This is a follow on from my 2002 modifications to a QUAD405 which I obtained together with an early model FM4 tuner and QUAD 34 pre-amplifier — The intention was to do a few modifications to the 34 that did not involve changing the op-amps like everyone else does — The 405 — 44 and FM4 in question all have DIN sockets — The QUAD34 had no formal CD input button and the AUX input sensitivity was fixed at 100mV for the nominal 500mV output at maximum volume — The TAPE input could be set to 100mV or 300mV by internal links so the idea was to drop the AUX input also by 10dB to 300mV similar to that of the TAPE input and use it for CD but things were not so straight forward as expected . . .

The Green and Red LEDs on this early model QUAD 34 were 5mm types but those on the FM4 were smaller 3mm types with brown plastic bezels — like the grey and black plastic bezels on the later 4 series equipment used to fit 3mm LEDs in the the 5mm holes — Both separately and especially together the QUAD 34 and QUAD FM4 looked ugly

As well as being different sizes the LEDs were different shades and brightness so I changed them all and matched them to the button colours and lowered the intensity as I had done on the QUAD 405 — Although I measured the bends in the leads some still did not sit well so LED panel mount holders were fitted to keep them level to the front panel but back a bit

The QUAD 34 uses different series resistors for the Red and previously Green LEDs — The resistors for the Red LEDs R109 and R116 were kept at 3k3 as this gives about 4mA and a good low level light output — The others now Yellow needed about 5mA so R95—R96 and R97 were changed from 1k to 2k7 (voltage across R ≈ 13.5V)

The QUAD FM4 (left) originally had a common 270R resistor R13 for all LEDs which was changed to 1k to give 5mA — The Red LED had a link which was replaced with "R13b" 200R to give 1.2k total and again about 4mA (voltage across R ≈ 4.8V)

This QUAD 34 pre-amplifier was an early version before serial number 8000 without the input buffers fitted on the later models and the QUAD 44 but it had single TL071 op-amps unlike the later versions which used some TL072 dual op-amps

TL072 and TL074 etc. have better than 120dB cross talk between amplifiers both on paper and in practice with good design and layout — Some modifications to the QUAD 34 and 44 suggest piggy back PCBs using single op-amps to reduce crosstalk but these are a waste of time as you will discover . . .

The RADIO input has a sensitivity of 100mV to match the FM4 tuner and an input impedance of 100kΩ through R11 and R12 to ground — The TAPE and AUX inputs are directly switched to the same busses and so also have a sensitivity of 100mV but have an impedance of 39kΩ through R19—R20—R12 and R14 to ground

On the TAPE input there is provision by links R16 and R18 to place 82kΩ resistors in series with the inputs to lower the sensitivity to about 300mV or -10dB which is suitable for a CD input to be balanced with the radio — At the time this amp was made BBC Radio 3 — a rather conservatively modulated (deviated) FM station here in the UK — was used by QUAD as a reference level for other sources

On the right C5 and C6 are changed for 1µF capacitors with 82kΩ resistors fitted in series to give 300mV sensitivity for a CD input — The TAPE input has C7 and C8 changed to 1µF and its sensitivity set to 300mV by R16 and R18, the TAPE output is also set to 300mV

I had only intended to do a few mods like fit the BC orange polyester capacitors and make the output impedance 600Ω for a project I was doing so the lid went on and the QUAD34 was connected in my system with the modified QUAD405 and I looked forward to listening to some CDs and seeing if the capacitors made a difference worthy of note but after a while it was clear something was not right . . .

Instead of having a well defined stereo image some music from CD sounded strange and where it had all sounded excellent with the modified QUAD 405 and a QUAD66 pre-amp or passive volume now the image appeared central and dull — Moving the CD to the TAPE input made no difference but the RADIO input — although the volume had reduced range — gave a good stereo image and sounded normal

So what was wrong ? — The change of capacitors could not be the problem, I have used this type for years and fitted them to many amplifiers including QUAD 34s and 44s — Moving the capacitors apart had no effect and rather than remove C5—C8 I fitted the same type capacitors for the RADIO input again to no effect but it showed the capacitors were not the problem — Swapping the FM4 to the AUX did not reveal much but on some programme material I noticed a slight lack of stereo image which I was now concentrating on

Everything pointed to a Monoing (new word ?) of the channels but not straight forward left meets right — This was strange! — It turned out to be frequency selective crosstalk but where from and why did it only show after changing the sensitivity — It wasn't noticed on the RADIO input or the AUX input before changing the sensitivity but was now very obvious on some CD tracks I was familiar with

