Gentlemen,

Anyone know of a suitable jellybean op-amp that would make for a low
distortion pre-amp for a broadcast quaility dynamic microphone?
(single supply preferably).
TIA.
-CD

On Saturday, May 15, 2021 at 9:29:03 PM UTC+10, Cursitor Doom wrote:
> Gentlemen,
>
> Anyone know of a suitable jellybean op-amp that would make for a low
> distortion pre-amp for a broadcast quality dynamic microphone?
> (single supply preferably).

There are a range of ways of making microphones. Presumably the broad-cast industry has standardised on one - and Phil Allison will probably tell us what it is - but without that information it's pretty silly question.

--
Bill Sloman, Sydney

On 5/15/2021 7:28 AM, Cursitor Doom wrote:
> Gentlemen,
>
> Anyone know of a suitable jellybean op-amp that would make for a low
> distortion pre-amp for a broadcast quaility dynamic microphone?
> (single supply preferably).
> TIA.
> -CD
>

What's "broadcast quality"? does that mean really good or like not as
good as "studio quality' or...?

I don't think any "jellybean" op-amp has the combination of noise, open
loop gain and distortion performance to make a really good dynamic mic
pre by itself. An NE5534 will work OK for some applications in something
like:

<https://i.pinimg.com/736x/44/59/f4/4459f4828fc78378bc28c817119d614d.jpg>

but the high-quality solid-state pres tend to use discrete transistors
or transistor/hybrid designs. Just because it's audio doesn't mean an
op-amp can slum it, a good dynamic mic pre should have up to 70 dB of
low-distortion gain out to like 15kHz around where the -3dB point of say
the SM57 is, that's a tall order for a jellybean.

A jellybean amp would be better with an input transformer, but then you
could afford a better op amp anyway...

On 5/15/2021 9:51 AM, bitrex wrote:
> On 5/15/2021 7:28 AM, Cursitor Doom wrote:
>> Gentlemen,
>>
>> Anyone know of a suitable jellybean op-amp that would make for a low
>> distortion pre-amp for a broadcast quaility dynamic microphone?
>> (single supply preferably).
>> TIA.
>> -CD
>>
>
> What's "broadcast quality"? does that mean really good or like not as
> good as "studio quality' or...?
>
> I don't think any "jellybean" op-amp has the combination of noise, open
> loop gain and distortion performance to make a really good dynamic mic
> pre by itself. An NE5534 will work OK for some applications in something
> like:
>
> <https://i.pinimg.com/736x/44/59/f4/4459f4828fc78378bc28c817119d614d.jpg>
>
> but the high-quality solid-state pres tend to use discrete transistors
> or transistor/hybrid designs.

I should qualify that as high-quality, but still built to a reasonable
budget, there are all sorts of amazing op amps out there that might work
on their own but they're not priced so you could build a whole mixer
input section with them and end up with something you could sell.

On 5/15/2021 9:36 AM, Bill Sloman wrote:
> On Saturday, May 15, 2021 at 9:29:03 PM UTC+10, Cursitor Doom wrote:
>> Gentlemen,
>>
>> Anyone know of a suitable jellybean op-amp that would make for a low
>> distortion pre-amp for a broadcast quality dynamic microphone?
>> (single supply preferably).
>
> There are a range of ways of making microphones. Presumably the broad-cast industry has standardised on one - and Phil Allison will probably tell us what it is - but without that information it's pretty silly question.
>

One of the few remaining areas in small-signal audio where if you need
cheap and good an op amp isn't the way to go, all the budget analog
mixers by e.g. Behringer use discrete transistor mic pres AFAIK.

A number of studio-types swear by 'em for micing stuff like guitar cabs
with dynamic mics, they're not bad! They stole the design from Mackie,
after all lol

On Sat, 15 May 2021 09:57:02 -0400, bitrex <user@example.net> wrote:

>> What's "broadcast quality"? does that mean really good or like not as
>> good as "studio quality' or...?

Fair point. I suppose a close approximation would be something the BBC
Radio 4 station would use for studio interviews on spots such as Today
and The World Tonight:

https://www.bbc.co.uk/programmes/b006qtl3/episodes/downloads

>> I don't think any "jellybean" op-amp has the combination of noise, open
>> loop gain and distortion performance to make a really good dynamic mic
>> pre by itself. An NE5534 will work OK for some applications in something
>> like:
>>
>> <https://i.pinimg.com/736x/44/59/f4/4459f4828fc78378bc28c817119d614d.jpg>
>>
>> but the high-quality solid-state pres tend to use discrete transistors
>> or transistor/hybrid designs.
>
>I should qualify that as high-quality, but still built to a reasonable
>budget, there are all sorts of amazing op amps out there that might work
>on their own but they're not priced so you could build a whole mixer
>input section with them and end up with something you could sell.

