BSB 74 Calibration

[jjgolden]

Hello,

Does anybody have documentation or working knowledge of how to
calibrate the left and right BSB -74 modules?
I'm assuming you need to power and calibrate them while removed from the chassis...?

I need to get my L&R thresholds equal.
The BSB left channel shows about 10db of gain reduction before the right kicks in. Also the gain drops about 1.5db between "bypass" and "ON"

Description of what the "P" trim pots are would be great.
(i.e. where's the threshold and where's the gain's?)

Step by step instructions with nice color photos would be invaluable! javascript:emoticon(':D')


Thanks for your time and info,

JJ Golden

[petermontg]

JJ,

There is a Neumann callbration in manuals in the lathe trolls wiki. Has calibration for the BSB74. Hope it helps. It's in the navigation bar on right hand side.

Peter

Edit: link - http://lathetrolls.pbworks.com/w/browse/#view=ViewFolder&param=Neumann%20VMS%20Calibration%20Setup

[opcode66]

That is the calibration instructions for the amp rack. Not for the BSB's. I don't think there is anything that you need to calibrate in the BSB's. If they are not responding equivalently send then pull them both and send the suspicous one back to Al. If something needs fixing in it, I don't think it is as simple as an adjustment to a pot or trimmer.

[petermontg]

Are last 2 pages not about BSB74?

[opcode66]

Yes, they describe the function of the BSB and some of the maths behind knowing what acceptable Amps and Accelleration are. But, no instructions on how to "calibrate" them. Unlike the first few pages of the document which do contain specific instructions on how to calibrate the amp rack channels.

[petermontg]

Jasus, ya learn something new everyday. Fair play for spotting that.

[jjgolden]

Thanks for chiming in guys.
Thanks Peter. I appreciate the effort.
If you remove one of these modules, you'll see all of the variable resistor trims pots there...all 13 of them!

If I had neumann extender cards and some info on the trim pots description, I believe I could do it... and hopefully learn something along the way.

JJG

[opcode66]

You bought your lathe from Al, correct? Call him. My guess is that he will say the same thing I did. Send the module to him.

[jjgolden]

Thanks Opcode,

Yes, Most roads lead to Grundy.

I like fishing for info here as well.

I'll keep you posted when I get it sorted.

Any additional info is always welcome!

JJG

[opcode66]

I have a copy of the BSB74 schematics. Though schematics don't really tell you everything you need to know about proper usage/calibration... So, not sure what help they would be. I can send you a copy however.

[opcode66]

I've been having better results doing the high frequency limiting elsewhere in my chain. I'm actually thinking about pulling mine out entirely.

[jjgolden]

Thanks Opcode,

I've got the paper and the pdf schematics.

I de-ess in the signal chain, but I like to have the A.L. in the chain as a safety percussion.


JJG

[opcode66]

I hear ya. I was the same way until recently.

Now, I just don't like the way things sound passing through the BSB. Maybe I am going overbaord here. But, I think I get better cuts doing the HFL elsewhere and not passing audio through the BSB's at all.

[jjgolden]

If you run a tone through it, do you get any level difference between "Bypass" and "off"?

[opcode66]

The documentation states that even in the Off position signal still passes through all the stages of the BSB. Why would I continue to do that if I'm not using them?

[audadvnc]

Depends on whether the BSB uses a passive optocoupler (ex: Urei LA4, Westrex 1700 hi freq limiter) or a some active electronic stage like a VCA as its gain element (ex: dbx 160, Brooke Siren DPR402).

A limiter I built recently (to adjust mono/stereo mix of bass by level) uses a Vactrol optocoupler, which becomes an open, multi megohm resistor when dark and drops out of the circuit; only a few resistors remain, which attenuate but do not degrade the signal to any noticeable degree. I'm a big fan of Vactols; if you don't need instantaneous response or tight control of all parameters, they do not degrade signal when used as intended, and are simple to implement.

Voltage controlled amplifiers are by their nature going to be in the circuit all the time, and do add an additional layer of hoops for the signal to jump through on its way to the disk.

[jjgolden]

@opcode
To see if it,s displaying level a difference....I,m not implying anything about how it,s used.
There,s a difference of about 1db between off and bypass on mine, which isn,t right.
Was wondering if your,s or anyone elses displayed similar differences.
I left a message with Al today.


JJG

[Nickou]

I recalibrated mine after having changed all the IC and other little things ...

you need a scope and a sine generator and a extternal PSU ... if you have the shematics , look at them , and you will see than the audio path is clear .
It is easy to do , any good tech can do that ... no need to send that to Flozky , AL , or any specialist ...

As said opocode ,if you cut from digital there is others way to do it ...
so exept if you cut from tape , the best place for this machine is out of the rack !!

[opcode66]

There is a trimmer labeled P12 (R53). It is a 5K linear. It sits just before R54 which is a 10k resistor. P12 sits between IC 5 and IC 9 which are N5556 op-amps. P12 is the Gain Trimmer. I think that is what you want to adjust.

[jjgolden]

Thanks for the input guys.

Thanks Opcode, that's more what I was looking for.
In the end, I'm going to send it over to a guy with experience in
calibrating them.

Thanks again,

JJG