REDD EQ, Helios 69 and Dick Swettenham

[ruffrecords]

I always assumed your design was 100% spot on Ian and you'd accounted for everything.  Hope it didn't sound like I thought otherwise.

No, you didn't sound like that. Back in the day, inductor dcr might well have been an issue because they did not have or use the high AL materials available today so basically you are right. Even today, dcr is important for the bass inductor (7 Henries) and you need to compensate for it in the design. Mid section you can more or less ignore it.

I figured it was an issue with a wrong resistor value, and short or wiring issue on the boost inductor etc. 

Good luck EWalter

I agree, that is the most likely reason.

Cheers

Ian

 

[Winston OBoogie]

...dcr is important for the bass inductor (7 Henries) and you need to compensate for it in the design. Mid section you can more or less ignore it.

OK got it.  I plan on using your scheme in a squalid-state pre I'm working on for my son, full credit will be given to the designer of course
I may incorporate active mids from Neve 1081 type, not sure yet, but I always thought that the high end lift on the Helios, Sound Techniques etc.  was sweeter sounding than a Baxendall high shelf.  Some of my fav.  sounding records recorded on Sound Techniques at Trident etc.

 

[ruffrecords]

OK got it.  I plan on using your scheme in a squalid-state pre I'm working on for my son, full credit will be given to the designer of course
I may incorporate active mids from Neve 1081 type, not sure yet, but I always thought that the high end lift on the Helios, Sound Techniques etc.  was sweeter sounding than a Baxendall high shelf.  Some of my fav.  sounding records recorded on Sound Techniques at Trident etc.

Of all the EQs I have designed I like the REDD the best. To my ears it is certainly the most musical. Despite the very broad mid boost it is remarkable how it can bring out individual instruments in a mix. Second place of course goes to the Helios.

Cheers

Ian

 

[Winston OBoogie]

Of all the EQs I have designed I like the REDD the best. To my ears it is certainly the most musical. Despite the very broad mid boost it is remarkable how it can bring out individual instruments in a mix. Second place of course goes to the Helios.

Cheers

Ian

Oh without doubt, that mid boost is great!  My comments were more about the high end shelf of Helios or EMI 'Classic' as incorporated in yours being a really sweet sounding one compared to a typical Baxendall of a decade later which would be imparted with the sonic attributes of the amps the tone control enclosed which, sometimes, weren't the best at being linear upat the top end to be honest.
But having the mid of the 'Pop' too is fantastic.  To look at the curves of the original EMI which were plotted for each implementation of boost and cut and the responses available as they reacted with each in boost and/or cut, you realise that a lot was possible with just two bands available on the desk.
I don't think the 'Classic' cassettes were ever employed in the Studio 2 REDD.51 desk and, possibly, neither were they in the Studio 3 one.
Classical was a Studio one thing with its REDD.37, and also the REDD.37 in Kingsway Hall which was used by both EMI and Decca.

Anyway,

 

[Winston OBoogie]

P.S.  To follow up on my thoughts re.  Dick Swettenham and the EMI EQ connection postulate, it seems he left Abbey Road just before they took delivery of the REDD.37 desks which incorporated the Pop and Classic EQ.  If there were any REDD desks there before he left, it would have been the REDD.17 with a similar, but not identical classic type tone control.
I think he took the Pultec basic scheme and scaled it up in impedance, adding bits of his own based on what he'd found out were useful while being a maintenance engineer.  We will never know for sure though so... 
Cheers.

 

[ruffrecords]

P.S.  To follow up on my thoughts re.  Dick Swettenham and the EMI EQ connection postulate, it seems he left Abbey Road just before they took delivery of the REDD.37 desks which incorporated the Pop and Classic EQ.  If there were any REDD desks there before he left, it would have been the REDD.17 with a similar, but not identical classic type tone control.
I think he took the Pultec basic scheme and scaled it up in impedance, adding bits of his own based on what he'd found out were useful while being a maintenance engineer.  We will never know for sure though so... 
Cheers.

