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My refurbished pre-amplifiers

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Here is picture of 70 kg pre-amplifier stack, I have refurbished and used. ARC SP3A power supply reached the best before date this spring. It has to be sorted out soon.

Top to bottom: Quad 22, Quad 34, ARC SP14, ARC SP6C, H/K Citation 11, ARC SP3A, Conrad Johnson Premier 7B.

Best Regards

Kimmo

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cool, nice stuff. Funny, I always think Audio Research stuff looks a little like bench test equipment. With it all stacked up like that, for sure.

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Funny, I always think Audio Research stuff looks a little like bench test equipment.

I have also wondered why most US hifi/high end gear has to look like distributor testing machine. I can not imagine that my or any other members wife will shout "what nice amp you have bought in" when I will show my new H/K Citation 17/16 combo. In my stack no amp can be considered "nice looking", but perhaps Quad 22 and 34 amps can be considered at least unobtrusive.

One nice thing in Quad gear is their easy serviceability. When I retubed my Conrad Jonson Premier 7B I had to unscrew almost 50 machine screws, remove side panels and top chassis from face plate. When retubing Quad 22 you have to unscrew 2 screws and remove chassis, this will take 1-2 minutes.

I have almost completed refurbishment of one most attractive looking amplifier ever. This mid 70`s integrated amplifier is Armstrong 621... and it is actually quite good. Sadly Armstrong history ended in early 80´s. In my photo you may note orange colored mains cable, it is not stock item but I did not change it to original colored item to be sure that mains switch does not get damaged when soldering mains lead to it.

Best Regards

Kimmo

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All beautiful! Don't know many preamp collectors ;)

In this thread http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=8257&page=10#entry103254

glenz66 wondered about a good preamp to use with his Thorens/Linn/Shure TT setup and an Adcom 555 amp with AR-2ax speakers.

Do you have some favorites to recommend?

My only preamp refurb project has been the excellent but little-known DB Systems DB-1A.

The attached photo shows the DB-1A preamp atop the DB-5 tone control. On top of that stack is a DBP-2J/5 switch box. Not shown are a DB-9 headphone amp and the separate DB-2A power supply (for preamp & tone controls).
The DB products, in their walnut cabinets, look good with the KLH Model Eighteen FM tuner and the Henry Kloss-designed Tivoli Model CD.

-Kent

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I did not have working audio system from early 90´s to 2008... so my experience is limited my own preamps. I have bought H/K Citation 17 after the pic.

CJ Premier 7B is propably best preamp I do have... but is quite rare as only 220 units were built between 1988 and 1996.

Quad 22 can sensibly be used only with Quad II so... Quad 34 can only be considered usable with other amplifiers. It is very well designed... line gain is suitable for 500MV sensitivity, phono section is quite quiet... performance is smooth, but lacking some detail with Quad 405... I have not tried use it with other amps.

ARC SP14 is smooth and quiet but nothing special, even it was considered to be quite good in 90´s. ARC SP3 refurbishment is not finished, so can not say nothing for sure. ARC SP6C is my favorite pre amp. It was not liked due cold sound quality when introduced. I discovered that 6DJ8 internal shield´s were left floating... when I grounded the shields, performance improved quite much. Line gain is a bit too high for CD input. Phono stage is OK for MM or high output MC.

H/K Citation 11 is a bit harsher sounding and noisier than Citation 17. Both Citation´s do have suitable line gain for CD but phono´s are too noisy for MC´s and gain is also short for MC operation.

H/K Citation´s and Quad´s are easy to refurbish and there is good supply for any of them. ARC:s are not too easy to refurbish... twist lock electrolytics are not available for SP3 and SP6C... SP14 do have double sided PCB so replacing volume pot is major work without special tools.

I hope that this helps...

Kimmo

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Thanks again, Kent! And thank you for that detailed information, Kimmo. This information most certainly will help guide me through the maze of options that lies ahead. Awesome stack by the way!

