Full Helios Pre Schematic

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Lerok

Active member
Joined
Jan 17, 2018
Messages
37
Location
Fort Wayne, Indiana
Hello! After a six month break from my tiny side project (switching jobs and houses), I'm back, and I just wanted to run this schematic by the helpful people here before I went and began making a PCB for it.

Anyways, I've combined the 15C1 and 22113 schematics on this website, made a few adjustments (125uf caps are hard to come by around where I live), and cleaned up the wiring mess as best I could.

Warning: It isn't the prettiest schematic you'll ever see (I'm more suited to graph paper and a good old pencil), but it gets the job done.

I'll probably use Cinemag in and out transformers, though I might look into some other ones (maybe a 1:2 10k Jensen for the output?) after the basic build is done.

Anyways, any thoughts/concerns are always welcome! I'm fairly new to electrical work, apart from basic console repair.

Schematics I used:

15C1: https://groupdiy.com/index.php?action=dlattach;topic=58393.0;attach=32789;image
22113: https://imgur.com/lzT8SKf
 

Attachments

  • LerokSchematic.png
    LerokSchematic.png
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You don't need 125 or 200uF, they are for psu filtering and decoupling, not for time constant. Use 220uF, that'll do.

Your 3707 (first transistor in preamp) is drawn wrong or oriented wrong? Or is it the other two? Something dosen't compute for running off a -24V supply  (we almost only use negative supply for pnp-germanium technology)...

Jakob E.



 
I agree with Jakob, use the nearest next larger available cap size for the electrolytics.

I think the first stage is drawn correctly but the power supply is the wrong way round. It should be +24 like the output stage. I think they only made a few of the -24V versions in the very early days.

Edit: don't forget to turn the appropriate electrolytics round as well. Also, you do not need the 600 ohm resistor across the output. It is only there to make sure the output cap gets charged up. Change it to 10K. And lastly don't use a 1:2 10K transformer for the output; you need a 1:2 with a 600 ohm secondary i,e, 150:600

Cheers

Ian
 
ruffrecords said:
I think the first stage is drawn correctly ....

Hmm.
2N4058 is PNP, so it can't be used for second and third transistor if +24V is used for PS.  Also, there is lower 50uF string which shunts output to gnd.  And R13 and C9 are questionable, upper 50uF string can be smaller.
 
moamps said:
Hmm.
2N4058 is PNP, so it can't be used for second and third transistor if +24V is used for PS.  Also, there is lower 50uF string which shunts output to gnd.  And R13 and C9 are questionable, upper 50uF string can be smaller.

Nice catch. The schematic is still correct but drawn as for the +24V version with  the transistors are labelled from the -24V version. So use the 2N4058 for the first stage and something like the modern equivalent of the BC184L for the  two NPNs.

Cheers

Ian
 
So, How did this turn out for you Lerok?
If succeeded, I am Very interested to see your final schematic, as I am eager to try this as well.

Thank you.
//Ted
 
Sorry for going awol for a long time, I actually bought everything I was missing, but just haven't had time to assemble, as I've been house hunting and moving from Minnesota to Indiana - I'm hoping to have it put together soon, with the updated schematic and PCB design, so I'll report back in a month or so when that gets all finalized - thank you all for the help!
 
I just noticed another error on the schematic. The 47K gain pot in the first stage does not need one leg connected to ground. It is just a variable series resistor so you only use two of its pins.

Cheers

Ian
 
I've ordered the PCBs after a large amount of simulation and experimenting on bread boards - I wound up going with a similar design to the schematic at https://groupdiy.com/index.php?topic=77314.msg986132

The first stage is simulating at around 40 dB of gain, you could push it higher by lowering the 450r resistor in the feedback loop, but the breadboard simulations seemed to be best around 40dB max (pushing it higher resulted in a noticeable increase in distortion and noise).

I'm considering adding a second 22113 stage to act as a makeup gain for the EQ I added, and then just using the original as a +20dB switch (I don't have the schemo for it at the moment, but it's somewhere in technical documents)

The other option would be to use the 22113 itself as a preamp. This isn't exactly "conventional", but I vaguely remember Rocinante doing this by mistake and it turning out fairly well.

