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Quad 405 uppgrades

 
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iso
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PostPosted: Tue Apr 22, 2014 10:14 am    Post subject: Quad 405 uppgrades Reply with quote

Audiolabtower has mentioned that he has tried most Ludwig and Snook Quad 405 mods. Especially Ludwig seems to think that modern faster acting op-amps and output devices do not offer any advantage on PCB:s, that were made for slower devices. They might actually be inferior due their higher speed. Any comments about devices that have been found to be enough but not too much.

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Kimmo
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audiolabtower
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PostPosted: Tue Apr 22, 2014 7:28 pm    Post subject: Reply with quote

A modern op-amp was definitely the most noticeable improvement to me, probably due to better signal to noise.
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iso
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PostPosted: Wed Apr 23, 2014 5:57 am    Post subject: Reply with quote

Which op-amp types and output devices you found to give most balanced performance... and how much current limiting you did find necessary?

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Kimmo
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audiolabtower
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PostPosted: Wed Apr 23, 2014 9:51 am    Post subject: Reply with quote

OP134 was a good upgrade. I would be wary of very high spec op amps with huge bandwidth as they may get unstable and I no longer had easy access to a scope to check exactly what was happening at very high frequencies. This is one area where real progress has occurred since the days of Texas TLO devices.

Output devices were still the original ones I bought direct from Quad in 1977 for £12 a set! With this distortion cancelling design they are not too critical.

I never found current limiting a problem since only used Kef, Spendor and Harbeth models with true 8 ohm specs, ie nothing much below 6 ohm where it matters and not highly reactive in those days.

I never got round to upgrading to the -2 mods, which would have meant buying the surface mount modules from Quad and probably making a new pcb, since by that time had started on Audiolab amps (integrated and later stacked power amps) and found these somehow more resolving than the 405 even with mods.
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iso
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PostPosted: Fri Apr 25, 2014 7:59 am    Post subject: Current limiting Reply with quote

audiolabtower wrote:
I never found current limiting a problem since only used Kef, Spendor and Harbeth models with true 8 ohm specs, ie nothing much below 6 ohm where it matters and not highly reactive in those days.


I have also found this high current thing gone little overboard due to the American way... if something is good, then more must be better and if more is better, then too much must be enough.

Multiple paralleled output devices makes it possible to use less feedback for given output impedance... but good question is how low output impedance is needed. My H/K Citation 12 seems to have no trouble controlling sensible speakers with one pair late 60`s RCA 40636 (audio 2N3055) and crude output current limiting.

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Kimmo
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audiolabtower
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PostPosted: Fri Apr 25, 2014 9:38 am    Post subject: Reply with quote

Ah, the 2N3055... how many years did that do ok in the Quad 303... Smile
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iso
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PostPosted: Tue Jun 17, 2014 12:24 pm    Post subject: 405 Reply with quote

I have now replaced the original electrolytic caps of my Quad 405. This unit was built in 1978 and it was circuit wise the last original 405. Current limiting was slightly liberated, but but more sensible limiting circuit arrived later with mkII.

I used Kemet/Aerovox 30 series 15000UF/63V caps for main reservoirs as they are correct size for original clamps. Other electrolytic´s were good quality ones. I replaced original loudspeaker terminals with Deltron ones, so I do not have to use special cables with 405. Output terminal replacement was 30 minute upgrade that took roughly 8 hours to complete. The original LS terminal holes were approx 8 mm dia and Deltrons were made for 6 mm cut out. Terminal spacing was critical as clamp board is soldered directly to terminals. Therefore there was quite much trouble to find suitable material for spacer between hole and terminal.

I added slightly heavier gauge earth bus between main reservoirs and increased gauge of wire that connects earth bus to heat sink ground terminal. No major mods yet and 405 is performing quite nicely with Quad 34 as pre amplifier. Sense of space is reproduced quite well, even HF seems to be slightly tizzy compared to my tube gear.

I will most likely recap 34 tomorrow. Dada electronics seems to recommend to replace most coupling caps of 34 with wire links, as there is practically no bias for signal coupling electrolytic´s. They suggest that caps at tape and line output are good idea. I experimented couple of years ago with my CD payer output. Then it turned out that output electrolytic´s produced better sound quality than plastic film cap or even DC-coupling the output (there was very little DC offset at output).

Has anybody experimented with Quad 34 or coupling caps in similar situation?

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Kimmo
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proffski
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PostPosted: Tue Jun 24, 2014 8:47 am    Post subject: Re: 405 Reply with quote

As good as the Quad series were they had a few design faults, MKI 405 is well known. You have done some interesting work, I wish I still had the time...
Keep us informed and thanks for sharing!
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iso
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PostPosted: Fri Jun 27, 2014 10:10 am    Post subject: Re: 405 Reply with quote

proffski wrote:
Keep us informed and thanks for sharing!

Thank you for your kind words Proffski.

About year ago, I shared at Audiokarma what I did discover when I refurbished Harman Kadon Citation 11 http://www.audiokarma.org/forums/showthread.php?t=528615 . I found quite much to be improved in layout of circuit without destroying the basic design. It was quite odd to find out that there was no response for this kind of information. Information like how to fit gold plated input RCA:s or which brand of caps I should use seems to be more popular.

