Manley Labs STEREO EQ owner manual Line Drivers and Interfacing

Line Drivers and Interfacing

Most balanced line drivers are one of a few variations of cross- coupled mixed feedback circuits. You know, the ones where you can plug in balanced, or unbalanced pin 2 or pin 3 grounded gear and are supposed to allow fool-proof transformer-like interfacing. Two problems - most use positive feedback at just under oscillation levels which may mis-behave with reactive loads (like long cables). The second problem is that this type of output stage generally reacts to differences in the input's impedance balance and changes the source impedance balance, which can mess up common mode rejection. In fact, even if the next device may have a great common mode rejection ratio spec like 80 dB across most of the audio spectrum, the biggest requirement is that the source impedance also be balanced or equal on both legs. For example, if the source impedance of one leg is 100 ohms and the other is 105 ohms (5% resistors) the common mode rejection could end up being 30 dB. But cross-coupled outputs can amplify an impedance imbalance and make a bad thing worse.

The Mini takes a different approach, which uses that I/O switch on the back panel. Output impedance remains constant and near ideally balanced and about 30 ohms in all situations. As balanced outputs, they will reach about +30 dBm, and in fact will drive 50 ohms or headphones all day long.

The I/O switch and line driver circuit came about from a little email challenge from George Massenburg, while discussing the issues and problems of typical cross-coupled output circuits, the desirability of an ideal transformer-like output, but avoiding the transformers. We came up with a number of alternatives and this one used in the Mini Massive was one of the best. The only downside was that it does require the switch, which makes it less than brainless and automatic. But the switch allows yet another benefit, which is the possibilty for true consumer or semi-pro "-10 dBu" interfacing, and this becomes the third position on the toggle. Yet another possible benefit is that the switch may provide the means for a user to reduce headroom and use premature clipping as an effect.

Most gear in most studios has balanced inputs and outputs. The Mini Massive happliy accepts both balanced or unbalanced sources and either XLR or phone plugs can be used - easy, no problem. If the source is consumer level (or semi-pro) "-10 unbalanced" (probably uses RCA phono jacks) then you may need a phono to phone plug adapter, and you should select "-10 dBv" on the back panel to ensure the best signal to noise performance. Most synths and electronic instruments should also be used with the "-10 dBv" setting unless the signal is particularly hot and you hear distortion. In that case try the "+4 unbalanced" or "+4 balanced".

For most situations the default setting will be "+4 balanced" which is that toggle in tthe middle setting. This setting has the best headroom too, because it drives the signal on two legs so there is another 6 dB of drive available, which in this case maxes out at a respectable +30 dBm.

A small percentage of gear has unbalanced inputs and some mastering studios are wired unbalanced (and some even remove input transformers or extra circuitry used for balancing, towards a goal of minimalist electronic path). For these rare situations the "+4 unbalanced" setting should prove ideal. So if the default "when in doubt" setting for the toggle is "+4 balanced" and you patch an unbalanced piece of gear after the Mini, then what happens is that the basic level seems 6 dB too low. Then swith to "+4 unbalanced".

And then there is the transformer output option, which remains the most forgiving and automatic method to interface given that the ouput is floating and either leg can be grounded without a loss of headroom and with less groundloop difficulties. Technicalities aside, the transformer also provides a bit of fashionable transformer warmth and richness or fatness. The switch associated with the transformer option, then allows transformerless direct outputs, the transformer, or a slightly exagerated transformer color labelled vintage because it biases the transformer similarly to vintage British console circuits. 3 sounds, your choice.

We may be to blame for suggesting that transformers were the biggest factor for the desirable colors of vintage gear but you should also be aware of potential downsides. Most importantly typical transformers have frequency responces of 5 - 10 Hz to 50-100 kHz and while one might assume that this should be inaudible, in reality, it is accompanied with occasionally audible phase shift which might diminish some tightness and impact or effortless treble extension. Always a trade-off - warm and phat or tight and deep, smooth or incredibly fast. The effects are generally subtle and depend both on the how much exteme lows or highs are present in the sound and how important accurate imaging is for this source. Most people are not super-sensitive to phase and time smear issues and generally report differences in imaging and depth. Actually, in the case of the Mini Massive, simply adding the transformer was almost inaudible because it is a pretty good transformer, and we had to exagerate its characteristics to provide the expected audible difference and colors. It is to be expected that the better the part or circuit the less that it will affect the sound and more transparent it has to be. However, it is generally easy to provide less than optimum drive conditions (if transparent is considered optimum) for a transformer, or put another way, easy to set up conditions where the transformer color becomes subtle or significantly audible. Alternatively we could have designed a less than ideal transformer with particular sonic character, but we chose to use the same transformer that the Massive Passive uses to maintain some of the same flavor and desirability and drive it with similar characteristics, given the fan-base of the Massivo.

Which brings up another possible issue with transformers. Because their characteristics depend on both the source and destination, there may be conditions that cause unexpected results. In the case of an output transformer, what it feeds may change its frequency response slightly. Input transformers are generally much more of a wild card that way, because a transformer is even more dependent on the source, which may be anything - we don't know what you will plug into it. Anyways for a typical line output transformer, we tend to expect, that a heavily resistive load like 600 ohms may reduce both the extreme highs and level slightly. A very long cable may reduce highs. A very capacitive cable may affect the frequency response as if one bell boosted a few dB at 30 or 50 kHz. It happens rarely but about once a year we get a call reporting some unusual symptom which ends up being caused by an unusually long cable or strange piece of gear. This may be expected with any transformer coupled gear. At least with the Mini Massive, it is easily solved by switching to "transformerless".

The I/O toggle can be considered to be before the "transformer option" toggle and does affect the drive to the transformer which means, 95% of the time you'll still want the I/O toggle set to the middle position "+4 balanced" for maximum headroom. The exception would be with "-10 dBv" gear or instruments. However, one doesn't need to worry about the 6 dB loss feeding unbalanced gear with the transformer in use.