~ BBC LS5/8 AM8/16 Amplifier modified for lower noise ~

This 1979 BBC report states in a very BBC way ~ Of late, the need has arisen in the BBC for new studio monitoring loudspeakers in two distinct fields of application. One is the balancing of pop and light music, for which high sound levels are required; the other is the balancing of serious music, in which the main requirement is for clean uncoloured sound quality

The British Broadcasting Corporation BBC internal Research and Design Department produced the design which was initially built and tested by the internal Equipment Department using Swisstone Electronics Ltd [aka Rogers] drivers ~ It was a natural progression to get Rogers to make the entire LS5/8 with amplifier

So in the 1980s the British company Rogers [not the 'Roy Rogers' they are today along with LEAK] took on manufacture of the BBC derived low level active crossover fitted in the QUAD 405 amplifier to power the BBC LS5/8 programme monitoring loudspeaker but it appears the original 1979 concept and the final product were a bit different

The BBC report describes their intention behind the design and gives scientific reason for the choice of only 2 drivers with measured responses in the chosen cabinet ~ It has schematics of the circuits chosen to equalise the drivers in the cabinet which are almost unchanged in the Rogers LS5/8 information & Maintenance document ~ Except …

The AM8/16 crossover as built to fit the 405 case has transformer balanced inputs with a 3 pin XLR and a PO316 jack wired in parallel

A switched Bass Boost at 35Hz was added for different cabinets used in outside broadcast vehicles and some other BBC locations

A useful input attenuator was fitted but unlike some other professional studio amplifiers it had no position indicator or a lock

Pictured right one of many variants of the crossover ~ A Rogers built AFL102 with the Bass Boost switch mounted on the PCB

Another variant with the Bass Boost switch mounted to the right of the XLR and wired to the PCB but here removed and bypassed by the BBC for a fixed flat response and to prevent misuse

There were only a few hundred AM8/16 made but many variants of mounting and positioning of the parts but the electrical design remained the same

As you can see and as you know if you own them ~ There is little room to safely fit the crossover in the QUAD 405 case

Monitoring in a control room is not the same as listening to music at home and many people who have 'obtained' an LS5/8 or two ~ especially if they paid far too much on ebay or at a BBC auction ~ are surprised and disappointed with the high level of noise from the High Frequency [ HF or tweeter ] driver when sitting close to the speaker

This higher than desirable tweeter noise is partially due to using a 405 PCB for the HF [ treble ] amplifier with its standard 0.5V for 100W output sensitivity which is limited to 18W output and uses much less in practice ~ You may have read here how I found the noise of the QUAD 405 at 1W made distortion measurement difficult

The HF noise is also due to the design of the crossover but not because it uses discrete transistors ~ The crossover design unchanged from the 1979 BBC report uses complimentary transistor [ Sziklai pair ] emitter followers as unity gain amplifiers to keep the distortion low and the 0dB gain [x1] of each stage does not amplify or add any significant noise

With unity gain filter sections the bass boost gain is provided by the pre–amp section and the low frequency selective boost is achieved in a very interesting very BBC way ~ But it is never required for 'normal' room or most studio listening and as seen on the rear of most AM8/16s it was 'removed' or in practice just bypassed by the BBC

The pre–amp gain [ not specified in the 1979 BBC report ] was increased by about 8dB and for a flat response the gain of the Low Frequency [ LF or Bass ] path is reduced by 8dB with attenuator R14 and R15 which is decoupled via C28 because it also caries the bias from the pre–amp output to the 600Hz dip filter and the low pass [ LP ] output filter

For the bass boost of +5dB R13 R18 provide a fixed attenuation of about 3dB and with R15 R16 to ground via the input impedance of the HP filter Tr4/Tr7 the total is 8dB ~ For +8dB bass boost R14 and R15 once again provide 8dB attenuation but now with only R15 to ground via the input of Tr4/Tr7 which is a high impedance at 35Hz

The AM8/16 pre–amp has a gain of 24dB and for most domestic listening the input needs to be attenuated using the control on the back to keep the source volume control within a reasonable range ~ This can reduce the signal to noise but is not the main cause of the noticeable hiss from the LS5/8 when used with a standard AM8/16

Because the pre–amp gain was made higher to achieve the bass boost the signal to the HF driver high pass filter [HPF] Tr12/Tr13 is 8dB too high but rather than attenuate the input by 8dB the correction was made between the high pass filter output and 405 PCB input using R46 and RV2 as an attenuator with the 405 PCB 20kΩ input impedance

As the high pass filter path is 8dB higher the attenuation with R46 and RV2 should not affect the signal to noise into the 405 PCB but the 405 amplifier noise is already bad with its own series 22kΩ resistor and the extra ≈155kΩ of R46 and mid RV2 in series adds both excess resistor noise and additional noise due to the op–amp input current

Bypass or Remove the Bass Boost ?

