Inside the 710 Twin-Finity

The 710 Twin-Finity is an all-new preamp design featuring both tube and solid-state preamps and the ability to blend between the two. The solid-state section is all new and uses the latest technology available, resulting in an audio image with exceptional clarity and transient response. It is very open and true to the source audio. The tube section is all new as well, and is capable of ranging from clean tube tones that just smooth the signal to all-out harmonic saturation.

Like a lot of the UA staff, I am a musician (guitarist) and have a studio of my own. In my studio I am fortunate enough to have a collection of great preamps to work with, each with its own flavor. A couple of years ago, I finally switched from using a console to using discrete mic preamps for tracking. I really love working this way because it gives me so much tonal flexibility. Even when we start with tape we use discrete pres and transfer to DAW. Having a lot of different preamps to choose from is an absolute luxury, and has taken years to achieve, but obviously not everyone is in this position. So to give more users access to this type of work flow, we wanted to create a single box that can cover everything from the “straight wire with gain” paradigm to all-out tube overdrive, and all the flavors in between. The Blend knob, combined with Gain and Level controls, gives the 710 the ability to cover so much sonic territory.

The basic topology of the 710 is as follows: The mic, line, and DI inputs are coupled to the first stage, a transimpedance amplifier. In the 710 we used WIMA polypropylene caps, instead of the transformer found in some of our other gear, to couple the mic inputs. Transformers and caps block DC (like phantom power) from the amplifier's inputs. This was important given our design goal of having one path of the preamp be as transparent and clean as possible.

The DI path is a little different in that it is single ended, and it goes through a discrete JFET buffer for impedance and level conversion before hitting the first gain stage. Access to the gain settings of this first stage is via the Gain pot. The total range is almost 60 dB.

After this first gain stage is the high-pass filter. It is an active Bessel filter at 74.1 Hz. We chose the Bessel configuration because it has the best group delay specs (keeps the phase linear). Bessels are often used for crossovers because of their linear nature. We placed the high-pass here because it's the first place it can inserted, and the sooner you can get rid of the low freq energy you don’t want, the better. This is also the same point in the circuit the drive meter “reads.” At this point, the audio single is split and drives two different set gain amplifiers. One is solid state, which adds no coloration, and the other is the tube section.

The solid-state section of the 710 was the easier of the two sections to design and build. When you are going for as clean and transparent as possible, there are a lot of tools available to assist you in quantifying exactly what a piece of gear is doing to an audio signal. It comes down to empirical data to see if you are moving in the right direction. The initial incarnation of the solid-state section had slew rates around 15V/uS, frequency response flat through 200 kHz, THD of 0.005% @ 40 dB of gain, and noise specs around -95 dB. This is no small accomplishment, considering the circuit shares a box with a high voltage (300+ VDC) power supply. I say "initial incarnation" because the frequency response had to be lowered slightly in order to get the tube and solid-state sides to have matching phase and frequency response. Still, with razor-flat frequency response from 20 Hz - 100 kHz, +/- 0.2dB, the final incarnation is no slouch. The unit is flat and phase coherent, regardless of the Blend knob's position through the entire audio band.

You can measure things like transient response, THD, noise floors, etc., but creating good tone is part science, part art, completely subjective, and was the major challenge for the tube circuit.

Let me start the tube section by saying I really love driving gear hard and seeing what it has to offer. The very first thing I do with a new piece of gear (well, second thing I do — first thing I do is take it apart to see what is in there :) ) is plug a guitar in and drive it into distortion. This really tells me a lot about the character of the unit. Some things are just not designed to be driven hard and I like to know that up front. Some of the most classic and sought-after gear behaves really nicely when driven hard.

Si in his home studio.

