Basic opamp based preamp - Page 8

Michael Bean · 4th July 2011, 10:24 PM

Here's what I use in my actice 3-way system.

INPUT-PREAMP.JPG

Mike

Stormrider · 4th July 2011, 10:52 PM

That's interesting Mike, thanks. I don't think I've seen anything like that before. Is that basically a buffer inside of the feedback loop for the first stage? One thing I really don't get is the 49.9R basically jumpering the buffer stage.

jcx · 4th July 2011, 11:35 PM

two 50 Ohm esistors are making sure the output current divides equally between the 2 op amps - a way of doubling the output current

I'd use a genuinely high output current op amp in a multiloop composite
CFA op amps designed for A/DSL line driving make excellent outputs for audio line levels

Michael Bean · 4th July 2011, 11:39 PM

Hi Stormrider,
Yes, it's kind of a "poor man's" buffer. I needed a little more current drive capability for the three active filter inputs than a single LM4562 could deliver and didn't want to mess with a separate buffer chip or discretes, plus there are already two Opamps right there in the chip ready to use. The 49.9 ohm resistors are there to equalize the output currents from each chip so they won't "see" each other's outputs, and because they're both in the feedback loop, only penalty is slightly reduced output voltage. It works really well, sounds good.

Mike

Stormrider · 4th July 2011, 11:58 PM

Cool. Like I said I've never seen anything like that, but then I've only been at it for a few years now.

The DRV134 has more than enough drive capability for what I need, so I'm good with the circuit I'm working with now. They are kind of expensive though, ~5$ea at mouser, but not bad compared to signal transformers.

Stormrider · 6th July 2011, 12:45 AM

Come on guys, this has got to be far from an ideal design. I know there is room for improvement. I'm open to ideas.

Michael Bean · 6th July 2011, 12:58 AM

What do you consider "ideal", what are you trying to accomplish?

Mike

4th July 2011, 10:52 PM	#72
Stormrider diyAudio Member Join Date: Apr 2006 Location: Connecticut	That's interesting Mike, thanks. I don't think I've seen anything like that before. Is that basically a buffer inside of the feedback loop for the first stage? One thing I really don't get is the 49.9R basically jumpering the buffer stage. __________________ AJ

4th July 2011, 11:58 PM	#75
Stormrider diyAudio Member Join Date: Apr 2006 Location: Connecticut	Cool. Like I said I've never seen anything like that, but then I've only been at it for a few years now. The DRV134 has more than enough drive capability for what I need, so I'm good with the circuit I'm working with now. They are kind of expensive though, ~5$ea at mouser, but not bad compared to signal transformers. __________________ AJ

6th July 2011, 12:45 AM	#76
Stormrider diyAudio Member Join Date: Apr 2006 Location: Connecticut	Come on guys, this has got to be far from an ideal design. I know there is room for improvement. I'm open to ideas. __________________ AJ

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4th July 2011, 10:24 PM	#71
Michael Bean diyAudio Member Join Date: Sep 2004 Location: Minneapolis, Minnesota, USA	Here's what I use in my actice 3-way system. INPUT-PREAMP.JPG Mike

4th July 2011, 11:35 PM	#73
jcx diyAudio Member Join Date: Feb 2003 Location: ..	two 50 Ohm esistors are making sure the output current divides equally between the 2 op amps - a way of doubling the output current I'd use a genuinely high output current op amp in a multiloop composite CFA op amps designed for A/DSL line driving make excellent outputs for audio line levels

4th July 2011, 11:39 PM	#74
Michael Bean diyAudio Member Join Date: Sep 2004 Location: Minneapolis, Minnesota, USA	Hi Stormrider, Yes, it's kind of a "poor man's" buffer. I needed a little more current drive capability for the three active filter inputs than a single LM4562 could deliver and didn't want to mess with a separate buffer chip or discretes, plus there are already two Opamps right there in the chip ready to use. The 49.9 ohm resistors are there to equalize the output currents from each chip so they won't "see" each other's outputs, and because they're both in the feedback loop, only penalty is slightly reduced output voltage. It works really well, sounds good. Mike

6th July 2011, 12:58 AM	#77
Michael Bean diyAudio Member Join Date: Sep 2004 Location: Minneapolis, Minnesota, USA	What do you consider "ideal", what are you trying to accomplish? Mike

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