(Really Basic) Questions on Helios Mic Pre

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Lerok

Active member
Joined
Jan 17, 2018
Messages
37
Location
Fort Wayne, Indiana
Hi, first time posting - I've been reading around the older helios pages, and before I went and ordered components, wanted to make sure I wasn't missing something totally obvious.

The layout for the pre is

XLR In > Cinemag cmmi-10c > 2128 Mic Pre > 22113 (Helios Line Out Stage) > Cinemag cmoq2s > XLR Out

The 2128 schematic is https://groupdiy.com/index.php?action=dlattach;topic=58393.0;attach=32789;image

The 22113 schematic is https://i.imgur.com/lzT8SKfl.jpg

My primary question is what is the OV? I'm not an electronics whiz, but I've done a few around the house projects, but I don't quite understand this (i assume it means Output Voltage).

Looking at the Helios 22113 schematic, I'm going to guess that I hook it up to the second input of the 22113, which eventually goes to the ground (as per the schematic).

Thanks in advance! Ronan
 
0V is zero volts or ground to some people or more correctly analogue 0V.

You will need to set the gain of the 22113 by adjusting R8.

I can see only one input to the 22113 - the one labelled IN.

Cheers

Ia.
 
Quick question: Does anyone know where to connect the feedback output? Once again, I'm not used to reading audio schematics :)

I'd assume it goes to either ground or is left unconnected, but I'm at a loss, thanks!
 

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In that specific schematic it is connected to 0V via a resistor. The ratio of that resistor to the 47K feedback resistor is what sets the gain.

Cheers

Ian
 
Lerok said:
The layout for the pre is

XLR In > Cinemag cmmi-10c > 2128 Mic Pre > 22113 (Helios Line Out Stage) > Cinemag cmoq2s > XLR Out
Beware that, as shown, the output stage has 40dB of gain. That would result in a minimum overall gain of 60dB. That is too much for most recording situations. You need to address this, either by inserting a volume control, such as a potentiometer/variable pad/fader between the preamp and the output amp or providing ways to alter the output gain, by switching R8 to different values. Typically, 68r, 680r and open circuit would result in 40, 20 and 0dB of gain for the output stage.
 
Just looked at your original post again. The 2128 is not the amplifier usually used for the mic pre. It is the one  labelled 15C1, the left hand schematic. of the two you posted where the 2128 is on the right. I am not certain but I think the 2128 is meant to be used as a virtual earth summer.

The 15C1 seems to have been typically configured by a gain switch to give up to 50dB of gain, which is rather pushing things for a simple three transistor amplifier. Combined with the additional 20dB of gain from the 1:10 input transformer, this provides an overall gain of up to70dB. This is normally followed by a passive EQ and then another 15C1 to make up the loss in the EQ and then fed to the channel fader and buses

To make a stand alone mic pre with a balanced output you have correctly identified you need to replace the second amp with the 22113 line driving card. Since you have all the gain you need in the first stage, the 22113 needs little if any gain. I would be tempted to run the output of the first amp at a nominal -10dBu which will give you about 24dB of headroom and add the additional 10dB in the output stage. To reach pro loutput levels you will probably need to use a 1:2 transformer at the output which will provide 6dB of the 10dB needed. You might also like to add another 10dB of gain to the 22113 and place a level trim control at its input with say a plus or minus 10dB range.

Cheers

Ian
 
ruffrecords said:
Just looked at your original post again. The 2128 is not the amplifier usually used for the mic pre. It is the one  labelled 15C1, the left hand schematic. of the two you posted where the 2128 is on the right.
Apart from a marginal capacitor value, both these circuits are identical, only the PCB's are different.

I am not certain but I think the 2128 is meant to be used as a virtual earth summer.
I don't see anything in either of these circuits that would make them particularly suitable for a VE summer; a suitably placed FB resistor would be adequate, though.
 
abbey road d enfer said:
Apart from a marginal capacitor value, both these circuits are identical, only the PCB's are different.

Look again. The right hand circuit has been modified to include and additional feedback pat. It makes no sense as drawn for the left hand input to be an input. So all I could think of was the f/b point might be used as a summing point. Here are the scehmatics again:

https://groupdiy.com/index.php?action=dlattach;topic=58393.0;attach=32789;image
I don't see anything in of these circuits that would make them particularly suitable for a VE summer; a suitably placed FB resistor would be adequate, though.

I agree.

Cheers

ian
 
ruffrecords said:
Look again. The right hand circuit has been modified to include and additional feedback pat. It makes no sense as drawn for the left hand input to be an input. So all I could think of was the f/b point might be used as a summing point.
I don't really see it.
Either the RH schemo has at least one blatant error, the direct connection between the 3rd transistor's emitter and the base of the 1st, which shorts the terminal identified as i/p, which in turn defeats any sensible use as a mic pre, which the legend claims it is  :eek:, or it is indeed a common-base VE summing amp, but then the addition of a 1k resistor is very detrimental to the noise figure.
I know that Mr. Swettenham's designs were idiosyncratic, but I think we are clearly facing a mistake in the drawing. The fact that the RH schemo is wholly had-drawn suggests it is less "refined" than the LH one.
 
abbey road d enfer said:
I don't really see it.
Either the RH schemo has at least one blatant error, the direct connection between the 3rd transistor's emitter and the base of the 1st, which shorts the terminal identified as i/p, which in turn defeats any sensible use as a mic pre, which the legend claims it is  :eek:, or it is indeed a common-base VE summing amp, but then the addition of a 1k resistor is very detrimental to the noise figure.
I know that Mr. Swettenham's designs were idiosyncratic, but I think we are clearly facing a mistake in the drawing. The fact that the RH schemo is wholly had-drawn suggests it is less "refined" than the LH one.

