Qs re Helios 2182 —> 22113 Microphone Preamp

[mdcmdcmdc]

Hi folks,

Apologies in advance for what are sure to be a lot of questions. I'm trying to make the leap from basic stompbox circuits to balanced audio, and the Helios preamp seems to be a nice place to start. I had initially planned on building a few channels of Dogears' fantastic Stay At Home Type 69 project in 19ru format, but I have a more immediate need to add preamp channels than I do for outboard EQs.

I've dug through a lot of forum threads on similar projects, but very few of them seem to follow through with a completed project and a finished schematic. So, forgive me for going over well-worn ground here - trust, I did search and try to find a handy ZIP of gerbers to save myself some trouble.

All that said, my general idea is a simple 500-series mic preamp with a few doodads on the front end per the Jensen design notes. The block diagram being:

• Input circuitry (per: https://www.jensen-transformers.com/wp-content/uploads/2014/08/as016.pdf and Pad before microphone input transformer )
• Hammond 1140-MN-A input transformer
• Helios 2182 amp stage w/ 11-position gain switch (per: IA-H9 and stay at home type 69 project )
• Switchable HPF (1-transistor sallen key?)
• Helios 22113 line driver

I have a mishmash of initial questions, sure to be followed by more questions...

1. Is there any better/best practice when it comes to attaching the +48V LED indicator to either the +48V or +24V supply? Switch pop is always an issue with stompboxes, but maybe less of a problem in this kind of circuit as you're not really switching on the fly, I guess. Is there a ballpark CLR value for either +48 or +24V?
2. I'm also using this project as an excuse to make the switch from EagleCAD to KiCAD; it's mostly been fine/intuitive... just curious if there's a custom library for these kinds of audio projects with commonly used parts? For example, I'm unsure how to make the grayhill switch in KiCAD...
3. I haven't been able to find a handy online calculator for a 1-transistor sallen key (as I believe is seen in dogears' helios project); I'm happy to just crib the values from that schematic but I'd like to see the math if possible.
4. Based on other threads... it seems like without the EQ section the 2182 puts out enough gain for most purposes; would it then make more sense to keep the gain of the 22113 stage fixed and add an attenuator at the output (something like the CAPI/Hairball/Bourns 600r t-pad maybe? Classic Audio Products, Inc. ). I'd like to be able to drive the second stage and then control the overall output so as to not overload the front end of my AD interface.

I've drawn out as far as the input and 2182; I'm not worrying too much about the 500-series connections at this point, nor the PCB layout. I'd like to make sure the schematic is sound and then move onto the physical reality of everything.

In any case, big thanks to dogears, ruffrecords, whoops, twentytrees, rocinante, and others who have been so helpful in other threads in making all this info accessible.

 

[Dylan W]

RE question 1, you can see how CAPI does it:

https://www.capi-gear.com/catalog/images/gallery/VP26/Rev_C/VP26-Rev-C-Schematic.pdf
The only thing I would watch out for if you hang the LED off +48 is the current draw; depending on the 500 series chassis it may not have enough power to power eg 8x LEDs and 8x mics at full current. The LED in the CAPI scheme draws about 4.5 mA, and the 500 series phantom spec is officially 5 mA per slot. In practice it should be fine, especially if Jeff @ CAPI has done it this way.

 

[Dylan W]

The other thing you have to figure out is how you want to derive the +24V from the +/-16V. There have been plenty of Neve-esque 500 series modules, so this is ground that has been tread.

Some info here:

https://groupdiy.com/threads/24vdc-from-15vdc.28577/https://groupdiy.com/threads/24v-from-500-psu.71572/

 

[mdcmdcmdc]

Oddly, I was looking around for CAPI schematics earlier and I couldn't find them on his site - in that VP26 schem you posted, what is R14 doing? It's a resistor between the cathode and 'audio common,' but the cathode also goes directly to chassis ground?

 

[mdcmdcmdc]

The other thing you have to figure out is how you want to derive the +24V from the +/-16V. There have been plenty of Neve-esque 500 series modules, so this is ground that has been tread.

Some info here:

https://groupdiy.com/threads/24vdc-from-15vdc.28577/https://groupdiy.com/threads/24v-from-500-psu.71572/

ty! I'm definitely aware that's on the horizon, but I'm also definitely putting off thinking about it for the time being...

