REDD EQ, Helios 69 and Dick Swettenham

[thakava]

I never actually posted final photos of these - I made a pair and they sound great!
I don't believe they (the RS127 only) are worth the effort and cost to build honestly - the full REDD EQ that Ian designed is not much more effort and you get a 3 band eq not just 1. Much better value for your time and effort. But they do work well and they also look pretty cool, haha.

View attachment 88811

Thanks! They look great...

 

[HerbertR]

I never actually posted final photos of these - I made a pair and they sound great!
I don't believe they (the RS127 only) are worth the effort and cost to build honestly - the full REDD EQ that Ian designed is not much more effort and you get a 3 band eq not just 1. Much better value for your time and effort. But they do work well and they also look pretty cool, haha.

View attachment 88811

Wow Mike! What a stunning artwork....Respect....

Then I should also share some pictures inside of my piece.
It is really a excellent sounding EQ. But the sound is close to Helios EQs - like I built with Igor Kapelevics 500 design - the abbey road roots cannot be denied. It has the typical "woody", clear, on the face, fine... character ...only the 2dB steps are sometimes a little bit to much... But what this piece can add on Vocals, Drums, Guitars....is really fantastic. +2/+4 dB Treble, +2dB Bass, -2dB Brilliance at 10k or 4.7K ... BOOM...+life, +3D, +firstclass character, ++++

 

[HerbertR]

As for the passive loss I made a small simple Fet TL072 Make up Gain stage that works great. This Make Up Circuit schematic is also provided from Ian. I ve also implemented 10k/10k Edcor for in and out which work fantasic in this build. There are for sure a lot of other good TX candidates ( I think LL5402 1:1 for In and Out will also do a sexy job for about 100 Bucks more than the Edcors...) But in some builds these cheap Edcors are doing an unbeatable good job. And here I think they are on the point. A perfect nunacing "irony" matching TX color in combination to the passive "woody" sweet-clear inductor design. As said - close to Helios 69... Its not Neve, its not SSL... Its EMI Sound....

 

[ruffrecords]

Wow Mike! What a stunning artwork....Respect....

Then I should also share some pictures inside of my piece.
It is really a excellent sounding EQ. But the sound is close to Helios EQs - like I built with Igor Kapelevics 500 design - the abbey road roots cannot be denied. It has the typical "woody", clear, on the face, fine... character ...only the 2dB steps are sometimes a little bit to much... But what this piece can add on Vocals, Drums, Guitars....is really fantastic. +2/+4 dB Treble, +2dB Bass, -2dB Brilliance at 10k or 4.7K ... BOOM...+life, +3D, +firstclass character, ++++

Is it OK if I copy these pics and send to Pierre who hosts a gallery of projects based on my designs?

Cheers

Ian

 

[HerbertR]

Is it OK if I copy these pics and send to Pierre who hosts a gallery of projects based on my designs?

Cheers

Ian

Hi Grandmaster Ian, for sure!
It is also a good thing to share the .fpd with everyone who needs it. It is simple to modify it with the free FP Designer Software. Change the initials, change the drillholes for the used rotary switches like Grayhill 71ADF or 71BDF....Lorlin or Elma..and go..its a durable UV Print on anodisised Alu. You can calculate less than 100 Bucks for this FP. Perfectly for Studio usage. Frank from Frontpanels.de or Schaeffer AG do a real great job.

 

[TimG]

Hi Grandmaster Ian, for sure!
It is also a good thing to share the .fpd with everyone who needs it. It is simple to modify it with the free FP Designer Software. Change the initials, change the drillholes for the used rotary switches like Grayhill 71ADF or 71BDF....Lorlin or Elma..and go..its a durable UV Print on anodisised Alu. You can calculate less than 100 Bucks for this FP. Perfectly for Studio usage. Frank from Frontpanels.de or Schaeffer AG do a real great job.

It’s gorgeous. Thanks for sharing the .fpd file!!!

 

[TwentyTrees]

Hi Grandmaster Ian, for sure!
It is also a good thing to share the .fpd with everyone who needs it. It is simple to modify it with the free FP Designer Software. Change the initials, change the drillholes for the used rotary switches like Grayhill 71ADF or 71BDF....Lorlin or Elma..and go..its a durable UV Print on anodisised Alu. You can calculate less than 100 Bucks for this FP. Perfectly for Studio usage. Frank from Frontpanels.de or Schaeffer AG do a real great job.

