Spectra Sonics 110 pre with Opamp Labs 109 EQ

[craigmorris74]

Hi folks,

I wanted to rack a couple of Spectra Sonics 110 cards to use as mic pres.  I also have the inductors and 325EQ amp modules to build a Opeamp Labs 109 EQ (I've attached both schematics). 

Since both run of +/- 24V, I was wondering if I could make a channel strip where the output of the 110 runs into the input of the 109 EQ.  Would there be any additional circuitry needed to get them to work together properly.

Thanks,

Craig

 

[craigmorris74]

Here's the Spectra Sonics 110A schematic.

 

[gridcurrent]

I wanted to rack a couple of Spectra Sonics 110 cards to use as mic pres.  I also have the inductors and 325EQ amp modules to build a Opeamp Labs 109 EQ (I've attached both schematics). 

Since both run of +/- 24V, I was wondering if I could make a channel strip where the output of the 110 runs into the input of the 109 EQ.  Would there be any additional circuitry needed to get them to work together properly.

how about a fader ?
you might want to add resistive isolation and a dc blocking capacitor on the output of the 325eq,
or a suitable transformer.

 

[craigmorris74]

how about a fader ?
you might want to add resistive isolation and a dc blocking capacitor on the output of the 325eq,
or a suitable transformer.

I will add a fader.  Don't know what value to use.  600 ohm?  10k?

I have some 600:600 ohm transformers here, would they be suitable?

 

[gridcurrent]

I will add a fader.  Don't know what value to use.  600 ohm?  10k?

I have some 600:600 ohm transformers here, would they be suitable?

the Spectra 110 specifications indicate ability to drive 600 ohms.
10k faders offer greater "kill" and easier to drive. 
would be useful to see the input stage of the 325eq amp re ideal fader impedance;
an inspection of its output stage should answer your question of 600:600 transformers.
rackmonkey might be the OpAmpLabs resource around here.

 

[craigmorris74]

the Spectra 110 specifications indicate ability to drive 600 ohms.
10k faders offer greater "kill" and easier to drive. 
would be useful to see the input stage of the 325eq amp re ideal fader impedance;
an inspection of its output stage should answer your question of 600:600 transformers.
rackmonkey might be the OpAmpLabs resource around here.

It seems like someone opened up the 325EQ can a while back.  I'll look.

 

[craigmorris74]

Here's where facility traced the input of the 325EQ.

 

[craigmorris74]

Here's some of the specs.

 

[rackmonkey]

There is already a 15uF tantalum cap on the output of the 325EQ. Input impedance of the module is 10k using the inverting input. I’ve attached a schematic of the 1204 console so you can see how it was used in practice.

The 325EQ is just a 425 with a couple of extras (10k resistor on the input and a cap on the output, IIRC) so it should drive a 600:600 transformer just fine.

 

[rackmonkey]

Module description.

One caution: OAL modules/consoles did run on +/-24V, but they were notorious for crapping out at that voltage from what I’ve read. I’ve run them anywhere from +/-15V - +/-22V and never had one die on me. I don’t know if the 325EQ was particularly susceptible to being run at nominal voltage or not. The mic pre modules and the 425 summing amp were the ones people who owned those consoles seem to mention, (yes, the 325EQ is a 425, so the lack of mentions might not mean much) but regardless it might not hurt to keep a spare or two around just in case.

 

[abbey road d enfer]

I will add a fader.

Performance of this EQ implies it being driven by a low impedance source. A 10k fader would be utterly inadequate. A 600r might work but that would be straining the mic pre. As can be seen from the mixer schemo, they put the fader at the very end, driving active modules (actually summing amps).
If you want to insert a fader between the mic pre and the EQ, you need an active buffer.

I have some 600:600 ohm transformers here, would they be suitable?

Transformers add losses, which translate as too high output impedance. It may "work" somewhat, but not perform.

 

[rackmonkey]

Transformers add losses, which translate as too high output impedance. It may "work" somewhat, but not perform.

Is it the specific application or gain config of the 325EQ in this situation that makes you doubt the wisdom of a transformer here, Abby? The OAL 425 is basically the same as the 325EQ, which was almost always used on outputs driving a 1:1 transformer in OAL designs. I use 425s the same way in a few preamps and see good performance, both measured and sound-wise. Curious to understand the thinking in this case.

 

[rafafredd]

Hey, do you have specs on those inductors in the schem???

 

[rackmonkey]

Hey, do you have specs on those inductors in the schem???

Yes. Everything I know about them is here: https://groupdiy.com/index.php?topic=74899.0

 

[abbey road d enfer]

Is it the specific application or gain config of the 325EQ in this situation that makes you doubt the wisdom of a transformer here, Abby? The OAL 425 is basically the same as the 325EQ, which was almost always used on outputs driving a 1:1 transformer in OAL designs. I use 425s the same way in a few preamps and see good performance, both measured and sound-wise. Curious to understand the thinking in this case.

As I wrote, perfect operation of this EQ requires a very low source impedance. Now, a good 600:600 xfmr would introduce about 50-100 ohms, that would result in an error of about 1dB worst case. This is certainly perfectly acceptable, though not "perfect".

 

[mjrippe]

I think the 600:600 would be fine at the output of the EQ, but not between the pre and EQ as Abbey explained.  The Spectra will drive the EQ just fine with nothing in between. 

 

[rackmonkey]

I think the 600:600 would be fine at the output of the EQ, but not between the pre and EQ as Abbey explained.  The Spectra will drive the EQ just fine with nothing in between.

I misunderstood Abby’s post. I thought he was referring to the output of the EQ. I think Abby was responding the idea of using one between the pre and the EQ, but looking back at the whole thread I’m not sure that’s what was asked about.

 

[craigmorris74]

I misunderstood Abby’s post. I thought he was referring to the output of the EQ. I think Abby was responding the idea of using one between the pre and the EQ, but looking back at the whole thread I’m not sure that’s what was asked about.

My question was should there be anything between the 110 pre and the 109 EQ to make things work better.  Gridcurrent suggested a transformer.  Abby didn't think that was a good idea.

 

[rackmonkey]

Got it.

Okay, Craig, what's your plan here? I'm interested in what you do with this. I was talking to Cyrus M (Mount Cyanide) a while back about collaborating on a project to make the OAL 109 EQ in 500 series format and maybe also as small boards that connect directly to dual switches. But I haven't found time to get a layout done and source modern replacements for the inductors.

 

[gridcurrent]

My question was should there be anything between the 110 pre and the 109 EQ to make things work better.  Gridcurrent suggested a transformer.  Abby didn't think that was a good idea.

grid suggested a transformer on the output of the 325eq section, or resistor isolation.
the concept of  fader after the Spectra preamp is apparently isn't such a good idea as the eq needs a low impedance source.