Three Band - Draft Schematic - Helios 69 Variation

[BluegrassDan]

Hey Gents. Here is a draft schematic of a variation on Ian's Helios 69 project.

The idea is to use a SINGLE 12 position, 2 deck, 1 pole rotary switch to control both low frequency attenuation and boost. The mid freq select and boost/cut are separate rotary switches. The high shelf is a separate rotary.

This would allow for just four knobs on the front panel. Of course, the low frequency boost would be fixed (I chose 60Hz, but it could be whatever you want).

Can you guys give this a once over and see if this makes sense? See any glaring issues? Any other concerns?

 

[ruffrecords]

The HPF deck positions corresponding to the boost need to be shorted together and connected to the input else the EQ has no input for bass boost. Other than that the bass cut/boost will operate fine as a fixed frequency.

The hi bands is exactly as the original Helios. The mid band is interesting. The Helios uses a complex switching system to maintain the correct potential divider but you have included it in a single pole  switch as I did in an early version of the REDDEQ. The problem is with the 5.1K resistor on the far side of the in/out switch. When the EQ is out, this provides the necessary attenuation with the 24K in bypass mode but it is still in circuit when the EQ is in. The 5.1K is therefore in parallel with the 5.1K formed by the bottom half of the mid switch so the attenuation increases by 6dB. There are several ways round this. First you could move the 5.1K to the other side of the switch so it is always connected to the 24K but never across the EQ output, The only problem with this is that whatever is driving the EQ sees the load  impedance halve in bypass mode. You could fix this by using a DPDT switch so you can also switch the input to the 24K.

It is a pity you still need a two pole switch for the bass.  A lot of other lower cost switches could be used if only a single pole could be used but I have not found a way of doing this.

Cheers

Ian

 

[BluegrassDan]

Ian, per my private message question...

What would be the best way to connect the EQ’s output to an LA2A circuit? Interstage transformer, or do I need a tube makeup gain stage?

 

[ruffrecords]

Ian, per my private message question...

What would be the best way to connect the EQ’s output to an LA2A circuit? Interstage transformer, or do I need a tube makeup gain stage?

You definitely need a gain make up stage, tube or otherwise. It needs an input impedance at least ten times greater than the 5K1 resistor. I think in the original it was over 200K.

Cheers

Ian

 

[BluegrassDan]

Okay. Here is my second stab at the schematic. How does this look?

(The low switch is a bit messy visually, but hopefully it tells the story.)

 

[ruffrecords]

Okay. Here is my second stab at the schematic. How does this look?

(The low switch is a bit messy visually, but hopefully it tells the story.)

I think I may have explained myself badly. All you need do is connect the cut segments together when the switch is in the boost positions. You do not need the messy individual connections to the boost section. Then it should be OK.

The in/out switch is still not right. You short the 5.1K to 0V for some reason.  Here is a link to a generic eq in/out scheme I used for the original poor man's pultec:

https://drive.google.com/open?id=0B_n67A1hN3qtb3lrdk53MXJsMTA

Replace the block that is labelled PMEQP1A with the Helios EQ. Replace the part labelled pad with the 24K and 5K1 resistors. It is very much like a bypass but with a pad in the bypass route set to equal the loss in the EQ.

Cheers

Ian

 

[BluegrassDan]

How does this look now?

 

[ruffrecords]

How does this look now?

That looks fine to me. As I mentioned it is very similar to my early design for the REDD EQ:

https://drive.google.com/open?id=0B_n67A1hN3qteUJTUDZ5MGkwUkk

and you can see I also made the mistake of retaining the 5.1K resistor.

Cheers

Ian

 

[BluegrassDan]

Ian, you’re awesome! Thanks for your help.

 

[ruffrecords]

Ian, you’re awesome! Thanks for your help.

Happy to help. Let me know how yo get on.

Cheers

Ian