On 12/21/2021 1:03 AM, amdx wrote:> On 12/20/2021 9:59 PM, bitrex wrote: >> On 12/20/2021 8:24 PM, Sylvia Else wrote: >>> On 21-Dec-21 11:41 am, amdx wrote: >>>> The only problem that I may have created is the transistor, I used >>>> BC547B with HFE of 200 to 450 vs BC549C with HFE of 420 to 800. >>>> >>>> All I had in my junk box. >>>> >>>> I built a low noise amplifier, original design was for a 30v supply, >>>> I chose to correct the values for a 15V supply. Designer does have a >>>> 12v version, my values are close. >>>> >>>> I did set an emitter resistor to have equal clipping. Maximum rms >>>> voltage before clipping is 6.2v vs 7v for his 30 volt version. I >>>> didn't expect that much for the 15 volt version (?) >>>> >>>> The designer says it has a gain of 40. My build had a gain of 35, a >>>> little low but... >>>> >>>> The designer also says by removing a feedback resistor and bypassing >>>> an emitter resistor, and adding a capacitor, the open loop gain >>>> >>>> will increase to 3400. When I remove the feedback I get no change in >>>> the gain. I'm assuming the feedback is the 100k going between >>>> >>>> Q1 base and the 3.9k and 390Ω junction and not the gain set pot. Do >>>> I have that correct? >>>> >>>> Opening the gain pot circuit raises gain to 56. >>>> >>>> Would the low HFE transistor cause low open loop gain? >>>> >>>>> Schematic and writeup, https://sound-au.com/project13.htm >>>> >>>> Thanks, Mikek >>>> >>>> >>>> >>> >>> Looks to me as if R6 is also setting the bias point of Q1, so you >>> can't just remove it. Indeed, I don't see how the circuit would work >>> at all without R6. >>> >>> Bypassing R5 would allow the bias to be set, but remove the feedback >>> through R6. >>> >>> I think the feedback resistor the designer was talking about must be >>> the "Set Gain" resistor. >>> >>> Sylvia. >> >> Yeah there are two loops, and if you bypass them all by removing the >> 50k "set gain" and bypassing R5 for AC the V/V gain becomes large but >> unpredictable, I think it will look like a lump maybe it will be 3400 >> somewhere, beats me. >> >> The inner loop is an I -> V transimpedance amplifier in isolation; >> with no external load the feedback is proportional to current flowing >> in Q2's emitter resistance so if you apply a test voltage on the left >> of R1 with the 50k disconnected, instead of injecting a test current >> into the base/R6 junction the "open loop" V/V gain will also depend on >> the source impedance if it's also large with respect to R1. > I'm driving it with 600Ω, as noted in the writeup. >> >> Add an external load and close the 50k around to the junction of R1 >> and then it functions vaguely like an inverting voltage-sensing >> feedback op-amp, with the junction of R1/50k looking vaguely like a >> virtual ground. >> > I haven't loaded the output. I can try that, Author says 22K, but > we'll see. > > Thanks, Mikek > >The gain with the 50k removed and R5 bypassed is a gee-whiz figure but I don't know exactly how he's calculating/measuring that and I don't think it matters too much with the load and outer feedback connected, providing only ~30 dB of closed-loop gain into 22k the circuit should work well enough and as described with the transistors specified that aren't defective.
Low open loop gain from a mic preamp-why?
Started by ●December 20, 2021
Reply by ●December 21, 20212021-12-21
Reply by ●December 21, 20212021-12-21
On 21/12/2021 00:41, amdx wrote:> > Would the low HFE transistor cause low open loop gain? > >> Schematic and writeup, https://sound-au.com/project13.htm >No, the gain is determined by other factors, mainly negative feedback. However you would be better off using a BC549 as it is lower noise and better suited to low current operation than BC547. All the voltage gain in that design comes from the first transistor and is classic transconductance amplifier K = Gm * Rc (where Rc is collector load), Gm is Ic/Vt (Ic is collector current and Vt is thermal voltage, roughly 26mV at room temp, more exactly kT/q) What this means is that when operating into the very high load impedance presented by the second stage an open load gain in the thousands is certainly achievable. That circuit configuration is very dependable and I have used similar variants of it countless times over the decades. If you are hell-bent on maximum gain then open circuit the "gain set" resistor and bypass R5. If R5 is 330ohm then a bypass cap of 22uF would have a -3dB point at 22Hz. piglet
Reply by ●December 21, 20212021-12-21
On Tue, 21 Dec 2021 06:55:55 +0000, piglet <erichpwagner@hotmail.com> wrote:>On 21/12/2021 00:41, amdx wrote: >> >> �Would the low HFE transistor cause low open loop gain? >> >>> Schematic and writeup, https://sound-au.com/project13.htm >> > >No, the gain is determined by other factors, mainly negative feedback. > >However you would be better off using a BC549 as it is lower noise and >better suited to low current operation than BC547.Given the crazy 1.2K input resistor, the choice of transistor is secondary. Really low beta would increase shot noise, but Johnson will usually dominate. Transistor Rbbs are crushed by the 1.2K and the 600r mic. The author's claim of 3.2 nV noise density isn't possible. The classic 2-transistor "GE" circuit was simple and made more sense. Today, a jfet or an IC makes more sense.> >All the voltage gain in that design comes from the first transistor and >is classic transconductance amplifier K = Gm * Rc (where Rc is >collector load), Gm is Ic/Vt (Ic is collector current and Vt is thermal >voltage, roughly 26mV at room temp, more exactly kT/q)Again, the 600r mike and 1.2K resistor are in series with the base. Base current hence gain depends on beta. But as the author correctly states, it's audio so nothing much matters. If you like the way it sounds, you're done. -- I yam what I yam - Popeye