Simply by–passing the 35Hz boost switch as done by the BBC Design and Equipment dept does not address the HF amplifier noise ~ So the answer has to be directly connect the pre–amp output to the 600Hz filter input and reduce the pre–amp gain by about 8dB then lower the value of RV2 and R46 used for the HF amplifier gain adjustment

If you wish you can remove the components no longer required as shown pictured and indicated on this schematic this will give you spare rare 2n2484 transistors

C26 is changed to 100nF film and C6 with a 1µF film capacitor ~ The circuit is now closer to the original BBC 1979/22 report and the noise is lower than what was actually built

The BBC Equipment department often fitted small ceramic capacitors for a.c. coupling like C26 in the picture and these should be changed

I have seen many AM8/16 crossovers but never 2 identical builds ~ Most used polystyrene and other poly film capacitors where required and MRS25 or other metal film resistors ~ There is little to be gained fitting esoteric components and little room to fit these often larger parts in the hope that the crossover will sound better or quieter

Although this PCB has 'ROGERS AM8/16 made under licence to the BBC' on the silk screen it looks like a BBC D&ED build or a joint proto–type?

The clue is components like the 1970s Philips MRS25 resistors and those not so nice for audio ceramic capacitors C21 C25 and C26

But then the silver RIFA film capacitors are more Rogers than BBC so maybe an early collaboration ~ Note R41 is hand written

The blue wire from C28 to R15 is the later BBC bypass of the bass boost switch and here R37 has overheated due to failure of C20

Testing a modified AM8/16 crossover with two 20kΩ resistors on 110 spade terminals at the outputs

Audio test gear like the FFT analyser I use has a high input impedance [1MΩ] and the outputs of the AFL102 require the correct load of 20kΩ

Using resistors mounted with long legs on 110 connectors makes connecting test leads and additional scope probes much easier

The original long wiring which allows the PO316 jack to be fitted after the crossover is mounted in the 405 chassis is still in place but these long wires often get burnt on the 405 PCB resistors R30 R31

When I do re-fit the PO316 I use shorter PTFE wire and the body is covered with Kapton tape to avoid short circuits as shown pictured

This is a modified early build AM8/16 which has the old style PO316 jack mounting and the crossover frame only mounted to the rear of the 405 case

Later models had a hole in the 405 MkII side plate to fit an extra screw that kept the crossover frame away from the output transistors but short circuits and burnt wires still occurred

If you have read my other pages about modifying the QUAD 405 amplifier you would imagine that I also modify the PCBs of the AM8/16s I have worked on but this is not always the case as some level of originality needs to be kept

I do change all the capacitors ~ especially C8 for polystyrene 160V 1% and have sometimes applied my move C8 modification but the input op–amp circuit topology is not changed so the amplifier PCBs are kept inverting although modifying and inverting both would not [should not] be a problem

I also change the 560Ω resistors R30 and R31 and mount them further away from the PCB [ and nearer to PO316 wiring ] as shown if they are not the stand–off type

In the picture my C8 move modification has been made with polystyrene capacitor and new electrolytic C10 mounted on the reverse of the board

The PCBs of the AM8/16 are specifically Bass and treble modified ~ they have holes drilled in the sides of the heatsinks to mount the perspex safety cover

The treble PCBs have additional Zener diodes mounted across D1 and D2 to reduce the op–amp supply and effect a voltage or power limit for the HF drive unit

I increase R7 and R8 on the treble PCBs to 4k3Ω to reduce the dissipation in them and the current in the zeners D1 and D2 and mark this on the PCB

As the treble [HF] PCB is already different to the bass [LF] I had considered modifying the input high pass filter [HPF] to reduce the LF gain of this HF PCB ~ This would allow a 'better' non electrolytic capacitor to be used for C2 but also LF feedback around the op–amp ~ I believe Wharfdale did this with 405 PCBs in an experimental active loudspeaker

With the bass boost circuit removed and other mods as described above the noise performance of the LS5/8 is improved and I have never been asked to modify a pair further ~ You could maybe apply my 405 Mod3 where the input impedance is still 20kΩ and the AFl102 crossover outputs are still correctly loaded so do not need changing

I have modified several AM8/16s and BBC QUAD 303 A2s for use in studios and have been asked to keep the BBC fitted Neutrik LNE mains input connectors which are still available from suppliers like Canford Audio and ebay if you want that original BBC sound

I prefer to change the AM8/16 LNE mains input for a safer IEC connector as pictured ~ The PO316 jack is a BBC requirment often not used and often the source of many problems so need not be fitted but a switched phono input can be useful especially for domestic use

The 4mm output sockets are also not needed as the amp connects to the LS5/8 cabinet via a 5 pin DIN so these holes can be used to mount a switch and a Neutrik NYS367 phono

There are extra capacitors inductors and a mains filter fitted to the AM8/16 because like many BBC designed amplifiers it could be used on transmitter sites as well as in studios

For domestic use [ including your home studio ] parts like the mains filter and the capacitors across the output socket can be removed as can the 10µH inductor L1 prior to the gain control RV1

Fitting wires [green/white and yellow] to L1 pads enables the unbalanced phono or the balanced transformer input to be switched to the gain control

The phono body is connected to chassis and 2 ferrite beads are placed on each wire at the L1 pads just in case RF pick–up is a problem

A common problem with an old original AM8/16 nowadays is the mains supply may have a residual current circuit breaker and the leakage of the old RF mains filter trips the power off ~ A quick fix if this happens is to simply cut away the filter close to the LNE connector ~ A better safer solution is fit an IEC mains input

" Got a feeling I've been here before ~ Won't you let me help you find the door "