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 -- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
Low open loop gain from a mic preamp-why?
Started by ●December 20, 2021
Reply by ●December 20, 20212021-12-20
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. >** The 549 is "low noise - the 547, not so much.> 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 (?)** Do you know what rms means?> 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.** Seems very high - not credible.> > Schematic and writeup, https://sound-au.com/project13.htm** Only one transistor is providing voltage gain. ..... Phil
Reply by ●December 20, 20212021-12-20
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.
Reply by ●December 20, 20212021-12-20
On 12/20/2021 7:06 PM, Phil Allison wrote:> 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. >> > ** The 549 is "low noise - the 547, not so much.I understand, It's what I had and wanted an amp to try. Also only had a few metal film resistors.>> 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 (?) > ** Do you know what rms means? You're probably asking, did I convert from P to P to RMS, I did, but I do have an error. That 6.2vrms was when I had Q2 backwards. Now with that corrected, I only have 1/4v rms before clipping. Gain of 25.>> 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. > ** Seems very high - not credible.Hmm, what about the version with the opamp buffer, it says 1,200,000 open loop gain.>>> Schematic and writeup, https://sound-au.com/project13.htm > ** Only one transistor is providing voltage gain. > > > ..... Phil Is transistor HFE going to affect the gain of this amp? Mikek -- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
Reply by ●December 20, 20212021-12-20
amdx wrote: ==========>> > > ** The 549 is "low noise - the 547, not so much.> I understand, It's what I had and wanted an amp to try. Also only had a > few metal film resistors.** MF resistors are in this circuit,> > ** Do you know what rms means?> You're probably asking, did I convert from P to P to RMS, I did,** No you didn't .> I do have an error. That 6.2vrms was when I had Q2 backwards.** ROTFL - what bullshit.> Hmm, what about the version with the opamp buffer, it says 1,200,000 > open loop gain.** How many angels can dance on the head of a pin ?> Is transistor HFE going to affect the gain of this amp?** Yep.
Reply by ●December 20, 20212021-12-20
On 12/20/2021 8:18 PM, Phil Allison wrote:> amdx wrote: > ========== >>> ** The 549 is "low noise - the 547, not so much. >> I understand, It's what I had and wanted an amp to try. Also only had a >> few metal film resistors. > ** MF resistors are in this circuit, > >>> ** Do you know what rms means? >> You're probably asking, did I convert from P to P to RMS, I did, > ** No you didn't . OK Phil you were there when I pulled up this calculator and adjusted the input for P to P and put 17.5 as my input and got 6.xxx which I round up to 6.2. But I will admit I made an error, I'm not sure if i was 10x probe error or what, but I did get it wrong.> >> I do have an error. That 6.2vrms was when I had Q2 backwards. > ** ROTFL - what bullshit. Ya that's what I said went I found the backward transistor. Yes, Phil I did have an error.>> Hmm, what about the version with the opamp buffer, it says 1,200,000 >> open loop gain. > ** How many angels can dance on the head of a pin ? > >> Is transistor HFE going to affect the gain of this amp? > ** Yep. > > >-- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
Reply by ●December 20, 20212021-12-20
On 12/20/2021 8:49 PM, amdx wrote:> On 12/20/2021 8:18 PM, Phil Allison wrote: >> amdx wrote: >> ========== >>>> ** The 549 is "low noise - the 547, not so much. >>> I understand, It's what I had and wanted an amp to try. Also only >>> had a >>> few metal film resistors. >> ** MF resistors are in this circuit, >> >>>> ** Do you know what rms means? >>> You're probably asking, did I convert from P to P to RMS, I did, >> ** No you didn't . > > OK Phil you were there when I pulled up this calculator > https://www.allaboutcircuits.com/tools/peak-to-peak-voltage-calculator/of course forgot the link. :-)> and adjusted the input for P to P and put > > 17.5 as my input and got 6.xxx which I round up to 6.2. But I will > admit I made an error, I'm not sure > > if i was 10x probe error or what, but I did get it wrong. > >> >>> I do have an error. That 6.2vrms was when I had Q2 backwards. >> ** ROTFL - what bullshit. > Ya that's what I said went I found the backward transistor. Yes, Phil > I did have an error. >>> Hmm, what about the version with the opamp buffer, it says 1,200,000 >>> open loop gain. >> ** How many angels can dance on the head of a pin ? >> >>> Is transistor HFE going to affect the gain of this amp? >> ** Yep. >> >> >> > >-- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
Reply by ●December 20, 20212021-12-20
> 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.That's how I see it. The original author said to bypass R5 with a "suitably large" capacitor, and I'd guess "suitably large" is maybe 10,000 uF or so. In order to see the increased open-loop AC gain, you have to open the "set gain" path completely, and also filter out all of the AC feedback through R4/R5/R6, leaving this loop to set only the DC bias on Q1. The bypass-cap/R5 time constant is going to set the low-frequency limit on the increased open-loop gain.
Reply by ●December 21, 20212021-12-21
On 12/20/2021 9:16 PM, Dave Platt wrote:>> 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. > That's how I see it. The original author said to bypass R5 with a > "suitably large" capacitor, and I'd guess "suitably large" is maybe > 10,000 uF or so. In order to see the increased open-loop AC gain, you > have to open the "set gain" path completely, and also filter out all > of the AC feedback through R4/R5/R6, leaving this loop to set only the > DC bias on Q1. The bypass-cap/R5 time constant is going to set the > low-frequency limit on the increased open-loop gain. > I used 100 uf. My testing was at 1kHz, where 100uf is only 1.6 Ω. But, I'll try it in the morning. Mikek -- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
Reply by ●December 21, 20212021-12-21
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 -- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus
