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#1
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Behringer Mic Preamp
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. |
#2
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Behringer Mic Preamp
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." |
#3
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Behringer Mic Preamp
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 |
#5
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Behringer Mic Preamp
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." |
#6
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Behringer Mic Preamp
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 |
#7
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Behringer Mic Preamp
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#8
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Behringer Mic Preamp
Huh? You want it to have a sound?
I don't . I want it to sound like a lifless wire, but with gain. Mark |
#9
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Behringer Mic Preamp
"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 he http://sound.westhost.com/articles/cmpd-vs-darl.htm ... and applied he 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-cont...00-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 |
#10
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Behringer Mic Preamp
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 |
#11
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Behringer Mic Preamp
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