an audio question about impedance matching
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From: Jerry Avins on 30 Jul 2010 13:04 On 7/30/2010 11:48 AM, robert bristow-johnson wrote: ... > i understand how level is changed when a transformer is tossed in. my > primary question has been to how is *tone* changed? is it because > higher levels push the guitar amp into a little bit of distortion and > that makes the tone "better"? the issue with some of these custom > pickups is that their initial level is lower and, when going into a > guitar amp, the pickups "just ain't hot enough" and *tone* is part of > the subjective experience (because cranking up the volume knob doesn't > get us to the same sound). putting a transformer in between and > "matching impedance" reportedly fixes this. Is it any properly rated transformer, or a particular few? Transformers have inter-turn capacitance, leakage inductance, and an effective equivalent circuit that can look like a 4th-order analog filter. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Steve Pope on 30 Jul 2010 13:11 Jerry Avins <jya(a)ieee.org> wrote: >On 7/30/2010 11:48 AM, robert bristow-johnson wrote: >> i understand how level is changed when a transformer is tossed in. my >> primary question has been to how is *tone* changed? is it because >> higher levels push the guitar amp into a little bit of distortion and >> that makes the tone "better"? the issue with some of these custom >> pickups is that their initial level is lower and, when going into a >> guitar amp, the pickups "just ain't hot enough" and *tone* is part of >> the subjective experience (because cranking up the volume knob doesn't >> get us to the same sound). putting a transformer in between and >> "matching impedance" reportedly fixes this. >Is it any properly rated transformer, or a particular few? Transformers >have inter-turn capacitance, leakage inductance, and an effective >equivalent circuit that can look like a 4th-order analog filter. At least in the distant past, a guitar pickup wanted a capacitive load instead of a hi-Z load to achive a normal (as opposed to super-bright) tone balance. Some amps (or guitars) would have a pot and cap in series right at the amp input (or across the pickup), and that was the "tone control". Loading a pickup with a transformer, which both transforms the load on the other side and has an intrinsic reactive load of its own, can undoubtedly do interesting things to tone, in at least some cases. Steve
From: Mark K on 30 Jul 2010 13:13 > now, if, instead we connect that custom pickup to a transformerless > booster amp that is perfectly linear, high input impedance, low output > impedance, and bring the level up to whatever the transformer did, > then will *tone* suffer? if so, then since the levels are the same, > if must be something with the source impedance of the pickup and/or > the input impedance of the guitar amp. no? > > > Audio transformers are used in professional equipment to provide > > galvanic isolation and common mode rej where you NEED it to work the > > first time without any fuss despite the fact that the guy next to you > > is controlling 10,000 Watt worth of lighting equipment and his cables > > run next to yours, or your audio signal runs to the next building > > whose "ground" is 10V different from your ground. > > i understand how transformers are useful for isolating grounds and for > providing a natural differential input (and high CMRR) besides the > function of changing voltage level and reflecting impedance. > > > The folks at rec.audio.pro may have additional comments.. > > i was thinking of posting there, but i hadn't really been hanging > there since the Gabe Weiner days and it seemed to have gotten really > noisy. maybe i'll post this question there. i *did* post it to (the > newly created) sci.physics.acoustics (the alt newsgroup is moving to > there). > > L8r, > > r b-j- Hide quoted text - > > - Show quoted text - If the context of your question is adding a transformer between a guitar pickup and a guitar amp, then the answer is a typical guitar pickup is very high impedance and lots of inducatnace and is typically "designed" to operate into a 1Meg or so resistive load. If you put any other load on it, it will effect the "tone" because it will effect the frequency response at the very least. Mark
From: glen herrmannsfeldt on 30 Jul 2010 17:30 Steve Pope <spope33(a)speedymail.org> wrote: (snip) > At least in the distant past, a guitar pickup wanted a capacitive > load instead of a hi-Z load to achive a normal (as opposed to > super-bright) tone balance. Some amps (or guitars) would have a pot > and cap in series right at the amp input (or across the pickup), and > that was the "tone control". The OP seems to be connecting a microphone to a guitar amp... The effect of such a "tone control" would depend on the source impedance, though, and may be enough to cause the effect seen (or imagined). > Loading a pickup with a transformer, which both transforms the > load on the other side and has an intrinsic reactive load of its > own, can undoubtedly do interesting things to tone, in at > least some cases. -- glen
From: Steve Pope on 30 Jul 2010 17:32
glen herrmannsfeldt <gah(a)ugcs.caltech.edu> wrote: >Steve Pope <spope33(a)speedymail.org> wrote: >> At least in the distant past, a guitar pickup wanted a capacitive >> load instead of a hi-Z load to achive a normal (as opposed to >> super-bright) tone balance. Some amps (or guitars) would have a pot >> and cap in series right at the amp input (or across the pickup), and >> that was the "tone control". >The OP seems to be connecting a microphone to a guitar amp... I could swear the OP had talked about a guitar pickup, initially, but my memory plays tricks on me these days. Steve |