On Wed, 22 Dec 2021 19:01:22 -0600, amdx <amdx@knology.net> wrote:>On 12/22/2021 12:37 PM, John Walliker wrote: >> On Wednesday, 22 December 2021 at 17:23:15 UTC, jla...@highlandsniptechnology.com wrote: >> >>>> ** Most are a JFET with drain as output needing a +DC source >>> and a series resistor of about 4000 ohms >>> I'd guess that the best load on the jfet would be constant-voltage, >>> namely a cascode NPN or the inverting input of an opamp. That >>> minimizes d-g feedback and likely distortion. >> Yes, connecting the mic output to the inverting input of a low-noise >> op-amp via a capacitor works very well. No series resistor is needed >> or even desirable. The electret mics being used >> by the OP are intended to operate at 0.5mA drain current. The pullup >> resistor can be chosen according to the supply voltage to give that >> current when the output dc level is about 1.9V. These are the conditions >> under which the mics are characterised by the manufacturer. >> Running at much lower current would result in the drain voltage falling >> which reduces the dynamic range. There are advantages in using >> a fairly high bias voltage of up to a maximum of 10V with a >> correspondingly higher pullup resistor to keep the the output at >> about 1.9V and the current at 0.5mA. A 9V battery makes an >> excellent low-noise bias source. The higher pullup resistor which, >> for a 9V bias supply would be 14.2k, reduces the noise gain of the >> op-amp. >> As the acoustic input modulates the drain current of the FET in >> the mic, all the audio current ends up in the feedback resistor of >> the op-amp and the drain voltage stays almost constant. >> With a feedback resistor of 17.4k and using the PUI Audio >> AOM-5024L-HD-R microphone the output of the preamp will be >> about 0.5Vrms/Pa. >> >> John >> > �Isn't there a trade off? Does the higher 14.2k pullup resistor have >it's own noise vs the reduced noise off the opamp. > > �I experimented the as much as 20V on the mic FET from a 25v supply. It >only gave me about 30% more signal over 1.5V > >on the mic FET. What other � effects would a high pullup voltage and >resistor have? > > ��������������������������������������������� MikekImpact ionization current into the jfet gate. -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
Low open loop gain from a mic preamp-why?
Started by ●December 20, 2021
Reply by ●December 22, 20212021-12-22
Reply by ●December 23, 20212021-12-23
"Phil Allison" wrote in message news:877d3735-4091-44fe-a5eb-48138179ccb6n@googlegroups.com... Kevin Aylward wrote: =================> > > > The 1k2 input resistor dominates the noise. > > > > >** Maybe, but only just. BC549s are non ideal for low source > > >impedances. > > >> The data sheet for the BC549 has a noise figure of 1.2 dB with a 2k > >> source. > >> This calculates to rbb' = ~ 52 ohms > > >** Please cite. My info is the ideal is 50k. > What do you mean by 50k ?>** I see no cite of the noise data.There is this wonderful modern technique to gain information. its called Google.> > >> Note: the input capacitor should be increased to 100uf so that the > > >> 1/f current noise of the input transistor is also shorted through the > > >> mic resistance. > > > > >** Has no effect, all the white noise energy is concentrated above > > >1kHz. > >> > It most certainly does have a significant audible effect... > > >>** Bollocks. Try really answering my obvious point. > >> The audible noise from an input loaded mic pre is all above 1kHz.>> As I explained, it is a real problem, known to all in the industry.>** What is ??1/f and RTS/Popcorn noise.>> I guess you haven't sat in a test room actually listening to LF noise.>** Another desperate red herring, plus absurd.You are alleging that 1/f noise isn't a problem in mic amps. This is ludicrous. Its an on going battle to locate low (1/f, RTS) noise transistors. A listening test in a pro company's quality room proves otherwise. End of.> >> The early Studiomasters used 2N4403, > >> >** Really? Never seen a bad new one in my life.>> Well... it was certainly a fact in 1980.>** Nope, it was not.You are just pissing the in wind in totally ignorance. Its a proven fact. The bags of transistors that failed the quality noise test proved it. Process variations are a fundamental issue in transistor manufacture. 1/f and RTS noise are fundamental issues in semiconductor fabrication. Some of us actually design low noise, large transistor count analog ASICs that are sold in large numbers, and actually have extensive measurement data on this issue. Those 2N4403s had major RTS/Popcorn noise. Period.> >** Low frequent noise is not audible at low levels - at all.