I will upload the final schematics as soon as I have some time to put them together - I'm in college right now, and the last few weeks are quite hectic.
 
Here is where I'm at on the schematic (excuse the messy layout, I was more concerned with PCB design than making a easy to read schematic). There are headers on the PCB to connect to:

- XLR In
- Phantom Switch
- Polarity Switch
- Mic Input Transformer
- -25dB Input Pad
- -20dB Output Stage Pad (I like to think of it more as a +20dB Output Boost)
- Output Transformer
- XLR/Line Out
- +24v and +48v Inputs

OSxvgI5.png


With a +5dB transformer like the CMMI-2C, this preamp theoretically sits around a maximum of +65dB (40 from the preamp, 20 from the output, 5 from the transformer) and a minimum of -15dB (-25dB Pad + 5dB Trafo + 5dB Minimum Pre Gain)

It's labeled as +24v, but people have had success up to +32v. I stole the phantom power application schematic from the internet, but the logic checks out to me.

I guess at this point it's a final check before I go order PCBs (previously I just had the pre stage which sounded great). Anyone notice anything crazy? The only thing that's jumping at me is the B Ground and A ground. I'm guessing I can just attach these together. The XLR grounds should go to Chassis, and the circuit ground should go to PSU 0v (they should all get tied together at some point).

Sorry for my rambling of thoughts, hope someone else gets use out of this schematic!
 
I guess I can't be of any help for this interesting project at the moment. I still wanted to express my interest and wish you good luck. Maybe this is my next project...?

Keep going and keep us informed.👍

Edit:

I don't know this circuit, but isn't the polarity of C11, C13  C19, C21 and C22 backwards?
 
If you use an input transformer you don't need the input protection. The transformer won't be bothered by the phantom power.

Add some series resistance after the phantom switch before C25, like 200R.

I don't think a 1:2 is the right transformer for noise for this circuit.

The R43/R44 pad is not the right impedance either, and will also do weird things to the impedance presented to the mic. Consider a U-pad on the input.

If you're using +24V for supply, flip the orientation on your electrolytics or they'll be nice firecrackers. Except C15, that one is fine.

R50/51 is only a ~600 ohm load for the output stage. Better to have it larger. Or use a 10k pot, simple.
 
dogears said:
If you use an input transformer you don't need the input protection. The transformer won't be bothered by the phantom power.

Add some series resistance after the phantom switch before C25, like 200R.

I don't think a 1:2 is the right transformer for noise for this circuit.

The R43/R44 pad is not the right impedance either, and will also do weird things to the impedance presented to the mic. Consider a U-pad on the input.

If you're using +24V for supply, flip the orientation on your electrolytics or they'll be nice firecrackers. Except C15, that one is fine.

R50/51 is only a ~600 ohm load for the output stage. Better to have it larger. Or use a 10k pot, simple.

Ah, spaced that on the electrolytics (the original schemos were for -24v), reasons why another set of eyes is useful haha...

Originally I had the pad after the output stage, so that's why the impedance was wonky - do you think something like 2k would be appropriate?

I'll switch to a U pad on the input, I remember Ian suggesting that in an earlier helios thread - and looking at the original mic transformer, it seems as though it was wired 1:10, while it's difficult to get the sowter recreation in my region, I'll try experimenting with a 1:10 from either jensen or cinemag once this project gets over the "does it work" phase.

Thanks!
 
1:10 Jensen and Cinemag work great. For output I used the Cinemag CMOQ-2S. I haven't tried it but Jeff's EA2503 would probably work too.
 
Rocinante said:
1:10 Jensen and Cinemag work great. For output I used the Cinemag CMOQ-2S. I haven't tried it but Jeff's EA2503 would probably work too.

That seems pretty close to my original plans - I'll probably switch the CMOQ-2s for an output xfmr from crimson or another less popular brand (have to cut some corners somewhere) and try that as I've heard good things. Thanks, you were a major inspiration for my interest in this project!
 
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