Behringer Mic Preamp

[mcp6453[_2_]]

Can someone tell me what is going on with T1 through T4 in this preamp schematic? Transistor input stages are normally
used to improve noise performance ahead of an op amp, but this stage appears to be attenuating. There is an argument
that T1 and T2 prevent VR1A from loading the mic input, I guess.

http://i.gyazo.com/8c219f9a60813599f47306062846bad7.png

It's also peculiar the way the emitter of T3 and the collector of T2 (through R30) are connected to the inverting input
of IC6B, and the emitter of T4 and the collector of T1 (through R29) are connected to the non-inverting input.

Inquiring minds want to know.

[Scott Dorsey]

In article >,
mcp6453 > wrote:
>Can someone tell me what is going on with T1 through T4 in this preamp schematic? Transistor input stages are normally
>used to improve noise performance ahead of an op amp, but this stage appears to be attenuating. There is an argument
>that T1 and T2 prevent VR1A from loading the mic input, I guess.

No. This is a poorly-drawn schematic that makes it very hard to see what
is going on, but this is just another version of the classic 4-transistor
op-amp circuit. The PNP is giving you voltage gain, the NPN is acting as
a follower. The "Gain" control is just providing a leakage path between
the two halves of the circuit.

Get a copy of the Mackie 1202 or 1604 schematic, it uses the same design
but the Mackie schematics are laid out by someone who knows how the circuit
works and lays the two halves out symmetrically so you can see what it
going on.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Les Cargill[_4_]]

mcp6453 wrote:
> Can someone tell me what is going on with T1 through T4 in this preamp schematic? Transistor input stages are normally
> used to improve noise performance ahead of an op amp, but this stage appears to be attenuating. There is an argument
> that T1 and T2 prevent VR1A from loading the mic input, I guess.
>
> http://i.gyazo.com/8c219f9a60813599f47306062846bad7.png
>
> It's also peculiar the way the emitter of T3 and the collector of T2 (through R30) are connected to the inverting input
> of IC6B, and the emitter of T4 and the collector of T1 (through R29) are connected to the non-inverting input.
>
> Inquiring minds want to know.
>


If I didn't know any better, I'd say they have symmetric Darlington
pairs - T1 and T3 are one, T2 and T4 are the other.


It looks like ICB6 then brings those differential signals back to a
single-ended output.

How that gain control is supposed to work must be related to being the
feedback path or something. It only affects the .... base? of T4;
T1 and T3 have to be being gain-adjusted through voodoo, unless
the differential @ ICB6 somehow forgives that sin.

I'm not gonna get the book out, but I was unaware that you could
use two different transistors for a Darlington pair. So they may
just be cascaded, not an actual Darlington configuration.


--
Les Cargill

[david gourley[_2_]]

mcp6453 > said...news:Xq6dnZJmCLJx63bOnZ2dnUU7-
:

> Can someone tell me what is going on with T1 through T4 in this preamp
schematic? Transistor input stages are normally
> used to improve noise performance ahead of an op amp, but this stage
appears to be attenuating. There is an argument
> that T1 and T2 prevent VR1A from loading the mic input, I guess.
>
> http://i.gyazo.com/8c219f9a60813599f47306062846bad7.png
>
> It's also peculiar the way the emitter of T3 and the collector of T2
(through R30) are connected to the inverting input
> of IC6B, and the emitter of T4 and the collector of T1 (through R29) are
connected to the non-inverting input.
>
> Inquiring minds want to know.

I agree, it's drawn poorly.

What really baffles me is how a 4580 opamp can spec so well, yet sound so
lifeless (or worse). I mean, there are far worse ones to use but also far
better ones.

david

[Scott Dorsey]

david gourley > wrote:
>
>What really baffles me is how a 4580 opamp can spec so well, yet sound so
>lifeless (or worse). I mean, there are far worse ones to use but also far
>better ones.

