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 Post subject: velleman preamp
PostPosted: 19 Jun 2010, 03:46 
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Joined: 17 Jun 2010, 05:47
Posts: 9
This is what i´w build so far. I used a neutrik transformer in in the input(i dont know Why i just felt like it) tested it Whit a sm57 mic and it was fine. it even had some sound to it, nothing like my dbx or focusrite pre amps, but definetly better than my behringer and yamaha mc802 pre´s.

I would be very thankful if you guys could point out some ways to make it better..? :dice:


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 Post subject: Re: velleman preamp
PostPosted: 19 Jun 2010, 05:59 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
Hi,

1) Use a split supply.
2) #1 will enable you to lose all the electrolytics.
3) Use it in non-inverting mode.

:2c:

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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 Post subject: Re: velleman preamp
PostPosted: 19 Jun 2010, 07:28 
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Joined: 17 Jun 2010, 05:47
Posts: 9
Thanks For The Help.
I have a "symmetrical power supply thing" is it the same thing??
does it mean that i can take all the capacitors off..?

-a true noob :blush:


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 Post subject: Re: velleman preamp
PostPosted: 19 Jun 2010, 17:52 
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Joined: 08 Aug 2009, 03:11
Posts: 2229
Location: Chilliwack, BC
There ya go! :)

Image

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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 Post subject: Re: velleman preamp
PostPosted: 20 Jun 2010, 02:15 
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Joined: 17 Jun 2010, 05:47
Posts: 9
Thanks Geek. What kind of benefits does a non-invertin amplifier have, i heve searched the net but foun no clear answer.


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 Post subject: Re: velleman preamp
PostPosted: 20 Jun 2010, 03:19 
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Location: Chilliwack, BC
Hi,

By using the non-inverting input, you increase the input impedance of an amplifier several fold. Most modern opamps are capable of 500Meg or higher input impedance, so for all practical purposes, your input impedance in my posted schematic is 100K. With the inverting amp you posted, the input impedance is ~2.2K. Though for a mic pre, it's kind of moot, but that brings us to noise performance.

The gain is set by the feedback resistor (220K) divided by the inverting input to ground (2.2K). So this circuit has a gain of 220K/2.2K=100.

To get say a gain of 100 with a 100K input impedance with an inverting amplifier, the resistors would have to be 10Meg on the feedback and 100K on the input. As resistances go up, so does noise. Also stray capacitance becomes a problem, making layout more critical and chances of instability higher. So keeping the feedback loop resistors a reasonable value (as either schematic shows), both noise and instability problems are kept at bay.

With a non-inverting amplifier, phase issues, especially with pro gear, are reduced.

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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 Post subject: Re: velleman preamp
PostPosted: 20 Jun 2010, 11:37 
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Joined: 17 Jun 2010, 05:47
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I think i´m gettin even more confused.. Why 100k I have always been told that 1-2k is a good input impedance for a mic pre, because most mics are 100-200ohms.


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 Post subject: Re: velleman preamp
PostPosted: 20 Jun 2010, 14:16 
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Joined: 17 Jun 2010, 05:47
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Does the pot i have conrol the gain or only the output..? if i add another pot here, would it controll the gain??


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 Post subject: Re: velleman preamp
PostPosted: 20 Jun 2010, 20:40 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
onionseller wrote:
I think i´m gettin even more confused.. Why 100k I have always been told that 1-2k is a good input impedance for a mic pre, because most mics are 100-200ohms.

I did 100K as something more generic. Use the loading recommended by the datasheet of your microphone cartridge.

Quote:
Does the pot i have conrol the gain or only the output..? if i add another pot here, would it controll the gain??

The pot only controls the output.

Where you have your pot will control the gain some by altering the NFB. Not the best way to control gain on a critical gain stage, but it works. Then I would eliminate the output pot completely (or ground one end and keep it out of the NFB loop as your drawing has it) and make your control 100K.

Cheers!

_________________
-= Gregg =-
* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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 Post subject: Re: velleman preamp
PostPosted: 21 Jun 2010, 10:57 
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Joined: 14 Oct 2008, 17:35
Posts: 904
Location: Dartmouth, Nova Scotia
Geek wrote:
By using the non-inverting input, you increase the input impedance of an amplifier several fold. Most modern opamps are capable of 500Meg or higher input impedance, so for all practical purposes, your input impedance in my posted schematic is 100K. With the inverting amp you posted, the input impedance is ~2.2K. Though for a mic pre, it's kind of moot, but that brings us to noise performance.

The gain is set by the feedback resistor (220K) divided by the inverting input to ground (2.2K). So this circuit has a gain of 220K/2.2K=100.

To get say a gain of 100 with a 100K input impedance with an inverting amplifier, the resistors would have to be 10Meg on the feedback and 100K on the input. As resistances go up, so does noise. Also stray capacitance becomes a problem, making layout more critical and chances of instability higher. So keeping the feedback loop resistors a reasonable value (as either schematic shows), both noise and instability problems are kept at bay.

With a non-inverting amplifier, phase issues, especially with pro gear, are reduced.

That is something new that I learned today, so it is ok if you don't understand the changes right away. :D

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My Mini 12AU7 Tube Preamp, Pioneer SX-D7000, JVC XL-V221BK, JBL L80T, DCM TF700


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