| RoBoBibbedyBob |
Can anyone suggest a replacement, upgraded rectifier for the Quad 34 pre amp?
It's a L7918CV as standard (1.5 Amp, -18v)... I expect this can be improved, but can't find anything that appears any better from my usual electronic suppliers.
Any suggestions would be appreciated. I have changed over all op-amps to burr brown, replaced all electrolytics with bipolar caps, and the pre is really singing - but just wanted to try and do some work on the psu side to finish the modifications off!
Cheers. |
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| Zombie |
Try 4 average shottkys, and change the big cap in the PSU for smth bigger and better, or all to smth bigger and better...
I have removed ALL caps between the different stages in the amp, too...and the input caps where I have DC under control.
In its current form my LINNish friend prefers my 34 to my Croft Super Micro...my jury still out
Don't know if changing the TL071 in the PSU for smth better helps... |
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| RoBoBibbedyBob |
Hi Zombie
That's good stuff, cheers. I'm no electronics expert, but I'm a total expert with a soldering iron which is why I'm so keen to have a go (have skills, lack knowledge).
I already changed the TL071 in the PSU to a OPA134 (I think). Is it ok to uprate all of the caps in the PSU "compartment" in the 34, or could changing certain ones have an adverse effect?
As for these shottky's - thanks for the suggestion, but not being a straight swap I'd need more info (sorry to appear kinda dumb here). I presume shottky's are kinda zenner diodes, and you're suggesting 4 in parallel?
I'm willing to try anything, at almost any cost, it's sounding so much sweeter already, but you know how it is when you start doing something...! |
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| Zombie |
Hello!
I am about the same, but have knowledge about the componanets etc, but lack deeper knowledge - that's why I pester this kind community :smash:
I misunderstood you, thought you were talking about the rectifier bridge - D34. If you are not familiar with these things, forget about the shottkys. Maybe you can change the 7918 for smth better, but check pin compability. A guy here at the forum says the PSU is very good without any modifications, that he can't see any noise on his scope...
I changed the C14 (1000uF) for an Elna 2200uF, the 22uF and 100uFs for Elnas - mind that the 100uF are not inside the "box", but at the 4066 switches.
Have you bypassed the +/- supply pins of the opamps with small 0,1uF film caps to ground? Quite necessary if you use faster opamps.
The 100uF caps I bypassed with a wire where the following (but check DC at them first): C89/90, C91/92, C30/31, C34/48, C72/78
Other caps I shorted: C85/86, C36/37.
I also shorted out the buffer opamps IC24/27 and replaced all the others with OPA627.
The 4066 chips are replaced with MAX4066A, also thinking about bypassing their supply with small film caps, but I am not sure if it will impove anything.
Cheers
Tom |
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| analog_sa |
| quote: | | It's a L7918CV as standard (1.5 Amp, -18v)... |
You may wish to rephrase your query as the L7918 sounds more like a regulator than rectifier. |
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| Zombie |
| It is a reg, I misunderstood... :smash: |
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| 405man |
It is probably not a good idea to bypass C30/31 and 34/48 as these provide DC blocking to the volume and balance pots and there removal will cause the pots to become noisy. I usually replace these with bipolar electrolytics
Stuart |
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| Zombie |
405man: thanks, do you mean audible scratching noise when turning the pots? Would bypassing the 100uFs with film caps be an improvement?
Would changing the pF and small nF caps to silver micas or teflons give any audible improvement? |
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| RoBoBibbedyBob |
| Ok I'll embark on changing out the psu caps shortly. As for the faster opamps - no I haven't installed any additional caps... Is this really necessary? |
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| jives11 |
Question for 405man.
Have you found a preferred bipolar ? The blackgate ones are vastly expensive, I have seen some smaller cheaper ones in yellow fron Nikai - are these what you use. I'm thinking along the same lines for the AC coupling caps in my Quad 44. There are a pair of 100uF electrolytics in the signal path just before the output RCA's |
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| 405man |
I use the nikai capacitors and am very pleased with them. Having read the articles in Electronics World ( Aug 2002 - Jan
2003 ) on capacitor distortion these appear to have lower distortion than the Blackgate and are readily available and inexpensive. I dont think the Blackgate must be better because they are expensive and conversly I dont think there is anything wrong with the nikia because they are inexpensive. The Black gate ones are probably better if you want to introduce some sort of colouration.
Stuart |
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| jives11 |
Many thanks 405man,
a further question if I may.
I will probably replace the 5 PSU decoupling caps at the same time. there are 2 1000uf 25v axials which I will swap for equivalent 105 degree long life versions. The 2 47uF smoothing caps after the descrete regulators are 63v on my 44. I'm guessing these only see +/- 15v. Is there any reason why such high voltage caps are used ? I wondered what the best replacement choice would be ? (Oscon, panasonic FC, Stargets).