Measurements showed the modified AUX input crosstalk [measured at maximum volume with the unused input shorted to ground] was -52dB at 1kHz R-L and -35dB at 1kHz L-R The RADIO input measured -74dB at 1KHz R-L and -68dB at 1KHz L-R — At 10kHz things were much worse for the AUX and TAPE inputs — a nice round 20dB worse in each case which was confirmed to be 6dB/octave suggesting capacitive coupling, but coupling that was greater from Left-Right than Right-Left so it was not between identical signal tracks or between the new large bodied capacitors, it was coupling that was on all inputs to some extent and a lot less with a low source impedance so it was around the input buss somewhere

It appears that from serial numbers 8001 the QUAD34 pre-amp was fitted with input buffers which suggests the introduction of CDs and the need to reduce the input level without undue loading had highlighted the problem but having discovered it the hard way I was not going to make a raft of input buffers especially like QUAD had done with 100µF electrolytic capacitors — As with all problems it is better to remove the cause than try to mask it, and why make the signal path unnecessarily complicated

Nice philosophy but where to start — First thing was to leave the 82kΩ/39kΩ dividers untouched as I had made my measurements with them in place and although lowering the total input impedance would reduce the crosstalk, and possibly provide a better load for my CD player, the mechanism would still be there — The obvious suspects were the analogue switches but these were found to be faultless — Note from the picture above that I fitted them in sockets and I tried all manner of swaps with the latest parts coming off worse

As I did not have a service manual and could find no suitable information on the Web — Especially about the unspecified crosstalk — I scanned the bottom of the 34 and traced out the wiring in PhotoShop — It was obvious when the tracks were coloured where potential problems lay but rather than write more I have drawn you a picture ... Also you can download the schematic which I have redrawn to show the various mods mentioned here

On the left is a marked up picture showing the modification to remove the causes of crosstalk — The tracks highlighted in red and yellow indicate Right and Left channels respectively — Coloured dots over Green show where cut sections of track were grounded to form a screen between channels and dots without green show where short pieces of track were cut and removed

In the top left a short piece of Green wire reinforces the ground and provides a screen between the Left input and output paths — The Red wire running under IC2-IC5-IC1 is the signal path that used to run past R73-R74-C61-C62 which was responsible for the worse Left to Right crosstalk. The Red and White wires at the bottom of the picture are to the mono switch

Click on image for larger picture

The above picture should explain what was done — Cross talk now measured about -70dB between all inputs which enabled the last potential problem areas to be investigated

The only other changes made with diminishing effect were a screen on the top side as shown opposite and screened cables from IC5 and IC6 to C30 and C31 — The PCB is double sided and has pins to link the two sides — Where connections such as the mono switch were rerouted, the PCB tracks were isolated by removing these pins and grounding the unused track as outlined in red in the picture opposite

The problem is once I start lifting stones and interesting things crawl out there is a temptation to lift more to find out what's underneath them — I now had a QUAD 34 with high input impedance at 300mV without buffers that sounded better than many other active pre-amps I have heard — The input buffers on the 34, 44 and other pre-amps mask the crosstalk problem but how ?

If the output impedance of a buffer is low to signal it's due to the feedback and if it is to appear low to an interfering signal or crosstalk then that signal must be present at the input of the buffer to be amplified in anti-phase to the signal on its output but the only way that works well with an op-amp buffer [output connected to inverting input] is if the non-inverting input has a low impedance to ground

The input buffers as fitted by QUAD use dual op-amps which is not a problem but when set for 300mV sensitivity as I wanted the buffers have a reduced bass turnover at about 5Hz, this can be corrected by increasing the input capacitors which was the starting point that led to this mod but having compared buffered QUAD34s and 44 with the modified 34 I definitely prefer the simpler single path

The QUAD 34 now sounded clean and transparent and a few other terms I normally try to avoid using in case I sound like a Hi-Fi magazine editor who has just been wined and dined — It was almost not there but a simple (passive if you wish) volume control still gave a more natural sound with slightly better image placement so I had it back on the bench for a final tweak

The distortion had always measured much better than the published specification so I had never measured any voltages around the op-amps but this time I found myself measuring the d.c. at the output terminals and the left channel had +11mV — Not much but not 0V — like it should be with the series capacitor C77 fitted

The original tantalum capacitor C77 was not just leaky — it had been fitted the wrong way around but that's how it was marked on the PCB silk screen — I have heard tales of C77 leaking on many QUAD34 models when wet electrolytics were fitted — Could they all be fitted the wrong way around?

