wants more current than the pickup can deliver. If you add a DI box that presents a hi-z load to the guitar, and a low-z source to the mic-preamp, you get a much better sound because everyone is happier.

 

 

 

 

 

 

 

 

 

 

 

 

[EveningSky]

Your responses are very helpful.

 

Just need to clarify two points:

Z = resistance or capacitance?

A guitar has high current output (hiZ),

while a mic (lowZ) has low current output?

 

Yours, ES

[Paully]

**Originally posted by EveningSky **

 

Just need to clarify two points:

Z = practically speaking, resistance or impedance

C = Capacitance

 

 

A guitar has high current output (hiZ),

while a mic (lowZ) has low current output?

 

Both have their own fixed output impedance. For either to realize the optimum current/voltage output, they need to plug into the proper higher load impedance. The theoretical perfect match is 10:1. Current flow will vary depending on the ratio. EG: If a guitar pickup has an output impedance of 1000 Z, it should plug into an amp input with a 10,000 Z input impedance.

 

Paul

[EveningSky]

Thank you Paul.

That is very helpful.

Yours, ES

[VockViel]

 

Originally posted by Kiwiburger I'm not 100% clear how to compare dbU with dbV. Something to do with the different voltages used. For practical purposes I don't think it matters. A lot of modern gear lets you use inputs and outputs as either balanced or unbalanced, using either TS or TRS cables.

 

 

dBu is referenced from .775 volts, whereas dBv is referenced at 1 volt, and yes, pro vs consumer audio line levels ARE compared with different references (silly, we know). It's a pretty moot point usually as most pro gear will let you adjust line input levels.

 

To get back to ES's question, yes, pro line signals just run hotter.

[paulskirocks]

for all practical purposes, Z (impedance) is AC 'resistance'... but it really involves reactance and resistance, which are out of phase... so, Z=R, if there is no reactance, or equal inductive and capacitive reactance (which cancels to none)... the thing is, resistors are not the only components that 'impede' electricity in an alternating current circuit... capacitors and inductors do, as well... and, to further that, the reactance of capacitors and inductors depends on the frequency of the AC signal!

 

anyways, you substitute Z in place of the R in ohms law in an AC circuit...

 

ok, that seemed confusing, if you don't know electronics! sorry 'bout that...

 

paulski

 

ps: i am no master of electronics, but almost 30 years in Amateur Radio (aka Ham Radio) definitely helps to grasp this stuff...

[EveningSky]

Thank you to you both. Your explanations help me better understand the concept of Z.

Yours, ES