Spectra Sonics 8x4 mixer repair/refurb project...

Hey guys,

so I recently got a really beautiful old Spectra Sonics 8 channel, 4 bus, mono mixer…

It's got 2 101 cards per channel, Triad a67j's on the inputs, the 2 band selectable Spectra Sonics 500 EQ's, 10K pots for faders, 4 position aux/buss selector, phase, phantom, a -10/-20 pad and the 3 position gain that all the old SS consoles/strips

So i had initially wanted to just add a stereo buss to it so I could sum drums from larger mixes and to use it as a small mixer in conjunction with my tape machine. I primarily do 8 track recordings, so it seemed perfect…

It made the trip from LA ok, and showed up in good shape, but from the looks of it, someone got into it to maybe repair a dead channel (or 3) and its all full of, bad wiring, inconstancies and weird little things. It has a few dead channels, and the aux/ buss routing doesn't seem to work, so since I have to take it apart, I am thinking about adding a few things to make it a little more useful in the studio.

From what I have read about the 101 cards, the EQ's are usually used in the feedback loop of the 101's - and in by doing so it sets the gain of that amp stage - in a few of the channels on the mixer - the eq is tied to between the + output of the first 101 card and the feedback return… the strange thing is that it actually sounds a little better that way, not sure why yet - might be that the other channels that have the 101's and the eq's hooked up correctly are sick in other ways…

So at this point I am just starting to plan out what I'd like to do, I definitely would like to add insert points, and balanced ones if possible, I'd like to make a stereo buss, with insert points and faders and a Control room Pot.

Id also like to fix the aux section, and maybe even add an insert point there for kicks!

here is a flow chart of what Im thinking for the input channels…

my proposed changes are in the outlined boxes...


So I wanted to come to you guys to get some advice on the insert points:

my question is this - in the literature for the 101 cards it says that they don't need to be matched impedance wise - but I was thinking that I would still add transformers there just because at that point it would be easy to maintain a balanced signal… so I would do 10k:10k on the send and 600:600 on the in?

does any body know how the old SS's did this? I know they had balanced inserts…

my second question at this point is about faders…

From the looks for the SS docs I can get my hands on, it seems like they have used 600R, 2.4K and 10K faders in different designs, this design has a 10K "fader" - is there any reason to stick with 10K? Bill from SS said they switched to the 2.4K to improve the noise response, is there any reason I couldn't go with 600R faders?

alright - thanks fella's - I'll attach some ore photos so you guys can see what were working with…

Dear mutterd - I have been off the GroupDIY for over a year now, renovating my house. Your SS console looks fabulous even if it needs some work. I don't have time to address your questions today but let me think about this because I am trying to get my own small SS mixer project going right now. I have nine 101 amp cards and a bunch of 103 summing cards and at least eight A-67J's. And I am now trying to gather the technical info needed to assemble these into a small 8x2 line mixer. If you want, you can contact me offline (tubemooley_at_yahoo.com). I want to call Bill Cheney at SS and I have found a couple other dudes who seem to know about the SS stuff. Thanks, Dana Walcott, Mendon, MA.

Ok, so i have gotten a few emails about this project lately so I thought I'd share my progress…

So the first revision of the mixer was done to be used as an 8 channel mixer to drive my 8 track 1/2" machine for a series of location recordings I'm doing…

I had been poking around trying to chase down some anomalies and just got sick of it - I was coming up against the recording date and was still having trouble so I decided I would tear the back end down - basically everything behind the input transformer - and rewire it all the same! and correctly…

this is the rats nest starting to untangle...



as things started coming out I found that someone had but caps across the inputs - maybe to stop osciallions cause of the lousy ground? who knows… Spectra Sonics was pretty specific about the grounding of these units, and if done correctly, and everything is terminated correctly, there shouldn't be any HF spinning around in there - but I guess this was done as a low pass on input.



kept on going till i got this:



At this point I started to make lengths of console cable to wire everything up.



In the literature for the 101's and the 201 card holder they talk about HF grounding by the use of .47uf ceramics. On the front end, these are put between pin 1 of the xlr in's and ground and between the shield of the 1/4" ins and ground. Also, the negative input of the second 101 in chain is also grounded with a .47uf cap.

Since i was building this back out using the same card edge connectors that were in there i had to do a little bit of work to get the grounding correct. On the 101 cards the ground is not continuous so both the Input - and the Output - must be grounded for the amp to work. Its not like other discrete opamps in that it can be utilized as an inverting amp - the 101's are always non inverting. So i left the power supply ground bus connecting all the cards, and on the first 101 in each channel i just made a big pool of solder to connect Input - and Output -, for the second 101 I grounded the input to the power supply ground with a cap.

after I get all the wires done I wound up sliding a folded 1/2" piece of copper foil down over the power supply ground to create a better grounding plane.




The SS 500 Eq's sit between the Output + of the 101 amp and the feedback point - unless they are terminated by an inductive load like a transformer - in which case the 500 sits between the DC Signal Output + and the Feedback point with 10uf tantalum cap between the DC out and the 500 to block the DC so as the caps in the eq circuit dont get charged, effectively killing all headroom of the amp.

so my mixer was initially set up like the channels in the high end SS's, which have a 101 with the 500 EQ between the output and the feedback as the preamp, which drives a fader, which feeds into another 101 to drive the line outs and busses.