Discretes eh? Well, that's a surprise. That would be better still from
my PoV; having less of the work done for you affords more scope for
learning something new.

On Sat, 15 May 2021 16:41:28 +0100, Cursitor Doom <cd@nowhere.com>
wrote:

>On Sat, 15 May 2021 09:57:02 -0400, bitrex <user@example.net> wrote:
>
>>> What's "broadcast quality"? does that mean really good or like not as
>>> good as "studio quality' or...?
>
>Fair point. I suppose a close approximation would be something the BBC
>Radio 4 station would use for studio interviews on spots such as Today
>and The World Tonight:
>
>https://www.bbc.co.uk/programmes/b006qtl3/episodes/downloads
>
>>> I don't think any "jellybean" op-amp has the combination of noise, open
>>> loop gain and distortion performance to make a really good dynamic mic
>>> pre by itself. An NE5534 will work OK for some applications in something
>>> like:
>>>
>>> <https://i.pinimg.com/736x/44/59/f4/4459f4828fc78378bc28c817119d614d.jpg>
>>>
>>> but the high-quality solid-state pres tend to use discrete transistors
>>> or transistor/hybrid designs.
>>
>>I should qualify that as high-quality, but still built to a reasonable
>>budget, there are all sorts of amazing op amps out there that might work
>>on their own but they're not priced so you could build a whole mixer
>>input section with them and end up with something you could sell.
>
>Discretes eh? Well, that's a surprise. That would be better still from
>my PoV; having less of the work done for you affords more scope for
>learning something new.

Some jfets get down to 1 nv/rthz noise levels. A few fairly expensive
opamps do too, but their current noise can be bad.

Current noise into an inductive transducer will make high frequency
noise, hiss.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On 5/15/2021 12:05 PM, jlarkin@highlandsniptechnology.com wrote:
> On Sat, 15 May 2021 16:41:28 +0100, Cursitor Doom <cd@nowhere.com>
> wrote:
>
>> On Sat, 15 May 2021 09:57:02 -0400, bitrex <user@example.net> wrote:
>>
>>>> What's "broadcast quality"? does that mean really good or like not as
>>>> good as "studio quality' or...?
>>
>> Fair point. I suppose a close approximation would be something the BBC
>> Radio 4 station would use for studio interviews on spots such as Today
>> and The World Tonight:
>>
>> https://www.bbc.co.uk/programmes/b006qtl3/episodes/downloads
>>
>>>> I don't think any "jellybean" op-amp has the combination of noise, open
>>>> loop gain and distortion performance to make a really good dynamic mic
>>>> pre by itself. An NE5534 will work OK for some applications in something
>>>> like:
>>>>
>>>> <https://i.pinimg.com/736x/44/59/f4/4459f4828fc78378bc28c817119d614d.jpg>
>>>>
>>>> but the high-quality solid-state pres tend to use discrete transistors
>>>> or transistor/hybrid designs.
>>>
>>> I should qualify that as high-quality, but still built to a reasonable
>>> budget, there are all sorts of amazing op amps out there that might work
>>> on their own but they're not priced so you could build a whole mixer
>>> input section with them and end up with something you could sell.
>>
>> Discretes eh? Well, that's a surprise. That would be better still from
>> my PoV; having less of the work done for you affords more scope for
>> learning something new.
>
> Some jfets get down to 1 nv/rthz noise levels. A few fairly expensive
> opamps do too, but their current noise can be bad.

You can parallel up op amps to get lower equivalent input-refereed
voltage noise but as it only goes down as the square root of the
sum-of-squares I think that exercise applied to jellybeans soon becomes
silly and you destroy your current noise performance well before you get
down to the performance of the best low noise transistors.

> Current noise into an inductive transducer will make high frequency
> noise, hiss.
>

In olden times a step-up transformer of say 1:10 was the only way to get
70 dB of gain across the full audio band with reasonably low noise, out
of a couple tubes. I don't think most tube mic pres used global
feedback, or any feedback at all for the cheaper ones, but if there was
feedback I think it was usually around the driver and output stage,
putting it around the input triode or pentode was likely hard to
stabilize, didn't add much in the way of performance anyway, and leaves
the question of where do you put the input gain control if there is one,
after the input transformer and first voltage amp is a logical place for it.

Using an input transformer with a solid state pre I think is silly
unless you need the isolation; a low-noise discrete diff pair to
dominate the noise performance of the chain, followed by a FET op amp is
a good compromise. It's OK without global feedback also, wrapping global
feedback around the whole chain is also a bit tricky and is explained
more in the book "Small Signal Audio Design" by Doug Self.

