73P Mic Preamp Design Log October 24, 2023 13:41

In 2018, I published a post about a “Mystery Project” that involved researching, building up, and listening to various circuits from vintage Neve consoles. In the five years since then, that project has evolved into a 500-series mic preamp kit, coming in early 2024. What follows is a preview of the “Design Notes” that will accompany the kit.

Picking a Circuit

Our intention with this project was to recreate the mic preamp from the 1073 preamp/EQ module as authentically as possible. (Neve actually had their own module with just the mic preamp called the 1290. But since 1073 is the model most people are familiar with, we’ll call it that.) In surveying the current field of 1073-esque preamps, we noticed two distinct approaches; we’ll call these the “three-stage” and “two-stage” designs.

The original 1073 (and 1290) used three stages. This approach uses two high-gain preamp stages (yes, I know it’s confusing for a preamp to contain preamps, but that’s what they’re called), and one low-gain output stage. Neve split the gain between stages this way to keep distortion consistently low at all gain settings. Most modern clones are three-stage preamps.

The two-stage approach has only one preamp stage, followed by the output stage. This is based on yet another Neve module, the 1272 line amplifier. Around the 90’s people started racking these modules and adding gain switches to convert them into mic preamps. Since then, several companies have made two-stage preamps, often named after the 1073 even though they are closer to the 1272. These can still sound great, and some are highly regarded.

The two-stage approach is appealing because it reduces cost and complexity. This is especially true for a 500-series module and DIY kit. However, after researching the circuit and evaluating a vintage 1272, we decided against it. In addition to being more faithful to the 1073, the three-stage design allows for more gain and better performance.

The preamplifier circuit, of which there are two in the 1073 and one in the 1272.

This is clear if we look at the original modules. The 1073 provided up to 80dB of gain, the 1272 only 50dB. From 0-50dB the preamps are identical, but from 55-80dB, the 1073 engages the third amplifier stage. Modern, two-stage preamps achieve more than 50dB by increasing the gain of each stage. This pushes the amplifiers beyond what they were asked to do in the original designs. This isn’t necessarily a bad thing, but it’s not how Neve designed their mic preamps. It’s telling that Neve never used the 1272 as a mic preamp, and instead went to the expense of the third stage. For this reason we decided that the three-stage design was the clear choice.

Adapting to 500-Series

The first challenge was how to fit the 1290’s many parts, including two big transformers and one giant transistor and heatsink. However, this was a straightforward problem that just required some grunt work. We also aimed to replicate the grounding scheme of the original. Neve managed ground currents carefully between amplifier stages to keep noise low. We recreated this on the 500-series module by giving each stage its own ground pour, which is tied to the others via a bus wire which runs along the bottom of the module back to the card-edge connector.

Our most recent prototype, with everything fitting in a single 500-series slot. We have at least one more round of prototyping to go, with an improved layout and the TIP3055 transistor.

The more complex problem was how to handle the power supply. The 80-series consoles ran on a unipolar 25v power supply. The 500-series on the other hand uses a bipolar +/-16v supply. Therefore, some form of conversion needs to happen to power the 1290 correctly from the 500-series rack.

We considered several options. First, we looked into adjusting the 1290 circuit to run on +/-16v. A bipolar +/-16v supply can also be used as a unipolar 32v supply by treating -16v as ground. In this scheme, some resistor values would just have to be adjusted to make the transistors bias correctly. This has the benefit that no regulation or DC-DC conversion is required, and some high-end 1290-style preamps have used this approach. However, we quickly realized that scaling the bias resistors would also require scaling gain setting and feedback resistors, which would in turn require scaling capacitors, and so on until the circuit barely resembled the original.

The next approach was to regulate both power rails down from +/-16v to +/-12v, thereby creating 24v between the two rails. This is the most common route taken by existing clones. It has the drawback of requiring two regulators or an integrated DC-DC converter, but it does not require any changes to the 1290 circuit.

We were set on this path when we realized that we could also achieve 24v with a single regulator, by using -16v as ground and regulating +16v down to +8v. This freed up some precious PCB space and eliminated a potential failure point. So we decided on this scheme, but with an important twist: we “deregulated” the +8v rail to get even closer to the original.

