Neve 1260. (31267>B108>B104>1166) as a mic pre/ hiss problems

[Arkestra]

To begin with, I'm not sure you can use these 1260's as mic pres but am trying.

I have read this thread:

http://groupdiy.com/index.php?topic=7126.20

and other posts referencing the 1272/BA283/31267 as a mic input. I am going into a B108 which looks very similar to the BA283.  This drives a B104 in standard configuration

I am having an issue referenced in the thread above, there is a loud hiss, NOT hum, in all transformer configurations ONLY when a mic is plugged into the input. I have tried a Jensen JT-115k in place of the 31267 with the exact same result. I have wired the 31267's in almost every possible recommended configuration with no change in hiss. Pin 6 is grounded.

I am guessing this is either an impedance issue having something to do with the input xfrm loading (?) or a ground issue. I feel I have observed all proper grounding techniques I use in any other project. There is NO hum at 60/120 even with the amps wide open. With the mic attached there is still no hum but the hiss begins and increases as you adjust the B108 output (louder hiss with less attenuation) You can hear the signal but it seems for lack of a better term, "one legged"

I have rebuilt and bench tested all cards and am certain they are working properly. I am wondering if the B108 just does not have the gain for a mic, but I feel it should. On the bench I could "saturate" the B108 output with the 31267 wired up with a mic attached and monitoring through a scope terminated 50ohms.

Thoughts?

 

[PRR]

So many numbers. WHAT is your mike transformer going into? 1260, 1272, BA283, 31267, B108 ...?

Is there a schematic for YOUR thing? (Not just a "very similar" thing.)

 

[Arkestra]

Yes sorry its a bit of a rambling post.
The module is a 1260. This module contains one B108 card , one B104 card, an externally mounted OC22 and a 1166 output transformer.

The input transformer is a 31267 and is external to the module.
The 31267 is connected to the B108 preamp card via point B on schematic. This card feeds a B104 via a pot, and the 104 feeds an externally mounted OC22 driving a 1166 output transformer.

I have been unable to find any schematic for the 1260, I am told it is a line amp. I have the schematics for the B104 and B108

http://www.technicalaudio.com/neve/neve_pdf/0108_preamplifier_D10011.pdf
http://www.technicalaudio.com/neve/neve_pdf/0104_line_amplifier_D10004.pdf

on the B108, A and J are strapped with a 880 ohm resistor (I think) on the card connector side.

 

[ruffrecords]

The B108 is different to the later Neve 3 transistor pre-amps in that there is no NFB from the emitter follower. This means that unless you apply some NFB externally you will have very high gain which could explain the hiss. What is connected to pin H?

Cheers

Ian

 

[Arkestra]

Hello Ian, thank you for the reply. I will look tonight after work but I believe H is connected directly to the trim pot on the front panel. The wiper of this then goes to the input of the 104 card.

Forgive my ignorance, I'm learning about Neve as I go, but wouldn't pin J supply the NFB to the 108? Similar to connecting pin  S to U on a BA284? (per a 1073)

I miss typed my earlier post, presently there is a resistor strapping J and B (not A) on the cards I have. I assume this is currently providing a feedback path.

Thanks again, Max

 

[PRR]

> module contains one B108 card , one B104 card

Bizarre. B104 seems to be Positive Ground, B108 appears to be Negative Ground.

Two supplies? Or is there more trickery?

And which is the mike-transformer feeding?

B104 is shown with an output transformer, so I *ass*ume* is the output.

B108 *appears* to be run "infinite gain". I would assume it is a Mix-Amp, that a resistor from J to B (and mix input resistors to B) set the mix gain. (The 5K trimmer sets DC levels, not audio gain.)

> a loud hiss ... ONLY when a mic is plugged into the input.

This will be expected if there is a NFB resistor J to B. Voltage gain tends toward Rnfb/Rin. When nothing on the input, Rin is infinite, gain is unity, hiss is teeny. The instant something is connected to input, Rin has some medium or low value, gain goes up.

> presently there is a resistor strapping J and B

as I suspected.

The B108 can be severely modified to make a conventional mike amp, but....

> one B108 card, one B104 card, an externally mounted OC22 and a 1166 output transformer.

Without knowing which connects to what, and how this OC22 fit in, it is hard to see the full picture. --Ah, the OC22 may replace the OC36 shown outside the dot-line on the B104 drawing? The 1166 is also over there.

However the 250 Ohm emitter resistor seems odd since the 1166 appears to be connected 200:600 Ohms. Usually an emitter resistor is much smaller than the load, not larger.

 

[ruffrecords]

Hello Ian, thank you for the reply. I will look tonight after work but I believe H is connected directly to the trim pot on the front panel. The wiper of this then goes to the input of the 104 card.

Forgive my ignorance, I'm learning about Neve as I go, but wouldn't pin J supply the NFB to the 108? Similar to connecting pin  S to U on a BA284? (per a 1073)

You change gain on the BA284 with a resistor between pins T and V. The B108 is quite different and does not have the secondary feedback path of the BA284.

I miss typed my earlier post, presently there is a resistor strapping J and B (not A) on the cards I have. I assume this is currently providing a feedback path.

What value is this resistor?

Cheers

Ian

 

[Arkestra]

Thank you both for the replies, this is good information. Seems to confirm some suspicions. I got tied up doing live sound all weekend so when I get a chance I can post a "block diagram" from my notebook. Hopefully that can move this forward.

Yes this looks like a strange module.  As far as I can tell, the 108 references the "return" as ground, and the 104 references "+24" as ground. So im referencing the return (psu -) as chassis/ground/earth. PRR, you are correct about the OC22 being in place of the OC36.  I can try and include some pictures as well.

Thanks again