>> It most certainly is audible. Its a fact.>** See Flectcher Munsen curves.Oh dear.....>White noise is predominately high frequency energy, masking any low >frequency component.Not when the music goes quiet it don't. I note that you have all book theory, and no appreciation of reality regarding pro audio. I guess you haven't sat in a test room actually listening to LF noise from a mixer.. You just have no f'ing idea, particularly about recording mixers. You are only embarrassing yourself now. Music has dynamic range. Mixers have lots of channels (8-24) all added up. Lots of the channels have say 15 dB 40 Hz boost. 30 Hz noise (1/f) on a good transistor might be around 3 times that of flatband. Bad random samples might be 10 x flatband, or larger. It all adds up such that its easily audible. Drum tracks have single impulses. They go quiet continuously when the noise breaks through. Vocal tracks go quiet thought-out a song. Indeed, Studiomaster had probably the first MIDI muting desk on the market in the early 80s, specifically to allow for timed programmed muting of tracks to avoid noise build up. I guess all that electronics was put in for no reason? I guess those getting paid to select out the bad LF noise devices were not worth the money?> > >** FFS take look at P66. > > > It should look VERY familiar .... ;-) > > I checked out https://sound-au.com/project66.htm > > To the best of my knowledge, this compound pair was first used in a mic > amp > by Studiomaster. A guy named Alan Pound introduced it, prior to my > arrival. > >** The same or closely similar topolgy is used in many desks. > But you don't get to see them.Sure, but apparently, they were copied from Studiomaster. The pair had been used before, this appears to be the first use in a ic amp. Studiomaster moved on to include the diff input pair in a main feedback loop. Its a cascode stack. -- Kevin Aylward http://www.anasoft.co.uk/ SuperSpice http://www.kevinaylward.co.uk/ee/index.html
Reply by ●December 23, 20212021-12-23
Kevin Aylward wrote: ==================.> > > > >> The data sheet for the BC549 has a noise figure of 1.2 dB with a 2k > > >> source. > > > >** Please cite. My info is the ideal is 50k. > > > What do you mean by 50k ? > > >** I see no cite of the noise data. > > There is this wonderful modern technique to gain information. its called > Google.** Then post it.> > > >>** Bollocks. Try really answering my obvious point. > > > >> The audible noise from an input loaded mic pre is all above 1kHz. > > >> As I explained, it is a real problem, known to all in the industry. > > >** What is ?? > > 1/f and RTS/Popcorn noise.** Red herring - not audible in audio band white noise.> >> I guess you haven't sat in a test room actually listening to LF noise. > > >** Another desperate red herring, plus absurd.> You are alleging that 1/f noise isn't a problem in mic amps** Errrrr = yep.> A listening test in a pro company's quality room proves otherwise.** Very smelly horse manure. Or is it adctually Kev manure?> > >> The early Studiomasters used 2N4403, > > > >> >** Really? Never seen a bad new one in my life. > > >> Well... it was certainly a fact in 1980. > > >** Nope, it was not. > > You are just pissing the in wind in totally ignorance.** Kev has no evidence so just make shits up. I used the same types to make MC head amps - after Mr Leach. Full RIAA EQ followed one of them. Lots of LF boost.> Its a proven fact.** Proven by whom, to who and where ? By you to yourself ?? ------------------------------------------------------------ ( snip sickening, self agrandising crap )> > >** Low frequent noise is not audible at low levels - at all. > > >> It most certainly is audible. Its a fact. > > >** See Flectcher Munsen curves.> Oh dear.....** Oh shit ...... the above gents are turning in their graves. Whirrr, whirrr ...> >White noise is predominately high frequency energy, masking any low > >frequency component. > > Not when the music goes quiet it don't.** ROTFLMAO !!! There is no crazy lie this pommy charlatan will not post. ==============================================> I note that you have all book theory,** Pot v Kettle ?? Your BS is all pseudo theory with no evidence. Anywhere.> Lots of the channels have say 15 dB 40 Hz boost.** ROTFLMAO !!!!!!! Do you actually expect me to believe in the tooth fairy too ??> Drum tracks have single impulses. They go quiet continuously when the noise > breaks through.** Do they - -how interesting ......> Vocal tracks go quiet thought-out a song.** Singers gotta breathe ...> I guess all that electronics was put in for no reason?** Nah - just fills up the bos actually.> I guess those getting paid to select out the bad LF noise devices were not > worth the money?** Staff do as told - there's is not to reason why. Just need to get paid at the end of the week. BTW: What ever happened to Graham Stevenson ? AKA " Pooh Bear" ?? Think he came along after yourself at Studiomaster. He was full of it up to the eyeballs here as well. Then he suddenly disappeared. The lad seemed to have a few screws loose at the time. Like you. ..... Phil
