High Input Impedance Single Supply Amp ?
in [Design]
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From: Jan Panteltje on 14 Jun 2010 11:08 ABORT RETRY IGNORE ^
From: John Larkin on 14 Jun 2010 11:28 On Mon, 14 Jun 2010 15:08:37 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >ABORT RETRY IGNORE > ^ ABORT RETRY IGNORE ^
From: Jan Panteltje on 14 Jun 2010 11:37 On a sunny day (Mon, 14 Jun 2010 08:28:11 -0700) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <3jic1610r90lfqbtdkdldbk07fqs7fk353(a)4ax.com>: >On Mon, 14 Jun 2010 15:08:37 GMT, Jan Panteltje ><pNaonStpealmtje(a)yahoo.com> wrote: > >>ABORT RETRY IGNORE >> ^ > >ABORT RETRY IGNORE >^ > > ABORT RETRY IGNORE ^
From: «Leo» on 14 Jun 2010 18:03 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... To all. So I guess I'll use high enough imput impedance, like 10 times or more...The application is neither critical nor Hi-Fi, but it would be good to make the best of the components I have available...and that excludes any of those JFETs mentioned before. So long as the circuit's noise isn't audible enough compared to the actual signal it's OK. the idea is to plug it in a 100W guitar power amp for band rehersal, and mix it with the guitar signal...I know it's not the best solution but I can't afford another Amp.
From: JosephKK on 16 Jun 2010 01:20
On Sat, 12 Jun 2010 11:14:59 -0700 (PDT), «Leo» <leo2100(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 @ 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... >> >> 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 Thank you for that information. This mic is of the low-Z dynamic class. high-Z inputs for such are not a correct application. Actually, your best result will be from a low-Z input discrete mic amp. You can get reasonable performance from ultra low voltage noise opamps. Much simpler to work with. The load impedance presented to this mic should not be much above 1k to 2k, and it works reasonably well into about 750 ohms to as much as 10k. If i can find circuit to the low-Z mic inputs to my Ampex AX-300 i would post it. |