Yet Another Helios Mic Pre Thread

[Lerok]

Hi, I hate to beat a dead horse, but I've been circling back to some of my early interests before I knew anything about electronics - I've been putting together a Helios Mic Pre, wanted to share what's working for me, and ask for some suggestions - everything I've been able to find on the forum so far either uses the 2128 pre into an EQ (Rocinante and ruffrecords have been massive sources of inspiration), or uses the 22113 output card as the preamp (which apparently doesn't sound half bad, once again, thanks Rocinante!)

I've been trying to integrate the two cards into one simple preamp, but hit a bit of a hiccup - my current schematic is this (R33 and R20 are both 5k revlog pots to control gain):

It simulates well, with one problem - there's way too much gain. Following the wise recommendations of my predecessors, I'm aware pushing the 2128 (first half of schematic) past 40dB isn't a great idea, which is why R34 is there (to act as a limit on the gain) but even cranked down to the minimum, I'm getting 20dB of gain after the 2128 stage, and 36dB gain after the 22113 - I'm planning on adding a 15dB pad at the start, which will offset the gain from the 2128, and I could always add a bypass switch on the output stage before the output transformer, but I'm a little concerned that the 2128 doesn't have enough juice to drive a 600ohm load...

Just to clarify, R14 is where the 2128 stage goes into the 22113 stage...

Any thoughts would be appreciated!

 

[dogears]

Increase the size of your gain pots. 50k for your pot max gets around 6 dB from the 2128.

 

[Squeaky]

You are probably aware but I'll start with some obvious stuff. With an 82K feedback resistor the 2128 offers just a few dB of gain. When channels were used in line level mode there was a pad on the line in at the front end of the 2128 (visible on schematics), and the 82k (sometimes 56k) resistor was selected by default. There is a 16dB pad (27k and 5k1) and a filter between the 2128 and the following line level stage (whether it is the 2128 or 22113). This pad is engaged when the eq is bypassed (there is a 16dB loss associated with the eq when it is engaged). If you leave the feedback for the 22113 disconnected (as shown in one of the router schematics) I think you get 10dB of gain, which seems to be the minimum for this amp (makes sense in the router schematic as it is a post fader amp and 10dB is right for that and Spice is showing me a minimum 10dB).

If you are using the two amps in line level mode I would use the 10k 4k7 pad upfront, use 82k for the 2128 gain resistor, use the 27k 5k1 pad in between the two amps, I would also include the 68p or 680p capacitor between the amps, and I would use a 1k3 resistor for the 22113. Obviously a different story if we are looking at mic level. If you want to use a potentiometer for the 2128 gain you could look at the way it was done with the echo returns.

 

[Lerok]

You are probably aware but I'll start with some obvious stuff. With an 82K feedback resistor the 2128 offers just a few dB of gain. When channels were used in line level mode there was a pad on the line in at the front end of the 2128 (visible on schematics), and the 82k (sometimes 56k) resistor was selected by default. There is a 16dB pad (27k and 5k1) and a filter between the 2128 and the following line level stage (whether it is the 2128 or 22113). This pad is engaged when the eq is bypassed (there is a 16dB loss associated with the eq when it is engaged). If you leave the feedback for the 22113 disconnected (as shown in one of the router schematics) I think you get 10dB of gain, which seems to be the minimum for this amp (makes sense in the router schematic as it is a post fader amp and 10dB is right for that and Spice is showing me a minimum 10dB).

If you are using the two amps in line level mode I would use the 10k 4k7 pad upfront, use 82k for the 2128 gain resistor, use the 27k 5k1 pad in between the two amps, I would also include the 68p or 680p capacitor between the amps, and I would use a 1k3 resistor for the 22113. Obviously a different story if we are looking at mic level. If you want to use a potentiometer for the 2128 gain you could look at the way it was done with the echo returns.

This answers my question perfectly - I'll fix an 82k resistor on the 2128, should be perfect! Just didn't know what the 'correct' solution was to keep it as close to the original helios as possible. Thanks!

 

[L´Andratté]

I don´t understand why you are reducing gain in the pre stage not in the output (R20). You are amplifying stage 1´s noise...?

 

[ruffrecords]

I think it is important to rememeber that this pair would be sufficient to build a channel amp including passive EQ. The EQ would have an insertion loss of about 15dB which you do not have in your version.

Also, it is worth remembering that the mic input transformer has 20dB of gain, so with up to 40dB of gain in the first stage you have a total of 60dB available before you even reach the output stage.  If you had EQ after the first page you would need 15dB of gain in the output stage. As it is, you only need 0dB.

Cheers

Ian

 

[Lerok]

I don´t understand why you are reducing gain in the pre stage not in the output (R20). You are amplifying stage 1´s noise...?

Sorry, brain failure, meant the 22113 - will be using the pre stage's gain haha...

I think it is important to rememeber that this pair would be sufficient to build a channel amp including passive EQ. The EQ would have an insertion loss of about 15dB which you do not have in your version.

Also, it is worth remembering that the mic input transformer has 20dB of gain, so with up to 40dB of gain in the first stage you have a total of 60dB available before you even reach the output stage.  If you had EQ after the first page you would need 15dB of gain in the output stage. As it is, you only need 0dB.

Cheers

Ian

I'm debating on adding the EQ - as it stands I have 8 of the Neumann W492 design floating around here I'm planning on using as channel EQs with these preamps, though I have heard nothing but good things about the helios EQ - I suppose it's worth exploring!

 

[Rocinante]

I'm debating on adding the EQ - as it stands I have 8 of the Neumann W492 design floating around here I'm planning on using as channel EQs with these preamps, though I have heard nothing but good things about the helios EQ - I suppose it's worth exploring!

Very much worth exploring. Also the w492 is an active eq. I'm fairly certain it wouldn't work with the Helios stages.
The w492 is an excellent eq btw.

 

[Lerok]

Very much worth exploring. Also the w492 is an active eq. I'm fairly certain it wouldn't work with the Helios stages.
The w492 is an excellent eq btw.

I was originally planning on building separate units to keep components sort of consolidated to their own boxes - that said, only having to put in one set of transformers (ie, using the helios EQ) is looking quite attractive, given how expensive transformers are in my area.

I'll probably wind up building a pair of w492s for fun, and using 8 to 16 helios EQs with the preamps.

Side note, a lot of your previous projects served as a major inspiration for me getting into the whole DIY pro audio world - thanks!

 

[Whoops]

everything I've been able to find on the forum so far either uses the 2128 pre into an EQ (Rocinante and ruffrecords have been massive sources of inspiration), or uses the 22113 output card as the preamp (which apparently doesn't sound half bad, once again, thanks Rocinante!)

Have a look in the Helios folder in the technical docs section, I post a while ago schematics of an Helios console, there you can see how the MiC amp circuit, EQ and makeup gain circuits are implemented

 

[mdcmdcmdc]

Hi Lerok - just curious if you'd made any progress with this project? I'm interested in building a few channels of Helios pres without the EQ section.