Quad 34 Mk II PS mods when using LME op-amps and Maxim switches?

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01-Mar-2019 05:07 PM

Fabian

Fabian

Posts: 72

Hi everybody,


in the past there have been occasional issues when replacing the op-amps with LME 49710/49720and the switches with Maxim 4066 types at the same time. The most prominent case was probably that of John (The Quadfather). Search for "LME & Maxim = hum in Quad 34" on this forum. In this case the final solution was to decouple all op-amps with small 100nF caps, not just the usual suspects IC9 and IC10 (or maybe also IC7 and IC8).

 

However in this very thread also some other modifications have been discussed and Joost confirmed that he had put them to good use.

 

The first is the replacement of C58 and C84 with 220uF instead of 22uF values to provide a lower internal impedance. I would like to implement this mod right away, because I think that it cannot do any harm. Is anybody not sharing this opinion?

 

The second mod, also mentioned in the "LME & Maxim = hum in Quad 34" thread and mentioned by Joost in another place, is the change of R123 and R124 to 10k and R84 and R85 to 220R. This is suggested to provide a symmetrical output voltage instead of the original output voltage of +8.6 and -9.4 V. When using the modern LME op-amps, is there anything to gain from this mod? Maybe just by means of the slightly increased total voltage? I currently don't have access to a scope, so I cannot verify if the clipping will be symmetrical in the end.

 

The third mod is not discussed in the other thread, but I think The Quadfather had mentioned in a different thread (similar topic though) that he had also replaced C74 with 2200uF instead of 1000uF. This is easy and still cheap to do. Todays 2200uF elcos are still smaller than the 1000uF original one. But I am undecided on this one. First off, the minimal reduction in noise might not be worth it at all. I wouldn't care about that. The more interesting question to me is, if the rectifier chip and transformer can deal with the additional current, in particular when switching on the amp. Who has tried this mod as well? Who has not and for what reason?

 

Thanks for reading and for your comments.


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04-Mar-2019 08:48 AM

EJP

EJP

Posts: 1559

It is only necessary to decouple IC9 and IC10. The rest of them don't have a stability problem.

Changing the capacitors you mention can't really hurt, but I doubt you will notice any difference. Save your time and money. It is already a regulated power supply.

There is nothing actually wrong with making the supply rails symmetrical, but there is no actual benefit. You can't get any signals through the 34 that are anywhere near the rail voltages, due to all the diode limiters at the inputs, which will clip the signal far sooner than the rail voltage limits.

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04-Mar-2019 11:01 PM

Fabian

Fabian

Posts: 72

Thanks for your clear explanation, Esmond.

I took the bit about decoupling ALL opamps straight from John's last post in the mentioned thread. Well possible that this strange problem was corrected incidentally when soldring in the extra caps.

These 220uF and 2200uF caps are so cheap (even buying Panasonic low ESR types in quantities of just 4 and 2) that I will use them anyway. And then forget about them.

Regarding the supply rails I had been momentarily taken away by the wrong assumption that +ve could be raised to +9.4V while keeping -ve at -9.4V. Even if this was true it would probably make no difference at all. So in this case I will listen and not even start tinkering.

Once again, thank you.

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17-Mar-2019 03:13 PM

The Quadfather

The Quadfather

Posts: 112

"It is only necessary to decouple IC9 and IC10. The rest of them don't have a stability problem."

This wasn't the case with my Quad 34 (later phono socketed version, issue 6 PCB). I had hum issues when the lid was slid back onto the unit. One other user (Joe) had the same problems which Joost managed to solve, and that was by putting extra decoupling on IC19 or IC20. The following was from an email between Joe, Joost and myself, and I hope they do not mind me posting it here:

"The hum problem was very annoying, especially because it increases with the cover in place. Also it increases with the volume in position 0 and 24. During testing I accidentally switched on the filter, hum gone! Decoupling Ic 20 in the same way as Ic 10 finally solved it."

and 24. During testing I accidentally switched on the filter, hum gone! Decoupling Ic 20 in the same way as Ic 10 finally solved it."

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19-Mar-2019 08:26 AM

EJP

EJP

Posts: 1559

Some of these have an extra undocumented 5.6pF capacitor across R66 and R70. It would help to stabilize IC12/13, which runs with rather enormous 820K input and feedback resistors, which later op-amps may not like. I'm trying this at the moment, will advise further.

