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Old 23rd December 2012, 01:39 AM   #61
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I just realized you may have a problem...can your meter read down to mV? If not, we are in the dark setting bias properly.
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Old 25th December 2012, 10:58 PM   #62
gilwe is offline gilwe  United States
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Hi !

I can read mV (have a Fluke DMM here) so no problem.

I rechecked everything and here's what I get:

The voltage across the "56R" (560ohm here) is about 0.650v (not 1.0v)

The refurbished channel's output reads 15mv and has quite loud noticeable buzz and hum noise to it. The "good" channel has some buzz too, but much quieter.

I noticed that the ZTX752 output transistor (BC640 here) gets much hotter than the ZTX one on the good channel. Not to a point that makes the bulb light on, but quite hot. Is that for using an improper transistor as output ?

I also noticed something really strange... After letting the amp run with the bulb limiter for half an hour without issues, I went to install it in my studio. It has always been connected to the NS10M monitors, so I connected both monitors to the amp and turned it on. The bulb lit up !!

I powered it off and disconnected the monitors - no issue. I then found that when connecting one of the monitors, it causes the bulb to light up - what probably caused the disaster in the first place ! (it happened while reconnecting the monitors once moving the amp to a new location). The monitor lead cable has no short between the terminals, nor they touch each other on the monitor input area... both monitors read between 7.5 and 8 ohm when testing the resistance at the cable endings... and both works well with my QUAD 303. So I just wonder what could cause it to overload or short ? Does it mean the monitor has some fault ? (bad crossover caps maybe?)

Also I wonder where the loud buzz is coming from - is this because I'm not using the original ZTX parts ?

Last edited by gilwe; 25th December 2012 at 11:20 PM.
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Old 26th December 2012, 09:04 AM   #63
gilwe is offline gilwe  United States
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Regarding the bias, it is currently set to the minimum the trimpot allows: 3mV across both 0.22ohm resistors...

Is the louder hum/buzz at the output terminal of the bad channel a result of the higher voltage reading there (15mv) and
is the high voltage is a result of bad matching of Q1 and Q2 ?

Last edited by gilwe; 26th December 2012 at 09:24 AM.
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Old 26th December 2012, 06:51 PM   #64
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Hi, I trust Christmas season goes great for you.
Well, you sure like to bring the problems on by leaping ahead. 'Lucky there were no disasters yet. Yes, as explained, the BC639/40 don't dissipate heat as well as Zetex types which is the same as saying they get hotter for the same duty. However, if you can hold the transistor for 6 seconds (ROT) it will be OK.

The bulb is there to severely limit current, so even the small components are protected to some extent. When you connect a load to the amplifier and there is a signal of any sort - even hum, noise etc - the current flows and the bulb glows because it is in the current path supplying the amp. That will happen with most speakers and naturally, it will occur first in the channel with most noise. What you probably didn't notice is that when the bulb glows, the power supply voltage has taken a nosedive and the amplifier probably ceased to function properly at some point when the voltages fell so low as to turn off the transistors in the signal path.

Please don't connect a load again until you have sorted the bias and got it stable, and proven that all components are working properly etc. You don't want to fry your speakers as well as the amplifier, surely.
15mV offset, BTW, is fine. It is not related to the noise.
0.65V across 560R is ~1.2 mA bias - very small but expected in this design.


First, crank up the output stage bias to read 10mV and watch the meter. Test the good channel too.
They will probably slowly drift from cold as they warm up and the bulb may come on briefly. If the voltage appears stable and only slowly drifting, close the case and recheck in 10 mins. Continue until you get a stable reading which might take anything up to 30min. It should stabilize at a similar rate to the good channel and that's another reason to use similar transistors.

If this proved OK you can, if you wish, set it to the same measurement as you read on the good channel.

Hum and Buzz. Apart from input problems with components and connections, the chief problem is dead caps. With a severe event such as the amp. had, C3 is on my list for replacement. It doesn't strictly need to be a Kemet tantalum type, any good quality 100 uF electrolytic >16V rating will do, if it physically fits. I prefer Panasonic but Elna, NCC, Rubycon, Nichicon are also respected brands. Replace the others, (C6 and C7) as insurance too. Higher voltages are fine (though physically larger) but lower are not. If you replace the input cap with an electrolytic, It should be a non-polarized type, which won't be easy to find in typical parts stores. Speaker caps (Bipolar) aren't quite the same and not really suitable.

When you test the amplifier, short the inputs so that no spurious source noise confuses the test. If you have some 1-10k resistors, solder or wedge that across the input to check that noise is still acceptable at higher impedance.

Keep us posted on noise, it should be about the only difficulty after bias setting. It can be an input transistor problem and the originals were very low noise. Generic BC 549 can be rubbish. I try to use BC550C which is from Fairchild or On semi and well specified. Semi brands can be very important.

