[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

[Strat96]

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

Thanks for the information. I have been taking notes, including the Studio Sound article for the PPM driver circuit. There is little information in the Helios original schematics, so it is a matter of figuring out what works. I will be building these eventually and bought some meters many months ago without even having a clue to how compatible they would be. The Helios PPM circuit has 24 volts running straight into the meter followed by a 10K resistor.

[ruffrecords]

Thanks for the information. I have been taking notes, including the Studio Sound article for the PPM driver circuit. There is little information in the Helios original schematics, so it is a matter of figuring out what works. I will be building these eventually and bought some meters many months ago without even having a clue to how compatible they would be. The Helios PPM circuit has 24 volts running straight into the meter followed by a 10K resistor.

That is right. The meter is a 1mA movement so applying 10 volts to the 10K resistor results in 1mA through the meter. Fairly standard practice.

Cheers

Ian

[ruffrecords]

I looked again at the Studio Sound article and I noticed they said the transformer is made by Transformer Equipment Ltd in Sandwich, Kent. Turns out they are still in business. I contacted them and a very helpful guy called Luke looked up the transformer in their archives. It is a 1:5 step up transformer. Primary winding is Black and Red, secondary is Green, Blue and Yellow. And get this - it was originally supplied to Olympic Sound Studios dated 21st Feb. 1966. The correct part number is XUOL.038A (it looked like XV on the Studio Sound schematic but in the parts list it is correctly listed as XUOL.038A).

Hope that helps.

Cheers

Ian

[ruffrecords]

I have done some more research and found David Robinson's original mixer design series from 1964. Lo and behold the PPM design is the same only it use germanium transistors. Even the transformer is the same and this time it is marked on the schematic as 1:2.5:2.5 which ties in with what Luke told me.

Cheers

Ian

[Strat96]

I looked again at the Studio Sound article and I noticed they said the transformer is made by Transformer Equipment Ltd in Sandwich, Kent. Turns out they are still in business. I contacted them and a very helpful guy called Luke looked up the transformer in their archives. It is a 1:5 step up transformer. Primary winding is Black and Red, secondary is Green, Blue and Yellow. And get this - it was originally supplied to Olympic Sound Studios dated 21st Feb. 1966. The correct part number is XUOL.038A (it looked like XV on the Studio Sound schematic but in the parts list it is correctly listed as XUOL.038A).

Hope that helps.

Cheers

Ian

Thanks Ian, you are awesome!

I gave them an email, maybe I will hear from Luke? We can see what they say. Over the weekend I was looking at Edcor 10K 1:1s based on what we previously were talking about. They were really well priced for a 20-20KHz transformer. It's a good thing I didn't order.

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