jensenmann
Well-known member
Thanks to christmas, lockdown, weather and my wife´s mood I had time to realize a project which will hopefully end as a 500 series PCB available to the group. I´ve breadboarded the circuits roughly and everything seems to work as it should. Schematics are attached. It´s basically a Tilt EQ with HP/LP filters, polarity reverse and an output circuit switchable between electronically balanced and a choice of two transformers. Here´s some thoughts why it is what it is:
First of all I haven´t seen such a thingy before and I don´t believe that anybody really needs it. That´s a good reason for me to make it and hopefully no manufacturer will clone it and make money out of my work
The input circuit is pretty much standard, filtering HF garbage out before entering the circuit (R1,R3,C2 and R2, R4, C1).
The second stage is reversing polarity, which can be bypassed if wanted. This can create some funny results if used in parallel to a given signal. The input impedance of the following stage can be really low hence a chip is needed which is able to drive 600 Ohm easily.
Next is a Tilt EQ, right out of Douglas Selfs book. I was not too thrilled from my breadboard version and thought it needs some tweaking to fit my taste. That´s why I´ve added optional trimmers (P2-P5). So everybody can modify the endstop resistance for both LF boost/cut and HF boost/cut to own taste. I´ll add space for trimmers and resistors on the PCB. Since the circuit inverts phase another phase reversal stage is necessary.
Next is a high- and lowpass filter. It´s our Harrison Ford Filter project with some modifications to the filter frequencies which I posted in the according project thread.
The output stage relies on a DRV134 output driver. I´ve had not so good results with direct coupling these. That´s why I´v added coupling caps. It´s output signal is sent either directly out of the box or into a switch to determine which of the two transformers is used. Both trannies indeed sound different and are the result of listening tests with several other contenders. The good thing is that they are on the cheap side. I wanted the transformer´s artefacts to be obviously audible. So they will both bring their own character to the party. The Edcor PC600/600 has a steel core while the OEP is 50/50 Ni/steel. That gives them different distortion characteristics, mostly depending on the level sent through them. Unfortunately I have no clue if a Zobel network is needed, so I´ll add space on the PCB for that purpose. Any values will have to be determined. Same applies to a load resistor.
To supply the LEDs with constant current I´m using a LM317 in current limiting mode. It´s 1,25V reference voltage across R30 will force roughly 10mA through the LEDs, no matter how many are on or off. The LED supply voltage is decoupled from audio rails by R28/Cpsu.
Please feel free to add comments, hints, suggestions and let me know if there are errors.
First of all I haven´t seen such a thingy before and I don´t believe that anybody really needs it. That´s a good reason for me to make it and hopefully no manufacturer will clone it and make money out of my work
The input circuit is pretty much standard, filtering HF garbage out before entering the circuit (R1,R3,C2 and R2, R4, C1).
The second stage is reversing polarity, which can be bypassed if wanted. This can create some funny results if used in parallel to a given signal. The input impedance of the following stage can be really low hence a chip is needed which is able to drive 600 Ohm easily.
Next is a Tilt EQ, right out of Douglas Selfs book. I was not too thrilled from my breadboard version and thought it needs some tweaking to fit my taste. That´s why I´ve added optional trimmers (P2-P5). So everybody can modify the endstop resistance for both LF boost/cut and HF boost/cut to own taste. I´ll add space for trimmers and resistors on the PCB. Since the circuit inverts phase another phase reversal stage is necessary.
Next is a high- and lowpass filter. It´s our Harrison Ford Filter project with some modifications to the filter frequencies which I posted in the according project thread.
The output stage relies on a DRV134 output driver. I´ve had not so good results with direct coupling these. That´s why I´v added coupling caps. It´s output signal is sent either directly out of the box or into a switch to determine which of the two transformers is used. Both trannies indeed sound different and are the result of listening tests with several other contenders. The good thing is that they are on the cheap side. I wanted the transformer´s artefacts to be obviously audible. So they will both bring their own character to the party. The Edcor PC600/600 has a steel core while the OEP is 50/50 Ni/steel. That gives them different distortion characteristics, mostly depending on the level sent through them. Unfortunately I have no clue if a Zobel network is needed, so I´ll add space on the PCB for that purpose. Any values will have to be determined. Same applies to a load resistor.
To supply the LEDs with constant current I´m using a LM317 in current limiting mode. It´s 1,25V reference voltage across R30 will force roughly 10mA through the LEDs, no matter how many are on or off. The LED supply voltage is decoupled from audio rails by R28/Cpsu.
Please feel free to add comments, hints, suggestions and let me know if there are errors.
