A&H Spectrum Mixer Refurb 4

Overview During the summer of 2020 not much progress was made. This was down to the fact that whenever I turned the Spectrum on, it was noisy and there was no getting away from how noisy it was. Whilst I had bought effectively another Spectrum as spares and started to refurbish some cards, I had to sort the noise out! Looking at the noise floor in the original A&H Spectrum brochure and a new ZED428, I knew I had a hard job on my hands and I nearly gave up and bought a ZED428.

I decided to buy a second hand Allen & Heath RPS14 power supply to see if we can get to a lower noise floor. It will also provide lots more current for the upgraded op amps and maybe an extra 8 channels. The RPS14 eventually arrived and what a beast it is! A massive and heavy toroidal transformer and very nice voltage regulation with large capacitors. Time to make a custom cable to power the Spectrum up and see whether a modern power supply can get the Spectrum back in the game.

 

A&H Spectrum Mixer Refurb 3

Second Master/Group Section

Overview During March and April 2020 I had more time to complete the refurbishment and was lucky to find a set of two 8 channel Spectrum input channels with panels and frame on eBay. This enables me to refurbish a full set of channels outside the mixer, as well as having a full set of spare pot and switch buttons. The panels were more rusted than my original, so I am only transferring the channel cards in.

In October I located a Master/Group section (made 30 April 1991) with rather nice wood ends, some different coloured pot caps and a rusty front panel. This enables me to restore a set of master and group cards whilst keeping the working Spectrum in the Studio. A previous owner had started to modernise the Left and Right master channels with nice Nichicon caps, OPA2134’s and unusual Roederstein EKU Bakelite Bipolar Caps. Unfortunately cheap IC sockets were used and the Op Amps move about in them, so these will be replaced with turned pin versions.

Master Section Refurb The two master channels have the highest level of noise in the mixer even with the groups and channels off. The L and R signals are buffered by a 5532 Op Amps and then it is TL072’s downstream from there on FX Sends. The 5532 buffer was replaced with bi-polar OPA1612 Op Amps and the FX return TL072 with OPA1642. Both the new chips are SMD and are fitted with PCB adapters and 100nF SMD power decoupling capacitors.

The OPA1642 was chosen due to its low noise (5nV) and low power consumption at 28% higher than the TL072. It also has low THD and reasonable offset at 1mV. The 5532 is also used in the five balanced output drivers where it is replaced by a new PCB and THAT 1646 chips. The usual electrolytic recap was completed at the same time and the slate oscillator fixed. The ten Op Amp upgrade cost was around £45.

This was the original plan, but the second mixer had already been upgraded with OPA2134 and OPA2604 so I will try with the cards initially on the Left and Right channels.

Original EBOS Board

Master Balanced Outputs The Master section has five balanced outputs that use a separate EBOS PCB with Main Left & Right, Mono and Aux 3 and 4 outputs. It is an important part of the mixer and I want to ensure it has high performance.

The circuit uses NE5532 Op Amps and a trimmer to get the offset correct. Whilst this design was fine in the 1990’s, the rest of the PCB needs updating with new caps, 1% resistors and new cermet trimmers. The PCB is single sided and poor quality, with traces breaking, not at all like the main PCB’s. So rather than restore the old PCB, I have designed a new PCB that uses modern THAT1646 chips which provide an uplift in performance and reliability.

The new PCB has RFI suppression added to each circuit, with 100pF caps and inductors on the outputs. The THAT chips reduce the power consumption from 40mA to 30mA, which isn’t much, but any power reduction is worth having! The slew rate and THD also improve, so we should hear a better sound.

Group Refurb The eight group circuits have a high noise floor and a couple of the groups are not working correctly. The design has five sections:

  • Group bus summer
  • FX Sends for the lower and upper channels
  • EQ for the lower and upper channels

The bus summing is done with NE5532’s which have been replaced with bi-polar OPA1612 Op Amps. The FX and EQ originally used TL072 Op Amps which I have replaced with JFET OPA1642’s which reduced noise, and improve slew and THD. The upgrade cost is £22 per Group, with 10 IC’s replaced per card.

Channel Refurb The channels are a lower priority for upgrading due to the reduced cost/benefit, and the priority is to get all 16 channels working. Each channel has 4x TL072 Op Amps and a NE5532 as the combined Mic/Line pre-amplifier. Replacing all the Op Amps across 16 channels would cost £350, and I have to be careful not to increase power consumption. Upgrading the TAPE inputs to balanced is a priority.