Reply by ●December 21, 20212021-12-21
"amdx" wrote in message news:spr7qt$2n0$1@dont-email.me...>The only problem that I may have created is the transistor, I used BC547B >with HFE of 200 to 450 vs BC549C with HFE of 420 to 800.>All I had in my junk box.>I built a low noise amplifier, original design was for a 30v supply, I >chose to correct the values for a 15V supply. Designer does have a 12v >version, my values are close.Fundamentally, its a poor design. Its inherently NOT low noise, and loads the mic way to much, effecting the frequency response. The 1k2 input resistor dominates the noise. For a standard mic of 200/150 ohms it destroys the inherent noise performance of the source resistance due to thermal noise of the resister, and because of the input current noise dropped across the input source resistor. The issue is that the design is of an inverting feedback amplifier. Low noise amplifiers need to be non inverting if a feedback topology is used. This allows having a standard input bias resistor setting the mic load of say 6k8. This resister gets "shorted" by the mic resistance so does not contribute to the noise. Additionally the mic shorts the input current noise. 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. I suggest topologies such as here :-) https://www.kevinaylward.co.uk/ee/micampdesign/MicAmpDesign.html -- Kevin Aylward http://www.anasoft.co.uk/ SuperSpice http://www.kevinaylward.co.uk/ee/index.html
Reply by ●December 21, 20212021-12-21
On 12/21/2021 10:04 AM, jlarkin@highlandsniptechnology.com wrote:> On Tue, 21 Dec 2021 06:55:55 +0000, piglet <erichpwagner@hotmail.com> > wrote: > >> On 21/12/2021 00:41, amdx wrote: >>> Would the low HFE transistor cause low open loop gain? >>> >>>> Schematic and writeup, https://sound-au.com/project13.htm >> No, the gain is determined by other factors, mainly negative feedback. >> >> However you would be better off using a BC549 as it is lower noise and >> better suited to low current operation than BC547. > Given the crazy 1.2K input resistor, the choice of transistor is > secondary. Really low beta would increase shot noise, but Johnson will > usually dominate. Transistor Rbbs are crushed by the 1.2K and the 600r > mic. > > The author's claim of 3.2 nV noise density isn't possible. > > The classic 2-transistor "GE" circuit was simple and made more sense. > Today, a jfet or an IC makes more sense. > >> All the voltage gain in that design comes from the first transistor and >> is classic transconductance amplifier K = Gm * Rc (where Rc is >> collector load), Gm is Ic/Vt (Ic is collector current and Vt is thermal >> voltage, roughly 26mV at room temp, more exactly kT/q) > Again, the 600r mike and 1.2K resistor are in series with the base. > Base current hence gain depends on beta. > > But as the author correctly states, it's audio so nothing much > matters. If you like the way it sounds, you're done. > >I built another circuit today, I went with the 30V design, it works. Almost 20Vpp output signal. I used the same BC547B transistors, upgrades at a later date. I have a signal generator set at 1000Hz, I'm connected to the 600Ω output. If I put a another 600Ω resistor in series with the output, 8 units displayed on scope (end of the 600Ω resistor), when I connect it to the input it drops to 4 units on the scope. This tells me the input impedance is about 1200Ω (600Ω + 600Ω). I had to pick a few resistors before I got it to read 4 units. If you think this method is invalid let me know. I also went open loop and I did get the 3400 gain the author discussed, however the input impedance then went up to 40K. What type of problem does connecting a 2.2k output impedance mic to a 40k input impedance preamp. (very short cable) Does it contribute to noise? I would like to do the best I can with an electret mic driving a low noise high gain preamp. I don't really know what mic impedance I'm matching yet as it's subject to change. Mikek -- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
Reply by ●December 21, 20212021-12-21
amdx wrote: ==========> > I built another circuit today, I went with the 30V design, it works. > Almost 20Vpp output signal. > > I used the same BC547B transistors, upgrades at a later date. >> I have a signal generator set at 1000Hz, I'm connected to the 600Ω> output. If I put a another 600Ω resistor > > in series with the output, 8 units displayed on scope (end of the 600Ω > resistor), when I connect it to the input it drops to 4 units on the scope. > > This tells me the input impedance is about 1200Ω (600Ω + 600Ω). I had > to pick a few resistors before I got it to read 4 units. If you > > think this method is invalid let me know. I also went open loop and I > did get the 3400 gain the author discussed,** I spoke with Rod Elliott on the phone yesterday - he is a long time friend. I have a number of articles on that same site. The circuit works as claimed, when correctly built. FYI: For a seriously *low noise* and balanced mic pre-amp, see my article - Project 66.> What type of problem does connecting a 2.2k output impedance mic to a > 40k input impedance preamp. (very short cable)**None,> Does it contribute to noise?** No.> I would like to do the best I can with an electret mic driving a low > noise high gain preamp.** Electret capsules have *self noise* that would swamp that of most pre-amps. You are worried about nothing. ..... Phil
