XLR to RCA Adapters

Al, you're exactly on the right path regarding the performance of input vs. output transformers. Put another way, output transformers require tighter primary-to-secondary power coupling to maintain low output impedance, and the resulting low distortion and low noise figure. A side-effect of this is the necessity of a larger core, and higher leakage reactances between the primary and secondary. Input transformers on the other hand need to transfer very little power, and so can make effective use of Faraday shields and have lower leakage reactances, at the cost of the requirement of needing carefully controlled secondary impedances for good performance. But in both cases, the distribution of these reactances across the two windings can be controlled in the design of the transformer, and frequently an input transformer will work best with its secondary grounded on one side, or the primary with an output transformer.

A huge complicating factor is the fact that the design and performance of "balanced" inputs varies wildly in high-end audio . . . I would divide them into two "worlds", depending on whether the circuit after the balanced input is balanced differential, or conventional unbalanced. Both have myriad potential design issues.

The main issue with balanced-line-to-differential-circuit input stages is that most of them really offer no common-mode rejection at all, that is, a common-mode voltage on the input translates into a common-mode voltage on the output . . . hopefully (but not always) the common-mode voltage gain is less than the differential mode. The result is that any tiny gain or impedance imbalance within the equipment or cables (and in the following equipment, if it's of similar design) will result in the common-mode (noise) voltage becoming differential-mode (signal) voltage. It's also frequently more suceptible to RF interference than an unbalanced input (there are two input paths), and under no circumstances will the circuit work correctly if fed from an unbalanced source. An input transformer can improve things tremendously on all fronts.

The problem with balanced-line-to-unbalanced-circuit input stages is usually that many of the simpler designs have an impedance balance that's maintained by the open-loop gain of the input circuit, and the critical matching of resistors and circuit trace capacitances . . . and since this is never perfect, the CMRR is poor and usually falls as frequency increases. This can be improved by the buffered "instrumentation opamp" topology, but still all of these approaches almost always result in increased noise over an unbalanced input, as a result of the Johnson noise in the resistors forming the differential subtraction. Here again, a high-quality input transformer almost always performs better, especially because input RFI networks aren't required.

When interfacing with source imbalances or an unbalanced output, CMRR is determined by the ratio of the differential input impedance to the common-mode impedance. In the overwhelming majority of both types of input stages, the common-mode impedance is one-quarter that of the differential-mode impedance, making the impedance balance VERY critical, with very little noise rejection from an unbalanced source. There are two ways of dealing with this . . . raise the common-mode impedance, or lower the differential-mode (signal) impedance. Atmasphere advocates the latter with a 600 ohm terminating resistor . . . the obvious disadvantage is that the overwhelming majority of equipment on the market will perform more poorly into the lower impedance load.

Transformers do the opposite, they raise the common-mode impedance . . . which is why they still work well from an unbalanced source. Input transformers will generally have a higher common-mode impedance than output transformers as a result of the lower leakage reactances mentioned above.

Sorry Atmasphere, I didn't mean to imply that CMRR was the raison d'etre for your inclusion of the termination resistor. It usually only shows a handful of dB improvement for the CMRR in practice, sometimes a little more at higher frequencies. A certain REL subwoofer input circuit comes to mind, and this is a single-opamp differntial input stage.

I do think that it's highly inaccurate to characterize line output transformers, as a group, as requiring a substantial or specific load on the secondary to acheive proper transient response. While there have certainly been vast quantities of poor-quality transformers produced over the decades, tolerance of a wide range of secondary load impedances is one of the fundamental differences between output and input transformers. On a modern line output transformer, it is in fact the tight coupling between the windings and their higher inter-winding capacitances that allow the primary to provide damping for the secondary. This of course requires that the primary be driven from a very low impedance source, as indicated on transformer datasheets.

Sonically, I feel that the best input transformers are virtually transparent, with the main audible artifacts a result of huge, measureable improvements in bandwidth control and noise rejection. For output transformers, I think I usually hear do have a very slight discernable signature, but it tends to be mainly at low frequencies, and frequently a rather welcome coloration. But one example of the astounding potential of output transformers can be found in any of the Audio Precision analog-based generators . . . maybe as the patents for this famous circuit near expiration we might see something similar in an audio product that we actually listen to.

All of the line output transformers I have ever seen need some kind of load, but its not so much for transient response as for flat frequency response.

Proper transient response and flat frequency response of course go hand in hand, especially in the HF/ultrasonic region, where a poor quality transformer tends to resonate.

But for a bit of perspective, I pulled out a John Hardy M-2 mic preamp and ran a couple of sweeps. This uses a Jensen JT-11-BMCF output transformer, and its response deviation between 150, 600, and 100K loads were all within +/- 0.1 dB of each other from 20Hz to 50KHz. Measuring the AP System One itself (Bruce Hofer's patented transformer output circuit) delivers virtually indistinguishable performance into all three loads (less than +/- 0.01 dB of each other) from 20Hz to 200KHz.

Pretty good for a bit of iron, eh?

It is indeed! Did you get a chance to check the bandwidth at 10Hz and 5 Hz also?

I just ran an existing scripted procedure on the M-2, and it started at 20Hz. The Gen-Mon results I did manually, but I neglected to change the LF end of the sweep. And the System One only goes down to 10Hz . . . for subsonic results I have to use a function generator, then measure with a 'scope . . . big pain. THD results are even tougher; I need to use a DSO sync'd to the generator to acquire the waveform, average 64x to reduce noise, perform an FFT with a rectangular window sized to the exact generator frequency, then calculate the THD from the sum of the harmonics. Ugh.

Jensen's datasheets are probably more accurate at subsonic frequencies than my measurements . . . Deane Jensen was actually a pioneer in many of the techniques used to acheive accurate results in this region.

Hi Talk2me . . . Al managed to find much more information about your amp than I did, and his advice is spot-on as always. If the Burmester amp indeed only has a balanced input with a 2K input impedance, then there's a chance it's only a 1K impedance with their adapter, depending on the design of the input stage. It'd be a good idea to get Burmester's recommendation, and verify with the preamp manufacturer that it's comfortable driving a 1K-2K load.

Another transformer manufacturer that deserves recommendation is Lundahl, and the owner Per Lundahl has been very responsive to technical inquires in years past. They manufacture some input transformers that can be configured in many different ways . . . some configurations could work well for a small step-down ratio to a lower-impedance input such as the one on your amp, and present a less demanding load to the preamp. I think his offerings in pre-packaged plug-in solutions may be a bit more limited, but definately worth checking into.