On Sat, 15 May 2021 12:38:03 -0400, bitrex <user@example.net> wrote:

>On 5/15/2021 12:05 PM, jlarkin@highlandsniptechnology.com wrote:
>> On Sat, 15 May 2021 16:41:28 +0100, Cursitor Doom <cd@nowhere.com>
>> wrote:
>>
>>> On Sat, 15 May 2021 09:57:02 -0400, bitrex <user@example.net> wrote:
>>>
>>>>> What's "broadcast quality"? does that mean really good or like not as
>>>>> good as "studio quality' or...?
>>>
>>> Fair point. I suppose a close approximation would be something the BBC
>>> Radio 4 station would use for studio interviews on spots such as Today
>>> and The World Tonight:
>>>
>>> https://www.bbc.co.uk/programmes/b006qtl3/episodes/downloads
>>>
>>>>> I don't think any "jellybean" op-amp has the combination of noise, open
>>>>> loop gain and distortion performance to make a really good dynamic mic
>>>>> pre by itself. An NE5534 will work OK for some applications in something
>>>>> like:
>>>>>
>>>>> <https://i.pinimg.com/736x/44/59/f4/4459f4828fc78378bc28c817119d614d.jpg>
>>>>>
>>>>> but the high-quality solid-state pres tend to use discrete transistors
>>>>> or transistor/hybrid designs.
>>>>
>>>> I should qualify that as high-quality, but still built to a reasonable
>>>> budget, there are all sorts of amazing op amps out there that might work
>>>> on their own but they're not priced so you could build a whole mixer
>>>> input section with them and end up with something you could sell.
>>>
>>> Discretes eh? Well, that's a surprise. That would be better still from
>>> my PoV; having less of the work done for you affords more scope for
>>> learning something new.
>>
>> Some jfets get down to 1 nv/rthz noise levels. A few fairly expensive
>> opamps do too, but their current noise can be bad.
>
>You can parallel up op amps to get lower equivalent input-refereed
>voltage noise but as it only goes down as the square root of the
>sum-of-squares I think that exercise applied to jellybeans soon becomes
>silly and you destroy your current noise performance well before you get
>down to the performance of the best low noise transistors.

Paralleling opamps improves voltage noise and wrecks current noise.
Large-geometry bipolar transistors with bias current cancellation can
have a lot of current noise.

Paralleling jfets works better.

>
>> Current noise into an inductive transducer will make high frequency
>> noise, hiss.
>>
>
>In olden times a step-up transformer of say 1:10 was the only way to get
>70 dB of gain across the full audio band with reasonably low noise, out
>of a couple tubes. I don't think most tube mic pres used global
>feedback, or any feedback at all for the cheaper ones, but if there was
>feedback I think it was usually around the driver and output stage,
>putting it around the input triode or pentode was likely hard to
>stabilize, didn't add much in the way of performance anyway, and leaves
>the question of where do you put the input gain control if there is one,
>after the input transformer and first voltage amp is a logical place for it.

>
>Using an input transformer with a solid state pre I think is silly
>unless you need the isolation; a low-noise discrete diff pair to
>dominate the noise performance of the chain, followed by a FET op amp is
>a good compromise. It's OK without global feedback also, wrapping global
>feedback around the whole chain is also a bit tricky and is explained
>more in the book "Small Signal Audio Design" by Doug Self.

There was some triode, three characters like 6xx, that got down to
around 1 nv/rthz. It was a populat mic and tape head amp.

Diff pairs mostly add noise, another reason to not use opamps.

The "GE Circuit" started with an NPN that had very low impedance
feedback into its emitter. Single-ended of course.

In my NMR amps, I used a low noise opamp and a brute follower, to let
me use absurdly low values of feedback resistors that the opamp
couldn't drive.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On 5/15/2021 11:41 AM, Cursitor Doom wrote:
> On Sat, 15 May 2021 09:57:02 -0400, bitrex <user@example.net> wrote:
>
>>> What's "broadcast quality"? does that mean really good or like not as
>>> good as "studio quality' or...?
>
> Fair point. I suppose a close approximation would be something the BBC
> Radio 4 station would use for studio interviews on spots such as Today
> and The World Tonight:
>
> https://www.bbc.co.uk/programmes/b006qtl3/episodes/downloads
>
>>> I don't think any "jellybean" op-amp has the combination of noise, open
>>> loop gain and distortion performance to make a really good dynamic mic
>>> pre by itself. An NE5534 will work OK for some applications in something
>>> like:
>>>
>>> <https://i.pinimg.com/736x/44/59/f4/4459f4828fc78378bc28c817119d614d.jpg>
>>>
>>> but the high-quality solid-state pres tend to use discrete transistors
>>> or transistor/hybrid designs.
>>
>> I should qualify that as high-quality, but still built to a reasonable
>> budget, there are all sorts of amazing op amps out there that might work
>> on their own but they're not priced so you could build a whole mixer
>> input section with them and end up with something you could sell.
>
> Discretes eh? Well, that's a surprise. That would be better still from
> my PoV; having less of the work done for you affords more scope for
> learning something new.
>

