quad eight mic pre [pics]

[b3groover]

Thanks, Joe! This is exciting!!!

 

[b3groover]

Would this bad boy work for the power transformer to get the +/-28vdc needed for the Quad Eights?

http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=129-080

 

[JLM Audio]

That 28vac Toroid will work but give you slightly high unreg voltage and more heat.

I would use this transformer below as it is a bargain and fully shielded for $7USD but it will only just fit in a 2 rack high case.
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=129-088

It is also worth considering the wall mount or wall wart type external transformers as no mains wiring needs to be done and the problem of hum due to the power transformer being in the same case as the audio transformers is solved. MT7132-ND will work fine. Or if you need a metal case ground wire use MT7116-ND.
http://dkc3.digikey.com/PDF/T051/1377.pdf

Joe

www.jlmaudio.com

 

[b3groover]

Thanks Joe. I do want to keep the PSU inside the rack if possible. Should I worry about noise with that non-toroidial transformer you recommend?

 

[soundguy]

when using those wall wart power transformers, does that illimate the need for a fuse on the circuit? Whats the proper way to wire those, just run the AC to the power supply and thats it?

dave

 

[JLM Audio]

I do want to keep the PSU inside the rack if possible. Should I worry about noise with that non-toroidial transformer you recommend?

Toroids are not usually as quiet hum inducing wise as copper strapped EI or encased EI. They are definitely quieter than non strapped or non shielded EI. But it always get down to don't bolt up a transformer until you have the pre going so you can turn it and see where the hum is at a minimum. Also the biggest leakage from a toroid is where the mains wires leave the toroid ring. So usually keep these closest to the edge of the case.

when using those wall wart power transformers, does that illimate the need for a fuse on the circuit? Whats the proper way to wire those, just run the AC to the power supply and thats it?

Wall warts have a temp or current fuse usually built into the transformer. So if you overload it it will stop and never go again. So a fuse after it is a good idea as fuses are cheaper than wall warts. When using a wall wart with the JLM power supplies wire one of the AC wires to 0v and the other to only one of the AC ins and leave the other unused. If your wall wart has a ground wire as well just wire that to the metal pre amp case.

A trick if you have a power switch on the front panel near some low level audio signal and using a wall wart is to switch the wire that goes to the 0v on the JLM power supply instead of the other wire going to the AC in because that way when the switch is on the wire going to and from the power switch is just at 0v with no large AC noise on it Also the wire with the AC on it can go straight to the AC in on the JLM PCB (or with a fuse in line) which with the right layout will be a very short path.

Joe

www.jlmaudio.com

 

[b3groover]

A trick if you have a power switch on the front panel near some low level audio signal and using a wall wart is to switch the wire that goes to the 0v on the JLM power supply instead of the other wire going to the AC in because that way when the switch is on the wire going to and from the power switch is just at 0v with no large AC noise on it Also the wire with the AC on it can go straight to the AC in on the JLM PCB (or with a fuse in line) which with the right layout will be a very short path.

Now that's a good idea!!!

Thanks, Joe!

 

[soundguy]

I had mine out of the rack this week, took some photos for you guys. Stock ca127's with jensen 110K input transformers.

http://www.glideonfade.com/images/tech/qe127.jpg

http://www.glideonfade.com/images/tech/qe127panel.jpg

dave

 

[b3groover]

Nice, Dave! How would you describe the sound? I'm planning on using mine for drums, since I've read that they excell in that area.

I've got all the parts for mine except 6 more trannies (I have 2) and a rack chassis. I'm going to try and wire some up this weekend just to make sure they work and to hear what they sound like!

I'm also going to try and figure out how to mount them vertically in order to get all eight cards into a 2 space rack....

 

[CJ]

Coolsville!
I gamma tweaked the pic.
Are old school opamps in the heat sinks or big transistors?
I can't remember which circuit this is.

 

[gang of elk]

very nice! looks like you have those 10-transistor jobbies.. i've heard those referred to as ca127a.. (??) i love em.