But are there any less expensive ones?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[PStamler]

On Wednesday, August 13, 2014 2:44:25 PM UTC-6, Scott Dorsey wrote:
> david gourley > wrote:
>
> >
>
> >What really baffles me is how a 4580 opamp can spec so well, yet sound so
>
> >lifeless (or worse). I mean, there are far worse ones to use but also far
>
> >better ones.
>
>
>
> But are there any less expensive ones?

Spec, schmec. Look at Samuel Groner's actual measurements of the 4580, and you'll see it's thoroughly mediocre, especially in non-inverting mode. Or, presumably, differential mode, as in the Behringer circuit.

Peace,
Paul

[david gourley[_2_]]

PStamler > said...news:a81d1d2e-9cfa-46c7-aec5-
:

> On Wednesday, August 13, 2014 2:44:25 PM UTC-6, Scott Dorsey wrote:
>> david gourley > wrote:
>>
>> >
>>
>> >What really baffles me is how a 4580 opamp can spec so well, yet sound
so
>>
>> >lifeless (or worse). I mean, there are far worse ones to use but also
far
>>
>> >better ones.
>>
>>
>>
>> But are there any less expensive ones?
>
> Spec, schmec. Look at Samuel Groner's actual measurements of the 4580,
and you'll see it's thoroughly mediocre, especially in non-inverting mode.
Or, presumably, differential mode, as in the Behringer circuit.
>
> Peace,
> Paul

Yes, thanks. They help maintain a price point and and an upgrade market.

david

[[email protected]]

Huh? You want it to have a sound?

I don't . I want it to sound like a lifless wire, but with gain.

Mark

[Sean Conolly]

"mcp6453" > wrote in message
...
> Can someone tell me what is going on with T1 through T4 in this preamp
> schematic? Transistor input stages are normally
> used to improve noise performance ahead of an op amp, but this stage
> appears to be attenuating. There is an argument
> that T1 and T2 prevent VR1A from loading the mic input, I guess.
>
> http://i.gyazo.com/8c219f9a60813599f47306062846bad7.png
>
> It's also peculiar the way the emitter of T3 and the collector of T2
> (through R30) are connected to the inverting input
> of IC6B, and the emitter of T4 and the collector of T1 (through R29) are
> connected to the non-inverting input.
>
> Inquiring minds want to know.

It's a compound pair circuit, as described here:
http://sound.westhost.com/articles/cmpd-vs-darl.htm

... and applied here:
http://sound.westhost.com/project66.htm
(Credits to Phill Allison and Rod Elliot - and a big thanks to Rod for
putting so much info on his site.)


Details are thoroughly discussed on Rod's site, so all I'm going to add is
this.

The emitter of T3 is putting out an inverted signal from the input, so it
goes to the inverting input of the opamp to preserve the polarity.

What doesn't make sense to me is the PNP collector is crossed to the
*opposite* out of phase channel (R29 & R30). I'd expect then to be tied to
the in phase channel, like so:
http://blog.newyorkbrass.com/wp-content/uploads/2010/07/ADA8000-input-2.png

In the simulator, having them crossed gives slightly less gain, more
distortion, and more current through those resistors - it may be a mistake
in the schematic.

Sean

[[email protected]]

>
>
>
> In the simulator, having them crossed gives slightly less gain, more
>
> distortion, and more current through those resistors - it may be a mistake
>
> in the schematic.
>
>
>
> Sean

I agree, it looks like an error.

Mark

[david gourley[_2_]]

said...news:8413c7cf-6793-41dd-9e82-2d4a6235db78
@googlegroups.com:

> Huh? You want it to have a sound?
>
> I don't . I want it to sound like a lifless wire, but with gain.
>
> Mark
>
>

I fully understand your premise and agree for the most part.

I'm just talking about how well the component works within it's design
context. It's been demonstrated that the 4580 performance is mediocre at
best. I think that would qualify as another type of 'coloration,' albeit
certainly not one that I would want.

david