Altec 9470A / 9475A rack-up

[Piotr]

Hi !

So I've finished racking up four Altecs 9475A, there's still a couple of things that need to be taken care of (like channel 4 buzzing a little, will have to screen the toroid, clean the cabling etc...) but in the meantime here are a couple of pictures, please react !!!

I've used both JLM Audio's AC/DC power supply and Go-Between input switches, a toroid that's probably too big (50 VA, each channel needs 130 mA at full output, + 27dB...), the unit is dead silent except for channel 4, close to the transformer...

I haven't recorded with it yet but it seems promising from what I've heard so far !!!

If you have experience in screening toroids, I'd really like to hear it... Thanks !!!

 

[emrr]

looking good.  Put a solid sheet a metal between the PSU and preamp areas.  leave only the smallest access hole for the wiring to pass.    That big excess loop of mains wire is awfully close to channel 4 input circuitry. 

Wow, you had to wire it the hard way, no trays!

 

[Piotr]

Hi Doug !

Actually I had two trays...but four preamps !!! And the trays wouldn't have made it in the rack, so yeah, wired it the hard way. I've deliberatly left too much wire so that I could open the rack and test and debug it, which I should be able to do in a couple of days, then trim everything aftertwards. I'll probably try a smaller toroid like 30 VA, which should be more than enough (this one was a spare from another project) and maybe reduce the buzzing. I'll try to screen this one anyway and see what happens !!! Cheers, Piotr.

 

[lassoharp]

My Reaction is:

Awesome job!!!  These look great - I really like the faceplate layout/build.  Altec all the way.

Congratulations on a job well done!!

Thumbs

 

[Piotr]

Thanks Lassoharp !!!

 

[Piotr]

Hi !

I'm still in the testing/refining phase of this rack up. Here's a frequency response measure of the four preamps that I hope is right as I'm still learning to use FuzzMeasure correctly for this type of tests. The magnitude scale is irrelevant here as I haven't calibrated it precisely. You can see all four preamps, with and without the -20 dB pad engaged. More news soon !!! Cheers, Piotr.

 

[MM1100]

Love it!! Thought I'd join the party with another 9475A rack-up.

 

[coniglius]

I just racked one of mine (9470A) up over the weekend finally. Doesn't look anything special as I just mounted it into an empty LA-2A case for the time being, but will post pics soon.

I've got a question though... Forgive me if this is a newbie question, but that's what I am in this case... I've only tested this with a couple of mics so far, but if I use a dynamic mic like an SM57 I get hum/buzz, but if I use a powered condenser mic I don't. I've basically imitated what I've seen here as far as racking it up goes. I'm using the Power One power supply as pictured in emrr's builds. No DI or anything just straight in/out of the preamp using the low power setting.

 

[Piotr]

Hi,

it's probably just an I/O grounding issue, check if all your I/O XLR pin ones 1 are connected to your audio ground with a multimeter. Might be that simple, we'll see... Piotr.

 

[emrr]

Study the schematic, identify all the various grounds, and then be sure they're all tied to something....correct.  It's easy to miss one with these. 

 

[coniglius]

Thanks emrr. I went back and gave the schematic a careful look. I didn't have pin 8 connected to shield gnd and J to case. I connected them and voila! No improvement whatsoever. As far as I can tell from rack-ups other people have posted, other people aren't even doing this... I just see pin 1 on the in/outs tied to the shield tab on the xlrs, which is presumably tied to the chassis ground. That being said, it's very hard to tell in any of the pics because the connections are so close on the card edges... I'm at my wit's end with this project. The schematics and connections in the Altec manual are clear enough but I must still be doing something wrong as it's just not working for me. I still get low level hum and or simpathetic buzz. I've tested all other possible sources (microphone, cable, mixer, etc) with a "reference" preamp and I don't have these issues. I'm using the International Power IHB24-1.2 power supply. I have two Altec 9470a units and swapping them in the same setup yields the same results. I would really appreciate it if someone could post or send me good close-up pics of their card edge connections and what they're tied to. I've gone through the dozen or so rackup pics I could find here and online and taking the union of all the clearly visible connections in the pictures still leaves me with some that are unclear. I don't really have access to a digital camera for now or I would post pics of mine. I'm working on that. Any help in the meantime would be hugely appreciated.