With more channel cards available from buying another Spectrum I can experiment with different op amps.

PC Noise My Spectrum lives above a Windows PC used for audio recording, which puts out a lot of high frequency junk onto the mains which the mixer picks up rather loudly. This has been sorted out, but moving the mixer onto a clean power supply did not make much impact. I have added an external mains EMI filter to the PC along with ferrite rings on the PC and Mixer power cables. I also have an EMI internal filter for the RPS power supply and I will check the mixer frame ground is sound.

 

 

A&H Spectrum Mixer Refurb 1

A&H Spectrum

Overview In December 2019 I acquired a 16:8:16 Allen & Heath Spectrum Mixer as the center piece of my studio. Why would anyone use an analog console dating back to 1991! Well it enables me to use a hybrid recording setup, with my Cubase DAW acting as a digital multi track recorder and the analog desk is used for tracking and mixing out of the box. I gain a hands on workflow with outboard hardware but at the cost of some signal noise.

The Plan I had been looking out for a medium sized 8-bus analog mixer that could be upgraded and therefore used THD components. An Amek would have been fantastic but too big, and outside my price range. The Spectrum (sometimes called Sabre 8) is a well built desk that is smaller than a full sized Sabre with 24 or 32 channels and has only a 3 band EQ, but it has all the important routing I need and the addition of MIDI mute automation. Most of the schematics are available (200 Series) and show basic TL072’s in the channels and buses, with the NE5532 in the pre amp and bus master.

Connections The Spectrum is connected to Cubase Pro (Windows 10 PC) by a Focusrite 18i20 and OctoPre Dynamic, with 8 inputs from the mixers 8 group outs, 2 from the Main Mix and 16 outputs back to the tape inputs, all on balanced cables. There are six AUX busses with five hardware effects connected (PCM70, PCM80, REV7, Alesis Midiverb 4 and SDE2500) and AUX 1 used as a input to my Roland S-50 Sampler. My synths and samplers are connected to the 16 channel line inputs and the 16 Mic XLR inputs that have been converted to line inputs, I can switch between the two line input groups on each channel.

Tape Machines A Tascam DA30 DAT is the main tape machine connected to the Control Room analog outs, and replays into the Tape 1 input and the Focusrite SPDIF digital input. A Yamaha MT4X 4-track cassette tape machine records from the duplicated Bus Outs 1-4 and plays back stereo into the Tape 2 input.

Refurb Plan The Spectrum needs some minor repairs, a clean and refurbishment, although it powers on and works. There is one broken GAIN pot and four broken or missing control knobs. The power supply is rather beaten up and needs a service and clean. I suspect some of the MIDI muting thats uses JFET’s maybe dead. The plan is to strip the mixer and refurb the Master section first, followed by one input channel and one monitor channel, to see if the Op Amp upgrades and recap produces a better sound and noise floor.

The project objectives are:

  • Recap and restore the power supply
  • Change the Tape Inputs to balanced
  • Change the Group Outputs to balanced
  • Upgrade the existing balanced outs (EPOS module)
  • Possibly upgrade the Op Amps in the signal path

Op Amps I have to be careful not to spend too much on the Op Amp upgrades as there are over 130 of them! I can afford to use a high quality bi-polar SOIC Op Amps with adapters to replace the NE5532, but with over 100 TL072’s in the mixer, I need to keep to a THD Op Amp for these. Options include; OPA1642, LME49720, LT1358 and OPA2134. With the use of low offset Op Amps it may be possible to remove some of the 14 electrolytic caps in the signal flow to open up the sound or at least fit bypass caps. The EQ capacitors could be replaced with high quality Wima 2.5% polypropylene, and the TL072’s in the metering circuits will be left alone.

Battered PSU

PSU The power supply is the stock Allen & Heath RPS3 which was also used in the larger Sabre consoles and is rated at +/-16V at 3A. I will replace all the capacitors, IC’s, power transistors and upgrade to toridial transformers. I aim to use larger reservoir capacitors as in later power supplies, to reduce ripple. A new PCB may be needed. I also need to bend the front panel back into shape and clean it. I decided not to replace the PSU with a Blue Dog version as it would give little or no benefit in audio performance, but a cheap RSP11 would be useful.

I will post progress updates as I work my way through this project and share the challenges and how it sounds when completed!

 

Jupiter One

Dusty Voice Card

Introduction  I picked up a Roland Jupiter 4 voice card on eBay in late 2017 in good condition. In June 2018 I started a project to build a complete single Jupiter voice based around this card. The synth will fit into a Moog 60 HP case (another eBay find) with an aluminium front panel that replicates (most of) the controls of the Jupiter 4.

