Upgraded rectifier for Quad 34 (currently L7918CV) - help please!

RoBoBibbedyBob · 8th August 2004, 09:55 AM

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.

Zombie · 8th August 2004, 09:12 PM

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...

RoBoBibbedyBob · 8th August 2004, 09:31 PM

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...!

Zombie · 9th August 2004, 07:09 AM

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

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

analog_sa · 9th August 2004, 07:39 AM

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.

Zombie · 9th August 2004, 07:49 AM

It is a reg, I misunderstood...

405man · 9th August 2004, 08:14 AM

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

Zombie · 9th August 2004, 08:30 AM

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?

RoBoBibbedyBob · 9th August 2004, 08:45 AM

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?

jives11 · 13th September 2004, 05:31 PM

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

8th August 2004, 09:55 AM	#1
RoBoBibbedyBob diyAudio Member Join Date: Aug 2004 Location: In a house	Upgraded rectifier for Quad 34 (currently L7918CV) - help please! 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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8th August 2004, 09:12 PM	#2
Zombie diyAudio Member Join Date: Feb 2003 Location: At home	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...

8th August 2004, 09:31 PM	#3
RoBoBibbedyBob diyAudio Member Join Date: Aug 2004 Location: In a house	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...!

9th August 2004, 07:09 AM	#4
Zombie diyAudio Member Join Date: Feb 2003 Location: At home	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 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

9th August 2004, 07:49 AM	#6
Zombie diyAudio Member Join Date: Feb 2003 Location: At home	It is a reg, I misunderstood...

9th August 2004, 08:14 AM	#7
405man diyAudio Member Join Date: Dec 2003 Location: Near Glasgow Scotland	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

9th August 2004, 08:30 AM	#8
Zombie diyAudio Member Join Date: Feb 2003 Location: At home	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?

9th August 2004, 08:45 AM	#9
RoBoBibbedyBob diyAudio Member Join Date: Aug 2004 Location: In a house	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?

13th September 2004, 05:31 PM	#10
jives11 diyAudio Member Join Date: Mar 2004 Location: Hampshire	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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