 

[emrr]

Do you have B- of the supply grounded?  For both 24 and 48 supplies?  H, J, and 8 all need to see ground somewhere, and Altec gives you all these points for you to choose, depending on your system requirements.  XLR input pin 1 truly grounded?  You're getting ground with phantom somehow, and not w/o.  Any pics?  Hard to guess everything you're doing, since we can't see it. 

Check this:

http://farm4.static.flickr.com/3122/2628696450_065a4805e8_o.jpg
http://farm4.static.flickr.com/3014/2627878963_3da28e5fcb_o.jpg
http://farm4.static.flickr.com/3039/2628696562_5092a7519f_o.jpg
http://farm4.static.flickr.com/3153/2701122989_1771be5050_b.jpg
http://farm4.static.flickr.com/3273/2701125311_e7e3ca84f2_b.jpg

 

[coniglius]

Do you have B- of the supply grounded? For both 24 and 48 supplies? 

I believe so, in the following image, the lug on the power supply marked "S-" is connected to the chassis via one of the green wires.

http://www.creepingrabbit.net/Altec_9470A/altec02.jpg

I'm Not using phantom power... yet.

H, J, and 8 all need to see ground somewhere, and Altec gives you all these points for you to choose, depending on your system requirements.   XLR input pin 1 truly grounded? 

Pin 1 of the XLRs is tied to the shield lug. Underneath the XLR jacks I've grinded away the paint to make contact with the chassis. Connecting J and 8 to ground via any permutation of the 3 grounds you see in the images (green wires = ground), including leaving one or both completely unconnected, yields no difference at all in the hum level. In the images they are not connected.

Note this is just a temporary case until I get some of the right size. If it looks like it's in an LA-2A case, well that's because it is

http://www.creepingrabbit.net/Altec_9470A/altec00.jpg
http://www.creepingrabbit.net/Altec_9470A/altec01.jpg
http://www.creepingrabbit.net/Altec_9470A/altec03.jpg

I was already using those images you linked as reference, they were very helpful in getting me this far I did the best I could from them but it's still very hard to impossible to tell what's going on on the card because of ambiguity of the colors and overlapping wires and pins projected all projected into 2D space. In the first set of your most basic rackup images (which most closely resemble mine, minus the switches), it appears to me that there are only 3 pairs of red/white wires coming off of the card edge connectors and those are obviously spoken for by the in/outs and +/-. I can't tell at all where J and 8 are going; I can't make out any connections to those at all... Thanks again for all the help.

 

[emrr]

My first bit of concern is how close the audio transformers are to the power transformer.  I'd flip the module 180 from where you are.  You see how close I have my connector to the power trans in the panel builds; not a problem.    But, I'm guessing this isn't your main issue, if at all. 

I'd be sure the shielding on the in/out wires is also connected to xlr pin 1; as is, it's not doing anything, if I'm seeing it correctly. 

You should strap 5-6 for high power; headroom moves up 9 dB. 

In your case, you should tie 8-J-H at the connector.  It sounded like you had tried it, but I'm looking at unsoldered tabs in the pic. 

Anything else I'm missing?

 

[coniglius]

My first bit of concern is how close the audio transformers are to the power transformer.  I'd flip the module 180 from where you are.  You see how close I have my connector to the power trans in the panel builds; not a problem.    But, I'm guessing this isn't your main issue, if at all. 

Flipping the module 180 degrees so that the connecters were on the power supply side added a sh*tload of buzz, even if pulled it as far back from the ps as it would go. 

I'd be sure the shielding on the in/out wires is also connected to xlr pin 1; as is, it's not doing anything, if I'm seeing it correctly. 

You should strap 5-6 for high power; headroom moves up 9 dB. 

In your case, you should tie 8-J-H at the connector.  It sounded like you had tried it, but I'm looking at unsoldered tabs in the pic. 