Ah,  hoisted by the timeline petard. If he left before the 37s arrived he would have known much less. I am not sure about Pultec with scaled up impedance. I did that with my poor man's EQP1A project. The topology is somewhat different to the Helios but an interesting thought nonetheless. After all, they all consist of a pot divider with frequency selective components on each arm.

Cheers

Ian

 

[EWalter]

I don't have his latest/last revision schematic on hand but, looking at the boost plots you made @2K7, is it possible there's a short on the last winding of your inductor? 
I'm not exactly sure why the cut would be off in Q, it's using a different inductor than boost and is located as a shunt reactance.  But it could be widened by inserting some resistance in series with that particular tap of the inductor.  Maybe wait for Ian before changing anything there though. 
Check the boost inductor tap for problems there in the meantime.
Persevere, you're almost there! 
Whereabouts can I find Ian's up to date schematic?

Here is all the info on the build: https://drive.google.com/drive/folders/0B_n67A1hN3qtfjNQX0Z3Y1V3UTFYelRJZWp6UXQ5dmhWcXVXOFZ3Z0VtOWotemNjbHo3dnM

I checked all of the inductors in the cut path and they all check out just fine. L9 is the only one that is unique to the 2k7 circuit and it measured at 10.44mH. The rest of the inductors checked out, and are all used on other bands that don't have this same problem.

I figured it was an issue with a wrong resistor value, and short or wiring issue on the boost inductor etc. 

I agree, that is the most likely reason.

I checked all resistors and there were a couple that were slightly off do to my having different values. R12 is 390 and I had put in a 419 & R17 is 5k6 and I had put in 6k2. I've since replaced these with the correct values but I'm it didn't solve anything. I believe these are being used in the cut circuit of all frequencies, so I'm not sure this was the issue seeing as though all other bands are working well.

I also double-checked the caps on the 2k7 band cut and they're all correct values. I even pulled them out and tested them to be sure and they checked out.

I also beeped through the entire signal path on the 2k7 switch and didn't find any issues.

I'm hitting some walls here, but I want to say that this EQ sounds amazing! I'd be more than happy with it as-is at this point, I just want to see if I can get this last bit sorted out. I'm really happy with the way this little channel strip project has turned out so far and will post some pics once I get it wrapped.

Cheers,
Erik

 

[ruffrecords]

HI Erik, just to be clear, the only part you now have a problem with is the 2K7 Hz cut - is that right? And you are using my PCBs?

Cheers

Ian

 

[EWalter]

Hey Ian,

That's correct! And yes, I'm using your PCB.

Thanks,
Erik

 

[ruffrecords]

Hey Ian,

That's correct! And yes, I'm using your PCB.

Thanks,
Erik

OK, does the 2K7 cut work at all?

Cheers

Ian

 

[EWalter]

OK, does the 2K7 cut work at all?

Cheers

Ian

Hey Ian,

It does work, it’s just extremely narrow while the boost is extremely wide.

Thanks,
Erik

 

[ruffrecords]

Hey Ian,

It does work, it’s just extremely narrow while the boost is extremely wide.

Thanks,
Erik

Is this true of all cut frequencies or just the 2K7?

Cheers

ian

 

[EWalter]

Is this true of all cut frequencies or just the 2K7?

Cheers

ian

It is only the 2k7 that has this problem. All others are fine.

Thanks,
Erik

 

[ruffrecords]

OK, this is puzzling. If you look at the schematic, the only thing the 2K7 cut position changes is adding 10mH of inductance in series with other inductors and it changes the capacitance to 78nF. I can see no simple fault that would explain making the 2K7 cut very sharp. The only thing I can think of that might do that is if that switch position somehow had a low impedance path to 0V. Perhaps a solder bridge near the two caps making up the 78nF?