70 dB gain across ~20kHz with low distortion and low noise with cheap op
amps alone is one of those very tricky carpets that are 1 square foot
too big for the room I think, each way you come at it to tack three
corners down the fourth pops up.

Going discrete or at least hybrid shifts the whole burden onto the
designer in the first case, and takes some of the pressure off the cheap
op amp in the second.

On 5/15/2021 12:54 PM, jlarkin@highlandsniptechnology.com wrote:
> On Sat, 15 May 2021 12:38:03 -0400, bitrex <user@example.net> wrote:
>
>> On 5/15/2021 12:05 PM, jlarkin@highlandsniptechnology.com wrote:
>>> On Sat, 15 May 2021 16:41:28 +0100, Cursitor Doom <cd@nowhere.com>
>>> wrote:
>>>
>>>> On Sat, 15 May 2021 09:57:02 -0400, bitrex <user@example.net> wrote:
>>>>
>>>>>> What's "broadcast quality"? does that mean really good or like not as
>>>>>> good as "studio quality' or...?
>>>>
>>>> Fair point. I suppose a close approximation would be something the BBC
>>>> Radio 4 station would use for studio interviews on spots such as Today
>>>> and The World Tonight:
>>>>
>>>> https://www.bbc.co.uk/programmes/b006qtl3/episodes/downloads
>>>>
>>>>>> I don't think any "jellybean" op-amp has the combination of noise, open
>>>>>> loop gain and distortion performance to make a really good dynamic mic
>>>>>> pre by itself. An NE5534 will work OK for some applications in something
>>>>>> like:
>>>>>>
>>>>>> <https://i.pinimg.com/736x/44/59/f4/4459f4828fc78378bc28c817119d614d.jpg>
>>>>>>
>>>>>> but the high-quality solid-state pres tend to use discrete transistors
>>>>>> or transistor/hybrid designs.
>>>>>
>>>>> I should qualify that as high-quality, but still built to a reasonable
>>>>> budget, there are all sorts of amazing op amps out there that might work
>>>>> on their own but they're not priced so you could build a whole mixer
>>>>> input section with them and end up with something you could sell.
>>>>
>>>> Discretes eh? Well, that's a surprise. That would be better still from
>>>> my PoV; having less of the work done for you affords more scope for
>>>> learning something new.
>>>
>>> Some jfets get down to 1 nv/rthz noise levels. A few fairly expensive
>>> opamps do too, but their current noise can be bad.
>>
>> You can parallel up op amps to get lower equivalent input-refereed
>> voltage noise but as it only goes down as the square root of the
>> sum-of-squares I think that exercise applied to jellybeans soon becomes
>> silly and you destroy your current noise performance well before you get
>> down to the performance of the best low noise transistors.
>
> Paralleling opamps improves voltage noise and wrecks current noise.
> Large-geometry bipolar transistors with bias current cancellation can
> have a lot of current noise.
>
> Paralleling jfets works better.
>
>>
>>> Current noise into an inductive transducer will make high frequency
>>> noise, hiss.
>>>
>>
>> In olden times a step-up transformer of say 1:10 was the only way to get
>> 70 dB of gain across the full audio band with reasonably low noise, out
>> of a couple tubes. I don't think most tube mic pres used global
>> feedback, or any feedback at all for the cheaper ones, but if there was
>> feedback I think it was usually around the driver and output stage,
>> putting it around the input triode or pentode was likely hard to
>> stabilize, didn't add much in the way of performance anyway, and leaves
>> the question of where do you put the input gain control if there is one,
>> after the input transformer and first voltage amp is a logical place for it.
>
>
>>
>> Using an input transformer with a solid state pre I think is silly
>> unless you need the isolation; a low-noise discrete diff pair to
>> dominate the noise performance of the chain, followed by a FET op amp is
>> a good compromise. It's OK without global feedback also, wrapping global
>> feedback around the whole chain is also a bit tricky and is explained
>> more in the book "Small Signal Audio Design" by Doug Self.
>
>
> There was some triode, three characters like 6xx, that got down to
> around 1 nv/rthz. It was a populat mic and tape head amp.
>
> Diff pairs mostly add noise, another reason to not use opamps.
>
> The "GE Circuit" started with an NPN that had very low impedance
> feedback into its emitter. Single-ended of course.
>
> In my NMR amps, I used a low noise opamp and a brute follower, to let
> me use absurdly low values of feedback resistors that the opamp
> couldn't drive.
>
>