Best Regards,

Greg

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Some additional information...

ARC SP14 do have 44dB gain in low noise phono stage... so it is suitable for MC:s up to Denon DL103 variety.

Quad 34 do have optional MC input modules available for phono section. Other inputs can be configured for different gain structures, Here is link for detailed Quad review... it is a bit over enthusiastic but mostly accurate except for stating case being made of heavy gauge metal... at least my Quad do have light weight case which has been deformed.

http://kenrockwell.com/audio/quad/34-preamplifier.htm

Other Quad 34 link if interested for modifications

http://www.keith-snook.info/quad-34-mods.html

Best Regards

Kimmo

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Well guys I got a little sidetracked. I really wanted to stay in the ‘85-‘90 sweet spot for vintage preamplifiers, I wanted to, but I let myself fall in love again. I do hope this satisfies my Sansui fetish.

I am not sure if these specs are even close to respectable but I am going to rig up a blind test at some point.

Sansui CA-2000 Preamp:

Frequency response: 10Hz to 80kHz

Total harmonic distortion: 0.03%

Input sensitivity: 2mV (MM), 150mV (DIN), 150mV (line)

Signal to noise ratio: 75dB (MM), 90dB (DIN), 90dB (line)

Output: 150mV (line), 30mV (DIN), 1V (Pre out), 12V (Pre out Max)

Year: 1978

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Nice!

I'll admit I know nothing about Sansui equipment. I understand they made some excellent stuff in the 70s and those sure LOOK cool!

Since you have a "Sansui fetish" I suppose you'll be looking for that matching amp. Like you, my current amp is an Adcom 555 but I'm a sucker for big amps with VU meters!

Congrats and keep us posted.

-Kent

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Greatly appreciated, kind sir!

It was not my intention but after I lost out on a few single CA-2000 preamp units I pulled the trigger on the on the CA/BA-2000 set shown in the photo. The preamp has 2 outputs so I should be able let it control both the Adcom 555 and the Sansui BA-2000. Not really sure how that is going to help me at the moment…Unless I find the holy grail and join you all in AR-3(a) land. I could let the Adcom control the 3a’s and the BA-2000 run the 2ax’s and so forth and so on. At the least, I figured it would give me the push I need to open the AR-2ax’s for I will not be able to use the newly found items until that main quest is complete.

Greg

Edit: It is well that I did grab the CA/BA-2000 for I just opened the Adcom GFA-555 for the first time since buying it used 2-1/2 years ago. I did not use it since that woeful morn' I lost my AR-2ax woofer.

In the photos we see, compared to the other three, the top board of the front left canister is blackened. I now wonder if this was the cause of my loss of woofer. With that thought in the back of my mind I packed it up and reluctantly put it in the hands of my tech... O lente, lente currite, noctis equi!

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I think I have so much to learn about vintage equipment as this thread "Woofers not performing as usual" scares the hell out of me, yet confirms that I should most likely send the CA/BA-2000 over to join the Adcom555 with my tech for a long once over. I am starting to rue the day that I ever stepped foot in the realms of vintage electronics but it is always darkest before the dawn.

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Yes, trusting some of this stuff can be difficult unless you know it has been gone thru. Of course that isn't always cheap either. Sometimes those sales that seem expensive but the piece has been serviced is actually the cheaper route.

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As simple as proper recap seems to be, it may cost as bit. Recap I made to my loved Armstrong 621 http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=8432 took almost 2 full days to perform. Measuring cap dimensions and figuring suitable voltages (stock voltages ratings are sometimes way off from optimum voltages) will take some time. Choosing suitable replacements, for 46 caps in this case from Mouser, online will take some time... finding correct pitch for radials and dimensions for axials will take some time. Caps do cost something, especially axials. Good quality 100uf/100v Vishay 021 series electrolytic is considerably more expensive than 100uf/100V NPE from ERSE... and this is much as 100uf NPE is actually 2 pcs 200uf caps connected back to back.