The Sound of the Power Supply

An overlooked aspect of the original console is the design and performance of the power supply. Nothing in analog audio is truly “outside the signal path.” Everything in a piece of gear from the chassis to the power supply affects the sound. The power supply affects audio in two ways: by being quiet or noisy, and by being “stiff” or “saggy.” The first is straightforward—we want to keep noise as low as possible. The second describes the supply’s ability to deliver current while keeping the voltage stable. A “stiff” power supply never drops below its rated voltage. A “saggy” power supply will momentarily drop below the rated voltage when faced with a big spike in the demand for current. These spikes come from especially loud, low-end-heavy sources and increase with gain and level. The sag creates distortion and compression which are part of the sound of the preamp.

Modern regulator chips and DC-DC converters are invariably as stiff as possible. So in order to recreate the sag of the 80-series power supply, we simply used the same regulator technology Neve used. Our +8v rail is generated utilizing a zener shunt regulator with a large pass transistor. Think of it this way: if you had a vintage 80-series console, would you replace the power supply with a modern one?

Selecting Parts

Transformers were the most straightforward component to choose. Carnhill transformers are the de facto standard for anything vintage Neve. The blue input and red output transformers have even become visually iconic in their own right. We tested some options from OEP, Sowter, and Ed Anderson, but found no reason to deviate from the Carnhills.

All of the transistors we auditioned for the 73P.

Transistors do the work of amplifying the signal. The 1290 card uses two types: eight small-signal transistors, and one power transistor to drive the output transformer.

Our process for choosing them was to compare several currently-available options to a vintage set taken from an 80-series console. These were BC109C for the small-signal, and a Newmarket 2N3055 for the output.

Our takeaway was that transistors are important to the sound, but only when they are saturating. That is, when the preamp was delivering a clean signal with no noticeable saturation, we heard little-to-no difference between the transistors. But once they started to clip, audible differences did emerge.

However, even in saturation the differences are not night-and-day. We’re talking about the kind of differences that can only be positively identified with good monitoring and careful flipping back and forth between samples. This is confirmed by Neve’s own approach. When they were making the 80-series consoles, Neve used various small-signal transistors, including the BC107C, BC109C, BC184L, BC184C, BC114 and various brands of 2N3055. Additionally, manufacturing tolerances were much looser in those days, so batches of the same transistor might be as different as one transistor was to another. The takeaway, however, is not that every 80-series module sounded significantly different. Rather, it’s that the circuit is designed to be highly forgiving so that almost any transistor can drop in and sound basically right.

Many of the transistors Neve used are still in production, including the BC107/BC109, BC184, and 2N3055. However, manufacturing processes have changed so much since then that there’s no guarantee that these would sound anything like their vintage counterparts. So we also tested several other transistors that have been used in this circuit, including the BC550, BC548, 2N4401, MPSA06, MPSA18 for small-signal and the TIP3055 for output. We spent several days exclusively listening to transistors on various samples and at various gain levels deciding on the BC109C and TIP3055.

Capacitors often get a bad rap as something to avoid in audio circuits (have you ever seen “capacitorless signal path” in ad copy?), but they are key to the sound of this circuit. The 1290 uses three types: polystyrene, tantalum, and aluminum electrolytic.

We know from our listening tests for previous designs that polystyrenes and tantalums have their own sounds which cannot be achieved with more modern options. As such, we didn’t consider replacing them.

Electrolytic capacitors required a deeper dive. In our experience, different manufacturers, series, and even voltage ratings of the same value electrolytic capacitor can sound different. So we wanted to make sure to do our due diligence on this front.

The main difficulty lay in finding a gold standard to measure against. Electrolytic capacitors have a shorter shelf life than polystyrene and tantalum, so any that could be pulled out of an 80-series console would have long gone bad or at least changed their performance too much to be a gold standard. Additionally, any consoles still in use have been recapped (had the electrolytic capacitors replaced) several times. This further complicates the search for a gold standard. That is, should the reference point the caps that “Rumours” passed through in 1977 or the ones that recorded “Nevermind” in 1991?

So the best we could do is to trust the techs who have been recapping these consoles for years. They generally recommend the BC 031 series for both sound and form-factor. So we compared a set of 031s against our favorite Panasonic FR series on the same BA283 card. The result was that we found the FR series to be practically indistinguishable from the 031. Based on this, we opted for the FR series as the radial (vertical) mounting style fits better in the 500-series space constraints.

Coming Soon: The 73P Preamp Kit

As you can tell by this point, we’ve come a long way with this project and are just putting on the finishing touches. We plan to release the 73P Mic Preamp in early 2024. The exact details, including pricing and launch date will be announced this winter. Please subscribe to our newsletter via the footer below to find out immediately when the kit is ready to order. And thank you for following along!