 

[soundguy]

those are transistors.

think they are ca127a's.

to mount them vertically, just get a small L bracket and put it through the bolt that mounts the transformer to the card, I mounted 325's like this in a mixer thats taking me until the end of time to finish. Its not very road worthy, but if you know you just need it to sit in the rack and move once every 4 years, it will be fine. Here's an older picture, can barely see the bracket but you should get the idea-

http://www.glideonfade.com/images/tech/apimixer.jpg

so far as the sound, they are big and clean sounding. Snare drums are gonna like these...

dave

 

[b3groover]

Those are the same cards I have, the CA-127a.

I got them off eBay back in 2003 for $40 a piece. I got eight of them because that's all I could afford... I wish I had picked up 10 or 12... I could sell the others for a handy profit nowadays. But hey, that's why I'm not an investor.

Dave, good idea on mounting. I'm going to try out a couple of different ideas. They don't necessarily need to be road-worthy, but it would be nice to be able to use them outside the studio.

I'm really excited to hear these bad-boys...

 

[b3groover]

[quote author="gang of elk"]the input trafos are cinemag cmqe-3440a. my intention is to have all 3 input impedances selectable but i've only been able to figure out the switching logic to use 37.5 and 150 so far. [/quote]

Any luck with this?

 

[gang of elk]

no luck, the logic for accessing all 3 is pretty complex. in the end i like 600 best so despite having a switch and panel labeled accordingly, mine are wired for straight 600. it's easy to wire back to the switch for 37/150 but i doubt i'll ever need to. anyway, that's what i get for not making a proto. onward and upward! :grin:

 

[b3groover]

I'm getting ready to rack my Quad Eight cards. The following is a picture of what I worked on tonight:

10 of JLM Audio's Go-Between PCBs which add phase, pad, and phantom to each channel, 2 DI circuits to add a DI jack on two channels, and the five rail Powerstation!

I just gotta say that Joe's boards are extremely well laid-out... a stroke of genius! And they are a lot of fun to put together! Thanks, Joe!

:thumb:

Tomorrow, I'll be stripping my Hammond/Leslie (oh, I've got plans... I'll reveal that later!) during the day, but in the evening I'll be re-capping the Quad Eight cards and wiring up the transformers.

 

[JLM Audio]

:grin: Thanks for the kind words b3 and great photo.

Joe

www.jlmaudio.com

 

[MikoKensington]

Does someone have their pinout for the CA127A that we're all using? I just want to make sure I have the info right. I'm using two for line amps in the Gyraf pultec and two will get input transformers for mic preamplification.

Sneak peak, almost done.

 

[gang of elk]

hi miko, lookin good. here's some info from ca127 application notes...

[snip]

INPUT:
14 HIGH
13 LOW - COMMON
--------------------------

OUTPUT:

for 600 Ohm +25dBm:

7 HIGH
5 LOW
---------------------------
for 600 Ohm +25dBm:

6 HIGH
4 LOW
---------------------------
for 600 Ohm +30dBm +6dB Voltage Amplification:

7 HIGH
4 LOW
Strap 6-5
---------------------------
for 150 Ohm +30dBm

7 HIGH
4 LOW
Strap 7-6, 5-4

_____________________
VOLTAGE AMPLIFICATION - Connect resistor to terminals 9 + 10 (see chart below)

VOLTAGE AMLPIFICATION Rg (Ohms)
UNITY NONE
5 dB 36K
10 dB 15K
15 dB 6800
20 dB 3600
25 dB 1800
30 dB 1000
35 dB 540
40 dB 330
40dB voltage amplification to a single output winding is the maximum
recommended. There will be a 6dB increase in voltage amplification if
the output windings are connected in series.

[/snip]

 

[b3groover]

Ok, after getting majorly side-tracked with other things, I'm back to trying to complete my Quad Eight rack. Here's my biggest hurdle: How to mount these things.

I like soundguy's L-bracket idea, however there is not quite enough height in my rack case to do that. So how did you guys mount them horizontally?

Man, I need to get these things done! I have ALL the parts, I just need to mount them, and do the wiring.