LME & Maxim = hum in Quad 34?

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07-Jun-2018 06:06 PM

The Quadfather

The Quadfather

Posts: 95

Hi there.

I’ve just gone and replaced all the op-amps in my grey Quad 34 with sockets, and have installed a full set of LME op-amps which I purchased from DaDa some time ago.

The RIAA/Phono section op-amps were replaced with the ‘tin hat’ versions of the LME op-amps a few years back, which have functioned perfectly. Also back at that time (2015) I replaced the 4066 switches with the MAXIM types.

I have added the recommended 100nF decoupling caps between pins 3 and 4 and 3 and 7 on IC9 and IC10.

It works and sounds great on CD, but when I switch to phono I now get some hum on both channels. Curiously, if I alter the position of the balance control away from centre the hum vanishes in both channels.

If I switch to the Radio input (there’s nothing actually connected to the 34 here) I get the low level hum there too.

I’m at a loss. Is the combination of all LME op-amps AND the MAXIM switches too much for the 34s power supply, or does this hum mean more decoupling is needed? Strange how the CD input is hum free though.

Any suggestions?

Thank you.

John.


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08-Jun-2018 06:09 AM

EJP

EJP

Posts: 1484

John

The bypass capacitors across IC10/11 are at the volume control, and affect all channels, curing a hum which is at its worst at zero volume.

If you only have hum on a couple of channels, the problem is elsewhere, and earlier. It may be mechanical. Check the flat ribbon connector from the phono PCB, both ends; check that the screws securing the phono insert card are tight; and check the soldering on the Phono and Radio input socket(s). These are known to crack over long periods of time, and develop a ring of poor or non-conduction between the pins and the PCB.

The 4066s don't have anything to do with it, and the power supply on my 34 hasn't cracked up under the strain either.

EJP

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12-Jun-2018 05:50 PM

Joost Plugge

Joost Plugge

Posts: 957

In some cases it is also necessary to decouple the op-amps in the phono section. It is a inverting stage so connect the 100nF between pin 3 and 4 on Ic7 and between pin 3 and 7 on Ic8. Two caps in total will do the trick. Quad used long and complicated routed power lines for the Ic's and ignored the advice of every op-amp manufacturer, even in those days!

Joost

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13-Jun-2018 03:15 PM

The Quadfather

The Quadfather

Posts: 95

Thanks for the replies EJP & Joost - I appreciate your help :)

Checked the phono connectors and all fine there. Then I noticed that I'd put 100nF caps ACROSS pins 4 and 7 on each phono op-amp (following the advice on another audio forum 3 years back - I'd forgotten about this). In light of Joost's comments about the inverting phono stage, I simply removed those two 100nF caps, and the hum has gone. It wasn't an issue before I added the other LME op-amps, but obviously was once they were in place. So it was probably never working 'by the book' before. So thanks for the advices. I'll likely follow Joost's advice and add the single decoupling cap on each phono op-amp just as an extra precaution.

By the way, in addition to the upgrades in your Quad 34 manual, and capacitor bypasses and op-amp changes, I can recommend another (cheap!) tweak. Replace C77 and C78 electrolytics with Panasonic 6.8uF 250V ECWF polypropylene caps. A worthwhile and very cost effective modification, so those caps would never have to be touched again so good for longevity as well as sound. Somebody did the maths for me and the bass response is not degraded by the far smaller capacitance of the film caps. I can confirm this by listening too :)

Best wishes,

John

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13-Jun-2018 03:17 PM

The Quadfather

The Quadfather

Posts: 95

https://uk.rs-online.com/web/p/polypropylene-film-capacitors/7270054/
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13-Jun-2018 04:48 PM

The Quadfather

The Quadfather

Posts: 95

Joost I had a look at the RIAA stage on the Quad 34 and cannot see the inverting part of the phono stage. If it’s not too much trouble would you be able to indicate this area on the schematic for me? Thank you.

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13-Jun-2018 07:00 PM

The Quadfather

The Quadfather

Posts: 95

Curiouser and curiouser!

I thought the hum was cured, but that was listening with the cover off. Wih the chassis cover on, I get hum on phono and radio (not on CD). If I start to slide the cover off, the hum disappears when the cover is about 1/3 off. Or completely removed.

Any ideas on this weird issue? Usually it’s testing with the cover off that can cause a little hum. This is the opposite.

