From Brian Willoughby Sent Fri, Jan 12th 2018, 21:50
From Brian Willoughby Sent Fri, Jan 12th 2018, 21:50
This is a great explanation, Tony, and very thorough. Another explanation that stuck with me goes as follows: Older systems = that were based on transmitting power were very focused on =E2=80=9Cimpeda= nce matching.=E2=80=9D Thus, the ancient telephone and telegraph systems = used 600 ohms for both output impedance and input impedance. This = reduced the unwanted reflection of signals as well. A large part of the = motivation for this was that amplifiers were not available to boost the = signal in those early days. In these systems, the output and input = impedances literally matched. Modern systems use voltage signals, not power signals. We also have = amplifiers that cost pennies. So, output impedance is often less than = 100 ohms while input impedance is over 1 megaohm. The voltage is almost = identical from output to input given such impedances. What little = voltage loss there might be (0.01%) can be corrected with an op-amp. = Modern systems never have identical impedances for output and input, = although some people use the term =E2=80=9Cmatched=E2=80=9D when they = really mean that the impedances are coordinated to be complementary. As Tony explained, it=E2=80=99s all about the voltage levels. If an = output device produces more voltage than an input device can handle, = then you=E2=80=99ll need some way to reduce the level. Even when devices = vary significantly from the usual impedances, it=E2=80=99s not really = the impedance that needs to be corrected but the voltage level. = Transformers can reduce the level of an AC signal; passive resistor = dividers can reduce the voltage of AC and DC components; and active = circuits can also adjust levels, provided they have the headroom for the = strongest signal. Brian On Jan 10, 2018, at 1:52 AM, Oakley Sound <xxxxxxxxxxx@xxxxxxxxxx.xxx> = wrote: >=20 > > https://www.diyrecordingequipment.com/products/l2a >=20 > This is a useful device but not because of its impedance transforming = abilities. >=20 > Hi-Z or high impedance generally refers to input resistances of over = 47,000 ohms or 100K. For a guitar amplifier input it is generally a lot = higher, perhaps even a few millions of ohms. Lo-Z is usually lower than = 2000 ohms. It's not that common on anything other than microphone = pre-amp inputs. >=20 > Most mixers and audio interfaces will be around 47,000 ohms. High but = not as high as a guitar amp input or a pedal input. >=20 > Passive pickups and microphones will work their best when plugged into = amplifiers/pedals that have an impedance that the pick up is designed = for. So for most electric guitars they'll be wanting a high impedance of = over 1 million ohms. Any less and the volume pots will start working = strange and it'll not sound right. >=20 > But we're synth players and most synths will have low 'impedance' = outputs because they are driven by active electronics. That means the = synth won't care a damn whether it's plugged into a high resistance or = low resistance input. You'll get a little less output level when driving = even a low-Z input but it should not affect the sonics. >=20 > So what's with the need for these re-amp units? When being used to = drive guitar FX pedals they are being used as volume changers. So that's = nothing really to do with resistance but signal level. The reason a = guitar FX pedal distorts when you plug a synth into it is almost always = because it is being driven too hard. Turning the volume down on the = synth will usually have the same effect as will adding a -10dB in-line = pad. >=20 > There are, I'm sure, some curious vintage pedals behave the way they = do when driven from a passive pick up... maybe some 60s wah pedals. But = the rest are quite happy being driven from a low impedance source if the = volume level is correct. After all, consider what happens when you have = two pedals in series with another. The second pedal doesn't start to get = all distorted when driven from the low impedance output of the first. >=20 > Re-amp units like the one shown are useful for breaking ground loops, = adding some transformer saturation, adding a bit of low frequency cut as = well as volume changing. Handy things to have for sure, but when all you = need is a passive volume control, there may be easier ways to get what = you want. >=20 > Tony >=20