Reply by ●December 23, 20212021-12-23
amdx wrote:> On 12/22/2021 12:37 PM, John Walliker wrote: >> On Wednesday, 22 December 2021 at 17:23:15 UTC, >> jla...@highlandsniptechnology.com wrote: >>>> ** Most are a JFET with drain as output needing a +DC source >>> and a series resistor of about 4000 ohms >>> I'd guess that the best load on the jfet would be constant-voltage, >>> namely a cascode NPN or the inverting input of an opamp. That >>> minimizes d-g feedback and likely distortion. >> Yes, connecting the mic output to the inverting input of a low-noise >> op-amp via a capacitor works very well. No series resistor is needed >> or even desirable. The electret mics being used >> by the OP are intended to operate at 0.5mA drain current. The pullup >> resistor can be chosen according to the supply voltage to give that >> current when the output dc level is about 1.9V. These are the conditions >> under which the mics are characterised by the manufacturer. >> Running at much lower current would result in the drain voltage falling >> which reduces the dynamic range. There are advantages in using >> a fairly high bias voltage of up to a maximum of 10V with a >> correspondingly higher pullup resistor to keep the the output at >> about 1.9V and the current at 0.5mA. A 9V battery makes an >> excellent low-noise bias source. The higher pullup resistor which, >> for a 9V bias supply would be 14.2k, reduces the noise gain of the >> op-amp. >> As the acoustic input modulates the drain current of the FET in >> the mic, all the audio current ends up in the feedback resistor of >> the op-amp and the drain voltage stays almost constant. >> With a feedback resistor of 17.4k and using the PUI Audio >> AOM-5024L-HD-R microphone the output of the preamp will be >> about 0.5Vrms/Pa. >> >> John >> > Isn't there a trade off? Does the higher 14.2k pullup resistor have > it's own noise vs the reduced noise off the opamp. > > I experimented the as much as 20V on the mic FET from a 25v supply. It > only gave me about 30% more signal over 1.5V > > on the mic FET. What other ± effects would a high pullup voltage and > resistor have? > > Mikek >JL's suggestion of a cascode is a good one. The drain curves of JFETs are horrible--nearly as bad as a triode--so you want to nail the drain still to prevent distortion. The soft drain characteristic looks like a shunt resistance to leading order, so from a gain POV the drain load is effectively the drain resistance in parallel with the pullup resistor. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●December 23, 20212021-12-23
On Thu, 23 Dec 2021 14:56:31 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>amdx wrote: >> On 12/22/2021 12:37 PM, John Walliker wrote: >>> On Wednesday, 22 December 2021 at 17:23:15 UTC, >>> jla...@highlandsniptechnology.com wrote: >>>>> ** Most are a JFET with drain as output needing a +DC source >>>> � and a series resistor of about 4000 ohms >>>> I'd guess that the best load on the jfet would be constant-voltage, >>>> namely a cascode NPN or the inverting input of an opamp. That >>>> minimizes d-g feedback and likely distortion. >>> Yes, connecting the mic output to the inverting input of a low-noise >>> op-amp via a capacitor works very well.� No series resistor is needed >>> or even desirable.� The electret mics being used >>> by the OP are intended to operate at 0.5mA drain current.� The pullup >>> resistor can be chosen according to the supply voltage to give that >>> current when the output dc level is about 1.9V.� These are the conditions >>> under which the mics are characterised by the manufacturer. >>> Running at much lower current would result in the drain voltage falling >>> which reduces the dynamic range.� There are advantages in using >>> a fairly high bias voltage of up to a maximum of 10V with a >>> correspondingly higher pullup resistor to keep the the output at >>> about 1.9V and the current at 0.5mA.� A 9V battery makes an >>> excellent low-noise bias source.