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Old 11th September 2011, 04:49 PM   #451
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Some heat discolouration is normal, but all that white stuff doesn't look so good, especially near the input on the left of the picture. I agree that it looks like someone's worked on it. I wonder if any of that mess is conductive....

I recommend that you get yourself a new multimeter as your first order of business, even before you try powering this on again, as something is definitely not right. You don't want to run the amp with the bias current unknown as you'd only risk damaging it. I'd start by pulling the board and check the dodgy-looking soldering. Some of those joints probably need redoing and there might even be a slight short circuit somewhere.

Once you have checked it and cleaned it up, check and set the bias. The pots should start off at the centre point. I run my amp at 20mV on stock heatsinks.

If you go to an electronics store, pick up a couple of those clip-on transistor heatsinks, as a couple of those run quite hot anyway.

I recommend that you get yourself a one-handed solder sucker as well, if you don't already have one. I find the easiest way to clean up those old joints is to put some more rosin core solder on top to melt whatever's there, suck it all off and then re-solder from clean.
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Last edited by Ben Hanke; 11th September 2011 at 05:11 PM.
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Old 11th September 2011, 11:23 PM   #452
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Sorted, Ben. I'll get all that done.

What I'm hoping to do, once both amps are fine, is split the crossovers in the Tannoys and fit two pairs of binding posts to each, then run them bi-amped, using the volume control on the CDP. Next step, if the Tannoys' crossovers are amenable, is to go active, probably with a basic in-car active crossover first. Once that's done, I'll start modding the amps themselves.

I like the idea, which one previous poster mentioned, of a separate PSU for the front-end IC in the PM-66 KIS. I have a couple of pairs of Trio/Kenwood L-08M monoblocs, and each monobloc has two transformers, one for the input/driver stage, the other for the output stage. Even mine, which aren't properly refurbed and have failing electrolytics, have a superbly open, three-dimensional sound (though presumably their PSRR is unlikely to be as good as the IC in the Marantz).

Another idea might be to do what Kenwood did in the L-09M monoblocs, where the two large mains trannies feed one rail each. Of course, going the whole hog would involve a small supply for the Marantz's front-end, and two toroids wired together, or with one for each of the rails, or one for each of the power stage's channels.

Interesting as well was what another previous poster wrote about E/I vs toroid as far as rejection of mains-borne junk goes: E/Is have a far narrower bandwidth in terms of allowing junk from the primary to get into the secondary. I've also got a low-power R-core sitting in the cupboard at home, which might be worth bolting into one of the amps to power its front-end.

Cheers, Jon.
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Old 12th September 2011, 03:47 AM   #453
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Hey Jon,

Your speakers are two way monitors, am I right? I'm wondering what benefit you expect to get from bi-amping them? My B&Ws are two-way also, and have two sets of binding posts each for bi-amping, but I thought tweeters take so little power that it wasn't really worth it. I presume if you're planning on doing different mods for each amp then you're thinking of using a whole amplifier just to drive your tweeters rather than one amp per channel?

I don't want to put a downer on your plans just as you're getting started, but I have to give you a heads up about the volume control on the CD player - you'll have to listen to it yourself and make your own mind up, but I personally think it degrades the sound quality quite badly. Before you lose your pot, try comparing the sound of the CDP at full volume versus any other level and I think you'll hear a difference. If you hear what I hear then you might be better using a separate passive pre-amp or find out a way to use the volume pot in one of the amps.

Cheers,

Ben
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Old 12th September 2011, 04:45 PM   #454
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My amp has 30mV DC offset per channel on the output. I wonder if there's any way to get that down...?
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Old 13th September 2011, 11:40 PM   #455
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Hi Ben.

I've bi-amped before, and it's always brought improvements, sometimes very obvious, sometimes subtler. Don't forget, the amps aren't driving the tweeters or mid/bass units directly - they have to go through the crossovers, which, in conjunction with the drivers, tend to be relatively tough loads compared to a simple, purely resistive load. Separating the amps also reduces intermodulation distortion, AFAIA: ie, where the big bass peaks modulate, to a certain extent, the more delicate high-frequency signals.

In terms of arranging the amps, I was going to try one per speakers as well as the alternative, one per pair of the same driver type, to see which works best, Anyway, all of this is only a stepping stone towards going active, which, if done properly, invariably yields very clear benefits.

I'll have another listen to the CDP's volume control (not something I've done in any critical way so far, as the system as it currently stands is in my girlfriend's living room). I could always keep an eye out for a pre, and tweak that too (I'm thinking stepped attenuator, certainly).

Cheers, Jon.
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Old 14th September 2011, 07:06 AM   #456
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Hey Jon, thanks for the explanation. I just assumed most of the power of the tweeter amp would end up getting dissipated as heat.

A stepped attenuator sounds like a neat way to get two amps to work together in tandem. I've seen some DACT ones that have several channels stacked together.

Cheers,

Ben
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Old 19th September 2011, 03:04 PM   #457
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Hi folks,

I'm concerned about DC offset. Can anyone help me out?

Firstly, I measured across the output of my amp with the speakers connected and no music playing. Is that a valid test?

Secondly, assuming I measured correctly, does anyone know how I can trim my DC offset at the output? B&W advised me that it should not exceed 25mV, and I think within 20mV is the generally accepted allowable limit.

Thanks in advance,

Ben
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Old 19th September 2011, 03:34 PM   #458
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Hi Ben,

I think your test was sensible but I would also turn the volume through the normal range and measure the DC output of each channel as this is done.

I don't know how to trim it, sorry.

Simon
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Old 31st December 2011, 11:28 PM   #459
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Hey guys, I finally got around to doing the Schottky bridge. Wow, what a great mod! It's made a huge improvement! I've had the diodes just sitting there for more than a year.

I also have the beefy-looking 100 watt transistors sitting in my box of bits. Any opinions on whether I should attempt swapping those in too? I don't think they're the same manufacturer - pos are Tosh and neg are Magnatec or something - don't know if that matters at all.
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Old 1st January 2012, 02:59 AM   #460
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Hi Ben,

My best guess is that it does matter a great deal. It has been said that transistors are the single most important component in terms of quality vs the effect on sound. Even if that effect is being exaggerated I would make sure they're all good ones because they'll be quite a fuss to change.

Simon
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