 

[ruffrecords]

You would probably be well advised to add a trim control just before the line amp. Bear in mind that consoles of that era running on 24V power rails often had an internal operating level of about -10dBu in order for them to have sufficient headroom. So you should assume the gain of the first stage is arranged to be sufficient to reach -10dBu at its output. Since there is 20dB gain in the output transformer, to get a -70dBu input up to -10dBu requires only 40dB gain from the amplifier. You could then allow 10dB loss in the trim control which means the line amp can have a fixed gain of 20dB.

Cheers

Ian

 

[mdcmdcmdc]

Would you suggest a simple potentiometer/voltage divider or something more like the CAPI t-pad?

Also, I wasn't planning on using an output transformer after the 22113 stage - I was hoping to use a similar arrangement to the stay-at-home project. Any reason why that wouldn't work after the 22113?

 

[TwentyTrees]

A simple way to get most of the requirements you set out in your first post might be to get a few of the Dogears Stay-at-Home Type 69 PCBs and just not populate the EQ section. You could always wire a gain control rotary switch off board for more granular control than the standard 10dB steps.

If you prefer to design and lay out your own PCBs (and I see the attraction of getting exactly what you're looking for), the advice I often read is to design your own footprints for parts rather than using other people's libraries. That said, you can often find footprints on vendor sites - eg Mouser will often have a footprint link as part of an item listing.

 

[mdcmdcmdc]

Probably a sensible idea, but I do actually enjoy the whole process of laying out the PCB, etc. I'm using this as a bit of an excuse to finally get into 500-series equipment, and to make the switch over to KiCad from Eagle.

A few more questions:

Aside from matching pairs/repeatability/tactile satisfaction, is there any reason I couldn't sub a 50K pot for the grayhill switch and resistors?

Is the 22113 module capable on its own as an output stage or would it also benefit from the buffer that's used in the stay-at-home project?

 

[MidnightArrakis]

For building up your own PCB-footprint library, check out these PCB-footprint websites:

https://componentsearchengine.com/examples
https://github.com/Digi-Key/digikey-kicad-library
https://www.pcblibraries.com
https://www.samacsys.com
https://www.snapeda.com
https://www.ultralibrarian.com
[find a handy ZIP of gerbers to save myself some trouble] -- I have and use an industry-standard GERBER-file -- photo-plotting -- program that is also typically used by many PCB-fabrication shops to create their PCB films and I use it to check my own GERBER files before I send them out to be fabricated. After I have imported and have analyzed a couple of dozen of the GERBER files offered for use by the various members of this forum, I can safely say to you that probably 98% of the GERBER files on this forum are complete garbage and not to be trusted!!! So.....with you not being able to find "a handy ZIP of gerbers", you have actually saved yourself way more than "some trouble" had you found some!!!

[make the switch from EagleCAD to KiCAD] -- You should make the switch -- N---E---WAY!!! -- Apparently, ever since AutoDesk/AutoCAD has taken over EAGLE, it has become a really shit program!!!

https://www.kicad.org/download/
[I do actually enjoy the whole process of laying out the PCB] -- Yeah..........it is a kind of really fun thing to do!!!





/

 

[ruffrecords]

Would you suggest a simple potentiometer/voltage divider or something more like the CAPI t-pad?

If you use a trim before the 22113 then a regular 10K pot will do the job

Also, I wasn't planning on using an output transformer after the 22113 stage - I was hoping to use a similar arrangement to the stay-at-home project. Any reason why that wouldn't work after the 22113?

It has a pretty meaty output stage so using it on its own would be OK although the headroom would be limited to about +18dBu by the 24V rail. This is another reason for not having a trim control at the output because it only worsens headroom. If you don't want a transformer but do want a balanced output then use simple impedance balancing.

Cheers

Ian

 

[dogears]

Aside from matching pairs/repeatability/tactile satisfaction, is there any reason I couldn't sub a 50K pot for the grayhill switch and resistors?

Is the 22113 module capable on its own as an output stage or would it also benefit from the buffer that's used in the stay-at-home project?

no, you can use a 50k pot - needs to be rev log. 100k would work well too, or even 25k. Using 25k vs 50k will just limit the minimum gain.

The 22113 doesn't need the buffer. I have had issues getting the 22113 stable in the past - that's why I ended up going with the simpler earlier design of the same stage twice plus an output buffer.