High Input Impedance Single Supply Amp ?
in [Design]
|
From: John Larkin on 12 Jun 2010 14:56 On Sat, 12 Jun 2010 11:32:47 -0700 (PDT), Fred <byrdlegges(a)gmail.com> wrote: >On Jun 12, 1:14�pm, �Leo� <leo2...(a)gmail.com> wrote: >> On 12 jun, 14:27, John Larkin >> >> >> >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> > On Sat, 12 Jun 2010 10:09:33 -0700 (PDT), �Leo� <leo2...(a)gmail.com> >> > wrote: >> >> > >I need to make a high input impedance amplifier for a mic preamp with >> > >a single supply. I was thinking of using a TL071 (non-inverting) opamp >> > >with high resistance bias resistors (to bias the opamp to half the >> > >supply voltage) so the input impedance results is �aprox. 1MegOhm. But >> > >I don't know if such a scheme would work. I figured since the input >> > >bias currents in the JFET input opamps are very low (max 200pA @ 25�C, >> > >7nA full range) I could bias it with two 2.2MegOhm resistors. Also I >> > >think adding high value resistors increases noise at the input...and >> > >the overall gain of the circuit would be kind of large (~1000, or >> > >perhaps larger), so it would give me a large noise at the output. The >> > >input from the mic is in the micro-volt range. >> >> > >I don`t know if a discrete bipolar solution would be better...or any >> > >other clever circuit configuratios for that matter... >> >> > What sort of microphone is it? What's its impedance? Is it DC >> > continuous? Do you have access to both ends? >> >> > John >> >> I don't have the specs, but it is a SM58 kind of mic. Cheaper, with >> worse specs I imagine. Here's the SM58http://www.shure.com/americas/products/microphones/sm/sm58-vocal-micr... > >http://sound.westhost.com/project66.htm >http://www.jensen-transformers.com/apps_sc.html >http://orca.st.usm.edu/~jmneal/preamp1.htm >http://www.zen22142.zen.co.uk/Circuits/Audio/dyn_mic/dyn_mic.htm 4 out of 4 horrible circuits. That's about average in the audio world. John
From: Jan Panteltje on 12 Jun 2010 15:39 On a sunny day (Sat, 12 Jun 2010 11:14:59 -0700 (PDT)) it happened <leo2100(a)gmail.com> wrote in <97bdfe1c-2545-4d6d-a157-1e8abb0ebdc0(a)g19g2000yqc.googlegroups.com>: >On 12 jun, 14:27, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Sat, 12 Jun 2010 10:09:33 -0700 (PDT), �Leo� <leo2...(a)gmail.com> >> wrote: >> >> >I need to make a high input impedance amplifier for a mic preamp with >> >a single supply. I was thinking of using a TL071 (non-inverting) opamp >> >with high resistance bias resistors (to bias the opamp to half the >> >supply voltage) so the input impedance results is �aprox. 1MegOhm. But >> >I don't know if such a scheme would work. I figured since the input >> >bias currents in the JFET input opamps are very low (max 200pA @ 25�C, >> >7nA full range) I could bias it with two 2.2MegOhm resistors. Also I >> >think adding high value resistors increases noise at the input...and >> >the overall gain of the circuit would be kind of large (~1000, or >> >perhaps larger), so it would give me a large noise at the output. The >> >input from the mic is in the micro-volt range. >> >> >I don`t know if a discrete bipolar solution would be better...or any >> >other clever circuit configuratios for that matter... >> >> What sort of microphone is it? What's its impedance? Is it DC >> continuous? Do you have access to both ends? >> >> John > >I don't have the specs, but it is a SM58 kind of mic. Cheaper, with >worse specs I imagine. Here's the SM58 >http://www.shure.com/americas/products/microphones/sm/sm58-vocal-microphone That is a dynamic mike, with 300 Ohm impedance, datasheet page 2, read it. No high impedances involved anywhere, (I hope Phil agrees else I am ....)
From: Phil Allison on 12 Jun 2010 23:01 "�Leo�" I don't have the specs, but it is a SM58 kind of mic. Cheaper, with worse specs I imagine. ** The SM58 is a LOW impedance mic !! The input referenced self noise of a TL071 is more than a few micro-volts. The output signal from an SM58 is normally in the range of tens to hundreds of millivolts. What planet are you living on ??? .... Phil
From: Phil Allison on 13 Jun 2010 00:10 "Jan Panteltje" > >>I don't have the specs, but it is a SM58 kind of mic. Cheaper, with >>worse specs I imagine. Here's the SM58 >>http://www.shure.com/americas/products/microphones/sm/sm58-vocal-microphone > > That is a dynamic mike, with 300 Ohm impedance, > datasheet page 2, read it. > No high impedances involved anywhere, > (I hope Phil agrees else I am ....) ** The SM58 mic and its many clones have a source impedance of 200 to 300 ohms and a self noise of about 0.25uV in the audio band - which in practice equates to an SPL of about 15 dB. A speaking voice at a range of say 1 foot will cause the mic to generate about 10mV of signal. Yelling into the mic with it close held as possible to the lips will generate upwards of 1 volt rms. The mic can tolerate SPLs of up to 150dB even at low audio frequencies. A genuine Shure SM58 has a very low impedance diaphragm of about 14 ohms followed by an internal 4:1 auto-transformer (magnetically unshielded ) wired to step up the impedance to 270 ohms, once winding resistances are taken into account. The output signal floats and is connected across pins 2 and 3 of a male XLR socket at the end of the handle. However, I have seen a Chinese made clone of the SM58 where there was no transformer, the impedance was 600 ohms and the output signal was not floating. ..... Phil
From: John Larkin on 13 Jun 2010 00:26
On Sun, 13 Jun 2010 13:01:47 +1000, "Phil Allison" <phil_a(a)tpg.com.au> wrote: > >"�Leo�" > >I don't have the specs, but it is a SM58 kind of mic. Cheaper, with >worse specs I imagine. > > >** The SM58 is a LOW impedance mic !! > >The input referenced self noise of a TL071 is more than a few micro-volts. > >The output signal from an SM58 is normally in the range of tens to hundreds >of millivolts. > >What planet are you living on ??? > > >... Phil > > The SM58 datasheet says 1.85 mV output at 94 dB spl. And 300 ohms impedance. What's a reasonable equivalent SPL noise level for a mic amp? At some point, even in a good studio, there must be some background noise. How does that compare to typical electrical noise? And what do you think is the equivalent noise temperature of a dynamic mic? Does brownian motion matter? John |