EQ with Gain Make Up

[ruffrecords]

As most of you know I have a few passive EQ designs available on PCBs. As they are passive they all need a gain make up amplifier and usually some EQ in/out switching.

I get lots of questions about how to wire up EQ in/out circuits and how to attach a gain make up amplifier to the EQ. To make this easier for everyone I have been planning new versions of these EQs with the gain make up and EQ in/ouit switching already built into the PCB. The plan was that the boards would be EUrocard size with a 32 way connector so that they would be 100% compatible with the EZTubeMixer project.

None of this was particularly difficult except the gain make up amp. To be compatible with the EZTubeMixer it needed to run from 12V heaters but that limited the choice of tubes pretty much to the 12AU7 which is not renowned for low distortion. However, after some experimentation I was able to get a 12AU7 mu follower working with reasonable distortion figures by operating it at a higher current (I reported this here some months ago).

Bottom line is I have just completed building and initial testing the prototype of the first of these new EQ boards which is a 3U version of my 3band Pultec EQ and it seems to work well. The board has provision for an input transformer and it will drive a10K bridging load vai an external  600:600 transfomer just like the poor man's tube gain make up amp. Connections are also brought out to the 32 way connector for a pre EQ fader for use in msxer designs. Although the intention is for these boards to be used in 3U racks with all the controls within one of my standard 2.8 inch wide modules,  all the pots are on flying leads connected to Molex connectors on the PCB so they can be fitted to any size panel. The only thing that is fixed are the positions of the three frequency selections switches.

I plan to produce Helios 69 and REDD EQ versions of this board (the Helios PCB is currently being manufactured). The long term plan is to replace all three current passive EQ boards with the new ones with built in gain make up and bypass. I have attached a picture of the Pultec prototype.

Cheers

Ian

 

[salomonander]

Very cool. Do you have a frontpanel design you used for it?

 

[ruffrecords]

Very cool. Do you have a frontpanel design you used for it?

Attached is the front panel design I used for the prototype. The high boost frequency scale is wrong - there should only be 6 frequencies - you can tell I just copied the mid range to save time. As you can see the front panel is pretty crowded. I am using 9mm pots in the prototype and 13mm knobs.

I'll post a pic of the front panel in the next post.

Cheers

Ian

 

[ruffrecords]

Here's a pic of the front prototype front panel.

Cheers

Ian

 

[leadbreath]

VERY nice Ian!!  May I ask where you got the knobs for the frequency select??

 

[ruffrecords]

VERY nice Ian!!  May I ask where you got the knobs for the frequency select??

They are standard Elma collet knobs fitted with a nut cover.

Cheers

Ian

 

[G-Sun]

Great work Ian!

Do you plan to make a pcb for stepped pots and double-width-design?

 

[ruffrecords]

Great work Ian!

Do you plan to make a pcb for stepped pots and double-width-design?

My initial plans are to complete versions of this design for the Helios69 and REDDEQ. Both have fewer controls and will more easily fit in one of my standard modules sizes. In parallel with this I am also working on a four band mastering EQ with stepped pots. For that EQ the plan is to have none of the controls on the PCB to give builders the maximum flexibility in front panel layout and switch selection. All connections to frequency selection controls would be by 16 way (IDC) headers and to pots by 3 way (Molex) headers. I have yet to decide it this will be made EZTubeMixer compatible but it would be nice if I could fit it onto a single Eurocard.

The big problem with stepped pots is they take up a lot of room. From out previous discussions you know just how hard it is to squeeze the Pultec EQ controls into a standard module. I suspect squeezing a version with stepped pots into a double width module would be just as challenging. A triple width module might be possible and have enough room for a gain make up amp as well. You could then fit a pair in a 19 inch sub-rack to give you two channels of mastering EQ.

Cheers

Ian

 

[G-Sun]

My initial plans are to complete versions of this design for the Helios69 and REDDEQ. Both have fewer controls and will more easily fit in one of my standard modules sizes. In parallel with this I am also working on a four band mastering EQ with stepped pots. For that EQ the plan is to have none of the controls on the PCB to give builders the maximum flexibility in front panel layout and switch selection. All connections to frequency selection controls would be by 16 way (IDC) headers and to pots by 3 way (Molex) headers. I have yet to decide it this will be made EZTubeMixer compatible but it would be nice if I could fit it onto a single Eurocard.

The big problem with stepped pots is they take up a lot of room. From out previous discussions you know just how hard it is to squeeze the Pultec EQ controls into a standard module. I suspect squeezing a version with stepped pots into a double width module would be just as challenging. A triple width module might be possible and have enough room for a gain make up amp as well. You could then fit a pair in a 19 inch sub-rack to give you two channels of mastering EQ.

Awesome Ian!

Could you elobarate on you 4band mastering eq plan?
Or is it to early?

Mm.. I'm scaling down on audio production and gear building. Yet these moduls of yours are so tempting

 

[ruffrecords]

Could you elobarate on you 4band mastering eq plan?
Or is it to early?

Mm.. I'm scaling down on audio production and gear building. Yet these moduls of yours are so tempting

The current discussion about what it might include is here:

http://groupdiy.com/index.php?topic=61945.msg784929#msg784929

I am currently working through the paper design.

Cheers

Ian

 

[swimtoview]

Can't wait to give the Helios makeup gain a go in my mixer project.  I think I'll get a few blank stares at my Christmas list this year including only resistors, caps, and tubes!  Thanks again for all your hard work Ian!

 

[ruffrecords]

Can't wait to give the Helios makeup gain a go in my mixer project.  I think I'll get a few blank stares at my Christmas list this year including only resistors, caps, and tubes!  Thanks again for all your hard work Ian!

It's on my ToDo list!

Cheers

Ian