Reddish 500 EQ

[JMan]

>>>UPDATE 4/11/24: orders for pcbs are now closed.<<<

>>>UPDATE 4/30/24: BOM, Schematic, and some initial build notes attached to this post<<<

>>>UPDATE 08/12/24: Build Guide attached to this post; replaces build notes previously uploaded<<<

>>>UPDATE 08/25/24: Build Guide V1.1 attached to this post. Minor notes added reflecting reports from other builders. Addendum added to end of document for matching lead colors when using either of the UTM transformer options that are currently out there.

Enjoy these photos of finished units:

HerbertR (using prototype PCB)
lukas (using his own self-wound transformers and inductors)
MasterTheSound
Volume11
slipperyflax
Kevin
creature.of.habit
Beatpeat
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Original Post:

Hi Everyone,

I'd like to share one of my latest projects with you all. I present the "Redd-ish 500" EQ, a 500 series equalizer based on @ruffrecords amazing work. After building a standalone "RS127 Brilliance Control" box awhile back, I wanted to build a more robust version of the eq in a compact format. This version of the passive equalizer features the Bass and Brilliance sections of Ian's own Redd EQ with an added frequency point in the Brilliance section at 7.2k, and a pushbutton option to engage a treble boost based on the treble control section of the original circuit (the amount of boost is selectable between 2 and 4 dB via a shunt on the circuit board). The makeup amplifier that follows the eq also comes from Ian, and is based around a Burr Brown OPA2134 driving a 600:600 output transformer. I also included a relay-controlled true bypass.

I've used a UTM 2571 on the input, along with their multi-tap inductors in the EQ circuit, and a Cinemag CMOB-3S output transformer. PCBs and mechanical layout are my own, and the front panel was designed by me and produced by the ever-outstanding Frank at frontpanels.de.

Lots of lessons learned along the way here, including "you can't always guarantee that a manufacturer's mechanical drawing and dimensions are 100% precise" and "holy cow, don't ever get yourself into a position where you need to remove an already-soldered Grayhill switch from a PCB."

I can't thank Ian enough for his patience and generosity in answering my questions and giving suggestions. He is a true gem of this forum.
__________________________________________________________________

 

[TwentyTrees]

Wow, great work - this looks absolutely brilliant! Congratulations. Really pro-looking, bet it works wonders on the stereo bus. I'd be very interested to hear how it sounds if you ever record any samples.

 

[john12ax7]

This looks great. What software did you use to design the front panel?

 

[JMan]

Thanks for the encouraging words, all!

This looks great. What software did you use to design the front panel?

I just used the free software offered by Front Panel Express. Frank can work from the default fpd file, so this helps simplify things for me.

 

[HerbertR]

Absolute fantastic. Did you do the passive loss with a FET make up gain amp? Any chance to get a pair PCBs?

 

[JMan]

Absolute fantastic. Did you do the passive loss with a FET make up gain amp? Any chance to get a pair PCBs?

Hey Herbert, thanks so much! Yes, the make up gain amp is based on an OPA2134. It's not the same TL072 amp that Ian suggests for many of his projects -- David from Cinemag indicated that that amplifier would not be the best for driving the output transformer in this case, so Ian provided this alternative. Still a nice, simple circuit that runs happily on the +/- rails of the 500 rack.

I would want to make sure it was okay with Ian before giving out boards, since so much of the project is based on his work. Additionally, I'm in the midst of a divorce and relocation at the moment, so even with Ian's permission to share boards, it might be some time before I could reasonably send them. That said, if you're still interested once I get settled and I can get sign-off from Ian, I'd be more than happy to share.

 

[thelivingroom]

That looks really awesome, and 100% pro. And I feel your pain about manufacturers’ mechanical drawings. I’ve gotten to where before ordering anything to have made that has a hole or footprint on it happens after I receive all of those parts, they have a date with my digital calipers, and I tighten or loosen up all the dimensions. Got tired of spending 5 minutes per pot trying to get it lined up perfectly centered in the dial print on the front panel because I went with the drawing’s sloppy tolerance suggested cutout, or having to file half a millimeter more out of a previously perfectly CNC’d IEC cutout haha.

What program did you design the pcb in, and where did you get it made? That’s the next phase of learning that I need to get into, at the very least to make little auxiliary pcb’s for transformers and other things I want to add to some of my builds.

 

[JMan]

Oh yeah, I need to invest in some calipers too. Actually I was frustrated to find that the transformers (both the UTM and the Cinemag) were among the biggest offenders as far as the items with a less-than-stellar translation from mechanical drawing to physical product.

I used KiCad to design the pcb. I found it the easiest entry point when I first started trying to design pcbs (free and fairly intuitive to use as far as CAD software goes) and I've just stuck with it since. I had the prototype boards made by jlcpcb because of the low cost (which, well, I think we all know the ethics of utilizing super low-cost Chinese manufacturing can be a little iffy, so there's that), but I'm glad I did because my V1.0 boards had a few spacing errors that I didn't catch and I had to get a second run of boards made, which would have been pretty hard on the wallet with some of the domestic fab houses. The good news is that the current version of the pcb, which was used to build the unit in the first post, works perfectly!