EJP

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19-Mar-2019 09:50 PM

Fabian

Fabian

Posts: 72

One of my 34s (S/N 23xxx) had those soldered across R66 and R70 as well, but the other one (S/N 35xxx) did not. I concluded (maybe overhasty), that those caps had been introduced at some point, but were finally considered obsolete, so I removed them during the revision of the 23xxx unit.

Both amps are M12730-666 PCBs and all components did look exactly the same prior to the revision. Given the age of these gems I cannot even be sure if those serial number stickers are still original, so maybe my reasoning wasn't all that smart.

However, the revised amp (sans the caps, using the LME opamps) was stable right from the start with decoupling just IC9 and IC10. I kept both capacitors, just in case. Looking forward to hearing of further findings.

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20-Mar-2019 05:13 AM

EJP

EJP

Posts: 1559

5.6pF across R66/70 helped a lot with this one. Still unstable but at a much lower level of THD, around 0.02%.

With this LPF the unit as a whole is -3dB at ~55KHz with the SL 2nd-order LPF (which is on when both filters are off) starting to kick in as expected at 60KHz.

I should note that this capacitor is indeed documented, as C93/94, in the last listed modification, and shown on the latest schematic; and that it should be 6.8pF, not 5.6pF. Note that the modification note incorrectly says R76 for R70. If I have any in stock I will change and re-test.

I would have to add that the recent deliveries of LME49710 I have had this year have exhibited much more instability than previously. It got to the point in one 405 where I just had to take them out and put in something else.

EJP

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20-Mar-2019 02:43 PM

The Quadfather

The Quadfather

Posts: 112

Interesting. The LMEs that I bought were a batch from last year, which needed the extra compensation on IC19 to solve. Somehow I still wasn’t happy with the sound, was a little ‘brittle’. I went to 2134 and 134s and was far happier. Now I’ve gone back full circle back to the TL071s. Far less technically impressive than modern op-amps, but I prefer the subjectively ‘warmer’ sound. I find it helps a little with many modern CD masterings that are too harsh/bright/thin if played ‘accurately’. I added sockets so easy to change :)
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23-Mar-2019 11:43 AM

Fabian

Fabian

Posts: 72

@EJP: After checking the file Quad-34-Service-Manual-V1.2.pdf I found the two places where C93/C94 are mentioned. Both leave me puzzled.

As you already mentioned, the addendum on page 21 incorrectly states they were fitted in parallel with R66 and R76, when it should read R66 and R70. So far, so bad. I must admit that I don't really understand the reason for fitting the caps as described on that page. Might be due to my lacking knowledge of electronics of the English language.

When looking at the modified circuit diagram, things don't get any better. C93 is drawn in parallel with R66 indeed, i.e. between pins 2 and 6 of IC12. But when looking at IC13, C94 is placed between pin 6 and the other end of C52 (68nF!), which connects to pin 2. This must be an error, right? The actual placement on my 34 was across R70.

That one of my two 34s that had C93 and C94 in place came with another oddity I don't understand. I am happy tonot find this mentioned in the latest service manual: On RV2b there was an additional 1M resistor soldered between the center tap and the common end of R69, R68 and R56. What on earth could this be good for? Looking at the diagram it might have a marginal effect on gain when tilt is set to maximum treble boost. But I'm not sure on this and it looked weird to me.

Even more so since on the on the left channel a resistor was also present between the center tap of RV2a and the common end of R65, R64 and R51, but it was 680k instead of 1M.

I through this garbage out.

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23-Mar-2019 11:51 AM

Fabian

Fabian

Posts: 72

PS: I can only guess that C93/C94 were 6.8pF in my preamp. My cheap LCR meter cannot measure such small values reliably and the color coding didn't help much as well:

green - grey - gold - silver - white

While green and grey would fit, I could not find any reference to a color coding schema that would make use of gold and silver for ring 3 and 4. From my understanding it should be:

green - grey - white - whatever ...

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23-Mar-2019 12:15 PM

Ton

Ton

Posts: 52

Many color code tables for capacitors exist,  perhapsthis one helps a little ? 


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