Hi folks,
Apologies in advance for what are sure to be a lot of questions. I'm trying to make the leap from basic stompbox circuits to balanced audio, and the Helios preamp seems to be a nice place to start. I had initially planned on building a few channels of Dogears' fantastic Stay At Home Type 69 project in 19ru format, but I have a more immediate need to add preamp channels than I do for outboard EQs.
I've dug through a lot of forum threads on similar projects, but very few of them seem to follow through with a completed project and a finished schematic. So, forgive me for going over well-worn ground here - trust, I did search and try to find a handy ZIP of gerbers to save myself some trouble.
All that said, my general idea is a simple 500-series mic preamp with a few doodads on the front end per the Jensen design notes. The block diagram being:
In any case, big thanks to dogears, ruffrecords, whoops, twentytrees, rocinante, and others who have been so helpful in other threads in making all this info accessible.
Apologies in advance for what are sure to be a lot of questions. I'm trying to make the leap from basic stompbox circuits to balanced audio, and the Helios preamp seems to be a nice place to start. I had initially planned on building a few channels of Dogears' fantastic Stay At Home Type 69 project in 19ru format, but I have a more immediate need to add preamp channels than I do for outboard EQs.
I've dug through a lot of forum threads on similar projects, but very few of them seem to follow through with a completed project and a finished schematic. So, forgive me for going over well-worn ground here - trust, I did search and try to find a handy ZIP of gerbers to save myself some trouble.
All that said, my general idea is a simple 500-series mic preamp with a few doodads on the front end per the Jensen design notes. The block diagram being:
- Input circuitry (per: https://www.jensen-transformers.com/wp-content/uploads/2014/08/as016.pdf and Pad before microphone input transformer )
- Hammond 1140-MN-A input transformer
- Helios 2182 amp stage w/ 11-position gain switch (per: IA-H9 and stay at home type 69 project )
- Switchable HPF (1-transistor sallen key?)
- Helios 22113 line driver
- Is there any better/best practice when it comes to attaching the +48V LED indicator to either the +48V or +24V supply? Switch pop is always an issue with stompboxes, but maybe less of a problem in this kind of circuit as you're not really switching on the fly, I guess. Is there a ballpark CLR value for either +48 or +24V?
- I'm also using this project as an excuse to make the switch from EagleCAD to KiCAD; it's mostly been fine/intuitive... just curious if there's a custom library for these kinds of audio projects with commonly used parts? For example, I'm unsure how to make the grayhill switch in KiCAD...
- I haven't been able to find a handy online calculator for a 1-transistor sallen key (as I believe is seen in dogears' helios project); I'm happy to just crib the values from that schematic but I'd like to see the math if possible.
- Based on other threads... it seems like without the EQ section the 2182 puts out enough gain for most purposes; would it then make more sense to keep the gain of the 22113 stage fixed and add an attenuator at the output (something like the CAPI/Hairball/Bourns 600r t-pad maybe? Classic Audio Products, Inc. ). I'd like to be able to drive the second stage and then control the overall output so as to not overload the front end of my AD interface.
In any case, big thanks to dogears, ruffrecords, whoops, twentytrees, rocinante, and others who have been so helpful in other threads in making all this info accessible.