Helios 2128 + 22113 build attempt

[beatnik]

Thanks. I had found more schematics with some little differences, but I guess that one should be the right starting point.

Regarding output transformer, what did you use in your build and what ratio ?

 

[Rocinante]

I have to get to my computer but I'll upload what I have in a few. For DI I believe I copied a newer Neve 73LB but im not positive.

 

[Squeaky]

Thanks Rocinante.

In respect of output transformers, I used a Carnhill 1:2 in the first unit I built. I am currently building a unit that has a little 1:1 50% nickel Hammond transformer. I will probably get hold of some CMOB-2S transformers at some point in the future. There really are a lot of options. I myself don't have the test capabilities to find the "best" transformer for the application. I believe that Ian once commented that a 1:2 output transformer will help you get to pro audio output levels.

I'm still very much (and very slowly) learning as I go (probably just out of kindergarten). I am currently stuck on where to put an insert point. I wanted to place it after the 22k resistor that follows the 0-80-40 filter (Musicland desk schematic) but (without really understanding the physics) this seems like a bad idea. The idea was to have access to the filter before a patched compressor. A lot of schematics show an insert point between the 10H inductor and the 50k pot, but I can't get my head around that. Something to do with the impedance of the inductor? One day I will understand impedance better and the clouds will open in the heavens.

 

[Rocinante]

Reviewing my notes it says API/Passive or the DI.  I made 5 builds.  Two had DI's but iirc the DI's worked very well. I believe this is what I used.



Below is a cloud link to a folder that contains a bunch of Helios schematics and whatnot. I have more stuff but I believe this is a good chunk.

https://mega.nz/#F!m3RFmaZb!GACwaGNrb1jaT6Y5HWHAVg

 

[ruffrecords]

Thanks. I had found more schematics with some little differences, but I guess that one should be the right starting point.

Regarding output transformer, what did you use in your build and what ratio ?

Like Neve circuits, Helios used a 24V power supply (as did most console manufacturers back in the 70s) The absolute maximum peak to peak output of the circuit us therefore 24V which is about 8.4V rms, In practice you can never get this much and 8V rms is the maximum you can expect which is +20dBu. In those days most manufacturers  balanced outs were capable of +26dBu into 600 ohms so to get that you need to add a 1:2 step ip transformer at the output. So one of the chunky carnill 150:600 transformers will do.


Cheers

Ian

 

[Rocinante]

Like Neve circuits, Helios used a 24V power supply (as did most console manufacturers back in the 70s) The absolute maximum peak to peak output of the circuit us therefore 24V which is about 8.4V rms, In practice you c7an never get this much and 8V rms is the maximum you can expect which is +20dBu. In those days most manufacturers  balanced outs were capable of +26dBu into 600 ohms so to get that you need to add a 1:2 step ip transformer at the output. So one of the chunky carnill 150:600 transformers will do.


Cheers

Ian

I used Carnhill 150:600 and it sounded great. I also used EA2503 and Cinemag  CMOQ-2S and those both were good.

I had read that original modules operated at + or -36v so tried that in one build. It definitely increased the headroom but I couldn't get rid of the additional noise. Several times I put it down and stepped away but when I returned the noise always seemed worse. It could have been the transformer but I didn't have any additional transformers that could provide +36v so gave up on that idea.

 

[Squeaky]

Thanks very much Rocinante for the DI schematic and the folder link. I like the look of your mastering version.

Thanks also Ian for chiming in.

I use 24V for the 2128 and about 32V for the 22113. The Carnhill has worked well, it just takes up a bit too much space. I have never noticed a great deal of noise (however my only test is to put headphones on and turn up the gain).

I am going to start experimenting with unbalanced out modules.

 

[ruffrecords]

Thanks very much Rocinante for the DI schematic and the folder link. I like the look of your mastering version.

Thanks also Ian for chiming in.

I use 24V for the 2128 and about 32V for the 22113. The Carnhill has worked well, it just takes up a bit too much space. I have never noticed a great deal of noise (however my only test is to put headphones on and turn up the gain).

I am going to start experimenting with unbalanced out modules.

Carnhills are pretty big and for this application probably unnecessarily so.

For the classic Neve class A output stage where current flows in the transformer primary you need a big core because you need to be able to cope with the signal flux PLUS the dc flux. The Helios line amp is ac coupled so there is no requirement to carry dc so the core can be physically smaller. I have confirmed this with recent experiments on my tube line amps. These used to use an ungapped Carnhill wired 2K4:600 (VTB2291 is memory serves). I have now been working with a local transformer manufacturer to make a smaller 2K4:600 transformer that will perform as well or better than the Carnhill up to +20dBu output but which is a quarter the volume. I am pleased to say that so far the results are very satisfactory.