 

[thakava]

So awesome! Well done

 

[JMan]

Hey Everyone,

Just an update here. My life has settled down a bit finally, so I'm thinking about how best to share this project with anyone who is interested. I did get Ian's blessing to do so, which was important to me since the circuit is based on his design.

First things first, I am sending some boards to @HerbertR so that he can build them out and make sure that my results are repeatable and that there are no little issues or quirks that somehow escaped me.

Once Herbert reports back, I will make any adjustments needed (hopefully none), and then I will make boards available to anyone who is interested. There will be a schematic, BOM, build notes (probably not a step-by-step guide, but who knows), and maybe a few other goodies, certainly enough to get you from start to finish.

So, I'll let you all know when I'm ready to start collecting names for boards. If you want to be notified when that time comes, consider clicking "Watch" on this thread so that you can be notified of new posts.

Thanks all for your messages and interest so far!

 

[NOON]

Could you share the Kicad file so we can make our own boards and you don't have to bother with shipping etc?

 

[JMan]

Thanks for the inquiry. For the moment, I think I am going to start with a run of boards for those interested (again, after Herbert has built his and made sure everything is repeatable; I'll post an announcement and begin collecting names at that time).

I am hesitant to share my KiCad files only because I spent a huge amount of time and energy on them, and I don't know how I feel about releasing that work to the outside world where we know from a multitude of experiences that are documented right here on this forum it can often be subject to misuse and breach of trust. I know there may be some differing opinions about this, and I apologize if that's a disappointing answer, but that's where I am with it currently. Hopefully the ability to get your hands on boards will provide enough satisfaction.

 

[Ricardus]

I would absolutely love to build a few of these in the future, but funds will not allow it at the moment. I'll be watching the thread though.

 

[thelivingroom]

Thanks for the inquiry. For the moment, I think I am going to start with a run of boards for those interested (again, after Herbert has built his and made sure everything is repeatable; I'll post an announcement and begin collecting names at that time).

I am hesitant to share my KiCad files only because I spent a huge amount of time and energy on them, and I don't know how I feel about releasing that work to the outside world where we know from a multitude of experiences that are documented right here on this forum it can often be subject to misuse and breach of trust. I know there may be some differing opinions about this, and I apologize if that's a disappointing answer, but that's where I am with it currently. Hopefully the ability to get your hands on boards will provide enough satisfaction.

I don’t blame you at all for not wanting to share the files. All the various info alone on this site is risky enough, and I feel lucky to have it available.

I know you’re not taking names yet, but just putting it out there that I’ll definitely be supporting the cause by building a couple of these beauties whenever you’re ready.

 

[JMan]

Well folks, I am happy to announce that Herbert has successfully built a pair of these EQs! Based on his feedback, I plan to make two minor adjustments - he caught a missing trace that will be dead simple to add (even on the prototype board, all that is needed to correct for it is to simply solder a short wire between two points); and he used a UTM transformer for the output, which has a slightly different sized bobbin and mounting holes from the Cinemag, so I will adjust the board so that either option can be used without modification.

I also want to note that Herbert played around with one of the resistor values on the input and had some interesting things to report. Basically, it works perfectly as stock, but he was able to get the Brilliance Control section's curves to respond identically to the Softube plugin version by playing with the value (which may or may not be a goal, depending on what you're looking for). So my question for the group is this: how much does this matter to you? Again, Ian's default value works perfectly and sounds amazing, so you don't need to play with this. I can think of two options here, and I'd like to know what you would like to have on the board: I could leave it as is, and you can decide if you want to use the stock value or play around with other fixed resistors; or I could try to make space to replace it with a trimmer, in which case you could exert more fine variable control, or you could simply pre-trim it to the stock value and set and forget (or you could still just use a fixed resistor in its place). What do you think?

Give me just a little time to get around to making these changes and then I will do a run of boards for those interested! I don't know exactly what the cost will be just yet, but I'll try to get an estimate for you all asap.

 

[ruffrecords]

I suspect I know which resistor Herber played around with. The reason it is there at all is that the EQ was originally designed to be fed straight from a 10K fader in a mixer. This means the actual source impedance feeding the EQ varies with the pot position and this does slightly affect the size and shape of the curves obtained. The maximum source impedance of a 10K pot fed from a low impedance source is 2K5 so the EQ was designed to give the right curves with 2K5 in series with the input. So you can either feed it straight from a 10K fader or from a low source impedance via a 2K5 series resistor.

If it is fed directly from an input transformer then you need to consider the output impedance of the driving source and also the dcr of the transformer. These of course are variable.

Cheers

Ian

 

[JMan]

Ian,

You are of course correct!

Since it is fed from the input transformer in this case, I suppose what I'm trying to determine is whether I should leave it as a fixed resistor or if I should make it easy for people to play around with the value by using a trimpot - the builder will know better than I what their variables are (even the simple question of whether they are using the Carnhill or UTM transformer, which each have slightly different DCR, I believe).