Experienced tech will be able to perform similar recap sooner, but I doubt that not in less than 4-5 hours. Going rate for repair is about 60E/h... so total cost of professional recap like this would be 290,00-350,00E including parts. This is not astronomical if you like the amplifier that needs proper rebuild. But if you are going to sell amplifier like this and going rate for original "working" one is something like 100,00E, proper professional recap does not make sense before sale.

Moral of this is... it pays off to ask what has been done in service and look inside amp if this is true. Usually I do find original working unit to be best option. This way proper rebuild is easiest to make as no un reversible modifications have been made.

Best Regards

kimmo

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Sorry to have turned this excellent page of yours to laments about my amplifier issues, Kimmo. After reading your comments and seeing your workmanship on that wonderful looking (and certainly sounding) Armstrong 621 I now aspire to learn the art of recapping and restoration of vintage equipment. Lots to study and prep-work ahead. For now I am still working on this Speaker Protection thing.

David? Have you tried your hands at recapping vintage amps and such? In most cases it is the only route to take for lack of experienced techs. And If one is lucky enough to find one the wait game is the killing factor.

Best Regards,

Greg

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Hi Greg

I've recapped a couple of receivers (Advent 300, MAC4100) and MANY radios. You'll need a good soldering iron and some type of solder sucker (the blue plunger type works well, or if you're really serious get a Hakko). Here's a thread by Whaleman: http://www.audiokarma.org/forums/showthread.php?t=176380 I hijacked it at Post #64 to show my MAC4100 recap but Whaleman provided all the information and inspiration.

The best source of caps is Mouser. You may have to inspect your originals to make a list of what's needed. Go with the same value in uF (+/- 10% is OK if it's an oddball size. For example, replace 5uF with 4.7). It's usually a good idea to go up a step in voltage (NEVER down) so a 5uF 16v cap could be replaced with a 4.7uF 25v.

Nichicon makes a variety of "Audio grade" caps and you can filter your search on Mouser or DigiKey fo Audio grade. Panasonic caps are good. For small values you may be able to replace electrolytics with film caps.

BE ABSOLUTELY SURE to observe correct polarity! Film caps are non-polar and Nichicon Muse caps are non-polar but those are the exceptions.

If you have any "can" twist-lock caps you "may" be able to find new ones at Antique Electronic Supply. Or you can just install individual caps under the chassis. Or you can "stuff" the old ones.

I wrote about recapping a radio here, attached to Post #1: http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=6387

Good luck.

-Kent

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Awesome! This is just the type of information I was looking for. Vintage electronics can become somewhat addictive and I find much enjoyment soldering things with molecular structure. Especially if the end result leads to enjoying music.

Best Regards,
Greg

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Sorry to have turned this excellent page of yours to laments about my amplifier issues, Kimmo. After reading your comments and seeing your workmanship on that wonderful looking (and certainly sounding) Armstrong 621 I now aspire to learn the art of recapping and restoration of vintage equipment. Lots to study and prep-work ahead. For now I am still working on this Speaker Protection thing.

Greg

No problem Greg...

I think that forums like this are important for two reasons. First one is information about old gear and working procedures. I feel also that sharing ideas is important. Talking about caps, absolute phase or polarity of mains plug may not change ones mind, but if some other person will at least try to find out if there is something real going on... this discussion has paid off.

Advice from Kent is solid. Factory cap ratings have usually been able to make amplifiers work for several decades, so they are safe to use. However, my only criticism for factory ratings fall in low voltage applications. Sometimes there is only fraction of volt across capacitor rated for 50 volts, even in short circuit condition. This may sound nit picking, but cap rated for 3 V or 6,3 V will live happier life in such place.