� The higher pullup resistor which, >>> for a 9V bias� supply would be 14.2k, reduces the noise gain of the >>> op-amp. >>> As the acoustic input modulates the drain current of the FET in >>> the mic, all the audio current ends up in the feedback resistor of >>> the op-amp and the drain voltage stays almost constant. >>> With a feedback resistor of 17.4k and using the PUI Audio >>> AOM-5024L-HD-R microphone the output of the preamp will be >>> about 0.5Vrms/Pa. >>> >>> John >>> >> �Isn't there a trade off? Does the higher 14.2k pullup resistor have >> it's own noise vs the reduced noise off the opamp. >> >> �I experimented the as much as 20V on the mic FET from a 25v supply. It >> only gave me about 30% more signal over 1.5V >> >> on the mic FET. What other � effects would a high pullup voltage and >> resistor have? >> >> ��������������������������������������������� Mikek >> > >JL's suggestion of a cascode is a good one. The drain curves of JFETs >are horrible--nearly as bad as a triode--so you want to nail the drain >still to prevent distortion. > >The soft drain characteristic looks like a shunt resistance to leading >order, so from a gain POV the drain load is effectively the drain >resistance in parallel with the pullup resistor. > >Cheers > >Phil HobbsOr go into the inverting input of an opamp that's a few volts off ground. -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
Reply by ●December 23, 20212021-12-23
John Larkin wrote:> On Thu, 23 Dec 2021 14:56:31 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> amdx wrote: >>> On 12/22/2021 12:37 PM, John Walliker wrote: >>>> On Wednesday, 22 December 2021 at 17:23:15 UTC, >>>> jla...@highlandsniptechnology.com wrote: >>>>>> ** Most are a JFET with drain as output needing a +DC source >>>>> and a series resistor of about 4000 ohms >>>>> I'd guess that the best load on the jfet would be constant-voltage, >>>>> namely a cascode NPN or the inverting input of an opamp. That >>>>> minimizes d-g feedback and likely distortion. >>>> Yes, connecting the mic output to the inverting input of a low-noise >>>> op-amp via a capacitor works very well. No series resistor is needed >>>> or even desirable. The electret mics being used >>>> by the OP are intended to operate at 0.5mA drain current. The pullup >>>> resistor can be chosen according to the supply voltage to give that >>>> current when the output dc level is about 1.9V. These are the conditions >>>> under which the mics are characterised by the manufacturer. >>>> Running at much lower current would result in the drain voltage falling >>>> which reduces the dynamic range. There are advantages in using >>>> a fairly high bias voltage of up to a maximum of 10V with a >>>> correspondingly higher pullup resistor to keep the the output at >>>> about 1.9V and the current at 0.5mA. A 9V battery makes an >>>> excellent low-noise bias source. The higher pullup resistor which, >>>> for a 9V bias supply would be 14.2k, reduces the noise gain of the >>>> op-amp. >>>> As the acoustic input modulates the drain current of the FET in >>>> the mic, all the audio current ends up in the feedback resistor of >>>> the op-amp and the drain voltage stays almost constant. >>>> With a feedback resistor of 17.4k and using the PUI Audio >>>> AOM-5024L-HD-R microphone the output of the preamp will be >>>> about 0.5Vrms/Pa. >>>> >>>> John >>>> >>> Isn't there a trade off? Does the higher 14.2k pullup resistor have >>> it's own noise vs the reduced noise off the opamp. >>> >>> I experimented the as much as 20V on the mic FET from a 25v supply. It >>> only gave me about 30% more signal over 1.5V >>> >>> on the mic FET. What other ± effects would a high pullup voltage and >>> resistor have? >>> >>> Mikek >>> >> >> JL's suggestion of a cascode is a good one. The drain curves of JFETs >> are horrible--nearly as bad as a triode--so you want to nail the drain >> still to prevent distortion. >> >> The soft drain characteristic looks like a shunt resistance to leading >> order, so from a gain POV the drain load is effectively the drain >> resistance in parallel with the pullup resistor. >> >> Cheers >> >> Phil Hobbs > > Or go into the inverting input of an opamp that's a few volts off > ground. >Yup. At audio rates that's better than good enough. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●December 24, 20212021-12-24
On Tuesday, 21 December 2021 at 13:07:45 UTC, amdx wrote:> I used a LM386 and found a gain of 200 didn't cut it. It worked but > didn't have any signal at low audio levels.could always add a little pfb ;)> I want a lot of gain so I can hear a voice at 100 or 200 yards. I also > either need a clipping circuit or a fast AGC, so I don't hurt my ears on > a load sound.If a sound is loud enough to hurt your ears, clipping it will make it hurt way worse. A very fast acting agc is the simple way to go, fast to drop gain, not so fast to raise it.
