Input gain modification

[hrn]

I have an old Spectrasonics 8-channel mixer built 1968 which I use as preamps. Two channels have gain controls with fixed positions (0, -10dB and -20dB...-15dB too I think).

I'm going to put gain controls on the other channels too since they have some problems with the signal levels of modern mics. Think it's about the Z value on the inputs which I've heard usually were different on older equipment.

Is there something special to think about qualitywise doing this. Are fixed positions to prefer, electrical components etc.

Hans,
www.hagen.nu

[gyraf]

Hi Hans,

That question cannot really be answered without seeing the schematics for both the original and the modified channel. If possible, scan them and post them here..

Jakob E.

[hrn]

Thanks Jakob,
I don't have any schematics available and really don't know where to find them. It's quite rare stuff and I did not find anything on Internet.

Guess I have to take a look at the fixed ones (it was 0dB, -10dB and -20dB by the way) and try to do it the same way...


Is there any other "safe" and high quality way to do it do you think? Possibly I could do an external "gain box" or "Z box" before going into the Spectrasonics. What do you think?

Hans,
www.hagen.nu

[Tim Farrant]

Copying what is already there would seem to be the best solution if you do not have the circuit diagram. You may also wish to consider an external pad if overload is a problem.

Tim

[hrn]

Tim,
Are there any usual ways of making an external pad. Isn't it just some resistors or one adjustable? Or does it need more components?
Hans

[gyraf]

There's several ways of making an external pad. The simplest is just two restisors in series with the +/- signal lines, loaded with a resistor across the two. This gives you fixed, balanced attenuation. Look e.g. at the input of the G9:

http://www.gyraf.dk/gy_pd/g9/g9_sch.gif

The other option is to make an adjustable pad. A bit harder, as the needed pots are not easy to find. Look here for different variable pad/attenuator circuits:

http://www.gyraf.dk/schematics/bridge-attenuator.gif

Jakob Erland

[hrn]

Many thanks Jakob (många tack). I will have a look into this. Think the external pad is the way to go because I've noticed that guitars (mic'ed electrics, even with "clean" sound) sound so sweet with the overload going just straight in with full power (vocals don't) and I don't want to change the magic in the box.

By the way, I did some vocal recordings using the padded channels. -20dB did a great work on a VERY loud singer. The best vocal sound for me so far....

Hans

[hrn]

...And thanks Tim....sorry
Hans

[hrn]

I've heard that Switchcraft and Shure makes small XLR/XLR connectors to hang on to the cable with built in pads. Do you think that there could be any matters to look out for with these. Soundwise primarily of course.

Hans,
www.hagen.nu

[Tim Farrant]

The Shure A15AS switchable pad looks like it would solve your problem. But if the console is supplying phantom power, you will loose a few volts thru the pad which may be a problem for some condensor mics, depending on the current draw.

Good luck
Tim.

[hrn]

Thanks Tim,

When I read the info about it on the Shure site it looks like it passes the phantom power through, or do they consider a few volts lost as "passed".

http://www.shure.com/accessories/a15...blem%20Solvers

Hans,
www.hagen.nu

[Tim Farrant]

I would say they are using low value resistors, so there should not be a problem with too much loss. I'm sure Shure have tested it !!

Cheers
Tim