When the amplifier proves stable running at 10mV bias voltage, you can try it without the bulb and playing at very low level. Keep the meter attached so that you can stop the input and check the bias periodically. If that's OK, and all semis are similar temperatures in both channels, I guess it's smoke test time.
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Last edited by Ian Finch; 26th December 2012 at 06:59 PM.
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Old 26th December 2012, 07:09 PM   #65
gilwe is offline gilwe  United States
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Thanks for the directions. All caps are new... What was your comment about the Q1/Q2 matching ? or could I have the louder buzz as the "bad" channel now just amplifies louder than the "good" channel (which also has some buzz noise, but much quieter)

Regarding the speaker issue, just to make clear -

What happens is that when I connecting the left speaker to the "bad" channel, there is not issue the bulb doesn't light). When connecting the right speaker to the same "bad" channel - it lights up. The first disaster happened while I moved the amplifier to a new location, and reconnected the speakers to it - which was connecting the right speaker to the bad channel.

The second time the bias transistor blown (it actually broke from the spark, exactly like the original ZTX384C) was after I put the new ZTX transistors and connected the speakers the same way.

What I mean to say is that is something in the specific monitor which makes the channel fry... When I connect my other Tannoy speakers to either of the channel - no issues.

So I wonder what is in that specific speaker that would make the channel get fried ?
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Old 26th December 2012, 07:52 PM   #66
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OK, I get it. Well, you won't know the answer without examination or perhaps another member who had the the same issue. NS10s were popular but they had issues. You could Google and sniff about the web but the options are a fried cap, shorted lead or voice coil in the speaker. You are going to have to prove what it is to yourself someday, I guess. You could start with DC resistance across the driver terminals. An 8R driver usually reads ~6.5R

Don't get sidetracked trying to trim offset for what are only aesthetic reasons. Some people ulcer over offset voltage but anything under 40mV is piffle. A new Marantz 7004 here reads 17mV and 8 mV. Trim it if its convenient and you have enough transistors to mix and match. #54 detailed matching but that will vary again when in circuit. Patience is also needed.
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Old 26th December 2012, 10:41 PM   #67
gilwe is offline gilwe  United States
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Hi Ian, thanks.

Well, I now have the NAP90 running for a few hours with no issues, playing some music from a CD player
I'm having it connected to a pair of old Tannoy Mercury-S speakers.

The bias is stable on both channel and is set to 10mV.

It is noticable that the "good" channel sounds better (cleaner and more detailed). I wonder why you said the J13009-2 were not suitable in that circuit ? The sound quality on that channel is really good.

Also, do you think the original BD743 NAIM used as NA005 will sound or perform better than the BD911 you suggested ?

As for the NS10M monitor, I really have no clue... it works well with my QUAD 303. No shorts between the wire leads. Ohm reading is as on the second monitor unit. Caps are not leaky inside. This is just strange.

As for the buzz/hum on the "bad" channel, considering the caps are new, could that be a result of the poor quality/manufacturer of the replacement transistors ?

Last edited by gilwe; 26th December 2012 at 10:57 PM.
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Old 27th December 2012, 02:46 AM   #68
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Don't assume this is must be due to output transistors. Genuine BD911 is as close as you will get to BD743 with current production transistors, AFAIK. Such an obvious difference will more likely come from the VAS transistors where the critical Miller compensation capacitor C4 will not match correctly to the BC transistor type. We only used these to save cash and get started. Minor effects can be heard from different electro. capacitors and possibly the input transistors. You could also experiment with removing just the limiter transistors again as this is what many Naim enthusiasts routinely do, whether they perceive a difference or not. Just check the sites I referred to but given your many types of use, I wouldn't recommended it.

It's interesting that you compare with a Quad 303 - I'm listening with one today as well.
These are AC coupled with a big cap. so shorts in the load can't cause a DC meltdown.
Only a high level AC output will stress them and being suited to ESL 57s, they can drive a dead short,
like the bigger, current-dumping models 405, 606 etc. Even the little 306 is mighty tough.
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Old 27th December 2012, 03:29 AM   #69
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Re: MJE13009 etc. The Safe Operating Area and linearity of these transistors is not as good for audio as BD911, 734 and similar family. In a nutshell, they will fail sooner and probably within the amplifier's max. power specification. It has happened when the limiter might otherwise have protected it already.
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Old 27th December 2012, 09:20 AM   #70
gilwe is offline gilwe  United States
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Thanks, I will make an inventory for the correct parts now so I can install them asap.

Just to make sure I don't fry the amp again:

Currently what happens is that if the Tannoy speakers are connected to the amp (both channels) and I power it on, the bulb will light, a loud hum noise will be heard on both channel and the amp won't amplify.

When the amp is turned on while the speakers are disconnected, the bulb doesn't light. When then I connect the speakers (while the amp is powered on), still the bulb doesn't light and I can hear the amplification on both channels.

The question is - is that how it supposed to work ? in other words - can I now safely connect the amp directly to the 230v supply with no bulb limiter in the circuit - and turn it on, with the speakers connected ? or am I going to fry it again ?

Thanks a lot !
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