One word of warning about changing to output transformers containing Nickel alloy laminations. The resultant flux density is much lower than for  the same core cross section using regular steel laminations. The result is that although the distortion at mid frequencies is much reduced, the core clips at a much lower level below 100Hz. If you want decent high level bass response stick to steel.

Cheers

Ian

 

[Whoops]

One word of warning about changing to output transformers containing Nickel alloy laminations. The resultant flux density is much lower than for  the same core cross section using regular steel laminations. The result is that although the distortion at mid frequencies is much reduced, the core clips at a much lower level below 100Hz. If you want decent high level bass response stick to steel.

Cheers

Ian

Thank you so much Ian

 

[Squeaky]

Thanks for the information Ian, it might be somewhat a matter of taste, but your comments agree with conclusions that I reached from reading various posts on this forum (gapped v ungapped, &c). In the end, I decided that I ultimately wanted to settle with steel core transformers and was going to try the CMOB-2S, which I guess is the basic bifilar Cinemag steel core transformer. Although the 600:600 Edcor transformers are less expensive, the shipping costs to NZ are a bit higher and so it is six of one, half a dozen of the other. Either way, I won't be moving forward for a little while yet. I am not sure how the little Hammond will perform (1140-LU-B), it really is quite tiny. I purchased the transformer (thrown into a Mouser order) to use as a test 1:1 output transformer for the circuit.

 

[Rocinante]

The last one I built was module sized (1u x 8 inches) that I built to be used vertically with an external psu.  It was a tight fit but worked well.  I got the case from some 80's half rack piece of gear.




Some folks have experimented with good results running the Helios filters through an API-type in out but I plan on doing my next one using 2128 into ba283.  I think a few modifications and it could possibly produce some good results.

Also I don't know about you guys but there's something very 'vintage' sounding about this circuit. Especially on electric guitars.  I do not remember what the original Helios I heard sounded like anymore, but I remember being surprised and thinking it retained that vintage tone.

 

[Squeaky]

That certainly looks packed in there Rocinante.

On my last project I began by using a Royer 121 with a TG-2 for electrics and ended up pretty much exclusively using my Helios home build with the Royer for the same. It could get a little scratchy but if I got the mic placement and the gain setting right then it worked really well.

I really liked the eq for acoustic guitars during mix down. Maybe roll off the bass a bit, add a bit of low mid boost (I forget exactly which frequency) and a bit of a crank of the 10k. Guitar sat nicely in the mix with an easy on the ear presence. Kind of glistened and was never lost.

 

[Whoops]

Thats a really nice build Rocinate, looks great.

Did you design the front panel yourself?

is it possible you share more pics of your build?

thank you so much

 

[alexc]

Super nice build and concept.

 

[Rocinante]

I cleaned it up a bit more than what you see.  I was going to build a bunch but life and projects demanded my attention. I hopefully will someday.  The FPD file is at the bottom. The toggle switch pcb and Helios filter pcb are Ian Thompson Bell's. I also made it so it can fit horizontally in FPD with the same exact whole locations







https://mega.nz/#!7zIz2awJ!HrKQQ4seHwL3khu7Dy_zn0U9JKLuVP8PPeCsyU5mjXk

 

[Whoops]

The FPD file is at the bottom.
[/quote]

Thanks you so much,
it really looks awesome.

I tried to access the FPD link,  but it asks a password...

I really want to build a similar unit, I also have the EQ/Filter boards from Ian, and have the input transformer for Cyril Jones and a preamp pcb also. Need to get some time of to put everything together

thanks

 

[Rocinante]

Fixed link (I hope)

 

[Whoops]

thank you,
I was able to download it with the new link.

Did you design the 2128 preamp and 22113 pcb yourself?

 

[Rocinante]

Yes i designed them but with the help of PRR who made a suggestion to use the bd139/bd140 at the tail end, as well as Ian who guided me through various parts.
I'll share what i have e.g. gerbers, etc...just pm me.

 

[beatnik]

I am really interested in trying this out.

I have an unfinished eq unit based on a Igor kit that was sold a few years ago and that needs the make up gain stage section.

I only have pcbs for the first preamp stage 2128, not for the 22113 needed for the balanced transformer out.

It would be great if you can share your verified schematic for the amp sections.

I will get a batch of pcbs made and  if anybody is interested I will probably end up with a few extra pcbs and can give them to other builders who want to try out this design.