Notes from Kevin about his Exodus amp


I'm aghast at how the listening trial came out. It is so far from the results of our installed user base that I immediately started questioning the results.

There is an absolutely HUGE installed user base of DIYers using the Hypex amplifiers. The amp you used was nothing more than a stock UCD-400AD built per the Hypex specifications. The almost universal feedback we get from these amplifiers, not to mention our personal experience using them at CES, building units for studios and using them ourselves, told us that something was not right.

My first thought was damage in shipping but then I remembered our phone conversation about cables and it hit me like a ton of bricks. I realized that you didn't use our single-ended to balanced adaptor cables during the evaluation. There is no magic in our cables. They are a simple twisted pair design with a shield. What is critical is how you wire the input stage of a balanced amplifier. When you failed to use our adaptor cables (as per our instructions sent... shame on you!) for the evaluation you inadvertently introduced noise at the input of the amplifier. I'm unsure of exactly how much but my guess would be the neighborhood of 10db-20db. I'll have the amplifier in question sent back here and do some quick noise measurements with Praxis to illustrate my point.

Let me explain a little about the issues of adapting a singled ended-source to a balanced load.

The Hypex amps have true balanced input stages. To use them with single-ended sources you have to tie the inverting input and ground reference of the amplifier together to match the source ground reference. Normally, you tie pins 1 & 3 of a typical XLR connector together to achieve this. There are a couple of ways of doing that for single-ended sources. I'll leave out the option of using transformers (a great method) for brevity.

The preferred method of converting a single-ended source is to use an adaptor cable that ties the inverting input & the ground reference together on the SOURCE end of the cable. In this manner you keep ground currents that may travel on the signal ground reference isolated to the source side of the connection.

The second and less desirable option is to tie the analog ground reference and the inverting input together just inside the RCA inputs on the amplifier. The reason this is less desirable has to do with grounding differences created in ALL unbalanced audio systems. Currents DO travel through ground references and the less we have flowing through the analog signal reference the better. Most OEMs do this with a switch on the rear panel that allows the user to select between balanced or single-ended inputs. The switch effectively ties the ground reference to the inverting input when switched for single-ended inputs. Our amplifiers do not use such a switch. We provide XLR shorting plugs which short those pins if customers require the less desirable conversion method.

With all of our demo amplifiers we build adaptor cables that utilize the first method of conversion. We do this because we want to minimize the chance that someone will have a problem with background noise due to grounding issues and we want to show off our amplifiers in the best possible light.

In the final analysis when you used your single-ended cables directly into the RCA inputs without the proper XLR shorting plugs you wired the input of the amplifier incorrectly. The result is very predicabe... noise and hence the poor performance in your listening trials.

I'll get you the Praxis measurements once I get the amplifier back.

Best regards,

Kevin Haskins



Sorry about that. Had I known that using a standard RCA (unbalanced) input was going to cause a grounding problem then I would have certainly sought a solution. As a mentioned before, and to be fair, I had to use the same cables for all of the amps. It sounds like XLR shorting plugs or a switch on the back panel when using RCA's will need to be included with each amp.

I look forward to re-evaluating this amp at a later date.



GR Research Inc.
910 North Jackson
Iowa Park, TX 76367