[letterbeacon]

Out of interest, how was the summing done in a Helios? I can't work it out from the Musicland schematics that Whoops kindly posted.

I was thinking of maybe building an unbalanced summing mixer, and I was thinking that perhaps basing it around the Helios schematics could be fun.

EDIT - looks like this this is the summing section attached?

So to build a Helio-esque unbalanced summing mixer you would go unbalanced in > fader> 2213 amp card> pan > then the summing bus with 10k summing resistors to the Master Mixing Card?

[ruffrecords]

What you have attached is the per channel routing module schematic but you are right it does include the summing bus feed resistors (10K).

The board that does the summing is the MMC board which has a virtual earth input:

https://groupdiy.com/index.php?topic=58393.msg948068#msg948068

Cheers

Ian

[letterbeacon]

Thanks Ian - so if I take the insert, pan and summing resistors from the channel routing schematic, and use the MMC board (one for L and one for R), I should have a fully functioning virtual earth, unbalanced summing mixer without the need for transformers?

That sounds like a fun, relatively cheap build!

One question about the fader: the Helios uses a 5k log P&G fader, which then goes into a line amp before going to the pan circuit. Why do you need a line amp after the fader? Is it just acting as a unity gain buffer?

[ruffrecords]

Thanks Ian - so if I take the insert, pan and summing resistors from the channel routing schematic, and use the MMC board (one for L and one for R), I should have a fully functioning virtual earth, unbalanced summing mixer without the need for transformers?

That sounds like a fun, relatively cheap build!

One question about the fader: the Helios uses a 5k log P&G fader, which then goes into a line amp before going to the pan circuit. Why do you need a line amp after the fader? Is it just acting as a unity gain buffer?

A couple of reasons. First because the fader usually had 10dB in hand - in other words the zero mark on the fader was at 10dB attenuation so you need 10dB of gain make up after it. The other is that , in the routing module at least, there is a fairly heavy load to drive. Remember all the bus feed resistors are affectively connected to 0V by the virtual earth. By the time you add up the two echo sends, the foldback send and the pan pot, the worst case load is not far off 1K. You cannot drive that from a 5K fader. Instead you need a meaty driver stage. The pan pot and its bus resistors alone worst case represent a load of about 2.65K.

Cheers

Ian

[letterbeacon]

Ah ok, that makes sense - thank you.

[ruffrecords]

In the Sound Techniques thread, someone posted a link to a 1970 Studio Sound magazine:

https://www.americanradiohistory.com/Archive-All-Audio/Archive-Studio-Sound/70s/Studio-Sound-1970-11.pdf

In that mag is an article by David Robinson with a PPM design. The circuit that drives the meter itself is almost identical to the Helios one posted in Tech Docs. Coincidence?

Cheers

Ian

[InterstellarObscure]

I was hoping to get an idea for the type of transformer called for in the PPM  circuit. Unless I missed it, there’s no caption on the Helios schem. The article does list a part number (xvol.038) but I’m not finding any reference to it online.

Is there something in the schematic that makes it fairly obvious what the transformer ratio is? In the article, it sort of looks like there’s twice the windings in the secondary, maybe it’s 1:2.

[ruffrecords]

I was hoping to get an idea for the type of transformer called for in the PPM  circuit. Unless I missed it, there’s no caption on the Helios schem. The article does list a part number (xvol.038) but I’m not finding any reference to it online.

Is there something in the schematic that makes it fairly obvious what the transformer ratio is? In the article, it sort of looks like there’s twice the windings in the secondary, maybe it’s 1:2.

Looking at the preamp that drives it I would say a 10K:10K:10K type is most likely or a 10K:10K with a centre tapped secondary.

Cheers

Ian

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