Before you rush in world of restoration of vintage amplifiers... please study carefully safety regulations and basic information about electricity. As you are living on 115 V side of pond, safety regulations were not as strict as on this side of pond especially in 70´s - 90´s. I have witnessed some very bizarre looking connections in high quality 70´s gear. Say... when I checked if there was any leak between amplifier chassis and mains plug live connection in my H/K Citation 15 tuner, I found 2M leak between plug and chassis. After some investigation I realized that 2M2 resistor placed between mains transformer primary and chassis... and this resistor was marked in H/K service manual, so it was no wiring error. Luckily lethality of 115 V mains is only aprox 20% compared to 230V mains, if current is limited only by human body.

Grounded 3 prong mains lead is probably best safety device I can recommend. I have taken habit to install grounded mains lead to all mains operated vintage equipment, if original does have only 2 prong mains lead. You should also check that ground connection from plug has been connected to amplifier chassis. This is not necessarily so... ARC SP6C plug ground was connected only to switched mains outlets and CJ Premier 7 plug grounds were connected only mains filter grounds isolated from chassis. I suppose that there are other similar connections too.

Sometimes mains hum problems have been blamed to be reason for using amplifiers without safety ground connection. Hum problems may sometimes to be difficult to sort due some fault in original design, but there has been no such problems that I have not been able to sort, even different ground lift schemes may have been experimented. I must point here that I am not professional or trained electrician, so some basic information, common sense and help from friends is all that is needed.

And... lastly thank you for your kind words about my Armstrong 621. It feels sometimes a bit odd to write nice thread without any feedback.

Best Regards

Kimmo

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This is becoming my favorite little corner as I try to get back to this page at least once or twice a week to reflect and chew on the wealth of the information, comments, and inspiration provided by you all. There is indeed a vast amount of knowledge that must be gained to enter this area of expertise and it gives me great courage to know that you fellows started your journey in a similar fashion. And cheers! on the careful study of safety regulations and basic information about electricity note! That 20% of lethality still is not favorable odds.

Best Regards,

Greg

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While still chasing the ever elusive definitive speaker protection system against DC, I came across a wonderful thread that really spelled out the power amplifier and why things go boom. So I would like to share the following link with others who are as befuddled as I in the lost art of this subject... http://www.harbeth.co.uk/usergroup/showthread.php?1738-Can-speakers-be-protected-from-amplifier-damage-How-do-amps-work

And below, as a sample of how illuminating this was, I copied post #2 from the link above:

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

A.S.

Harbeth UK

Join Date:Jan 2006Location:South of England, UKPosts:4,753

Default Thoughts about the workings of amplifiers ....

Quote Originally Posted by broadsword View Post

...I have read a few times comments on these forums from members (and AS himself) that if an amplifier fails it can destroy the speakers. What is it exactly that happens that can cause a speaker to be destroyed?...

First we have to consider what an amplifier actually is. It is nothing more than a high voltage reservoir - somewhere between 50 and 150 volts - which is sitting quietly minding its own business until some music appears at the input terminal and the volume control is advanced.

You can think of the amplifier as a water reservoir high in the mountains, filled with water and contained by a dam. The dam has an opening, a gate, and under electrical control that gate can be opened a little, a lot or fully opened. Think of the fast-moving gate as being controlled by the music signal. When the music becomes loud, the gate rapidly opens and we (the listener, via the speakers), downstream, receive a gush of water. When the music falls quiet, the gate closes-off and we receive a mere trickle. Pause the music completely and there is no need for any water to flow - the gate is completely closed. Dam operation here.

No, of course, there is far too much inertia in a heavy metal gate to be able to modulate the flow of water according to every note in the music, but the operational principal in the audio amp is exactly that. Rather than using moving parts, the 'gate' in the amplifier is literally the gate pin on the transistors and thermionic valves. See attached.

What can go wrong then? The danger inherent in the storage of any potential energy source (water, high voltage, explosive, nuclear, heat etc.) is that the gate will fail and there will be a runaway leakage of the energy from where you want it (the reservoir) to where you don't (the loudspeakers). The implication of 'runaway' is that once the gate can no longer hold back the power, the power reserve is completely out of control and only when all the energy has dissipated (the reservoir has run dry) will the danger be over. But there will be consequent destruction to put right.