For the 710, a number of different tube-design configurations were tried. I started with a very classic tube configuration with a good dose of negative feedback wrapped around it. Amplifiers with a lot of negative feedback tend to be linear and fairly clean. In the broadest of terms in the audio world, feedback applied to amplifiers means that a portion of the output signal is fed back to the input in some way, reducing overall level. By sending a portion of the output signal back to the input (usually either phase inverted or to the negative input) you are canceling out a portion of what’s happening inside the feedback loop. How much is “cancelled” out depends on the ratio. A device with 100% negative feedback (a buffer) gives you, in theory, an exact copy of the input signal at the output. Tonally it should be the same, but can have better capability, different impedance, etc. By changing the amount of negative feedback applied, you can control gain and can control the character of an amp to some degree. With no negative feedback (a.k.a. running open loop), the output will have all of whatever the amplifier did to it, good and bad. Guitar preamps often have little or no negative feedback, allowing for all the “tube goodness” to make it to the output. Tube designs with extremely low distortion, like hi-fi units, often employ a healthy dose of negative feedback. Negative feedback though is not a bad thing, and is a technique that is used constantly to great effect.

As a side note, positive feedback exists as well. Instead of decreasing the input, it adds to the signal causing it to increase, which increases the output, which increases the input and away you go. Next stop Oscillationville. A cranked Marshall and a Les Paul feeding each other, eBows, and squealing microphones are all examples of positive feedback in action.

We wanted to create a single box that can cover everything from the “straight wire with gain” paradigm to all-out tube overdrive, and all the flavors in between.

After a number of circuit iterations, the 710 ended up with very little negative feedback. It is of course running class A, as are most all tube preamps. The circuit looks a lot more like a section from a guitar preamp then it does a standard mic pre. The gain only increases 3 dB when the feedback resistor is removed. Instead of negative feedback, we placed a pad inside the feedback loop to “soak up” most of the unneeded voltage gain. The pad doesn’t alter the tone as negative feedback can, and all the tube goodness is present in the output. The trick then became setting the operating point so that good tube saturation can be achieved before the solid-state side clips, while still having the headroom needed for clean tube tones. The end result of a lot of bench time and even more time in the studio is that the tube section has incredible tone and texture.

The drive meter is the window into this world. It is set up so that a 1 kHz sine wave has 1.2% THD at 0 VU. This might seem high, but we picked this value by using our ears, not audio measuring equipment or a published recommendation. We tried a bunch of sources and found a spot where the tube was really starting to bloom, dialed in the meter, and then went and measured it to find out what it actually was. We did not want people to be afraid to push it, because it has the nicest tube distortion of any pre I have. People tend to see a meter in the red and back off. Don’t be scared. This is not an A/D converter. Along with tube saturation comes the subtle compression tubes are known for. This becomes really obvious when you drive the tube side and compare it to the solid-state side.

After the tube/solid-state sections comes the blending portion of the preamp. Blending is nothing new, has been around longer then I have, and is used on stages and in studios all the time. But the downside has always been phase alignment. The DAW has made this technique much easier, because you can simply grab multiple tracks and slide them around in time to get a phase coherent set of tracks. Depending on the number of tracks, this can range from mildly tedious to a total pain in the a$$. The 710’s two paths are phase coherent by design, so phase is a non-issue and having everything in one box with just a few knobs to control makes blending a lot more accessible.

The tube/solid-state sections then feed either end of a linear taper pot. The output is taken from the wiper, buffered and sent on to a passive attenuator that is the Level control. The Level control has absolutely no effect on the tone. All of the tone is generated using the Gain pot. Level is just there to allow you to send the correct level to the downstream device. From here the signal is sent to the output stage, which is a very traditional monolithic IC design. It provides great drive capability and very high headroom.

That pretty much sums it up. In parting I just want to say don’t be afraid to drive your 710. It was designed to take it. Use the entire range of the Gain pot. If really clean is what you are after, crank the Level pot and bring the Gain up until it sounds good. If you want dirty, find the drive level you want with the Gain pot, and set your output level with the Level pot. Enjoy.

— Si Moorehead