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“If you’ve ever popped the lid off our original P1 half rack channel strip then you’ll know it was already a very busy place inside. So deciding to update the design extensively with intent to add a whole bunch of new features was a decision taken with some trepidation … but we do like a challenge. First up was to revisit the mic amp design.

Many of the traditional Class A mic amp designs use input transformers coupled to single discrete op amps. This combination yields typical maximum gains of between 60 and 65dB, maximum input headroom of between +9 to +12dBu (so they usually offer a switchable 20dB pad to cope with hot inputs) and a useable frequency response of around 30kHz at maximum mic amp gains.

With many studios now installing A/D converters offering sample rates up to 192kHz, many recording set-ups now have achievable audio bandwidths up in the range of 50kHz to 70kHz. Taking account of the never ending debate about the importance of ‘out of audio band’ frequency response, we decided to design a mic amp capable of at least 50kHz ‘analogue’ bandwidth even at maximum mic gain. As our reputation is for low colouration products, we also decided to work with a transformerless front end. You can always add colour later, but it’s not so easy to take it away!

The design approach we settled on combines a Class A discrete transistor front end, coupled to two further stages of IC based amplification, with two current feedback circuits providing the hi-gain/wide bandwidth performance we wanted to achieve. There are a number of advantages to this approach;

The discrete front end stage is not attempting to provide all the mic gain on its own. This extends the available audio bandwidth at high mic gains and greatly reduces mic amp distortion especially at high mic gains.

Mic gains up and beyond 70dB are easily achievable. We could have gone further but working with 70dB of gain on a single gain control knob is quite enough, and we have another 21dB of gain available on the Toolbox’s line output stage should you need to offer high levels into your system’s A/D converters.

With the first mic amp stage working in Class A, we should be able to achieve a very low distortion performance with no measurable crossover distortion at the mic amp output.

So how did it work out in practice?

We’re measuring frequency responses of -0.5dB at 63kHz with the mic amp working at its maximum gain of 70dB, extending to -0.5dB at 150kHz at unity gain.

Distortion when measured at the output of the mic amp is less than 0.003% with no measurable crossover component and we’re seeing noise figures measured at an EIN of 128dB.

With no input transformer to saturate, maximum input level is in excess of +20dBu so there is no need to use a pad when connecting to very high output level microphones.

This multistage mic amp design also provides very convenient access points for balanced line level and hi-Z unbalanced inputs, with the ability to set the gain for these inputs within the range desired. For the tracking toolbox we have set the three gain ranges at;

Balanced Microphone input : -5dB to +70dB
Balanced Line input : -10dB to +20dB
Unbalanced hi-Z Instrument input : +4dB to 35dB

which we think matches typical user requirements very well.

The pre-amp can be accessed as an impedance balanced output, via the insert send jack, for routing directly to your recording device. So completely independent from the dynamics section, when compression during tracking is not required. (A switchable high pass filter operating at 80Hz, and a slope of 18dB/octave, is provided.) This arrangement also allows the dynamics section to be used completely independently if required.

Source Switching

We received plenty of feedback from folks who loved the P1 but didn’t want to spend so much time visiting the rear of the unit to switch sources (means plugging and unplugging). So we took the hint and (somehow) found space on the front panel for the Hi-Z instrument input and a very neat little mic/line selection switch. Source switching is now done internally using sealed gold-alloy contact relays and we’ve moved over to 100% Neutrik audio connectors.

The compressor

I designed peakride compression nearly ten years ago and it’s received a lot of critical acclaim for it’s ability to track dynamic sources very well, especially vocals. The design objective was, from the beginning, to provide effective control of dynamics with minimal pumping whilst retaining the vitality of the performance. It also had to be easy to set up!

The final design arrived at uses a VCA compressor controlled by three linked side-chains. A lot of experimentation was done with a variety of sources, especially vocals, but also with acoustic piano and bass. Focusing in on vocal compression, we configured the three side chains as follows;

Side-chain 1 : a fast attack, fast release compressor which catches these over excited vocal phrases but recovers very quickly.

Side-chain 2 : a medium attack, medium release compressor which smoothes out the release response of side chain one.

