Whoops

SSL 4K G-Series Mic Preamp
« on: January 19, 2017, 08:03:40 AM »
Hi,
doing a new thread 4K G-Series Mic Preamp related.

Although talked a lot in other threads a G-Series preamp was never done as a project, the 4K preamp developed was the E-series transformer input preamp. (82E001)

The G Series preamp is the 82E291, I posted the schematics in Technical Docs Section.

here are the links for the schematics:
https://groupdiy.com/index.php?action=dlattach;topic=45604.0;attach=48024
https://groupdiy.com/index.php?action=dlattach;topic=45604.0;attach=48026
https://groupdiy.com/index.php?action=dlattach;topic=45604.0;attach=48028
https://groupdiy.com/index.php?action=dlattach;topic=45604.0;attach=48030

Some pics of the original cards and switch:








Whoops

Re: SSL 4K G-Series Mic Preamp
« Reply #1 on: January 19, 2017, 08:10:14 AM »
Any objections to balancing the output via DRV134 line driver? Phase reverse via relay?

Balacing via DRV134 or That1646 is perfect.

Phase reverse could be at the input and not in the Output like the schematic shows. I would say there's no need to use relays

I really like the distorted sound of this mic pre, I used to do it on the console, bring the gain up and attenuating the fader output.
What could be used as an Output level attenuator?  Could a T-PAD be used?

Thanks

dustbro

Re: SSL 4K G-Series Mic Preamp
« Reply #2 on: January 19, 2017, 09:16:20 AM »
Phase reverse happens post preamp via pins 23, 16 and 17.  Putting phase reverse at the input would induce clicks and pops when switching.
Check out the redrawn schematic for errors.  It contains 2 channels, as well as bipolar psu with +48V.
Once verified, I'll complete the layout.

Does anyone have a scanned copy of the gain switch network?
EDIT  - I see the docs above :)
« Last Edit: January 19, 2017, 09:26:52 AM by dustbro »

dustbro

Re: SSL 4K G-Series Mic Preamp
« Reply #3 on: January 19, 2017, 09:31:21 AM »

What could be used as an Output level attenuator?  Could a T-PAD be used?


Voltage divider right before the DRV134?

Whoops

Re: SSL 4K G-Series Mic Preamp
« Reply #4 on: January 19, 2017, 11:14:17 AM »
Voltage divider right before the DRV134?

The option of having output atenuator would be great, thanks.

Voltage Divider before the 134 seems quite good.


Whoops

Re: SSL 4K G-Series Mic Preamp
« Reply #5 on: January 19, 2017, 11:36:54 AM »
Oh mate you're fast,
great work

Phase reverse happens post preamp via pins 23, 16 and 17.  Putting phase reverse at the input would induce clicks and pops when switching.

ok, do what you think is the best


Check out the redrawn schematic for errors.  It contains 2 channels, as well as bipolar psu with +48V.
Once verified, I'll complete the layout.

I will verify that, maybe just later on , as I'm still working.


Whoops

Re: SSL 4K G-Series Mic Preamp
« Reply #6 on: January 19, 2017, 12:08:43 PM »
Dustbro just to clarify why I talked about having and output level control.

I mixed with an SSL 4K G console for years, one tool I used and still enjoy is distorting a signal by sending line level into the Mic Pre. You achieve a distorted Fuzz signal that works nicely on snare, room mics, drums, bass, vocals. That signal can be used alone or in parallel with the dry signal. Having talked with other mixer engineers with SSL experienced over the years I found that I was not the only one liking that but actually that it was common practice in Rock/Pop mixing engineers to resort to that tool on the 4K G when needed.
On the console you attenuate the signal going to the recorded with the channel fader.

This feature was also included by Waves in their NLS channel SSL emulation, on the plugin you have a "Mic" button that you press and emulates sending Line level into the Mic Preamp distorting the sound.
Although not exactly the same, the effect you can get with the plugin is actually similar to the distortion achieved with the real console, and trying the plugin "Mic" feature and listening to the effect is enough to decide if it's something you would want to have on the G Mic Pre or not.


The NLS plugin was based on "The SSL 4000G belonging to Mark ‘Spike’ Stent (Radiohead, Björk, Muse, Maroon 5, Madonna)"

With a standalone Mic pre without the console's output fader, having a Pot with the ability to attenuate the output of the Mic Pre is a nice and useful feature.

Thanks



Whoops

Re: SSL 4K G-Series Mic Preamp
« Reply #7 on: January 19, 2017, 03:03:23 PM »
Check out the redrawn schematic for errors.  It contains 2 channels, as well as bipolar psu with +48V.
Once verified, I'll complete the layout.

Hi,
checked your redrawn schematic against the original one with highlighting pen marking both schematics.
I Didn't find any error, it looks mint!

dustbro

Re: SSL 4K G-Series Mic Preamp
« Reply #8 on: January 20, 2017, 10:03:27 PM »
one tool I used and still enjoy is distorting a signal by sending line level into the Mic Pre. You achieve a distorted Fuzz signal that works nicely on snare, room mics, drums, bass, vocals.

