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!

 

All about the BA662 chip

Overview  The BA662 is a custom made DC controlled variable transconductance amplifier (or OTA) that Roland had manufactured by Rohm from the late 1970’s and is now obsolete. There are two types of chip with different suffices; the A version has been selected for lower offset than the B version. They are further classified by gain/gm. The BA662B can only replace another BA662B, whilst an A can be used in either application.

To quote Roland from the 100M Service Manual: “Color has great importance in circuits of some models”. However there is nothing mystical about the sound of the BA662, it is just a reasonable good OTA chip which is often carefully matched and selected.

Factory Selection The BA662 was used in places where close matching of the gain (gm) of the chip is important. Roland measured each BA662 and categorised them into 9 grades, ranging from 1 (low gm) to 9 (high gm) . They colour coded each chip with a small dot of paint on the top of the chip, the codes are:

  1. Brown
  2. Red
  3. Orange
  4. Yellow
  5. Green
  6. Blue
  7. Dark green or violet
  8. Grey or black
  9. White

Roland required exact matching of colour dot in some applications (e.g. portamento in the Jupiter 4), and within +/2 colours for others. This need for measurement and matching was obviously costly and led to the BA662 being replaced by precision OTA chips like the IR3109, which were also smaller. The BA662 lived on in some Boss pedals but was ultimately retired.

BA6110  The more recent Rohm BA6110 OTA chip is similar to the BA662, albeit with a slightly different pin out, and it too is now obsolete. It is 3-7dB louder in the same circuit, has low offset (3mV) and low distortion (below 0.2% typically) when the linearizing diodes are used.

We use the BA6110 in many AMSynths circuits (often matched), as they are still available, work well and are low cost. Other designers have used the LM13700 or even dual transistor based clones of the BA662 (which are a lot more expensive than using the BA6110!).

BA662F The Roland 80017A chip array in the Juno 106 uses the IR3109 in SMD and 2x BA662F 14-pin SMD chips. The BA662F version has the same OTA and buffer as the SIP version but in a surface mount body with the following pin out:

  1. OTA Control Input
  2. No Connection
  3. Negative OTA Signal Input
  4. Positive OTA Signal Input
  5. No Connection
  6. Buffer Control Input
  7. Buffer Control Input
  8.  Negative Power
  9. OTA Output
  10.  No Connection
  11.  Buffer Input
  12.  Buffer Output
  13.  No Connection
  14.  Positive Power

BA662F Adapter PCB

BA662F Adapter PCB I have designer an adapter PCB which enables a BA66F SMD chip to be used as a BA662 SIP replacement. The PCB includes an SMD offset trimmer to enable the output offset to be trimmed. Typically we want this to be below 100uV.

The PCB is Open Source and shared on OSH Park here. The trimmer is a Bourns 3314G 50K and the SMD resistor is 1M 0805. The correct turned pin SIL headers can be bought from Rapid Online here.

Copyright AMSynths 2019