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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Installed Sound/Contracting => Topic started by: Tom Young on April 17, 2011, 08:41:36 am

Title: Speaker volume control changes frequency response
Post by: Tom Young on April 17, 2011, 08:41:36 am
Several years ago I bought a handful of Dayton Audio passive speaker volume controls with small, tapped transformers (these are "impedance matching" and not a simple resistive element, aka: L-pad) and ended up not needing them. So they got stored.

Recently I attempted to put 2 of these into service and even when full up there is a noticeable bump in HF response on the speakers these are in line with. As the control is rotated CCW the volume changes (not very smoothly) and so does the frequency response.

These have jumpers that are used to match the speaker load to the desired/needed load at the amp. The directions leave a bit to be desired as far as clarity so I called tech support and sort of got my head around how these should be jumpered. But the change in frequency response and irratic volume level changes remain.

In the product description they say "Creatively combining resistive and reactive characteristics into the design, the hand wound autoformers incorporate far more turns than any other manufacturer."

Here is a link to the currently offered device (no difference other than manufacturer name):
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=302-205

Anyone have experience with and insights into these ?

BTW - I tried several so it does not appear to be a malfunctiong device.
Title: Re: Speaker volume control changes frequency response
Post by: Ivan Beaver on April 17, 2011, 10:11:44 am
Maybe they have incorporated a "loudness" control into them-like in home stereos.

As you turn it down you get more and more highs and lows-but once you get past halfway (give or take) the loudness circuit is no longer affectling the freq response.

I have never seen that in an autoformer before.
Title: Re: Speaker volume control changes frequency response
Post by: Tom Young on April 17, 2011, 10:51:39 am
Maybe they have incorporated a "loudness" control into them-like in home stereos.

As you turn it down you get more and more highs and lows-but once you get past halfway (give or take) the loudness circuit is no longer affectling the freq response.

I have never seen that in an autoformer before.

Nope, I don't think so. It would say so. Plus the audible change in frequency response happens at full volume (0 attenuation).

I'm going to hook it up to a speaker (here at home) and make some impedance measurements. Maybe measure the transfer function.
Title: Re: Speaker volume control changes frequency response
Post by: Hal Bissinger/COMSYSTEC on April 17, 2011, 03:01:51 pm
I'd also like to see what happens with a resistive 8 ohm load and a sweep. I never liked those things.

-Hal
Title: Re: Speaker volume control changes frequency response
Post by: Tom Young on April 21, 2011, 08:13:41 am
Update:

It appears that I may have simply been boneheaded about this.

I have been testing the volume controls here at home and they behave as they should (and very nicely-in most ways). But I am driving them at much lower levels using a small test speaker.

So I may simply be driving them into saturation (non-linearity) when deployed in the actual installed system, where several hundred watts are likely to be present. The volume controls are spec'd for 100W in the specs but say "200W" on the transformers themselves. The only reason I am puzzled (and did not pick up on the power issue earlier) is that, with one set of speakers, I did run them at very low levels and the volume controls still "misbehaved".

Here at home I tested 2 of these and they both behaved the same. I measured Z with a resistive load (as Hal suggested) as well as a test loudspeaker. I then measured the transfer function of the loudspeaker at each setting on the stepped volume control, as well as with the volume control bypassed. These all look good.

I'm going to test further to confirm that it is a power-related anomaly.

And in the meantime I'll attempt to use common, everyday 100W L-pads in the installed system. With a fire extinguisher close at hand  ;-)
Title: Re: Speaker volume control changes frequency response
Post by: John Roberts {JR} on April 21, 2011, 10:00:22 am
This is always the fun part... reproducing what we hear on the bench so we can understand what is going on.

 Let us know what you find.

JR