Pultec EQP-1a CLONE | Short story!

Hi guys! Let me introduce a short story about my DIY Clone of legendary tube equalizer Pultec EQP-1a!

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As you know, I really love various DIY projects. And the tube equalizer "PULTEC EQP-1A" has always attracted my attention. Of course I made a clone many times according to the Gyraf schematic. But I was always tormented by the question why the equalizer curves do not coincide with the curves for example, the Waves plugin? I decided that I need to understand this issue. The "right" eq curves and 18Bb high -frequency boost did not give me rest. Certainly the whole thing lies in a passive filter in which the inductance coil plays the main role. This, at first glance, the magical element always seemed to me not achieved, and on the Internet there are a lot of different and conflicting information on this subject. But is it really that complicated i thought? And when I had free time, I took my Gyraf clone and began to experiment. Since I do not have a vintage original at hand, for a reference, I took the eq curves from the Waves. In the manual for this plugin, I read that to create it, Waves were based on the original vintage gear, so I discarded doubts and stopped at these curves as a starting point.
By trial and errors, after long -term experiments, I managed to understand the dependence on the value of the capacitor and inductance, as well as DC resistance of the inductor. For each band of high frequencies, I calculated the required number of turns for inductor and values of capacitors. And who could believe? This is incredible, but there is no magic! I managed to approach 99% to the Waves eq curves! 20DB of high frequency boost and accurate correspondence the shape of waveform! I was in the seventh heaven! When the most difficult part of the filter was done, I started the low and high cut frequency section. I gradually followed the reference from WAVES.
The filter was ready. But I couldn't stop halfway. There was an amplifier ahead
The amplifier circuit for the original "PULTEC" easily google on the Internet. The schematic is not too complicated, with the exception of audio transformers. I found all documentation on the input, interstage and output transformers on the Internet forum. Then bought iron, wire and bobbins and began to wind it!
The experience and knowledge that I gained during winding the inductance coil, gave me opportunity to make the rest of transformers. Input and interstage transformers was wound on 85% nickel plates. The main complexity is only the output linear transformer in which there is a tertiary winding.
After winding, I made the necessary measurements to make sure that linearity and distortion is normal. The result was amazing!
Next step was design PCBs and making sure that the prototype working well, it was necessary to take up the manufacture of the enclosure. Given the size of all parts and board, I decided to make the most ergonomic case. This took some time, as I had to remember how to use the AutoCAD software. Then I had to spend few days in Photoshop to design of the front panel.
In my opinion, when cloning anу equipment, the external design is as important as the electrical circuit. It is always nice to see a beautiful product and get aesthetic pleasure in the process of use. Therefore, I carefully recreate the font and detailed location of knobs. Everything should have been like the original without any changes or additions. "Pultec" style knobs as well as the screws with a straight slots, each trifle should be taken into account.
I found a factory where the aluminum parts for the case were cut for me and bent in accordance with the drawings. The front panel was powder coated. It was difficult to choose desired color, but still it turned out close enough in my opinion.
The last important step is Silkscreen. I have not yet been faced with such work, but I wanted to experience myself in this matter. After several unsuccessful attempts, the inscriptions were in place. Back plate is very modest but more than enough

My dream has come true, and now I have the same welcome analog equalizer PULTEC

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