Low open loop gain from a mic preamp-why?...

[amdx]

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



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[Phil Allison]

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

 

[Sylvia Else]

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.

 

[amdx]

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



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[Phil Allison]

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.

 

[amdx]

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.

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[amdx]

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.

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[Dave Platt]

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.

 

[amdx]

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



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[amdx]

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


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[bitrex]

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.

 

[piglet]

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

 

[server]

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

 

[Kevin Aylward]

\"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

 

[amdx]

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





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[Phil Allison]

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

 

[John Larkin]

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.


                      Mikek

Electrets 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

 

[Phil Allison]

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

 

[Jan Panteltje]

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 ;-)

 

[Phil Allison]

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