an audio question about impedance matching
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From: glen herrmannsfeldt on 29 Jul 2010 13:58 Steve Pope <spope33(a)speedymail.org> wrote: > robert bristow-johnson <rbj(a)audioimagination.com> wrote: >>why can't a low-impedance source simply connected to a high-impedance >>load suffice? why are matching transformers needed (unless level is >>important)? > Many audio connections are in fact a low-impedance source connected > to a high impedance load. It is usual that the microphone impedance is lower than the input impedance, such that the microphone isn't loaded down. A high impedance microphone with built-in amplifier might be about 1K ohms, and work well with a 10k ohm load. Low impedance, according to wikipedia, is between 50 and 200 ohms, well matched for the shielded twisted pair cable usually used. > Using transformers on some connections is thought to prevent ground loops. > Matched connections (e.g. a 600 ohm load), when they are used, are > thought to prevent hum on long lines. I'm not sure how scientific > either of these beliefs are. Well, not only prevent ground loops but remove any common-mode signals picked up along the way. The shield helps, but doesn't do much for low frequency magnetic (inductive) coupling. Note that transformers are also used on both ends of UTP ethernet to remove the common mode signals. -- glen
From: steveu on 29 Jul 2010 14:03 >Steve Pope <spope33(a)speedymail.org> wrote: >Note that transformers are also used on both ends of UTP >ethernet to remove the common mode signals. They do that, but their primary purpose is electrical isolation for safety. They aren't tested at 4kV for their common mode rejection qualities. :-) Steve
From: Greg Berchin on 29 Jul 2010 14:04 On Thu, 29 Jul 2010 10:53:36 -0700 (PDT), robert bristow-johnson <rbj(a)audioimagination.com> wrote: >> >other than that, i do not understand the why "impedance matching" is >> >considered so important in audio. �it's not like we need to maximize >> >power transfer (in micro watts) from the transducer or pickup into the >> >input of the amplifier. � >> >> That depends. > >on what? whether we have the capability of amplifying signals or not? Or upon how much we need to amplify the signal. For a microphone or phono cartridge or guitar pickup, we want to provide as much signal amplitude as possible to the gain stages because they amplify the noise (including thermal noise, with such low-output devices) as much as they amplify the signal, and then they add noise of their own. >i'm thinking a custom guitar pickup (with lower source impedance) >going into a guitar amp. With musical instruments, often it is not what makes sense that counts, but what sounds good. "Good" is in the ear of the beholder. If a transformer makes it sound good, who are we to argue? Greg
From: steveu on 29 Jul 2010 14:10 >i know it's not DSP, but i thought people on the list would have some >experience, opinions, and knowledge. > >whether it's a microphone going into a preamp or mix board, or an >electric guitar going into a guitar amp, how does impedance matching >(or mismatching) affect *tone*? > >the way i see it (as an EE) is: > >1. that if one of either the source or load impedance has reactive >components and the other is resistive, changing the resistance will >change the simple RC filter and the corner frequency of it, thus >changing the tone. > >2. if both source and load impedance are mostly resistive, changing >one or the other will only change the level, not the waveshape. if >the input to the pre-amp or amp has some nonlinearity, a louder level >will push that stage into the nonlinear region which will change the >waveshape and affect the timbre. > >other than that, i do not understand the why "impedance matching" is >considered so important in audio. it's not like we need to maximize >power transfer (in micro watts) from the transducer or pickup into the >input of the amplifier. and, at audio frequencies (and the length of >cables), reflections along the transmission lines (due to impedance >mismatch) should not be an issue either. > >why can't a low-impedance source simply connected to a high-impedance >load suffice? why are matching transformers needed (unless level is >important)? > >-- > >r b-j rbj(a)audioimagination.com > >"Imagination is more important than knowledge." As others have said, a huge number of audio connections are a pretty low impedance feeding a pretty high one, and RC filtering is the main thing you need to worry about in most cases. However, there are a few places were impedance really matters. A top quality mic and mic amp can get an amazing noise floor, but you'd better have the impedances achieving maximum power transfer if you don't want to spoil things. Most mics, LP cartidges and other electromechnical input devices have resonance issues, unless they are properly loaded. Most output devices - speakers and headphones - are best driven from a super low impedance, so the amp damps their bass resonances. This is why putting the amp as close as possible to a speaker, or even inside the cabinet, makes so much sense. A lot more sense than trying to minimise impedance with crazy thick wires from a distant amp. Steve
From: Omer Osman on 29 Jul 2010 14:27
On Jul 29, 2:10 pm, "steveu" <steveu(a)n_o_s_p_a_m.coppice.org> wrote: > >i know it's not DSP, but i thought people on the list would have some > >experience, opinions, and knowledge. > > >whether it's a microphone going into a preamp or mix board, or an > >electric guitar going into a guitar amp, how does impedance matching > >(or mismatching) affect *tone*? > > >the way i see it (as an EE) is: > > >1. that if one of either the source or load impedance has reactive > >components and the other is resistive, changing the resistance will > >change the simple RC filter and the corner frequency of it, thus > >changing the tone. > > >2. if both source and load impedance are mostly resistive, changing > >one or the other will only change the level, not the waveshape. if > >the input to the pre-amp or amp has some nonlinearity, a louder level > >will push that stage into the nonlinear region which will change the > >waveshape and affect the timbre. > > >other than that, i do not understand the why "impedance matching" is > >considered so important in audio. it's not like we need to maximize > >power transfer (in micro watts) from the transducer or pickup into the > >input of the amplifier. and, at audio frequencies (and the length of > >cables), reflections along the transmission lines (due to impedance > >mismatch) should not be an issue either. > > >why can't a low-impedance source simply connected to a high-impedance > >load suffice? why are matching transformers needed (unless level is > >important)? > > >-- > > >r b-j r...(a)audioimagination.com > > >"Imagination is more important than knowledge." > > As others have said, a huge number of audio connections are a pretty low > impedance feeding a pretty high one, and RC filtering is the main thing you > need to worry about in most cases. However, there are a few places were > impedance really matters. > > A top quality mic and mic amp can get an amazing noise floor, but you'd > better have the impedances achieving maximum power transfer if you don't > want to spoil things. > > Most mics, LP cartidges and other electromechnical input devices have > resonance issues, unless they are properly loaded. > > Most output devices - speakers and headphones - are best driven from a > super low impedance, so the amp damps their bass resonances. This is why > putting the amp as close as possible to a speaker, or even inside the > cabinet, makes so much sense. A lot more sense than trying to minimise > impedance with crazy thick wires from a distant amp. > > Steve I like whats been said on this thread. Omer Osman |