High Input Impedance Single Supply Amp ?
in [Design]
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From: Paul Keinanen on 13 Jun 2010 05:39 On Sun, 13 Jun 2010 13:01:47 +1000, "Phil Allison" <phil_a(a)tpg.com.au> wrote: >** The SM58 is a LOW impedance mic !! > >The input referenced self noise of a TL071 is more than a few micro-volts. At 14 nV/rtHz and 20 kHz bandwidth, the noise voltage is sqrt(20 kHz) x 14 nV/rtHz = 2 uV. The SM58 sensitivity is quoted as 1.85 mV(a)94 dB SPL, so the amplifier noise would correspond to 35 dB SPL. Assuming 0.3 uV microphone self noise (the noise of a 300 ohm resistor at room temperature), this is 2 nV/rtHz, thus amplifiers like LT1028/1115 would not degrade the system noise figure significantly. >The output signal from an SM58 is normally in the range of tens to hundreds >of millivolts. That requires quite a lot of screaming :-). 10 mV = 109 dB, 100 mV = 129 dB, 1 V = 149 dB with this microphone.
From: «Leo» on 13 Jun 2010 10:30 On 12 jun, 14:09, «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 @ 25ºC, > 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... In reply to all. I know there are mic preamps everywhere on the net. I just wanted to build my own. I have a mic that is a SM58 look alike, so the specs could be much worse than those of the SM58, and I don't have the mean to measure the specs. My ultimate question would be, what input impedance should I aim at for my preamp ?
From: John Larkin on 13 Jun 2010 11:25 On Sun, 13 Jun 2010 07:30:13 -0700 (PDT), �Leo� <leo2100(a)gmail.com> wrote: >On 12 jun, 14:09, �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... > >In reply to all. > >I know there are mic preamps everywhere on the net. I just wanted to >build my own. > >I have a mic that is a SM58 look alike, so the specs could be much >worse than those of the SM58, and I don't have the mean to measure the >specs. > >My ultimate question would be, what input impedance should I aim at >for my preamp ? High. You don't want to push much opamp current noise into the 300 ohms of the mike, so lots of transistors and "low noise" opamps are out. Consider a BF862 jfet maybe, or even a few in parallel, if you want extreme quiet. There are some jfet opamps in the 4 nv/rthz noise range, but a BF862 is 0.8. You could usually get by running one at Idss, especially if you'd be willing to trim a resistor. A pseudo-inductor drain load might work too. A thread on dynamic microphone noise, in the context of thermodynamics and conservation of energy, would be interesting. Or maybe it's been done in another group. John
From: Paul Keinanen on 13 Jun 2010 13:21 On Sun, 13 Jun 2010 08:25:42 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sun, 13 Jun 2010 07:30:13 -0700 (PDT), �Leo� <leo2100(a)gmail.com> >wrote: > >>On 12 jun, 14:09, �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... >> >>In reply to all. >> >>I know there are mic preamps everywhere on the net. I just wanted to >>build my own. >> >>I have a mic that is a SM58 look alike, so the specs could be much >>worse than those of the SM58, and I don't have the mean to measure the >>specs. >> >>My ultimate question would be, what input impedance should I aim at >>for my preamp ? According to the Shure SM58 specs, the rated output impedance is 150 ohms, actual 300 ohms (whatever that means). For best noise performance, power matching should be used i.e. a load resistance of 150-300 ohms. However, most audio transducers are not designed for power matched conditions, instead, the frequency response is typically measured with an input resistance at e.g. 10 times the source resistance (1-10 kohm). >High. You don't want to push much opamp current noise into the 300 >ohms of the mike, so lots of transistors and "low noise" opamps are >out. Consider a BF862 jfet maybe, or even a few in parallel, if you >want extreme quiet. > >There are some jfet opamps in the 4 nv/rthz noise range, but a BF862 >is 0.8. You could usually get by running one at Idss, especially if >you'd be willing to trim a resistor. A pseudo-inductor drain load >might work too. > >A thread on dynamic microphone noise, in the context of thermodynamics >and conservation of energy, would be interesting. Or maybe it's been >done in another group. > >John In RF design, power matching is the accepted practice for lowest noise figures (although some low noise devices have their lowest noise figures just above or below the power matching point). Some diode ring mixers (such as the SBL-1) require 50 ohm load resistance on the IF port. In direct conversion receivers, the IF is the final audio. The first audio amplification stage is typically one or more transistors in parallel (or a single TO-220 transistor) connected in common base configuration to create the 50 ohm load resistance.
From: John Larkin on 13 Jun 2010 14:03
On Sun, 13 Jun 2010 20:21:46 +0300, Paul Keinanen <keinanen(a)sci.fi> wrote: >On Sun, 13 Jun 2010 08:25:42 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Sun, 13 Jun 2010 07:30:13 -0700 (PDT), �Leo� <leo2100(a)gmail.com> >>wrote: >> >>>On 12 jun, 14:09, �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... >>> >>>In reply to all. >>> >>>I know there are mic preamps everywhere on the net. I just wanted to >>>build my own. >>> >>>I have a mic that is a SM58 look alike, so the specs could be much >>>worse than those of the SM58, and I don't have the mean to measure the >>>specs. >>> >>>My ultimate question would be, what input impedance should I aim at >>>for my preamp ? > >According to the Shure SM58 specs, the rated output impedance is 150 >ohms, actual 300 ohms (whatever that means). > >For best noise performance, power matching should be used i.e. a load >resistance of 150-300 ohms. However, most audio transducers are not >designed for power matched conditions, instead, the frequency response >is typically measured with an input resistance at e.g. 10 times the >source resistance (1-10 kohm). Of course one should use all the transducer power that's available. But driving a 300 ohm mic into a 300 ohm, 0.8 nV/rthz opamp circuit just throws away half the signal, not to mention the current noise; it would be better to dump that into a 0.8 nv/rthz jfet. Twice better, actually. The real way to get low noise is to increase the transformer ratio and use the jfet, or a jfet opamp, up to the point that the high-frequency response gets hurt. Or use a few jfets in parallel, again using more of the available signal power. At 10 pF each, and a 300 ohm source, rolling at 20 KHz, you could use 2650 BF862s in parallel, for a voltage noise density of 16 pv/rthz. Audiophools would line up for that. You would need a good, low-noise, 40 amp power supply. > >>High. You don't want to push much opamp current noise into the 300 >>ohms of the mike, so lots of transistors and "low noise" opamps are >>out. Consider a BF862 jfet maybe, or even a few in parallel, if you >>want extreme quiet. >> >>There are some jfet opamps in the 4 nv/rthz noise range, but a BF862 >>is 0.8. You could usually get by running one at Idss, especially if >>you'd be willing to trim a resistor. A pseudo-inductor drain load >>might work too. >> >>A thread on dynamic microphone noise, in the context of thermodynamics >>and conservation of energy, would be interesting. Or maybe it's been >>done in another group. >> >>John > >In RF design, power matching is the accepted practice for lowest noise >figures (although some low noise devices have their lowest noise >figures just above or below the power matching point). The best uncooled microwave amps are probably phemts, with noise figures around 0.3 dB. The matching networks are essentially step-up transformers, giving free, almost noiseless voltage gain before the fet gate. The mmics I've tested tend to have input impedances in the 30-40 ohm range; I've only found one that actually hits 50 ohms. I wonder if that's the optimum nf point, or whether there's some other reason to make them that low. John |