1290 Class A Style Preamp Project

[Potato Cakes]

For about a year I have been working on an easier, neater way to build 1290/1073 style preamps for my own use and maybe for future products. I have finally finished the first rack of 8 preamps. They can be powered on either 24V/0V or +/-24V. I had added a regulator to adjust the -V rail to forward bias the circuit per a recommendation, but whenever I use a linear regulator after a capacitance multiplier I get hiss like you would hear with a high noise floor. I have yet to figure that one out, so for this particular build I just stuck with +/-24V. The boards tested fine individually so the thing to find out is how they perform when they are all in the same case. I am waiting on parts to build the power cable then I can test and calibrate.

Thought I would share a success story after having so much trouble with some of my other projects.

Thanks!

 

[ruffrecords]

It is clear to be that a great deal of thought has gone into this project. Well done.

Cheers

ian

 

[Potato Cakes]

Thanks, Ian. I appreciate it. I do have a guy doing the board layouts but I had to cobble schematics together and do all of the testing and got to enjoy all of the "why does this part not work" moments. There are still a couple of minor things to be addressed but these turned out good enough to do a first round build.

Thanks!

Paul

 

[Whoops]

congrats, it looks very good,

They can be powered on either 24V/0V or +/-24V. I had added a regulator to adjust the -V rail to forward bias the circuit per a recommendation, but whenever I use a linear regulator after a capacitance multiplier I get hiss like you would hear with a high noise floor.

Could you please explain better the -24V?
The 1290 circuit only uses +24V, so why the -24V?

 

[Potato Cakes]

The the 0V rail can be substituted for a -24V rail, which I have done. This also allows me to more easily integrate things like an LED metering section (which I did on this one) and not have to figure out how to elegantly do a regulated split rail of the 24V, because I do not like dealing power supply issues since I usually open up a whole new can of problems because that's what happens on every creative electronic adventure I take. Also, more headroom. I know the Rupert Neve Designs consoles run on +/-24V power supplies and supposedly uses a similar Class A design for the preamp and EQ sections, so I wanted to see if I could do something similar. For one client I remember doing the biasing procedure and I was able to get about 32VAC at the output before I got to clipping. You can also regulate the -VDC rail to less than -24V if you want to forward bias the circuit, which was recommended to me by a knowledgeable member. I tried to do that with an onboard regulator but using this just added noise. I found that I have use a regulator before DC filtering to achieve the lower -VDC rail and desired noise performance.

Thanks!

Paul