What can you do to minimise the risk of the amplifier 'dumping' its entire power reserve on my speakers, and how long will they last if the amp's gate fails? From healthy to destroyed will take under ten seconds, too little time for you to realise what's happening and catapult yourself from your listening chair to pull-out the speaker plugs. Thankfully, electronics are very reliable and I've only seen this total destruction two or three times in the last 25 years or so. But there are some things you can consider which stack the odds for or against this happening to you.

1.The older the amp the more likely it is to fail. Components do not last forever, and capacitors are notorious for ageing. And the PSU reservoir is nothing but a bank of big capacitors!

2.Don't push your amp to the limits of its power ability. That will put strain on the components. So a 5W amp is just begging for trouble

3.Electronic components that run hot are inevitably less reliable than ones that don't. Solid state obviously is going to be more reliable than tube gear, all other things being equal

4.Budget equipment means that the specification of the electronic components has been made on cost grounds. That will inevitably impact on their durability

5.Cost into your amplifier ownership the need to have it properly serviced every 10-15 years or so, as recommended. Think of owning an amp as bringing the same responsibilities as replacing the timing belt in your car engine

6.Don't use an amplifier that is hugely more powerful than the speakers you are driving. You don't need the power reserve of the Hoover dam to fill one swimming pool: the more power the more destruction it can bring

7.Super-audiophile amplifiers that are capable of hundreds of watts and can hardly be lifted are just pure machismo. Their reservoirs have the explosive potential of a small bomb. When they fail the result is devastating, possibly combustion

8.For normal listening an amp in the 50-100W range is just fine and a good power match to most speakers

9.Turn off the amplifier when not in use at the wall switch. That lengthens the operational life of the components (which is stated in powered hours)

10.If you smell anything unusually hot in the amplifier or speakers, switch off immediately and seek professional advice

Common sense about heat and durability.

I've prepared an image of how an audio amplifier works. Any audio amplifier, from $50 to $50,000 - they all work this way. The circuitry could be a little less complex (but not much) and could be 100 times more complex (why?), but the result would be the same functionality: the reservoir high voltage is permitted in a controlled way to pass through to the loudspeaker. What you find under the lid is three basic blocks: the gate stuff, the reservoir capacitor (bank) and the mains transformer - that's the lot.

How it works: a second or two after you turn on the mains, the transformer charges up the capacitors. If they are in good condition and not old and leaky they will hold that charge. Every 25th of a second or so, another little pulse of power will be drawn from the mains supply and presented to the capacitor reservoir. If they have discharged a little since the last mains pulse (because they have delivered some music to the loudspeakers and that power had to come from somewhere) they'll greedily take-in that top-up. If they are already fully charged, that's it; the reservoir is full. So the capacitors are topped-up twenty five times a second or so if they need it.

The music signal is applied to the input of the circuit, the gate control arrangement. This particular mono circuit has four small signal transistors (Q1-4) and two really meaty output devices (Q5-6). Q1-6 are all three legged devices and I have marked the gate pin on each. Because the big old output devices are not very sensitive (but can really handle a lot of power, a trade-off) the small signal transistors incrementally boost the incoming audio from left to right through the circuit until it is strong enough to usefully drive the gate pins of the output bruisers, Q5, Q6. I've marked their gates in red. OK so far?

So the music signal instantaneously works its way from left to right through the amplifier until it reaches the gates on Q5 and Q6. Exactly as with the water dam, those gates let trough the voltage stored in the reservoir capacitors to the speaker. If the music at any one instant becomes louder, the gates progressively 'turn-on' more and really let the juice flow from the PSU to the speaker .... if the music drops in level, Q5-6 progressively shut-off and the voltage flow from the reservoir diminishes to a trickle as commanded by the much smaller music signal.