Reply by ●December 21, 20212021-12-21
On Tue, 21 Dec 2021 15:08:59 -0600, amdx <amdx@knology.net> wrote:>On 12/21/2021 10:04 AM, jlarkin@highlandsniptechnology.com wrote: >> On Tue, 21 Dec 2021 06:55:55 +0000, piglet <erichpwagner@hotmail.com> >> wrote: >> >>> On 21/12/2021 00:41, amdx wrote: >>>> �Would the low HFE transistor cause low open loop gain? >>>> >>>>> Schematic and writeup, https://sound-au.com/project13.htm >>> No, the gain is determined by other factors, mainly negative feedback. >>> >>> However you would be better off using a BC549 as it is lower noise and >>> better suited to low current operation than BC547. >> Given the crazy 1.2K input resistor, the choice of transistor is >> secondary. Really low beta would increase shot noise, but Johnson will >> usually dominate. Transistor Rbbs are crushed by the 1.2K and the 600r >> mic. >> >> The author's claim of 3.2 nV noise density isn't possible. >> >> The classic 2-transistor "GE" circuit was simple and made more sense. >> Today, a jfet or an IC makes more sense. >> >>> All the voltage gain in that design comes from the first transistor and >>> is classic transconductance amplifier K = Gm * Rc (where Rc is >>> collector load), Gm is Ic/Vt (Ic is collector current and Vt is thermal >>> voltage, roughly 26mV at room temp, more exactly kT/q) >> Again, the 600r mike and 1.2K resistor are in series with the base. >> Base current hence gain depends on beta. >> >> But as the author correctly states, it's audio so nothing much >> matters. If you like the way it sounds, you're done. >> >> >I built another circuit today, I went with the 30V design, it works. >Almost 20Vpp output signal. > >I used the same BC547B transistors, upgrades at a later date. > >I have a signal generator set at 1000Hz, I'm connected to the 600? >output. If I put a another 600? resistor > >in series with the output, 8 units displayed on scope (end of the 600? >resistor), when I connect it to the input it drops to 4 units on the scope. > > �This tells me the input impedance is about 1200? (600? + 600?). I had >to pick a few resistors before I got it to read 4 units. If you > >think this method is invalid let me know. I also went open loop and I >did get the 3400 gain the author discussed, > >however the input impedance then went up to 40K. > >What type of problem does connecting a 2.2k output impedance mic to a >40k input impedance preamp. (very short cable) > >Does it contribute to noise? > >I would like to do the best I can with an electret mic driving a low >noise high gain preamp. > >I don't really know what mic impedance I'm matching yet as it's subject >to change. > > > ��������������������� MikekElectrets usually have a jfet amp inside, so don't especially need a low noise amp. -- 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 21, 20212021-12-21
Kevin Aylward wrote: ==================> > Fundamentally, its a poor design. Its inherently NOT low noise, and loads > the mic way to much, effecting the frequency response.** Nonsense. A 1200 ohm load will not affect dynamic mics and only drop the level from an electret. > The 1k2 input resistor dominates the noise. ** Maybe, but only just. BC549s are non ideal for low source impedances.> For a standard mic of 200/150 > ohms it destroys the inherent noise performance of the source resistance due > to thermal noise of the resister, and because of the input current noise > dropped across the input source resistor.** Still low enough for a great many real life uses and quite OK with an internally FET buffered electret capsule.> The issue is that the design is of an inverting feedback amplifier. Low > noise amplifiers need to be non inverting if a feedback topology is used. >** Correct.> This allows having a standard input bias resistor setting the mic load of > say 6k8.** Not needed at all - 1.2K is fine and standard practice.> 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. > https://www.kevinaylward.co.uk/ee/micampdesign/MicAmpDesign.html ** Hmmm - my Project 66 is simpler and measured performance better. Plus it is REAL !! ============ ... Phil
Reply by ●December 22, 20212021-12-22
On a sunny day (Tue, 21 Dec 2021 15:08:59 -0600) it happened amdx <amdx@knology.net> wrote in <sptfpd$6ek$1@dont-email.me>:>I would like to do the best I can with an electret mic driving a low >noise high gain preamp.I think the electret mikes have just a JFET source follower. And quite a large signal output. good quality opamp is likely all you need. Ducks ;-)
Reply by ●December 22, 20212021-12-22
Jan Panteltje wrote: ================>> > I think the electret mikes have just a JFET source follower. > And quite a large signal output.** Most are a JFET with drain as output needing a +DC source and a series resistor of about 4000 ohms> good quality opamp is likely all you need.** NE5532, RC4560, TL072 or TL074 and a host of others. .... Phil