Yeah, this looks fab - lovely attention to detail. Great work!

 

[aaquilato]

Agreed. Absolutely stunning.
Well done!
And yes to the Edcor TXs as well.
Great choice.

 

[JMan]

Hey @mike (or anyone who wants to answer), I was looking at your edited schematic for the RS127 portion of the circuit, and I see that you note a 10k:10k transformer OR 2.4k resistor on the input, but I seem to see both in your build posts. I am hoping to follow in your footsteps on this project, and I'm curious about how that input should work.

 

[ruffrecords]

Hey @mike (or anyone who wants to answer), I was looking at your edited schematic for the RS127 portion of the circuit, and I see that you note a 10k:10k transformer OR 2.4k resistor on the input, but I seem to see both in your build posts. I am hoping to follow in your footsteps on this project, and I'm curious about how that input should work.

I designed this EQ so it could work in a mixer just after a 10K level control. The maximum impedance of a 10K pot is usually 2K5 so if the EQ is not fed from a 10K pot you should fit a 2K4 series resistor at the input instead.

Cheers

Ian

 

[JMan]

Ian, thanks for that! I have some naïve follow-up questions, because the notion of source impedance still confuses me a bit.

First, if the input is fitted with a 2.4k resistor, does that then simply mean that the source impedance is now 2.4k? Is it as simple as that? (Also, the notion that a 10k pot offers a typical maximum impedance of 2.5k is really hurting my brain...I'm just going to accept it and not try to understand it right now).

Second, for practical purposes, I'm still trying to understand how this interacts with the 10k:10k transformer. Does it replace it, or does it come before/after it?

EDIT to add third question: I saw in the manual on your site that this eq needs to be loaded with no less than 100k ohms. So if I'm looking to follow this with the 22113 amplifier for makeup gain (using mike's redrawn schematic from a page or two ago, attached below), will any adjustments need to be made to ensure that it loads the eq adequately? I see the 2.2k series resistor on the input (is this, once again, setting source impedance?). But I'm unsure of how to determine what the load is here...

Apologies if the answers should be dead obvious. I feel like I'm in a bit over my head, but really want to figure it out!

DISCLAIMER: The schematic below is not my own work, and is taken from an earlier post in this thread by user "mike."

 

[ruffrecords]

Ian, thanks for that! I have some naïve follow-up questions, because the notion of source impedance still confuses me a bit.

Lots of people struggle with this. The concept you need to grasp is looking at an output from the point of view of the input that it is connected to. So for example say you have a phono cartridge connected to a phono preamp. Looking out from the preamp input towards the cartridge, what you can see is the impedance of the cartridge itself. The cartridge is the signal source for the preamp so this impedance is also called the source impedance. Ghat is all there is to it. When you look out of the input of my EQ to the source feeding it, it wants to see an impedance of about 2.5K

First, if the input is fitted with a 2.4k resistor, does that then simply mean that the source impedance is now 2.4k? Is it as simple as that?

From the above I hope you will see that this is not the case. If you turn around an look into the input, the impedance you see is the input impedance which the source has to drive. In the circuit you posted, part of the input impedance is the 2.4K resistor but it also includes all the complex impedances between the base of the first transistor and 0V. This is not a trivial thing to calculate in a circuit like this but it is probably more than 100K.

(Also, the notion that a 10k pot offers a typical maximum impedance of 2.5k is really hurting my brain...I'm just going to accept it and not try to understand it right now).

The 10K pot thing is also not too complicated. Imagine the top and the bottom of the pot as an input. If the pot is fed from a very low source impedance, the top of the pot is effectively connected to 0V for audio signals (the same as the bottoM). So if we now assume the top and bottom of the pot are shorted together and look between the wipr of the pot and 0V, what do we see. If the pot is turned right down the wiper is connected to 0V so we see as source of zero ohms. Also, with the pot right at the top, we also see zero ohms since the top is connected to the bottom for ac signals. It should obvious that the biggest value of impedance looking into the pot will be at the 5K/5K position. Since the top and bottom of the pot are connected together we have two 5K resistors in parallel so the impedance looks like 2K5.

Second, for practical purposes, I'm still trying to understand how this interacts with the 10k:10k transformer. Does it replace it, or does it come before/after it?