A bridge-T type peaking circuit in the feedback loop can also work to
bump up the closed-loop gain at high frequency where the op amp output Z
starts going up as open-loop gain drops, the downside is the size of the
cap in the bridge-T goes up in proportion to how low value of feedback
resistors you use and for anything very critical you probably wouldn't
want to use electrolytics.

On 5/15/2021 1:01 PM, bitrex wrote:
> On 5/15/2021 12:54 PM, jlarkin@highlandsniptechnology.com wrote:
>> On Sat, 15 May 2021 12:38:03 -0400, bitrex <user@example.net> wrote:
>>
>>> On 5/15/2021 12:05 PM, jlarkin@highlandsniptechnology.com wrote:
>>>> On Sat, 15 May 2021 16:41:28 +0100, Cursitor Doom <cd@nowhere.com>
>>>> wrote:
>>>>
>>>>> On Sat, 15 May 2021 09:57:02 -0400, bitrex <user@example.net> wrote:
>>>>>
>>>>>>> What's "broadcast quality"? does that mean really good or like
>>>>>>> not as
>>>>>>> good as "studio quality' or...?
>>>>>
>>>>> Fair point. I suppose a close approximation would be something the BBC
>>>>> Radio 4 station would use for studio interviews on spots such as Today
>>>>> and The World Tonight:
>>>>>
>>>>> https://www.bbc.co.uk/programmes/b006qtl3/episodes/downloads
>>>>>
>>>>>>> I don't think any "jellybean" op-amp has the combination of
>>>>>>> noise, open
>>>>>>> loop gain and distortion performance to make a really good
>>>>>>> dynamic mic
>>>>>>> pre by itself. An NE5534 will work OK for some applications in
>>>>>>> something
>>>>>>> like:
>>>>>>>
>>>>>>> <https://i.pinimg.com/736x/44/59/f4/4459f4828fc78378bc28c817119d614d.jpg>
>>>>>>>
>>>>>>>
>>>>>>> but the high-quality solid-state pres tend to use discrete
>>>>>>> transistors
>>>>>>> or transistor/hybrid designs.
>>>>>>
>>>>>> I should qualify that as high-quality, but still built to a
>>>>>> reasonable
>>>>>> budget, there are all sorts of amazing op amps out there that
>>>>>> might work
>>>>>> on their own but they're not priced so you could build a whole mixer
>>>>>> input section with them and end up with something you could sell.
>>>>>
>>>>> Discretes eh? Well, that's a surprise. That would be better still from
>>>>> my PoV; having less of the work done for you affords more scope for
>>>>> learning something new.
>>>>
>>>> Some jfets get down to 1 nv/rthz noise levels. A few fairly expensive
>>>> opamps do too, but their current noise can be bad.
>>>
>>> You can parallel up op amps to get lower equivalent input-refereed
>>> voltage noise but as it only goes down as the square root of the
>>> sum-of-squares I think that exercise applied to jellybeans soon becomes
>>> silly and you destroy your current noise performance well before you get
>>> down to the performance of the best low noise transistors.
>>
>> Paralleling opamps improves voltage noise and wrecks current noise.
>> Large-geometry bipolar transistors with bias current cancellation can
>> have a lot of current noise.
>>
>> Paralleling jfets works better.
>>
>>>
>>>> Current noise into an inductive transducer will make high frequency
>>>> noise, hiss.
>>>>
>>>
>>> In olden times a step-up transformer of say 1:10 was the only way to get
>>> 70 dB of gain across the full audio band with reasonably low noise, out
>>> of a couple tubes. I don't think most tube mic pres used global
>>> feedback, or any feedback at all for the cheaper ones, but if there was
>>> feedback I think it was usually around the driver and output stage,
>>> putting it around the input triode or pentode was likely hard to
>>> stabilize, didn't add much in the way of performance anyway, and leaves
>>> the question of where do you put the input gain control if there is one,
>>> after the input transformer and first voltage amp is a logical place
>>> for it.
>>
>>
>>>
>>> Using an input transformer with a solid state pre I think is silly
>>> unless you need the isolation; a low-noise discrete diff pair to
>>> dominate the noise performance of the chain, followed by a FET op amp is
>>> a good compromise. It's OK without global feedback also, wrapping global
>>> feedback around the whole chain is also a bit tricky and is explained
>>> more in the book "Small Signal Audio Design" by Doug Self.
>>
>>
>> There was some triode, three characters like 6xx, that got down to
>> around 1 nv/rthz. It was a populat mic and tape head amp.
>>
>> Diff pairs mostly add noise, another reason to not use opamps.
>>
>> The "GE Circuit" started with an NPN that had very low impedance
>> feedback into its emitter. Single-ended of course.
>>
>> In my NMR amps, I used a low noise opamp and a brute follower, to let
>> me use absurdly low values of feedback resistors that the opamp
>> couldn't drive.
>>
>>
>
> A bridge-T type peaking circuit in the feedback loop can also work to
> bump up the closed-loop gain at high frequency where the op amp output Z
> starts going up as open-loop gain drops, the downside is the size of the
> cap in the bridge-T goes up in proportion to how low value of feedback
> resistors you use and for anything very critical you probably wouldn't
> want to use electrolytics.
>