All I can say is the schematic on the left is the one that appears most often labelled as a Helios mic pre so my advice to the OP stands.. The right hand one does not, and we agree there is a glaring error or some really weird design going on.

Cheers

ian
 
Hmmmm the Helios story continues to unfold , I read someplace the mic amp in the Helios is the sound you crave from the mid 60 to early 70 albums,not so much the output stage .
Why not ham together a Helios Pre and a good old Neve 283 output stage ,its bound to be a combination made in audio heaven.The Helios pre is way ahead of its time in audio ,makes Neve's mic amp look 'old hat',but Neve output I dont think its been bettered ,well maybe technically ,but in terms of niceness after overload ,the high current class A is the beastmaster .
 
Tubetec said:
The Helios pre is way ahead of its time in audio ,makes Neve's mic amp look 'old hat',
They are both a simple three transistor design. Neve is a straightforward dc coupled pair with an emitter follower on the end to provide better drive capability and buffer the feedback circuitry. The Helios has the emitter follower between the pair. Helios tried to get more gain out of the three than Neve did but, given the Helios were mainly use for pop music recording, this was rarely if ever needed.

As I have mentioned before, the Helios input bias is suspect. You would normally design such a stage so the dc output voltage is half the rail voltage in order to maximise voltage seing. The Helios bias is wuite a way off this point which leads to asymmetric clipping which produces (pleasing) even order harmonic distortion. If you swap the positions of the two bias resistors the output dc voltage is very close to half the rail voltage. I often wonder if this was simply a drawing error that went unnoticed.

Cheers

ian
 
ruffrecords said:
As I have mentioned before, the Helios input bias is suspect. You would normally design such a stage so the dc output voltage is half the rail voltage in order to maximise voltage seing.
This is true when the load is very light. However, in presence of a significant load, the drive capability is assymetric. The side that has the active element generally has the capability of pulling close to the rail, while the side that has the resistor Ra can drive only to [(RL/RL+Ra).Vrail]. In order to re-balance the clipping votage, the bias point must be altered so the voltage across Ra is higher than the voltage across the output device.
 
The 22113 was the summing stage. The 2128 and 15c are nearly identical and were often replaced by one another modifying the board to fit the mixer (remember they were all hand done by Dick and company)
What I used
Cmmi-10c or Jensen 115k
2128 with gain switch (insert eq if needed) to 22113 with 600r + (up to 1k) to cmmi-2qs.
I will one day throw a Neve output stage in there.
How's it sound?
Sounds good. Vintage but not thin. Its got meat but low headroom.
Ymmv
 
Rocinante said:
The 22113 was the summing stage.
That is weird. The typical bus impedance of a 24ch mixer is about 200-300 ohms. The 2.2k at the input and the relatively high value of the NFB loop (R3, R6) add at least 10-15dB to the noise figure.


The 2128 and 15c are nearly identical
That's what we figured out; one of the most significant difference, according to the schemos, is the wrong connection that shorts the input in the 2128.  :eek:


Sounds good. Vintage but not thin. Its got meat but low headroom.
Low headroom is quite often the result of too much gain to start with.
The low-ish single rail supply doesn't help.
 
I just remembered that I added two transistors to the output stage of the 2128/15c as per suggestions by PRR. And Ian. Ripped it right from the 990.  That added the appropriate amount of gain needed.  I also tried 36v power supply which gave it more gain accompanied by a little bit more noise.
I was very fortunate in chatting with a dude who had recently bought a Helios console. I'll see if I can find the conversation.
 
Hoping this is a useful contribution. I think that you need to consider the sign of the voltage rails. 15C1 is negative (early modules) and 2128 is positive? Otherwise they are practically identical (as has been mentioned but also probably depending on the schematic you look at as there are line amp/buffer and perhaps summing amp variants). Voltage sign dictates transistor selection. I think that the Helios mic pre typically has 1uf at the input of the first gain stage (polyester mustard then tantalums later). I think that 0,1uf was used for buffers.  It is recommended that you should have at least +30v for the high level line amp. The 22113 requires a +ve rail. There are obviously ways to drop down the voltage for the 2128 which is apparently a bit fussy about requiring +24v.  There is a thread on another site where this is discussed. I used a 48v source and generated separate 24v and 32v with a JLM Audio kit. Gave me everything I needed. I used a BC161-16/BC141-16 for the output of the 22113 (used BCs everywhere actually). Can't exactly remember why and I only know enough to be dangerous so make sure to read the recommendations of those with more educated suggestions. Watch the gain staging. The suggestions made around the 22113 gain are pretty relevant.
 
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