To create a fully working single synthesizer voice means a lot more electronics are required than just the voice card! The complex module controller card has to be replicated along with a LFO. The arpeggiator has been left off the synth as it requires the 8048 micro-controller, which means no programmabilty or presets either. The JP4 modulation capabilities have been retained and enhanced, with a 5-axis thumb joystick and a revised S&H circuit for the VCF modulation.

Voice Card This is an early 1979 D version of the card with a BA662 based low pass filter. The first job was to clean and dry the card and remove the metal guides. This reveals 4 mounting holes for the top of the PCB. The 4 green connectors are replaced with 9 pin SIP headers, so the card can be mounted to a new motherboard PCB. Some of the ceramic capacitors needed replacing and the trimmers will be replaced with high quality cermet trimmers.

A hack to the card to add a potentiometer to adjust the VCA Gain trim has been removed and a trimmer put back on the PCB. The card is spaced 12 mm from the motherboard. I am hoping the voice works, but I am sure there will be some repairs needed. The card has additional power supply caps mounted on the rear, which I have retained.

The card is a Revision D but with Revision E kludges (power conditioning capacitors, 100 and 2200 pF capacitors around the uA726). The FET Op Amps in the card are LF353 rather than TL082, but they exhibit the same low current at 3.6 mA and 13 V/us slew speed, so equivalent performance.

Mock Up

Module Controller The Jupiter 4 has a dedicated 5th card in the card frame to hold all the circuits that translate and mix the programmable CV voltages for the Voice Card. It also contains the CMOS based envelope clock generators. All these circuits have been put on the Motherboard PCB of the Jupiter One which measure 295 mm x 105 mm.

The module control circuits and Noise circuit has been retained, augmented with the S&H from the Jupiter 8 and revised LFO. Op amps have been upgraded to TL072’s and BA6110 OTA chips replace the BA662’s. This PCB is spaced 11 mm from the panel PCB and it contains the +/- 15V power rails, derived from a switched and precision LDO power supply.

Jupiter LFO The LFO in the Jupiter 4 is on the main board and is almost identical to the one used in the Jupiter 8. The JP4 version uses discrete FET switching and a two chip CMOS 2 to 4 address decoder, this is rather component intensive and a simpler approach is needed. I have taken the same approach as Roland did on the Jupiter 8 LFO and used a 4052 analog switch.

4-Way Switch The Jupiter 4 relies on 4-way slide switches which are difficult to source, but we think we have the right one. It may mean making sub PCB’s to raise them up to the right level, but its the simplest electronic solution. The JP4 micro-controller decodes them the 4-way into 2-bit binary signals, using pull up resistors, for use in the Controller and Voice cards, which we can copy for the Pulse Width Modulation, LFO Wave Form and VCF Keyboard Follow.

Panel PCB The front panel PCB is 295 mm x 105 mm and contains 19 slide potentiometers, 4 rotary potentiometers, 2 rotary switches, 11 slide switches, the joystick and 4x jack sockets. The PCB mounts to the Motherboard via a set of 9 pin SIP sockets and headers. It is spaced 11 mm from the front panel.

Joy Stick

Joy Stick The Jupiter One has a small thumb sized 5-axis Joy Stick mounted on the front panel. This controls the Bend (X axis) and LFO Depth (Y axis) with the integrated push switch acting as Gate On. This provides excellent control over 3 of the synths key parameters. The Jupiter 4 Modulation controls (next to the keyboard) have been retained so that the Bend Amount and LFO can be patched into the VCO, VCF and VCA.

Inputs & Outputs The external connections have been kept to a practical minimum to conserve panel space, so this is not a semi modular. There are inputs for CV and Gate, as well as the X and Y CV’s that over ride the joystick if plugged in (Bend and LFO Depth).  The 6.35 mm analog output jack is mounted on the rear of the 60 HP case, along with the 12V AC power supply input.

Missing Controls The lack of micro controller means there is no arpeggiator or presets or portamento. The Trigger Section of the synth therefore has not been included (like the Pro Mars). The transpose switch has been replaced by having a 4 octave range switch.

Commercial Option I may release the Jupiter One as a commercial project using new Jupiter 4 voice cards based on the BA6110 and a BJT expo generator to replace the ua726. DIY and completed synths may also be available. A two voice version is planned (same 60 HP case) with programmable presets and OLED display.

Copyright AMSynths 2019