jensenmann

Filterbox 500
« on: January 10, 2021, 04:28:31 PM »
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.
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort


john12ax7

Re: Filterbox 500
« Reply #1 on: January 10, 2021, 04:41:49 PM »
The output switching seems a little strange.  Without transformers +Out is AC coupled while -Out is not.

Also what transformers were you planning on using?

jensenmann

Re: Filterbox 500
« Reply #2 on: January 10, 2021, 04:47:40 PM »
The output switching seems a little strange.  Without transformers +Out is AC coupled while -Out is not.

Also what transformers were you planning on using?

Good catch, changed...
Transformers are Edcor PC600/600 and OEP A262A2E.
« Last Edit: January 10, 2021, 04:56:18 PM by jensenmann »
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

john12ax7

Re: Filterbox 500
« Reply #3 on: January 10, 2021, 06:51:43 PM »
One thing I've wondered is when switching transformers like this does there need to to be a load on the primary and secondary? A discharge path for any currents.

jensenmann

Re: Filterbox 500
« Reply #4 on: January 11, 2021, 02:36:50 AM »
I don´t think that there will occur problems at such a low power. Anyway, R26/27 are load resistors for the trannies, they should be good enough to prevent any inductive peak while switching. Their values need to be determined together with the Zobel network. I´ll do that when the prototype PCB is here.
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

Re: Filterbox 500
« Reply #5 on: January 12, 2021, 03:06:21 AM »
Cool concept, reminds me abit of the Louder Than Liftoff Chop Shop Eq. Cool to have a DIY alternative!

jensenmann

Re: Filterbox 500
« Reply #6 on: January 12, 2021, 10:17:14 AM »
I´ve not been aware of that product. Interesting to see a somewhat similar thingy. Thanks for the info.
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

Ricardus

Re: Filterbox 500
« Reply #7 on: January 12, 2021, 11:49:20 AM »
Following.
Audio mastering for hire..

boji

Re: Filterbox 500
« Reply #8 on: January 12, 2021, 08:05:01 PM »
Quote
Following.
+1  Thanks for posting Jmann

 

Related Topics

  Subject / Started by Replies Last post
49 Replies
12065 Views
Last post December 01, 2019, 11:57:41 AM
by kante1603
0 Replies
515 Views
Last post September 19, 2019, 04:12:45 PM
by elskardio
2 Replies
945 Views
Last post January 07, 2021, 07:29:56 AM
by mylesgm
33 Replies
2714 Views
Last post January 15, 2021, 12:08:35 PM
by elskardio