Side-chain 3 : a slow attack, slow release compressor (but not too slow!) to provide very low distortion on sustained vocal phrases which often cause problems due to ‘ripple’ of the vocal modulating the control chain.

It’s a critical aspect of the design that the three side-chains have different ratios and knee characteristics which are mixed together in varying levels of contribution to work in harmony providing a ‘peakride’ compressor response.

The result is a variable (slightly) soft knee adaptive compressor which provides the following key advantages;

Fast short attack transients are well controlled without over compression.
A programme related auto variable release time was possible making set up of the compressor much faster and simpler.
‘Ripple’ audio distortion has been almost totally eliminated without very long release times being necessary (slowest release time is only 500ms).

The limiter

The original P1 limiter was an attempt to design the perfect audio limiter (must have been a long winter that year!) The design did achieve true ‘look-ahead’ detection of clipping which controlled a dynamically variable threshold, so that, what was in essence quite a slow limiter, could prevent any audio excursion above the preset audio limiter threshold. It was incredibly complex but on the whole it worked extremely well. It’s main weaknesses were a tendency to slightly over limit on certain types of material, and it was not really suitable for using as effect when driven hard intentionally.

The limiter design used in the Tracking Toolbox retains a very small amount of predictive capability but this is coupled with dynamic control of attack time. This limiter design has the capability for short burst attack times as fast as 100us, but only when required to catch very fast transients. Most of the time it works with a ‘natural’ attack time of around 1ms which produces much lower distortion especially with complex low frequency sources such as synth bass.

You’ll find variants of this design in both compression and limiter applications within our other products.

The expander

The P1 expander was designed as an easy to use one-knob expander for a wide variety of uses and could turn its hand to drum gating when required. However the feedback we got from users of the P1 was that the expander was being used primarily by voice over artists, presumably working in less than ideal acoustic spaces with PC fans and the like causing all sorts of recording challenges. They complained (very politely) that the P1 expander operated too abruptly for the spoken work and that the ratio was too high.

The new expander design in the Tracking Toolbox goes a long way to solving these issues. I have lowered the ratio from 1:3 to 1:2 and have redesigned the auto hold and auto release circuitry so there is little tendency for the expander to kick in mid phrase.

Maybe it’s not so flexible as a general purpose gate but we wanted to react to the user feedback and that’s what we’ve done.

Monitoring and Main Output

We have lots of P1 users who record straight ‘into the box’ and don’t have separate monitor mixers, so the new Tracking Toolbox retains the stereo mix section and integral headphone amp with the ability to level blend your tracking source into the monitor mix.

We’ve also taken the opportunity to beef up the line output stage so it can now deliver a massive +27dBu into 600ohms, and we’ve added a ground lift switch which can help to take care of ground loop problems which occasionally arise.

And it’s still 100% analogue!

In bringing so many audio features into the half rack format, it is sometimes tempting to take the same path as many other audio designers and use an external ‘wall wart’ or ‘in-line’ mains adapter. They have many advantages; a large amount of space is saved within the case, they can be very power efficient, they have less tendency to suffer from ground loop problems, and …the list goes on

But the same temptation can lead you to source a low cost ‘off the shelf’ switched mode PSU which provides the required dc supply voltages but with the addition of large amounts of high frequency crap, typically in the region of 150kHz. This is very difficult to fully filter out within the audio unit, and all I can say is that the majority of these ‘stand-a-alone’ adapters which I’ve had ‘on the bench’ should be no where near a quality audio unit. Unfortunately many are!

So although it takes quite a bit of real estate, I’ve stuck to a traditional fully linear and integral mains PSU within the Tracking Toolbox. That means a very high quality toroidal mains transformer (manufactured by Nuvotem Talema for those who want to Google) and they are quite simply superb. Incredibly low radiated hum; small footprint for the power provided, and they’re blue! This feeds traditional linear regulators for the audio dual supply rails and, via an additional regulator, for the microphone phantom power.

As is often the case, this ‘short’ blog has rambled on a bit, but I hope it has offered a useful insight into my personal design journey from P1 to Tracking Toolbox. As always, I’m happy to hear your views and comments."

Robert