There is also a 100uF/25 v decoupling a further -15v supply |
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| jives11 |
well took the plunge and replaced all electrolytics ( 5 in the power supply and 6 AC coupling . I figured as I'd got it second hand it may have had at least 10 years heavy use, so they could do with replacing anyway. used standard 105 degrees 1000uf, but used Elna Stargets for the 2 * 47uF and 100uF/25v
I used the previously mentioned Nitai bipolars for the 6 100uF/10v. I gather from other previous comments that on older models there may only have been 2 Green tanatalum beads , but on mine there appeared to be 4 extra ac coupling caps around the volume control
See pictures. Too early to say, but it works fine and sounds good. Maybe a bit more imagery but it was to late to do much listening |
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| 405man |
You have one of the later 44s. I use the Nikai capacitors for all the electrolytic coupeling capacitors and for the PSU decoupeling as they are readily available. It is certainly worth changing the opamps in the 44 for OPA134s
Stuart |
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| jives11 |
Thanks 405man,
I think I'm going to leave it now. I'm reluctant to replace all the opamps as it's fiddly and it now sounds pretty good. I also like the idea of using TL071/72s as they appear to be so unfashionable :D
I am going to turn the Radio Module into a line level by changing the potential divider resistors. Have you done this ? I'm curious what the ideal settings are, I have heard what sounds like overloading on the Quad CD/Aux module so think this still might need some further attenuation. |
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| Zombie |
| Some like'm with the old opamps. But you will never get close to what you will get with some better ones, like OPA134, 604 or even 627... |
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| Blue |
Do you guys ever search the web or read elsewhere before you come to the cosy, snug, we're all amatures anyway so who cares what we say environmet of this forum ?? go to google enter QUAD34 and search.... Buy some test equipment and measure.... listen to some live music and compare....
But as you are here anyway let me save you some time with a few FACTS:
If you don't know the difference between a regulator and a recifier, the regulator is like your elbow and the rectifier is your.......
Schottky rectifiers generate sharper spikes than, perfectly OK, standard devices and lowering or changing supply capacitors often makes things worse. If any of you bothered to measure the effects you cause with such unnecessary changes you would be adding resistors in series to the rectifier to produce a lower noise supply. No it won't change the supply impedance, that's what the capacitors and regulators are for.
The QUAD 34 (and some other models) has asymmetric supplies that guarantee a small +ve offset at the ORIGINAL op-amp outputs, although only 10s of mV it is enough to bias low voltage tantalums or nowadays solid aluminum but not wet elctrolytics. The fact that someone at QUAD lost the plot and they later fitted wet electrolytics is..... If you mess with the supply or change the op-amps or 100uF capacitors you may well have a different sounding amplifier but accurate and long lasting NO.
Bipolar capacitors are worse than any correctly biased electroytics with twice the ESR and are no good at all with dc across them long term. In the 34 if you use higher voltage coupling caps they will not work as well as lower voltage ones after a while, 3-6V is ideal.
C77 in the QUAD 34 is often fitted the wrong way because it's marked wrong on the silk screen and the assembly drawing. if it is a later wet electrolytic it has probably leaked and your switching is a bit screwed up.
Capacitor tests in magazines are a misleading waste of time.
Almost every commercial design audio or otherwise has some hidden elements that are not shown on the plans or expressed in the service manual often to the extent that a new company who attempt to produce the product mess it up.
As DIYers (Dabble It Y-chromosome) you are entitled to mess up your own equipment up but please don't get others to do it for you.
Anyone who changes op-amps and capacitors in QUAD equipment and regularly sells the results on ebay for a living will not agree with me.
Anyone who is a man will not agree with me.
So off the soap box.....
Zombie: I hate to admit it but changing the 15pF ? caps at the volume control for polystyrene ones does make a difference to an otherwise unmodified later QUAD 34 also why not bridge the 1n across the 1k5 and 100uF before each volume control input. if you have an early 34 there is some interesting stuff on the web to try before you start changing capacitors.
Soon to be kicked off the bus I'm sure
BLUE |
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| johnm |
Oh it's nice being a know-it-all isn't it Blue! If all you want to do is start a war then - frankly - please leave. If, on the other hand, you want to help us by sharing information then please do it without the insults.
Regards,
- John |
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| Werner |
| quote: | Originally posted by jives11
There are a pair of 100uF electrolytics in the signal path just before the output RCA's |
These can be left out as the TL071 opamps in the 44 are seldom a source of any DC offset.
There are one or two other places in the circuit where AC coupling caps can be bridged safely. |
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| Blue |
Johnm
I will take your advice as you appear to be a spokesman for the forum. I wonder how many others think a woman should not have an opinion on matters audio.
BLUE |
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| jives11 |
Dear Blue,
Please don't depart the forum. I'm interested in your robust opinions. My 44 sounds better than ever despite me replacing 6 AC coupling caps with bipolars. Perhaps the PSU caps I changed at the same time are the cause and I should go to tantalums as Walker Intended ?
The Jury is out but please keep presenting the evidence...... |
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| johnm |
Whoa Blue! What ON EARTH has this to do with gender? I certainly made no references to you being a woman in the slightest did I?!?! I was merely rattled by the apparent rudeness present in your first post, the information you gave - on the other hand - was extremely helpful. If this was - in fact - meant in jest then of course I sincerely apologise.
Just seemed to me you were knocking the way we were/are going about experimenting with these great old pre-amps.
"Do you guys ever search the web or read elsewhere before you come to the cosy, snug, we're all amatures anyway so who cares what we say environmet of this forum ??"
...and YOU mentioned the word guys ;-)
Anyway, back to the topic at hand... |
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