Quad 405 & 34 journey has proceeded but not finished. I will post when I have something sound to share.

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Kimmo
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iso
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PostPosted: Thu Feb 12, 2015 6:18 pm    Post subject: Reply with quote

It has been quite quiet here recently... so it might be good time to report how this Quad 405 and associated 34 rebuild has been proceeded.

My first idea was to try all mods for both units... but after some time I considered that rebuild close to original would be better approach. If best possible performance had been my goal, some other amplifier combo may have been better starting point.

However I made the following mods for reason mentioned. I increased gauge of Quad 405 earth bus slightly between main reservoirs and heat sink. Op amp was replaced with LM604. This replacement made amp to oscillate when turned off, local decoupling cap at positive rail would probably cure this. I will add these when it is time to open amp for some other reason. Output terminals were also replaced with Deltron binding posts.

Like 405 Quad 34 received fresh electrolytics. 2 pc 2u2 electrolytics were replaced with Wima film caps, as low voltage 2u2 electrolytics were not available. Many inter stage coupling caps were replaced with wire link, as caps will not age too well without bias when amplifier is going to see only temporarily use. I replaced 680nF CD input coupling with 1u5 cap, as LF turn off point did look a bit high to me. So... nothing too special or radical here.

This set has been performing quite well and I have used it temporarily. Few weeks ago I invited good friend of mine to listen Harman Kardon Citation 17 I rebuilt recently and Citation 12 rebuilt few years ago. After we had been listening Citation combo for 2-3 hours I said, why we do not try Quad 34/405 for comparison. Signal source was ARC CD3MKII and speakers were Cantatas I rebuild few years ago.

Difference between these amplifiers was considerable. H/K combo was quite analytic and dynamic. Presence of players was clear and sound stage was there, even it was not the deepest I have experienced. Early solid state harshness was there, but it was not too obvious. When Quad 34/405 combo was switched in, our first impression was that we must have different mix of record playing. Gone was all hardness and performance was totally smooth. Unfortunately presence of players and sound stage depth was reduced considerably. One mid 80´s Quad add does describe quite well what we felt... Quad set and orchestra playing behind curtain, maybe too thick?

But this brings me to this question. Was Quad design supposed to be like this. Comparison to Citations looks fair to me. Citations were more expensive amplifiers, but H/K designs were 5-7 years older than designs of Quads... development of SS gear was quite fast in those days. Any ideas?

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Kimmo
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proffski
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PostPosted: Tue Feb 24, 2015 4:28 pm    Post subject: Reply with quote

Tricky, I am no audio expert yet if pushed can find deisgn improvememts in most SS amplifiers if given a schematic. Sometimes I ponder as to how much say accountants have in some manufacturers and how much if anything is changed in order to save a few $ per amplifier... I know this a case for certain with one but cannot say anything at the moment as it could get a friend into trouble. What I do also find staggering is the difference of sound from some models coming out of the same factory! The latest were driving a quite benign load but were worlds appart in terms of sonic delivery.
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iso
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PostPosted: Thu Feb 26, 2015 5:07 pm    Post subject: Reply with quote

Yes... question about good sound quality is tricky. Bean counters can destroy perfectly good design. You also mentioned that you can find usually few improvements to schematics... but even "practically" same schematics can sound totally different... say how many Mullard EL34 amplifier clones have been made with varying results. Citation 12 schematic (from RCA datasheet) has also been used quite often in PA amplifiers... some being good and some not too good.

Good manufactures do also make mistakes. ARC SP6C was considered downgrade from SP6B in early 80`s. I could not understand why... and bought one. I rebuild it and sound quality was cold as it was described when new. I opened amplifier chassis few times after rebuild and tried to figure what was wrong. This paid off quite well, when I noticed that internal shields of cathode followers were floating. This pre amplifier turned to be my favorite pre amplifier after shields were grounded. Grounding of internal shield is so obvious that very seldom it has been drawn on the schematic... and still pin of the shield was not connected anywhere on the PCB.

Still... I do think that Quad did have enough engineering know how to make "perfect" amplifier for given price. But...were ELS 57 and ELS 63 so high on the priority list when amplifiers were designed, that performance was not best possible with dynamic speakers?

Best Regards

Kimmo
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proffski
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PostPosted: Wed Mar 04, 2015 12:18 pm    Post subject: Reply with quote

I've just had some Quad MK1 405s in for repair.
Power supply electrolytics bulging and one has leaked badly.
Usual scorch marks under some ceramic resistors on the PCB.
Looks like a challenging weekend...
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iso
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PostPosted: Wed Mar 04, 2015 4:27 pm    Post subject: Reply with quote

Construction of the amp is nice... removal of PCB´s is not too complicated... maybe clamp board could be easier to remove as there are electrolytics on the board, that have passed best before dates too.

Kemet/Aerovox 15000UF/63V caps will fit in the original main reservoir clamps and are close enough to the original ones... and are not too expensive.

Best Regards

Kimmo
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proffski
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PostPosted: Wed Mar 04, 2015 6:33 pm    Post subject: Reply with quote

Yes, I've been there before, but many years have passed.
I'll certainly agree about the electrolytics.
Watch this space... 🔉 🔊
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