It worked OK for my isolated NTSC video buffer though, a $1
voltage-feedback op amp can't really do "DC" to a 10 MHz -3dB into a 75
ohm feedback network impedance but you can fake it.

On 15/05/2021 17:38, bitrex wrote:

<snip>

> Using an input transformer with a solid state pre I think is silly
> unless you need the isolation;
Keeps the mic balanced too, good if you have a long cable.

--
Cheers
Clive

On 05/15/2021 07:28 AM, Cursitor Doom wrote:
> Gentlemen,
>
> Anyone know of a suitable jellybean op-amp that would make for a low
> distortion pre-amp for a broadcast quaility dynamic microphone?
> (single supply preferably).
> TIA.
> -CD
>
Not single supply, but a couple of possibilities come to mind:

https://www.ti.com/lit/ds/symlink/ina217.pdf

http://thatcorp.com/1510-1512_Audio_Preamplifier_ICs.shtml

They're not cheap though.

jlarkin@highlandsniptechnology.com wrote:

> On Sat, 15 May 2021 16:41:28 +0100, Cursitor Doom <cd@nowhere.com>
> wrote:
[...]

>>Discretes eh? Well, that's a surprise. That would be better still from
>>my PoV; having less of the work done for you affords more scope for
>>learning something new.
>
> Some jfets get down to 1 nv/rthz noise levels. A few fairly expensive
> opamps do too, but their current noise can be bad.
>
> Current noise into an inductive transducer will make high frequency
> noise, hiss.

Page 539 of AOE shows a simple 70 picovolt per-root-hertz preamp
design using paralleled bipolars.

See Figure 8.42. Ribbon microphone preamp congurations.

"But it requires a huge input blocking capacitor (150,000F to
preserve the signal's very low source impedance down to a few hertz)"

I designed a DC-coupled low noise preamp to overcome this problem. I
posted the TinyUrl link here, but it has already gone 404. I'll post
a new link when I get my new web site up and running.

Multiple sections can be added in parallel to reduce noise, but this
quickly runs into practical limits of 1/sqrt(N). The exponential
increase in number of devices soon limits the amount of noise
reduction you can achieve. Gerhard also makes very low noise
preamps,and I'm sure he could elucidate further.

--
The best designs occur in the theta state. - sw

Steve Wilson <spamme@not.com> wrote:

> jlarkin@highlandsniptechnology.com wrote:
>
>> On Sat, 15 May 2021 16:41:28 +0100, Cursitor Doom <cd@nowhere.com>
>> wrote:
> [...]
>
>>>Discretes eh? Well, that's a surprise. That would be better still from
>>>my PoV; having less of the work done for you affords more scope for
>>>learning something new.
>>
>> Some jfets get down to 1 nv/rthz noise levels. A few fairly expensive
>> opamps do too, but their current noise can be bad.
>>
>> Current noise into an inductive transducer will make high frequency
>> noise, hiss.
>
> Page 539 of AOE shows a simple 70 picovolt per-root-hertz preamp
> design using paralleled bipolars.
>
> See Figure 8.42. Ribbon microphone preamp congurations.
>
> "But it requires a huge input blocking capacitor (150,000F to
> preserve the signal's very low source impedance down to a few hertz)"
>
> I designed a DC-coupled low noise preamp to overcome this problem. I
> posted the TinyUrl link here, but it has already gone 404. I'll post
> a new link when I get my new web site up and running.