Connected the shields on the in/outs to xlr pin 1. Should I just leave the shields on the connector ends unconnected? Strapped 5-6. I tied 8-J-H and it didn't help at all with the low level hum, but it did help in that normally if I'd touch the module case it would buzz, now it doesn't. What's the best way to tie 8-J-H? I was just going to strap across 8-J, then J-H. Should I then just tie H to ground?

I'm not going to be able to do much until after Thanksgiving weekend. After which I'm going to try and better isolate the power supply (maybe mount on the outside), but I did try just setting the module outside of the chassis, isolated from the ps and it didn't help... Is there a chance it could be the ps itself causing the low level hum? I have another one that I could swap out. I'm also going to try running it on a different location's power. Thanks again.

 

[emrr]

My first bit of concern is how close the audio transformers are to the power transformer.   I'd flip the module 180 from where you are.  You see how close I have my connector to the power trans in the panel builds; not a problem.    But, I'm guessing this isn't your main issue, if at all. 

Flipping the module 180 degrees so that the connecters were on the power supply side added a sh*tload of buzz, even if pulled it as far back from the ps as it would go. 

interesting....case by case basis, I suppose. 

I'd be sure the shielding on the in/out wires is also connected to xlr pin 1; as is, it's not doing anything, if I'm seeing it correctly. 

You should strap 5-6 for high power; headroom moves up 9 dB.   

In your case, you should tie 8-J-H at the connector.  It sounded like you had tried it, but I'm looking at unsoldered tabs in the pic. 

Connected the shields on the in/outs to xlr pin 1. Should I just leave the shields on the connector ends unconnected?

yes

Strapped 5-6. I tied 8-J-H and it didn't help at all with the low level hum, but it did help in that normally if I'd touch the module case it would buzz, now it doesn't. What's the best way to tie 8-J-H? I was just going to strap across 8-J, then J-H. Should I then just tie H to ground?

One piece of wire looped through all 3 terminals.  H is already tied to ground at the PSU, right?  That's all you want; one point. 

I'm going to try and better isolate the power supply (maybe mount on the outside), but I did try just setting the module outside of the chassis, isolated from the ps and it didn't help... Is there a chance it could be the ps itself causing the low level hum? I have another one that I could swap out. I'm also going to try running it on a different location's power. Thanks again.

It's always the PSU causing the hum, one way or other.  Are you sure the outlet is truly grounded?  Is it sitting on top of another powered box with a power transformer field near? 

I've probably done 25 racked sets of these over time, and have never had a hum problem if everything was grounded and physically oriented correctly.  Most were done with the same supply you are using. 

You could try 8, J, or both to XLR pin 1 rather than to H, but I don't think that's going to do it. 

 

[emrr]

I found my test jig, and the connector looks as I've described. 

 

[coniglius]

[quote author=emrr]

It's always the PSU causing the hum, one way or other.   Are you sure the outlet is truly grounded?  Is it sitting on top of another powered box with a power transformer field near? 
I've probably done 25 racked sets of these over time, and have never had a hum problem if everything was grounded and physically oriented correctly.  Most were done with the same supply you are using. 
You could try 8, J, or both to XLR pin 1 rather than to H, but I don't think that's going to do it. 
[/quote]

Well I had it on my wooden bench with nothing else around and nothing else plugged into any outlets except a small Mackie mixer to be able to listen with; that's the simplest setup I could get for troubleshooting. The mixer's quiet and I've never had issues with hum with it for anything else, so I'm assuming that's not the problem (but I've tried it with a couple other mixers and still got the hum). The room is on its own breaker. As far as I know, everything is grounded to a spike in the ground outside. Next step is to try it off-site, and if that doesn't work, swap out the power supply. I'll also try "8, J, or both to XLR pin 1 rather than to H" as you suggest. Thanks again.

 

[mata_haze]

Quick question as I am working on a couple of these:
what are the edge connectors called? anybody knows where to get them from?

Thanks,
Mattia.

 

[mata_haze]

GOT IT!

http://search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=EDC307200-ND+

anyone has a couple available? they are just to expensive to order throught digikey

Best,
Mattia.