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13-Jun-2018 08:31 PM

The Quadfather

The Quadfather

Posts: 95

Joost, I just tried your suggestions of the 100nF between pin 3 & 4 of IC7 and pin 3 and 7 of IC8. It sadly hasn't made any difference the the differing levels of hum on the inputs, IF the cover is in place.

I think some Quad 34s just aren't suited to a combination of the extremely fast LME op-amps with the MAX4066 switches, even with the 100nF caps. My unit is a 1986 grey edition, PCB version is M12730-666.

As I do not have any test gear I guess all I can do it try installing my old set of OPA134 and 2134 opamps and see if the problem vanishes.

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13-Jun-2018 11:23 PM

The Quadfather

The Quadfather

Posts: 95

My apologies for another update, but I’ve just noticed around IC12 and IC13 there’s two bypass capacitors (yellow, looks like a resistor) soldered across R66 and R70 that looks like a late addition by Quad, soldered to the underside of the PCB. Could this capacitance perhaps be causing some problem when used with an LME instead of the original TL071? Those caps are the only extra components I can see on my PCB compared to earlier versions.

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14-Jun-2018 03:29 AM

EJP

EJP

Posts: 1484

6.8uF looking into a power amplifier load of at least 10k is OK, but I wonder if you realize how big polyproplyene capacitors get at these values? The ones you linked to are 23x23x17mm, which is a good chunk of a cubic inch. You'll never fit that into an early 44, and you'll have trouble on a late one.

EJP

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14-Jun-2018 08:11 AM

Joost Plugge

Joost Plugge

Posts: 957

Pin3, the positive op-amp input is connected to ground, so Ic7 and 8, part of the phono stage are inverting, like op-amp Ic9 and 10, the volume Ic's.  The hum issue with cover on or off is strange, first solve that. Are all the screws on the chassis and Pcb mounts tightented? The extra caps accros R66 and 70 limit the upper bandwith, should give no problems.The LME's draw substantial more current than TLO's, you could try increase C58 and 84 too 220u. To limit the current you could remove Ic25 as a test, it is only there to drive the tape input. Also check the pcb for cold joints esspecially the connecting pins for the upper and lower Pcb tracks.

I use 100uf bi-polars these days for interconnecting the op-amps. The distortion created by the elektrolytes is proportional to the signal voltage across them. By increasing the value you drop the signal voltage, a trick Quad allready mastered. So the 6.8u foil is working ok, but is a bulky and expensive component, as EJP mentioned. Beware of hum pickup by such large caps.

But first, fix the cover hum issue, it is not ok!

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14-Jun-2018 01:58 PM

The Quadfather

The Quadfather

Posts: 95

EJP - Yes it sounds big from the dimensions, but they both fit into the C77/78 positions in a Quad 34 nicely and are not massive like many ‘audiophool’ brands. I do not own a Quad 44.


Joost - Yes the lid issue is very perplexing, and I only discovered it by chance. My soldering work is factory neat, I can see no errors anywhere, the pcb is in fine condition. I will check all the through-hold pins as you suggest. I’m wondering if I may have a defective op-amp somewhere. I can detect no op-amps getting very hot though, just slightly warm. i’ll try your other suggestions. To simplify things first I may just pull out all the LMEs and try my OPAs.

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14-Jun-2018 07:45 PM

The Quadfather

The Quadfather

Posts: 95

Removed all the socketed LME opamps (with the exception of the directly soldered tin hat LMEs in the phono positions) with OPA134s and 2134s. No humming. No weirdness at all with the cover on or off.

I can only assume therefore that one of the LME opamps was faulty and turning the 34 into an oscillator that was sensitive enough to the positioning of the chassis cover.

OR that the issue 6 PCB plus complete set of LME plus Maxims is too much. Looking through the archive here I reminded myself I had issues here 3 years back too. It would be interesting to know if there’s a correlation between pcb revisions (and changesof components Quad used) and the stability of using LMEs and Maxims together. 

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15-Jun-2018 09:14 AM

clearsound

clearsound

Posts: 20

Hi John,

I had a similar problem with the dip LME's back in March 2015. This was just hum on the channels and I don't know if it was anything to do with the cover. If I remember correctly was  partially solved / reduced by fitting the metal can versions to all of the placements. These 34's also had the Maxim fitted. Since then both 34's have been changed substantially but if you have any of the T0-99 ( ? ) version, you might want to give them a try before going back to the OPA? I think RS do them at a reasonable cost.

Joe

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