From the September 1947 issue of RADIOCRAFT magazine: an interesting ‘response equalizer’ project that looks to have great potential for use in the recording studio. Download the complete article with schematics:
DOWNLOAD: ResponseEQ1947
You will need to download and read the article for complete details, but essentially this circuit provides the following functionality, with 7 controls: 4-position high-pass.; variable low-end boost with 4 selectable frequencies; 4-position low pass; variable high-end boost with 4 selectable frequencies; 10db overall gain with variable output trim. And it does all of this with no inductors or transformers, and just one very cheap 6SJ7 tube.
To use this circuit in a complete studio-ready device, we will need to add several things: First, an output stage capable of easily driving a 15k:600 output transformer. I am going to try the very simple output circuit show above, taken from tubecad.com.
A 250-300v DC power supply (plus 6.3v filament power) will also be necessary; plus input and output transformers. Once I get this all together, I will also need to figure out a good, silent bypass-switching arrangement. Should be a good inexpensive tube eq project. The overall application is very similar to the Pultec EQP-1A; this unit will not have as precise control due to the lack of inductors; but it will cost a heck of a lot less (likely $250 of parts verses $800 to build a Pultec). Plus it allows one to boost and cut at different frequencies, which the Pultec does not offer. Expect some build-notes, images, and audio examples here soon.
8 replies on “Simple Tube Program EQ project”
I’ll be interested in hearing how this turns out if you come back to it one day. It looks like a build I can probably manage. Thanks for posting it.
Hi Matth. I did finish it, shortly after I wrote that follow up. It worked great, although it had way too much gain (around 40 db). It sold almost immediately so i was never able to post any audio clips of it. It’s happily employed now with a busy producer and it has found its way onto many records!
Hi Chris, I really enjoyed your Federal 864 clone article.
Your build notes helped me alot, thank you very very much.
I was thinking my next project would be this EQ.
Wonder you are kind enough to add some hints to pick trans.
I know you are fan of Edcor, did you use those? what about input? Again thank you for all those info.
Hi sho. I used a jensen 1:10 input trans and and edcor 15k:600 half-watt output trans. This resulted in a lot of gain. Enough to use the thing as a mic pre with a tube mic.
Thanks!
You rock!
i work on jukeboxs and rebuild the amps i`m thinking of using this circuit in place of the avc circuit in a seeburg c amp i`ll take photos and video clips of it it should make it sound better
Chris,
Thank u very much for posting this equalizer diagram ,i tried to do this… Works great!
Hello there
do you think you have pcb friends who think to make this circuit