On mine there was a 5 position grayhill to select the echo out, and a pre and post fader tap for the echo sends, but there were no bus resistors or any real summing at all. basically all of the pre/post fader taps just were connected thru a dpdt switch, to a 2.5k pot, and then to a 1/4" jack on the back - so you get sound, but its low output and full of distortion. Its basically missing one more 101 per send - if each of those taps were connected to a bus resistor and then summed at the input of a 101 i'd be golden, but for this revision there aren't enough card edge connectors to make it work (bus/summing will be included in Version 3!!!)

In the real SS consoles there were pre and post fader taps for the echo send, which is then driven to the bus by another 101 in each channel and then summed and driven out by another 101… then the return is thru yet another 101 with an eq in the feedback…

so i scrapped all of that for now.

I had thought that there was a bad eq or two, but once they were out they all tested great. These things are incredible. Not only do they sound amazing, but they are little feats of engineering genius…



At this point I started to wire it all back up… I had 10k pots as faders on the original set up but a bunch of them were bad, so luckily i had a bag of Allen Bradley 600R log pots which i subbed in adding a 5.1k resistor to the input of the 2nd 101 card to attenuate the signal and get a little control of the surplus of gain out of the first 101.

the 600R's are a little low for the circuit - 2.5K is best, but for now it works ok.



In addition to all of this, I added a mic/line input switch. The tape machine I'm using with this is an old Teac 80-8 that i absolutely love, but its -10 with Really Crappy Apparatus connectors so I built a pad on the line in puts to optimize the output of the Teac using 5.1K series resistors and 100R shunts. Now I can go between tracking and playback, and i can use the mixer to dump down into my DAW for ruff mixes and what not.

Here is the happy couple…




SO…

now I am starting on Version 3, which will be the last revision using this faceplate. I have since gotten 8 more channel strips with eq's and cards so once i get metal work sorted out I'll build this into a 16x8. But for now, these are my design objectives:

8 Channels which would each include:
500 EQ's
Mic level XLR input
-10 db Tape/Line 1/4" input
Pan for Stereo Out, Pan Defeat for Mono
5 Position Pad -6/-10/-20/-30
Fixed frequency high pass filter
Bus selector switches
Fader
1 Monitor/Cue send
Channel Mute
Mon/Cue Mute
Transformer isolated TRS Insert points with defeat switch
a67j input transformers
hs66/T66 Output transformers

Master section which would include:
501 Eq's on Bus 1 and 2
Bus Master Faders
Transformer isolated Insert Points on Bus 1 and 2 with defeat switch
Mon/Cue Master Fader
Hs66's on Bus 1, Bus 2 and Mon/Aux

I've figured out a way to employ all of that in the same topology that i have now, just by rearranging the switches and pots. I'll also build a master section on a blank piece of 3RU aluminum that I'll attach to the side of the current mixer.

With this I'll be able to use it to mix down my 8 track recordings to stereo 2 track, I'll be able to bounce tracks thru the mono busses during the recording sessions and i'll be able to use this to sum drums on larger mixes…

I'm working on the line drawings for that now - I'll post them once i get them a little more solid…

BUT - as of right now i have done one tracking session with the mixer and i'm completely blown away by the results - the pre's are fast and accurate and sound amazing - the eq's are just phenomenal - i even used them to tape! Where the SS stuff really stand above the rest are with high/fast transient laden signals such as drums, acoustic guitars, harmonic percussion, things like that…

In general the initial transient of a say, a cymbal hit is so high in amplitude that even though it happens in micro seconds, maybe even nano seconds, it rams the amp up to the top and into clipping, and that causes the amp to distort the signal that happens afterwards, and that distortion last as long as it take the amplifier to recover. So sound sources that have transients that are so drastically different than the fundamental are always the hardest to capture honestly.

The bass drum, for instance, the initial sound of the beater hitting the head is loud and fast, but the sound that you are trying to capture happens after that - its the resonance of the drum head, and the drum itself. But that first initial click often veils the signal and makes is hard to capture the rest.

with these SS pre's - for the first time ever, I'm getting results that actually approximate the sound of a real bass drum.

with OH mics its the same thing - the cymbals sound natural, the resonance of the snare cuts thru…

its really great. Its been a bear of a project so far, but it seems worth it. I can wait to have 16 channels of this!

Wow

Amazing console, and it's impressive what you are doing.

I have a picture of a custom console built in a  similar way, with the same modules.

.

.

Hey that's the Muscle Shoals console, isn't it?

Do you have that mixer? It's a beauty...

I'd love to see some photos of the inside!

Thanks for sharing...
Timothy

mutterd said:

Hey that's the Muscle Shoals console, isn't it?

Do you have that mixer? It's a beauty...

I'd love to see some photos of the inside!

Thanks for sharing...
Timothy

Click to expand...

It was the desk that was at Muscle Shoals Studio in late 2011, the studio was owned and maintained by Noel Webster at that time. The desk was was not there at the Black Keys recording, but was installed some time after that recording.
I just saw that Noel Webster sold the studio in 2013, and that the Foundation that bought it want to do a Museum.
I don't know if the desk is still at the studio or not, probably the desk still belongs to Noel Webster, but I don't really know.
But he is a really nice and helpful guy, you can contact him through facebook.
I know the Mic Pre amplifiers on that desk were Spectrasonics 101 or 110 , don't remember which one for sure. Ask him he will know.

Necro thread here, but if you ever took time to draw schematics of the EQs (and pres! ) I'd be super interested in knowing more about them.
Cheers & thanks !

Superior gear!
Just to let you know : I’m selling a Spectra Sonics PSU (with their swishy logo & all electrolytic révised)!!