R/E/P => R/E/P Archives => Bruno Putzeys (Designer) - Dave Hecht (Master Tech) => Topic started by: volki on February 07, 2011, 03:08:57 PM
Title: Proper O/P attenuation for old Neve modules
Post by: volki on February 07, 2011, 03:08:57 PM
I've got a couple of 1272 like amps with that single ended LO1166 output stage right here... many rackings for those seem to use a passive output attenuator, obviously for the option to overdrive the o/p stage without overloading downstream gear.
Would anyone care to disclose how this is to be set up exactly, for best results (read: original sonics)? Would I have to get into double bridged T business or something for proper loading, or would a double L-Pad comprising of 2 ganged pots work fine? As for pots, I'm not really concerned about CMRR degradation since the cable run to the successive inputs would be just a few metres at best.
Thanks a bunch
Title: Re: Proper O/P attenuation for old Neve modules
Post by: MagnetoSound on February 10, 2011, 08:04:23 AM
Hi Volker,
The proper way to attenuate the 1272 is to wire a fader (typically a 5k or 10k rotary pot) in series with the internal 5k trimmer between the two gain blocks (pins K and L at the rear connector).
Thus, both the load across the output is kept constant and distortion is avoided - that's the 'proper' way.
If you really want to overdrive the output stage, a single 1k pot across the output will work fine. CMRR is not a problem as long as the following device has a floating, balanced input, and loading will remain constant enough if this is of a relatively high 'bridging' impedance, say 10k or so.
Best,
Dan
Title: Re: Proper O/P attenuation for old Neve modules
Post by: volki on February 18, 2011, 12:01:40 PM
Thanks Dan,
I was well aware of the fader option between the two amplifier blocks - I'm including that, but I also wanted to have the option to control both Xformer LF saturation and line driver amp overdrive independent of O/P level. So in the end I did end up with two pots forming a double L pad after the Xformer - that way, the load impedance remains fairly constant at different attenuation settings and thus will not increase distortion due to loading down the O/P. A simple pot between hot & cold would have meant just that - increased distortion at higher attenuation settings due to considerable lowering of the load impedance.
Cheers
Title: Re: Proper O/P attenuation for old Neve modules
Post by: MagnetoSound on February 18, 2011, 12:22:52 PM
volki wrote on Fri, 18 February 2011 17:01
A simple pot between hot & cold would have meant just that - increased distortion at higher attenuation settings due to considerable lowering of the load impedance.
No, I think you misunderstood me. A single pot, wired with it's high and low ends across the O/P presents a constant impedance.
A bridging load (5 or 10 times higher than the pot in value), with the cold side in parallel to the low end of the pot and the hot to the wiper of the pot, presents a marginal variation in load to the O/P and will not upset the amp at all.
Neve sometimes used this arrangement coming off a 1271 when attenuating monitor outputs.
Still, you seem to have found a good solution anyway.
Title: Re: Proper O/P attenuation for old Neve modules
Post by: volki on February 21, 2011, 10:38:16 AM
Ah, you actually meant taking the hot end off the wiper... then it makes sense. I was just thinking in balanced terms and thus only had symmetrical attenuation schemes in mind.
Well, as it happened, I had double 2k2 pots lying about here anyway, so I was able to make double L pads instead of single ones...