Here is the ASC file:

Version 4
SHEET 1 1888 724
WIRE 544 32 480 32
WIRE 672 32 608 32
WIRE 480 112 480 32
WIRE 528 112 480 112
WIRE 672 112 672 32
WIRE 672 112 608 112
WIRE 720 160 576 160
WIRE 864 160 720 160
WIRE 576 176 576 160
WIRE 720 176 720 160
WIRE 864 176 864 160
WIRE 480 192 480 112
WIRE 512 192 480 192
WIRE 544 192 512 192
WIRE 640 208 608 208
WIRE 672 208 672 112
WIRE 672 208 640 208
WIRE 400 224 384 224
WIRE 544 224 400 224
WIRE 384 240 384 224
WIRE 480 256 480 192
WIRE 576 256 576 240
WIRE 784 256 576 256
WIRE 784 272 784 256
WIRE 864 272 864 256
WIRE 384 336 384 320
WIRE 480 368 480 336
WIRE 528 368 480 368
WIRE 576 368 528 368
WIRE 672 368 576 368
WIRE 720 368 720 240
WIRE 720 368 672 368
WIRE 784 368 784 352
WIRE 480 384 480 368
WIRE 576 384 576 368
WIRE 672 384 672 368
WIRE 480 464 480 448
WIRE 576 464 576 448
WIRE 672 480 672 464
FLAG 384 336 0
FLAG 400 224 Vin
FLAG 640 208 U1O
FLAG 480 464 0
FLAG 528 368 R1C1
FLAG 784 368 0
FLAG 864 272 0
FLAG 512 192 U1N
FLAG 576 464 0
FLAG 672 480 0
SYMBOL voltage 384 224 R0
WINDOW 0 33 22 Left 2
WINDOW 3 37 77 Left 2
WINDOW 123 35 103 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 10
SYMATTR Value2 AC 1
SYMBOL opamps\\lt1028 576 144 R0
SYMATTR InstName U1
SYMBOL res 624 96 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 4950
SYMBOL res 464 240 R0
SYMATTR InstName R1
SYMATTR Value 50
SYMBOL cap 464 384 R0
SYMATTR InstName C1
SYMATTR Value 10mf
SYMATTR SpiceLine Rser=25m Lser=2n
SYMBOL voltage 784 368 M180
WINDOW 0 33 22 Left 2
WINDOW 3 33 44 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value 15
SYMBOL voltage 864 160 R0
WINDOW 0 33 22 Left 2
WINDOW 3 33 44 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V3
SYMATTR Value 15
SYMBOL cap 608 48 M270
WINDOW 0 32 32 VTop 2
WINDOW 3 0 32 VBottom 2
SYMATTR InstName C2
SYMATTR Value 100p
SYMBOL cap 560 384 R0
SYMATTR InstName C3
SYMATTR Value 10mf
SYMATTR SpiceLine Rser=25m Lser=2n
SYMBOL res 656 368 R0
SYMATTR InstName Rleak
SYMATTR Value 1e6
SYMBOL diode 704 240 M180
WINDOW 0 24 64 Left 2
WINDOW 3 24 0 Left 2
SYMATTR InstName D1
SYMATTR Value 1N4001
TEXT 368 -96 Left 2 ;'Measure Power Supply, Zener, LED Noise
TEXT 368 -64 Left 2 !.ac dec 500 1m 1e8\n;.tran 2

--
The best designs occur in the theta state. - sw

On 5/15/2021 1:04 PM, Clive Arthur wrote:
> On 15/05/2021 17:38, bitrex wrote:
>
> <snip>
>
>> Using an input transformer with a solid state pre I think is silly
>> unless you need the isolation;
> Keeps the mic balanced too, good if you have a long cable.
>

Ya but you don't need a transformer for that when you have an op amp or
op amp + diff pair with 90dB+ of CMRR.

On Sat, 15 May 2021 09:54:03 -0700, jlarkin@highlandsniptechnology.com
wrote:

>On Sat, 15 May 2021 12:38:03 -0400, bitrex <user@example.net> wrote:
>
>>On 5/15/2021 12:05 PM, jlarkin@highlandsniptechnology.com wrote:
>>> On Sat, 15 May 2021 16:41:28 +0100, Cursitor Doom <cd@nowhere.com>
>>> wrote:
>>>
>>>> On Sat, 15 May 2021 09:57:02 -0400, bitrex <user@example.net> wrote:
>>>>
>>>>>> What's "broadcast quality"? does that mean really good or like not as
>>>>>> good as "studio quality' or...?
>>>>
>>>> Fair point. I suppose a close approximation would be something the BBC
>>>> Radio 4 station would use for studio interviews on spots such as Today
>>>> and The World Tonight:
>>>>
>>>> https://www.bbc.co.uk/programmes/b006qtl3/episodes/downloads
>>>>
>>>>>> I don't think any "jellybean" op-amp has the combination of noise, open
>>>>>> loop gain and distortion performance to make a really good dynamic mic
>>>>>> pre by itself. An NE5534 will work OK for some applications in something
>>>>>> like:
>>>>>>
>>>>>> <https://i.pinimg.com/736x/44/59/f4/4459f4828fc78378bc28c817119d614d.jpg>
>>>>>>
>>>>>> but the high-quality solid-state pres tend to use discrete transistors
>>>>>> or transistor/hybrid designs.
>>>>>
>>>>> I should qualify that as high-quality, but still built to a reasonable
>>>>> budget, there are all sorts of amazing op amps out there that might work
>>>>> on their own but they're not priced so you could build a whole mixer
>>>>> input section with them and end up with something you could sell.
>>>>
>>>> Discretes eh? Well, that's a surprise. That would be better still from
>>>> my PoV; having less of the work done for you affords more scope for
>>>> learning something new.
>>>
>>> Some jfets get down to 1 nv/rthz noise levels. A few fairly expensive
>>> opamps do too, but their current noise can be bad.
>>
>>You can parallel up op amps to get lower equivalent input-refereed
>>voltage noise but as it only goes down as the square root of the
>>sum-of-squares I think that exercise applied to jellybeans soon becomes
>>silly and you destroy your current noise performance well before you get
>>down to the performance of the best low noise transistors.
>
>Paralleling opamps improves voltage noise and wrecks current noise.
>Large-geometry bipolar transistors with bias current cancellation can
>have a lot of current noise.
>
>Paralleling jfets works better.
>
>>
>>> Current noise into an inductive transducer will make high frequency
>>> noise, hiss.
>>>
>>
>>In olden times a step-up transformer of say 1:10 was the only way to get
>>70 dB of gain across the full audio band with reasonably low noise, out
>>of a couple tubes. I don't think most tube mic pres used global
>>feedback, or any feedback at all for the cheaper ones, but if there was
>>feedback I think it was usually around the driver and output stage,
>>putting it around the input triode or pentode was likely hard to
>>stabilize, didn't add much in the way of performance anyway, and leaves
>>the question of where do you put the input gain control if there is one,
>>after the input transformer and first voltage amp is a logical place for it.
>
>
>>
>>Using an input transformer with a solid state pre I think is silly
>>unless you need the isolation; a low-noise discrete diff pair to
>>dominate the noise performance of the chain, followed by a FET op amp is
>>a good compromise. It's OK without global feedback also, wrapping global
>>feedback around the whole chain is also a bit tricky and is explained
>>more in the book "Small Signal Audio Design" by Doug Self.
>
>
>There was some triode, three characters like 6xx, that got down to
>around 1 nv/rthz. It was a populat mic and tape head amp.
>
>Diff pairs mostly add noise, another reason to not use opamps.
>
>The "GE Circuit" started with an NPN that had very low impedance
>feedback into its emitter. Single-ended of course.
>
>In my NMR amps, I used a low noise opamp and a brute follower, to let
>me use absurdly low values of feedback resistors that the opamp
>couldn't drive.

Just use a SSM2015 or SSM2016 mic preamp IC if they are still
made... Analog Devices I think bought the old SSM products line.

Here is an example of the circuit used in the old Symetrix Audio
broadcast microphone preamp as well as the Symetrix SX202 mic
preamp...

http://3r61fy4ckkky4bvae644ns07-wpengine.netdna-ssl.com/wp-content/uploads/2016/02/528E_A00-1995.pdf

If you don't need the 48V phantom ciruit, leave out the 4K99 resistors
and even the capacitors inline with that and the preamp IC.

They are still around evidently. Might be something newer of course.

http://www.oemstrade.com/search/ssm2016

On 5/15/2021 6:28 AM, Cursitor Doom wrote:
> Gentlemen,
>
> Anyone know of a suitable jellybean op-amp that would make for a low
> distortion pre-amp for a broadcast quaility dynamic microphone?
> (single supply preferably).
> TIA.
> -CD

 Sign on to the MicBuilders IO groups forum, plenty of info there.

                                                       Mikek

--
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https://www.avast.com/antivirus

Clive Arthur wrote:
==============
>
> > Using an input transformer with a solid state pre I think is silly
> > unless you need the isolation;
>
> Keeps the mic balanced too, good if you have a long cable.

** Dynamic mics all have a "floating" output, so can be used with unbalanced cable & pre-amps.
They actually work better that way and can use a longer cable.

A strange but true audio heresy.

...... Phil