December 5, 2010

First test: Keith W's tube/horn system

Keith was generous enough to tolerate the mess we made of his normally tidy music setup and to host an enjoyable day. Keith's system consists of Acapella horns, JL audio subs, Cary Valve monoblock amps, an SGR power amp and a tube pre.

No aspect of the system had been skimped on - in fact most of the system was made up of components that many could only drool over. The only compromise was a temporary one - he had a Behringer analogue active crossover while waiting for a Marchand unit to be built.

We had a listen to his system with the Behringer crossover to give us a baseline to compare. It was obvious to us all that it is a very revealing system and very high quality.

Compatibility issues

Unfortunately we hit a snare. All of the digital active crossovers seemed to clip when the volume was turned up. The analogue crossover didn't suffer this problem, but we suspected it was due to incompatibility between the valve preamp and the DSP units. As a result, I'll offer a warning to those with a valve preamp. See if you can borrow a DSP unit before buying or if you have the option to evaluate in your system and return it if you have problems. Otherwise I would not purchase any of these for use with a valve pre. Keith's solution was to work out the crossover with DEQX, then have a custom analogue crossover made up.

Crossover setup

We set up the system as hybrid active using the crossover settings Keith had found to work best. Linkwitz Riley 4th order at 80 and 600 Hz. The passive crossover between the super tweeter and mid horn was left in place, so we effectively tri-amped where valves powered the top horn section, the SGR power amp powered the woofer, then crossed at 80 to the active subs.

Measurement setup

We measured at two metres using the DEQX measurement system to ensure that each pair was level matched and that the filters were doing the same thing. We discovered that there were some differences in the transfer functions.

Behringer DCX vs DCX modded

Cyber Murphy was the owner of the modded DCX and the stock DCX unit was mine. Murphy commented:

" -Modded DCX v Stock DCX: 2 of us thought there was a slight improvement in top end and decay with the modded unit, (and I also thought the modded unit had a quieter noise floor, but it's hard to be impartial with your own unit  ). 3 of us thought there was no difference.."

DCX stock vs modded

You can see they were very closely level matched but with the stock unit there is a dip at the crossover and the bass levels weren't matched.

DCX vs MiniDSP

They were closely level matched, but you can see the stock DCX again has a dip at the crossover. MiniDSP has less top end extension.

Above: DCX stock vs MiniDSP

We had to use digital attenuation to level match so it gave DCX a clear disadvantage. MiniDSP won in this comparison but the test was invalid. We resolved this issue in the follow up tests.

Above: DCX vs DEQX
We didn't get the level matching done very well in this case. We had mixed impressions with this part of the test.

Original thread for the event

Next: Follow up test

In the follow up, we resolved some of the issues and tested the same units in a more compatible system.

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