audioteslallc
Member
I would like to thank GroupDIY forum for allowing me to share my rendition of the Pultec style EQP program equalizer. Although the project is not yet 100% complete, I thought I'd share what I have accomplished so far. Being an "Audio Apprentice", if you will, I'm always looking for something cool to build. The famous Pultec EQP-1A was no exception. I studied the filter section and thought "this LCR stuff is really, really cool" and, of course, worthy of further experimentation. I did not have the usual tube-type input and output transformers available but I did have a couple of SpectraSonic 101 cards (I think they're the early ones because they require only a single supply) and a pair of Triad A-67J input and Triad HS-66 transformers dated 1969 handy. My friend and brother from another mother Billy pointed out hat the purpose of the vacuum tube circuit was simply nothing more than "the recovery amp", or "make-up gain stage" required to bring the signal back to it's input level caused by circuit insertion losses, etc. So why not try using the SpectraSonics cards instead of tubes?. Apparently they are quite robust and are capable of driving a 600 ohm output transformer without difficulty. So I did.
To my astonishment the filter section and the make up gain stage work very well together. At the time, I had no idea how the original early Pultec units were assembled, so I tried my hand at some point to point wiring using a turret style board for the capacitors and a home made multi-tapped inductor of my own design for the inductor stage. I built two each ending up with better than 5% tolerance between them. I also wanted the feel of NOS rotary switches, potentiometers and switches.....no modern flimsy cheap trash, only the real deal.
At the time of my build, I was unaware that the original units appear to have the various reactive components inside of a metal box for purposes of electromagnetic shielding. One obvious problem with the turret style arrangement I chose is that the signal basically goes into one end of the board and exits the other end about a foot away. This could make the board act as an antenna possibly introducing undesired noise, hash, trash and buzzing. With this in mind, careful component placement and layout are extremely important. Certain wires are parallel while others are perpendicular according to their signals relationship. I think this approach really made a difference in taming unwanted gremlins from raining on my parade.
One thing I noticed is that no other EQ of this style seems to have the ability to use the recovery amp as a stand alone preamp. I found out that the SpectraSonics 101 preamps are being discussed in high regard. So why not make it available for use? I challenged myself to make this happen by adding an additional input transformer and a switch for just that purpose. Now you can have a stand alone high quality preamp at your fingertips with the simple flip of a switch. Separate mic jacks (XLR and TRS) were added for this feature. Also, for the purpose of versatility and experimentation, an insert jack was added, just in case you wanted to add your own personal flavor of EQ. The MEQ comes to mind, for example. One could build only the controls required for you personal touch and stab into the existing SpecrtaSonics preamp! How cool is that?
Enjoy and Kind Regards,
aka Hankenstein
To my astonishment the filter section and the make up gain stage work very well together. At the time, I had no idea how the original early Pultec units were assembled, so I tried my hand at some point to point wiring using a turret style board for the capacitors and a home made multi-tapped inductor of my own design for the inductor stage. I built two each ending up with better than 5% tolerance between them. I also wanted the feel of NOS rotary switches, potentiometers and switches.....no modern flimsy cheap trash, only the real deal.
At the time of my build, I was unaware that the original units appear to have the various reactive components inside of a metal box for purposes of electromagnetic shielding. One obvious problem with the turret style arrangement I chose is that the signal basically goes into one end of the board and exits the other end about a foot away. This could make the board act as an antenna possibly introducing undesired noise, hash, trash and buzzing. With this in mind, careful component placement and layout are extremely important. Certain wires are parallel while others are perpendicular according to their signals relationship. I think this approach really made a difference in taming unwanted gremlins from raining on my parade.
One thing I noticed is that no other EQ of this style seems to have the ability to use the recovery amp as a stand alone preamp. I found out that the SpectraSonics 101 preamps are being discussed in high regard. So why not make it available for use? I challenged myself to make this happen by adding an additional input transformer and a switch for just that purpose. Now you can have a stand alone high quality preamp at your fingertips with the simple flip of a switch. Separate mic jacks (XLR and TRS) were added for this feature. Also, for the purpose of versatility and experimentation, an insert jack was added, just in case you wanted to add your own personal flavor of EQ. The MEQ comes to mind, for example. One could build only the controls required for you personal touch and stab into the existing SpecrtaSonics preamp! How cool is that?
Enjoy and Kind Regards,
aka Hankenstein
Work of art there!
