What do you think of this preamp?

[dasbin]

http://sound.westhost.com/project66.htm


This relatively simple transformerless preamp schematic makes some pretty hefty spec claims. What do you think of this? Can you think of any improvements? I want thinking of swapping the opamp for an OPA627 or some such thing.

 

[Guest]

[quote author="dasbin"]
Can you think of any improvements
[/quote]
C1 which is elyt have zero voltage bias.
It will be source of failures.
Use short instead of C1.
xvlk

 

[dasbin]

Quote from the page:

"The 1000uF capacitor can be a normal electrolytic of 10 or 16 volts rating. There is usually no problem with zero DC bias on modern electros. All other electros should be 25V rating as a minimum."

 

[SonsOfThunder]

I think this one has been discussed here before or maybe over at the old place. Someone will soon be along to suggest the Green Pre... :roll: :green:

For this ckt, I think you could sub 2n4401 for the BC549, if that's more available to you. Then sub a 5534 for the 071 for more o/p drive...you could then rescale all the resistor values around the opamp and maybe reduce the noise some more. Don't do that for a FET opamp though. OPA627 or OPA134 would probably work with the current values though. If you look at that pic of the PCB, he has used a Burr-Brown dual chip! I can't quite make out the number but it looks like OPA2134? This would be a good sub as I have used this chip in other projects.

Don't ignore the notes about the gain pot...very important!

HTH!
Charlie

 

[Guest]

[quote author="dasbin"] There is usually no problem with zero DC bias on modern electros.[/quote]
Problem is here:
I work with no electros (elyt caps) in other parts
than power.
Then I do not know magic properties of
modern elyt caps.
I only know, that in older equipment caps with not
big DC bias was sources of failures after 10 years of
work.
I do not know, how long sample of amp from this schematic
works without changing C1.
xvlk

 

[SonsOfThunder]

[quote author="buttachunk"]it's ironic (or synchronous) that Sons posted that. :wink:[/quote] I have just been hanging out here long enough to see the trends! I actually started my reply before yours was posted...

Peace!
Charlie

 

[PRR]

This relatively simple transformerless preamp schematic makes some pretty hefty spec claims. What do you think of this? Can you think of any improvements?

It is actually more complex than most.

I'm sure it works about as described.

I would not do a mike-amp that way. On the other hand, I don't see anything that needs changing except wild ideas that take you far away from the original. The availability of a pre-made PCB is a great feature.

If you must drive 600 ohms, use 5534, the 071 won't hack it. Many other chips will work.

C1 looks wrong, but in fact modern Japanese electros will work this way for decades. And when if fails, it costs a buck to replace it. Big deal.

It is NOT set up for Phantom power. And there isn't a real perfect way to adapt it, except maybe a (gasp!) input transformer.

 

[rafafredd]

Can we really get away without any cap in C1 position? Maybe if the transistors are perfectly matched?

Also, he states that it can be used as a balanced line receiver. If it´s not that easy to use it as a phantom powered mic preamp (you would need to add big caps in there), then it looks like a nice balanced line receiver, if the gain is set lower.