The 10K:10K transformer would normally be at the input of the EQ in order to provide a balanced input (if that is what you want). Transformers do what they say on the tin. They transform voltages, currents and impedances. The 10K only tells you the kind of impedances they work best with. It is not an intrinsic property of the transformer. So a 1:1 transformer like the 10K:10K one you can think of as largely transparent. So if you look into its secondary, the impedance you see is whatever impedance you would see looking out of the primary.

Hopd that helps.

Cheers

Ian

 

[shabtek]

Interesting explanation Ian

 

[ruffrecords]

Interesting explanation Ian

Hopefully made sense too.

Cheers

Ian

 

[Cyril Jones]

Hi Re note by Rman. In the past I have experienced/found failures with the 22113 amplifier whilst in studios servicing |Helios consoles . Transistor Q5 failed thus destroying the output section. On the day I added series resistor to the base connection to limit current in output transistors, problem solved, I cannot remember 20 years on what the load happened to be. Not wishing to suffer further output transistor failures I added a 3k3 or thereabouts. So much to repair/cure at the time due to poor console condition. I leave it to Ian or Robert to comment.

Cyril

 

[JMan]

Hi Re note by Rman. In the past I have experienced/found failures with the 22113 amplifier whilst in studios servicing |Helios consoles . Transistor Q5 failed thus destroying the output section. On the day I added series resistor to the base connection to limit current in output transistors, problem solved, I cannot remember 20 years on what the load happened to be. Not wishing to suffer further output transistor failures I added a 3k3 or thereabouts. So much to repair/cure at the time due to poor console condition. I leave it to Ian or Robert to comment.

Cyril

Interesting to know. So you would add a ~3k3 resistor to the base of Q6 and Q7 (the output transistors)…or do you mean that you added the resistor to the base of Q5? Would this have any other impacts aside from protecting the output transistors?

Also, apologies for sidetracking the thread here. Any future questions about the 22113 amp I’ll put in a separate thread so I’m not distracting from the main discussion here.

 

[Cyril Jones]

Interesting to know. So you would add a ~3k3 resistor to the base of Q6 and Q7 (the output transistors)…or do you mean that you added the resistor to the base of Q5? Would this have any other impacts aside from protecting the output transistors?

Also, apologies for sidetracking the thread here. Any future questions about the 22113 amp I’ll put in a separate thread so I’m not distracting from the main discussion here.

Interesting to know. So you would add a ~3k3 resistor to the base of Q6 and Q7 (the output transistors)…or do you mean that you added the resistor to the base of Q5? Would this have any other impacts aside from protecting the output transistors?

Also, apologies for side-tracking the thread here. Any future questions about the 22113 amp I’ll put in a separate thread so I’m not distracting from the main discussion here.

Hello. Should have made the position of the 3k3 more clear. I added them in series with the connection to base of Q5. the preceding transistors caused Q5 to pass a serious amount of current to flow through the output stage thus destroying that section. Quite a problem when on site in Paris with limited number of original transistors. Bear in mind the console was in a rough state all round. Even forced to purchase new power supplies
Bear in mind this was around 1995 give a year or two. That's all I can contribute.
Cyril

 

[JMan]

I'm considering using a Jensen JT-11P-1 for a 10k:10k input transformer for this eq, as has been suggested earlier in the thread. For now, I'm just doing the brilliance control part of the circuit as a standalone box. I was looking at the transformer's data sheet and it appears to suggest a zobel network. It's easy enough to add these resistor(s)/capacitor, but I get lost in the technical considerations. I also read an old thread where PRR was basically downplaying the usefulness of a zobel network, while the guy who designed these very transformers for Jensen was talking about the process of determining the values of the network to effectively damp the correct ringing frequencies.

So, if a data sheet suggests something like this, should I take it as de rigueur that this damping network should be added [more or less] regardless of what comes after? If so, is RL needed, and is it possible to determine a reasonable value for RL without testing equipment? (I'm still in the "DAW/Interface and DMM" stage of this hobby). Would this have an impact on the subsequent EQ circuitry?

 

[ruffrecords]

The data sheet quite clearly states that with a 10K load you can omit the Zobel. You only need it for higher values loads.

Cheers

Ian

 

[JMan]

Thanks Ian! Yes, I did see that, but I wasn’t sure what to make of it. So I should add a 10k resistor to ground — RL on the schematic above — off of the yellow lead between the transformer and the eq input? (Understanding that the zobel network is then unnecessary).

Apologies if this should be implicit and obvious from your answer.