Personally, this is a little fussier than what I'd want as a builder and I'd probably just leave it at 2k4 and be done. But I can imagine someone who wanted a very particular response and knew they would almost always be feeding it from, say, their soundcard, that person might like to be able to easily adjust a trimpot upon testing to achieve that goal.

Hopefully we'll get some input from potential builders here! If not, I'll just leave it as is.

As always Ian, thank you - your expertise is deeply appreciated and it is why this project even exists at all!

 

[SIXTYNINER]

Hi Everyone,

I'd like to share one of my latest projects with you all. I present the "Redd-ish 500" EQ, a 500 series equalizer based on @ruffrecords amazing work. After building a standalone "RS127 Brilliance Control" box awhile back, I wanted to build a more robust version of the eq in a compact format. This version of the passive equalizer features the Bass and Brilliance sections of Ian's own Redd EQ with an added frequency point in the Brilliance section at 7.2k, and a pushbutton option to engage a treble boost based on the treble control section of the original circuit (the amount of boost is selectable between 2 and 4 dB via a shunt on the circuit board). The makeup amplifier that follows the eq also comes from Ian, and is based around a Burr Brown OPA2134 driving a 600:600 output transformer. I also included a relay-controlled true bypass.

I've used a UTM 2571 on the input, along with their multi-tap inductors in the EQ circuit, and a Cinemag CMOB-3S output transformer. PCBs and mechanical layout are my own, and the front panel was designed by me and produced by the ever-outstanding Frank at frontpanels.de.

Lots of lessons learned along the way here, including "you can't always guarantee that a manufacturer's mechanical drawing and dimensions are 100% precise" and "holy cow, don't ever get yourself into a position where you need to remove an already-soldered Grayhill switch from a PCB."

I can't thank Ian enough for his patience and generosity in answering my questions and giving suggestions. He is a true gem of this forum.

Please enjoy the photos! Just know that in the photos with the front panel, the panel is only loosely affixed (I was still futzing around with things), so you might notice that the metal parts look slightly askew.

Nice Job Jman !

With other 3.4 boards,
each with different frequencies it would become a very so cool multi-band eq.
cheers

 

[HerbertR]

Yes, project completed. As Ian mentioned, the 10k:10K TX Carnhill 9071 (175:175:230:230 DCR Ohm) and 10K:10K UTM 2571 (330:330:400:400 DCR Ohm) inputs are slightly different. So I couldn't get the 10dB gain at the max setting, only 8dB, also the curves were slightly wider compared to the RS127 Rack/Grey plugin. So I set the input resistance to 1K instead of 2K4 Ohm. At 0 Ohm/shorted I get about 12-13dB gain, a little bit too much. I think when you build such high quality EQs you should take the time and trim them to perfection.

I have all the transformers from Igor UTM, including the output Tx. Igor can deliver the fitting size, corresponding to the UTM3523LP laminate size, also with 600:600 ohms. The frequency response with the neutral setting and activated EQ of the entire circuit is excellent from 20-20 KHz. There is not much to say about the sound - an ultimate bomb... The "grey" plugin actually sounds pretty good and very similar, but the hardware here is clearly a step higher. The sun is rising... What else can you expect from this HQ component setup and Ian's scheme... Frank from frontpanels.de supplied the L-bracket and the front panel (my panels are UV-printed on black anodized aluminum), most of the mechanical parts,.. are from Digikey (all 71BDF Grayhills are in stock), the Stockli triangle knobs in black can be bought cheaply from Tayda Electronics in Thailand. I am personally a big fan of old NOS Styroflex Caps as you can see... Many thanks for porting Ian's Redd EQ to the 500 series ... Cheeers

 

[Murdock]

Yes, project completed. As Ian mentioned, the 10k:10K TX Carnhill 9071 (175:175:230:230 DCR Ohm) and 10K:10K UTM 2571 (330:330:400:400 DCR Ohm) inputs are slightly different. So I couldn't get the 10dB gain at the max setting, only 8dB, also the curves were slightly wider compared to the RS127 Rack/Grey plugin. So I set the input resistance to 1K instead of 2K4 Ohm. At 0 Ohm/shorted I get about 12-13dB gain, a little bit too much. I think when you build such high quality EQs you should take the time and trim them to perfection.

I have all the transformers from Igor UTM, including the output Tx. Igor can deliver the fitting size, corresponding to the UTM3523LP laminate size, also with 600:600 ohms. The frequency response with the neutral setting and activated EQ of the entire circuit is excellent from 20-20 KHz. There is not much to say about the sound - an ultimate bomb... The "grey" plugin actually sounds pretty good and very similar, but the hardware here is clearly a step higher. The sun is rising... What else can you expect from this HQ component setup and Ian's scheme... Frank from frontpanels.de supplied the L-bracket and the front panel (my panels are UV-printed on black anodized aluminum), most of the mechanical parts,.. are from Digikey (all 71BDF Grayhills are in stock), the Stockli triangle knobs in black can be bought cheaply from Tayda Electronics in Thailand. I am personally a big fan of old NOS Styroflex Caps as you can see... Many thanks for porting Ian's Redd EQ to the 500 series ... Cheeers

Really nice build! Congrats!
Did you buy directly from Tayda Electronics?
How was your experience with them?