Neve BA283AM questions(s)

[Gsrokmix]

Hi Ian,

I am powering the circuit with a JLM Audio Powerplant 2.  I moved a couple of things around & the circuit is working again. I'm not sure what the problem was, maybe a short in the breadboard?

Speaking of the power supply, should I adjust the output for 24V loaded, or unloaded? I will be powering 2 channels of the mix amp. When I first built the PS, I hooked it up to my DC electronic load & ran it at 750mA just to test it, and set the output at 24V with the 750mA load.

Thanks again for your time and invaluable advice!

George

 

[ruffrecords]

Looks like the JLM is regulated so once you it has been set up it should be OK. Good luck at school.

Cheers

Ian

 

[Gsrokmix]

Thanks again Ian. Just for fun I ran some THD & noise tests using the Digilent Analog Discovery 2. The circuit is spread out on the breadboard a little bit, and my wires to and from the transformers were fairly long for convenience, but overall it looks like I'm off to a good start?

Thanks,

George

 

[ruffrecords]

Difficult to tell anything about its performance from the image you posted. Several contradictory figures in the pop up and the noise floor, on what I  think is meant to be a spectrum analysis, is way too high. I have heard of this device but I am not familiar with it so it is difficult to judge.

Cheers

Ian

 

[opacheco]

Opacheco, I do have an ORCAD file. I will upload to my FTP site this evening & include a link to it.

Thanks

George

George, I am sorry I had not seen your message!.......Please let me to know your link to your ORCAD file please in order to download it.

Thanks
Opacheco

 

[Gsrokmix]

Hi again,

So I am in the final stages of assembling my project. Thanks again to everyone who commented previously. I was getting ready to solder the connections to and from the input & output transformers when I realized that I didn't know what to do with the shield & drains in the Mogami (Redco actually) cabling I am using for hook-up.

I have a star grounding scheme with my mains earth ground, chassis, and mix amp common all tied to the same point. The 2 input terms on the input trans primary are coming from the hot & cold signal busses. The mix amp output is going to the 2 terms on the output trans primary. What do I do with the shield & drain wires?

Thanks again!

George

 

[ruffrecords]

What do I do with the shield & drain wires?

Thanks again!

George

Connect them to pin 1 on their respective XLRs and of course you have already connected all XLR pins 1 to chassis at the connector, right?

Chhers

Ian

 

[Gsrokmix]

Hi Ian,

So if the shields / drains go to ground on one side, where do they connect to on the transformers? The 16 line inputs are coming in on (2) DB25s, and the Pin 1s are all tied to a common buss which is connected to my star point. So my Pins 2 & 3 are what are tying to the input transformer.

Thanks,

George

 

[ruffrecords]

Hi Ian,

So if the shields / drains go to ground on one side, where do they connect to on the transformers? The 16 line inputs are coming in on (2) DB25s, and the Pin 1s are all tied to a common buss which is connected to my star point. So my Pins 2 & 3 are what are tying to the input transformer.

Thanks,

George

They other end goes nowhere. Just make them off (trim them back and do not connect them).

Cheers

Ian

 

[Gsrokmix]

Ah ok. I thought you were never supposed to leave any end of the shield unconnected, that's why I was confused.

Thanks,

George

 

[ruffrecords]

Ah ok. I thought you were never supposed to leave any end of the shield unconnected, that's why I was confused.

Thanks,

George

You are right. It is probably best to connect the screen to chassis at both ends.

Cheers

ian

 

[Gsrokmix]

Sorry to beat this to death, but:

My 16 line inputs will be coming into the box via 2 DB25 connectors. Pin 1 of all of these connections will be tied to a "ground buss" and then connected to my star ground (mains earth ground, chassis ground, & DC supply common).

The Pin 2 & Pin 3 busses will be connected to terminals 2 & 5 of the VTB9071 line transformer. Terminals 3 & 4 are tied together.
If I tie the shield & drain wires to the ground buss, what do I do with them on the cables between the resistor busses, and the input trans?  I'm assuming I can tie the shields / drains on the resistor side to the ground buss? There is an Earth connection (Pin 6) on the VTB9071 trans, do I use that? I am screwing the inputs transformers directly to the bottom of my metal aluminum enclosure.

Similarly, terminals 5 & 8 (6 & 7 tied together) of the output trans VTB9049 will be tied to pins 2 & 3 respectively of the output XLRs. If I connect the cable shield / drains to pin 1 of the output XLRs, where do they go on the transformer secondary side?

I appreciate anyone who has done something similar to chime in & share your thoughts & results.

Thanks as always!

George

 

[ruffrecords]

The rules are:

All XLR pins 1 connected to the chassis at the connector. For a DB25, the commoned screen connection should also go direct to the chassis at the connector.

For balanced line inputs I connect the cable's shield to XLR pin1 (or the equivalent) at the input connector. The shield at the transformer end I leave unconnected. I do the same for balanced line outs.

The rest of your post confuses me. On the one hand you talk about VTB9071 line input transformers (10K:10K) and then you talk about the cables between the resistor busses, and the input trans. Can you post a schematic?


Cheers

Ian

 

[Gsrokmix]

Hi Ian,

Here is a picture (not of the final layout). I am using the Last stage of the 283 card as a mix amp for 16 mono (8 stereo pairs) of analog inputs via the (2) DB25 connectors. Summing with a resistor network, as shown, and then make up gain from the amp. I am using a 10k:10k line input transformer after the resistor summing busses.

Thanks again for all of your input, I am learning a lot here!

George

 

[ruffrecords]

OK, so it is a passive summer.

So what I said before is OK for you. Connect screen common of inputs to chassis at dB25 connectors. Leave the ends at the mixing PCB unconnected. For the balanced mixes themselves it would probably be simplest to connect the screens to chassis at the line input transformers. At Neve I often saw a metal solder tag under the top screw of the transformer used to connect a cable screen to chassis.

Cheers

Ian

 

[Gsrokmix]

Thanks for clearing that up Ian. I will be finishing assembly & hopefully running some sound through this thing tomorrow.

George

 

[Gsrokmix]

Hi,

So my project is all wrapped up. I presented it to my class last week. I want to thank everyone, especially Ian, for all of the invaluable advice along the way. There were a lot of bumps in the road, but I learned quite a bit, and now have the confidence to move forward with other projects. I have included a couple of pictures of the final working box. I auditioned 3 different kinds of summing resistors, regular off the shelf Dales, Holcos, and Shinkohs (plus some Audio Note ones because I couldn't source enough Shinkohs). I found the Holcos to be the most pleasing to my ears. I didn't notice much of a difference from summing in Pro Tools with the Dales, the Shinkohs had a very nice smooth top end, but cut a little of the low end, and the Holcos had nearly the same smooth top, without the dip in the low end. The Neve output stage itself gave a broad mid-range presence boost overall as expected from my experience with Neve pre-amps of that era. I am going to add variable gain, and an output trim so I can play around with driving the amp, and output transformer harder to see what that does.

Again thank you everyone for the help. This site is a valuable resource for a lot of people, & I am sure glad it's here.

George

 

[Gsrokmix]

The Front (with my crooked meter)

 

[Gsrokmix]

The rear.

 

[Gsrokmix]

The resistor busses.