What fails? Any of these components could fail at any time, but experience tells us that anything that's running hot, either because it is working hard (such as Q5 and Q6 when playing rock music very loud) and/or poorly ventilated is susceptible to fail. And we know that the reservoir bank of capacitors, upon which the whole beast depends, is very vulnerable to ageing. So you could argue, with good justification, that the circuit design is not much important for the sonics but that the design of the seemingly humble PSU is in fact, much more important to the sound we hear. And it's the one part of the design which is vulnerable to age which must imply, I suppose, that an amplifier with a linear PUS as I've shown, could indeed measure, perform and sound different on day one or day one thousand of its life.

That's really all you need to know about audio amplifiers. They are simple, dumb closed-systems with wholly predictable performance that can be software modelled to very fine degree. There really isn't any room for black arts in amplifier design if minimising long-term warranty claims is your commercial goal. Hope that helps. (3 hours)

Attached Images Attached Images File Type: jpg Amp_principle-sc.jpg (142.8 KB, 66 views)

Alan A. Shaw

Designer, owner

Harbeth Audio UK

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Best Regards,

Greg

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Original Quad 405 clamp circuit on page 9 on the following service manual do use quite clever DC protection device. Operation of clamp circuit is explained on page 9 of service manual http://www.keith-snook.info/amplifier-hifi-schematics/QUAD%20405%20Service%20Data.pdf. Circuit was improved later in some Quad amplifiers like professional Quad 240.This amplifier do use active earth PSU, which makes it possible to use only one fuse for ea PSU, but needs own PSU for each channel. http://www.quad-hifi.info/public/quad_240%5B1737%5D.pdf

It should be noted that this clamp circuit protects only against excessive DC in output and works only with current limiting stage in power amplifier, unless output devices are very robust. More is needed if protection is needed against spikes, instability or oscillation. Output relay with some sensing circuit is usually used to avoid this kind of disturbance entering in loudspeaker terminals. But relays do oxidize and have to be replaced sometimes for good performance. More sophisticated sensing system for clamp circuit looks best approach to me. But limits for safe operation must be factored with care. This was culprit in Quad 405 design as safe operation area was so limited in original design. However, I have not found this to be any problem with late production Quad 405 (not improved Quad 405-2) or in Quad 240 I do have, even in spirited listening sessions with sensitive 8 ohm speakers. When more power is needed with difficult loads, problems do arise.

Nice feature of this clamp circuit is that there is nothing additional to interrupt signal flow to loudspeakers, when everything is working perfectly.

Best Regards

Kimmo

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This Welleman K4700 speaker protection kin may be easiest way to add DC protection to amplifier without factory installed DC protection.http://www.dadaelectronics.eu/shop/velleman-kits-and-modules/velleman-k4700-loudspeaker-protection-delay-kit. It is made for 230 VAC mains... so mains transformer on the PCB should be changed to one with 120VAC primary, if you are living on the 120VAC side of pond. Heavier duty output relays can be used if you wish, but most likely they will not fit on the PCB.

If you like to add immediate switch off after turn off, the following Dada kit will be better and more advanced but more expensive protection circuit http://www.dadaelectronics.eu/shop/dada-high-end-boards/dada-electronics-loudspeaker-protection-delay. As far as I understand this one is powered from amplifier PSU... so it will need no mods for 120VAC mains.

K4700 does not cost arm and leg and is widely available from Dada and other sources. More advanced Dada unit is more expensive, especially as it looks that two are needed for stereo ... but they both do have relay protected outputs.

Here are some info about installation of such kit to some Quad amplifiers http://quadrevisionspot.blogspot.fi/2006/05/velleman-k4700-in-quads-ed-pushes-it.html .

I hope that this helps.

Best Regards

Kimmo

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Thank you, Kimmo! I got a little derailed from this subject but I will soon be back on track to find the perfect solution for DC protection. And your information is always greatly appreciated!

Best Regards

Greg

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