Reversing C77 did not resolve the output offset problem as it had become damaged and would not reform — You can and should reform tantalums that have little or no voltage across them in operation — A 1.5V battery connected for 30 seconds or more will often improve things

On the model I had C16 and C17 were also fitted wrong way and so were reversed — I fitted the TAPE outputs with 100kΩ resistors to ground to ensure they were not affected by charge sharing with other equipment — The output capacitors C77 and C78 were for some years changed to 150µF 6.3V tantalum bypassed with 1µF 63V polyester capacitors, see QUAD 34 circuit diagram but now (2011) they are bypassed

Some of the 100µF coupling capacitors can be shorted with negligible effect and since first writing this article and listening to this QUAD 34 for over a year I finally shorted C30—C31—C48 and C54 after confirming they had negligible voltage across them — The result is non conclusive and I do not intend to put the capacitors back or fit different capacitors to confirm this — It works just as well without the capacitors and as I always say "Less is More"

Some QUAD34 modifications on the Web advise shorting all the 100µF caps and claim a response down to d.c. but this is of course not possible unless the TILT/BASS controls are also bypassed ( C36 and C37 ?) — Without the Ambler tone control or "TILT" as QUAD call it why have a QUAD pre-amp at all ?

With the series path electrolytic capacitors bypassed or shorted the QUAD34 sounds as natural as any "passive" control but with the benefit of low impedance buffered outputs, input switching, tape monitoring, filtering and of course the Ambler "TILT" tone control

The QUAD 34 supply voltages are unbalanced by design at +8.6V and -9.4V which (with the components originally fitted) appears to ensure the output clips symmetrically — At one time I wondered if this was some clever distortion reducing scheme but the distortion changes little as the supplies are varied to +9.5V and -8.5V

With balanced (± 9V) supplies the output tends to clip negative before positive — The unbalanced supplies do not affect the offset voltages at each op-amp output which are determined by the op-amp parameters and d.c. source resistances at their inputs

If you have changed the op-amp type and/or bypassed any series electrolytics C30—C31—C48—C54—C77—C78 the above "clipping" statement may no longer apply and the supply voltages may require adjusting so that the QUAD34 once again clips symmetrically i.e. achieves maximum output with its "low" supply voltages

The QUAD34 supply voltages are low because the CD4066 switches can be damaged by peak voltages above ±7.5V — Early models used zenner diodes D3 to D14 on the inputs to protect the input CD4066s — the later model used the input buffers to also protect the input switching

If you change op-amp types while keeping the electrolytics in circuit check (as the input and filter buttons are selected) that they are still polarised correctly — The outputs of IC5 and IC6 are often -ve and the outputs of IC19 and IC20 +ve so even if in original condition check C30—C31 which may require reversing and C77 (see above)

Although the Red LEDs I fitted in 2002 to the QUAD 34 and FM4 above looked OK when run at low current it was obvious when in 2004 I got another early QUAD 34 with brown buttons that the Red buttons with LEDs was "not quite right"

Having sold the 34 shown above I modified the brown button 34 and fitted all Amber LEDs — I managed to get a brown TUNE button for the FM4 and fitted an Amber LED with R13b restored to a link as shown

Maybe not to everyone's taste but I now prefer the 1970s look of Brown buttons with all Amber LEDs to the Red and Amber

I prefer either option over the original Red and Green LEDs fitted by QUAD which look ugly on the Brown and especially the later Grey 3 and 4 series equipment even with the smaller 3mm LEDs and coloured plastic bezels

Like most people I probably will never use the filter buttons whether Brown or Red — Unless perhaps playing some old vinyl records with annoying surface noise — With the QUAD 67CD above or a QUAD 66CD this is a good retro look but with a modern sound and functionality — Could the filters be bypassed and the buttons used for additional inputs ? — Could or should the input switching be changed to relays ? — Probably yes but for now

Disc Input Module Tweaks

As with the QUAD 22 and QUAD 33 the QUAD 34 was fitted with a user friendly plugin "DISC" module to accommodate the variety of different cartridge sensitivities available — The plugin module is an input amplifier that precedes the RIAA equalisation stage providing cartridge loading and buffered gain ahead of what would otherwise be a noisy shunt feedback RIAA stage using a TL071 op-amp

The use of the buffer amplifier module gives improved signal to noise and sufficient extra gain to provide for 100uV sensitivity for MC cartridges — The PCB layout is common to all modules and the loading and gain are simply determined by component changes to this PCB see QUAD 34 MC Disc input mods.pdf