Cheers

Ian

 

[EWalter]

OK, this is puzzling. If you look at the schematic, the only thing the 2K7 cut position changes is adding 10mH of inductance in series with other inductors and it changes the capacitance to 78nF. I can see no simple fault that would explain making the 2K7 cut very sharp. The only thing I can think of that might do that is if that switch position somehow had a low impedance path to 0V. Perhaps a solder bridge near the two caps making up the 78nF?

I just measured the capacitance across these caps and got 77nF. The inductor that’s unique to the 2k7 cut is reading 10mH. 

Regarding the switch position having a low z path to 0V, I’m not sure how I’d check this, but am happy to do so.

I guess my question would be; could it have something to do with the fact that the boost seems extra wide? Once again, I’m not finding any issues with any values of any of the components in the cut or boost paths, just trying to spitball some ideas.

One good thing is that I don’t really see myself using that band much, so if this is the way it is, I wouldn’t be very upset. I can always look at the bright side of having a surgical cut band in an often harsh frequency range! Additionally I don’t mean to take up all of your time with tedious troubleshooting.

Cheers,
Erik

 

[ruffrecords]

Hi Erik,


The boost frequency selectively shorts out the top of the pot divider and the cut shorts out the bottom of it. So if one of them has a fault it could possibly affect the other. One test you could do it to remove the 10mH cut inductor that the 2K7 band adds in. You should then get a flat response.

Also try this test. With the cut/boost control a zero, check the insertion loss of the EQ for each of the three mid frequencies. If the one part 2K7 band is messing with something you should see it here.

Can you post a pic of the PCB showing the wiring to the mid frequency select switch?

Cheers

ian

 

[EWalter]

Can you post a pic of the PCB showing the wiring to the mid frequency select switch?
ian

Hey Ian,

I just figured it out! Mine didn't come with the Lorlin header PCB (not sure if it was supposed to come with or if it's an add on), so I just used a different switch and wired it point-to-point. I found that I had reversed the boost and cut on the 2k7 :FACEPALM: even after tracing these down half a dozen times or so... My sincere apologies for the goose chase and thank you very much for all of your help. This EQ is fantastic - I'm thinking about getting 2 more to build a stereo box for my 2 bus!

I've included some images of the final build below (just realized that I don't really have any good ones of the top side of the REDD though!). This is basically my first build, so it's not as pristine as I'd like it to be, but I think it turned out alright. I'm also planning to add more to this box since there's a lot of room, but haven't decided on what it will be yet. I'll definitely include a selectable HPF and potentially some kind of compressor kit or something. There are also some cosmetic things that bum me out a bit - most notably the knobs on the REDD pots are slightly crooked due to the flat end on the Greyhill switches being off center.

Details:
ADM 1668 Mic/Line card
- API/Melcor type op amp
- UTC 0-8 input xfo
- ADM custom output XFO

REDD EQ
- 3 pos switch for Brilliance freq (I didn't use all 4 frequencies cause this was the switch I had)
- Blue LED indicator light for EQ bypass (I'm a geek and this is really exciting to me)

Five Fish PSU2448 Power Supply

Functions:
Mic/Line
+48V
Polarity Reverse
Output pad

Thanks again for all the help! Now on to the next project!

Cheers,
Erik

 

[ruffrecords]

Hey Erik, result man! It is so gratifying when you finally get it to work.

There may have been a Lorlin adaptor PCB in the distant past but most people just hard wire the Lorlin so I have not made any for years and I have no stock.

Enjoy

Cheers

Ian

 

[EWalter]

There may have been a Lorlin adaptor PCB in the distant past but most people just hard wire the Lorlin so I have not made any for years and I have no stock.

Ah! I figured as much and just went for it with a switch I had laying around.

Thanks!

-Erik

 

[Winston OBoogie]

Hey, glad you got it sorted Erik, nice one.
Easy mistake to make.  I haven't turned on a piece of gear I made yet that failed to amaze me when audio comes out the other end.
Down some wires. 
Cool