Yes! This is exactly what adding a voltage divider before the output would achieve. It's like adding a fader post preamp :)


Whoops

Re: SSL 4K G-Series Mic Preamp
« Reply #9 on: January 20, 2017, 10:54:02 PM »
Yes! This is exactly what adding a voltage divider before the output would achieve. It's like adding a fader post preamp :)

Thats seems a winner!

;)


totoxraymond

Re: SSL 4K G-Series Mic Preamp
« Reply #10 on: January 25, 2017, 01:08:19 PM »
Hi i'm really interested in this project.

there are several things that i don't understand though.

First, on the original schematics, the pair of transistor are powered from a +17V Power supply, therefore, IC1 and IC 2 inputs are biased at +17V. In the redrawn schematics posted here, they're powered with +18V, wouldn't that saturate the ne5534?

I double checked and IC1 and IC2 are biased at 12,06V on the positive In and 0,33V on the negative In? (not sure about the negative)

Second, i really can't figure out how the gain switch works. According to the schematic, one half of the switch makes a variable resistor between the two emitters. And, it SEEMS that it's also connected to the ouputs of IC1 and IC2 via R26 and R27, but it's not clear in the switch schematics if there's really a net between M01.1 and M01.2 and between M01.4 and M01.5.

And i don't see what use they have for the other half of the switch. I can't find those Black Red and Whit wires back on the board.

I need to breadboard this i guess.

Any hint?

Thomas

« Last Edit: January 25, 2017, 01:15:18 PM by totoxraymond »

dustbro

Re: SSL 4K G-Series Mic Preamp
« Reply #11 on: January 28, 2017, 11:27:15 AM »
First, on the original schematics, the pair of transistor are powered from a +17V Power supply, therefore, IC1 and IC 2 inputs are biased at +17V. In the redrawn schematics posted here, they're powered with +18V, wouldn't that saturate the ne5534?

You are correct. I need to add R72 and C37 to power the LM394

dustbro

Re: SSL 4K G-Series Mic Preamp
« Reply #12 on: January 28, 2017, 11:29:47 AM »

Second, i really can't figure out how the gain switch works. According to the schematic, one half of the switch makes a variable resistor between the two emitters. And, it SEEMS that it's also connected to the ouputs of IC1 and IC2 via R26 and R27, but it's not clear in the switch schematics if there's really a net between M01.1 and M01.2 and between M01.4 and M01.5.

And i don't see what use they have for the other half of the switch. I can't find those Black Red and Whit wires back on the board.

I have to dig up some more documents to see what's going on. I assume the 2nd array of resistors that go to RedBlackWhite are for total recall.

Whoops

Re: SSL 4K G-Series Mic Preamp
« Reply #13 on: January 28, 2017, 10:02:56 PM »
I have to dig up some more documents to see what's going on. I assume the 2nd array of resistors that go to RedBlackWhite are for total recall.

I have to full service manual documents for the console.
I will send you by email

abbey road d enfer

Re: SSL 4K G-Series Mic Preamp
« Reply #14 on: January 29, 2017, 10:35:42 AM »
The option of having output atenuator would be great, thanks.

Voltage Divider before the 134 seems quite good.
DRV134 must be driven by very low Z; failure to do so results in compromised CMRR and possibly distortion. You can use a pot with a unityègain follower.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

dustbro

Re: SSL 4K G-Series Mic Preamp
« Reply #15 on: January 30, 2017, 08:24:10 AM »
DRV134 must be driven by very low Z; failure to do so results in compromised CMRR and possibly distortion. You can use a pot with a unityègain follower.
From what I can see, a DRV134 can be used without a buffer cause it has one built in.
It's comprised of 3 Opamps.
A buffer, inverting amp, and non-inverting amp.
But after reading the 134 docs again, I should probably add 10uF NP caps between the output and sense pins to reduce DC offset, as well as add a coupling cap at the input of the 134.
« Last Edit: January 30, 2017, 09:31:47 AM by dustbro »

dustbro

Re: SSL 4K G-Series Mic Preamp
« Reply #16 on: January 30, 2017, 08:30:17 AM »
Wait... I stand corrected.
The layout guidelines do recommended an opamp or buffer.

abbey road d enfer

Re: SSL 4K G-Series Mic Preamp
« Reply #17 on: January 30, 2017, 09:42:12 AM »
Wait... I stand corrected.
The layout guidelines do recommended an opamp or buffer.
That's because, when you look at the functional diagram, you'll see that the EBOS is referenced to the input and to the output of the input inverter (not really a buffer). Unbalanced impedance at these points result in poor CMRR and output unbalance.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

dustbro

Re: SSL 4K G-Series Mic Preamp
« Reply #18 on: January 30, 2017, 10:06:51 AM »
That's because, when you look at the functional diagram, you'll see that the EBOS is referenced to the input and to the output of the input inverter (not really a buffer). Unbalanced impedance at these points result in poor CMRR and output unbalance.
Right... it's expecting an external buffer.
Updated schematic on the way

dustbro

Re: SSL 4K G-Series Mic Preamp
« Reply #19 on: January 30, 2017, 10:12:16 AM »
Updated schematic:
- R9, R10, R13 changed to +17V (R72 + C37 added to PSU)
- C7 100uF coupling cap added
- U6 TL081 unity gain buffer added before DRV134
- C8, C12 10uF NP caps added to DRV134 for DC offset error (is this really needed??)