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Old 10th July 2011, 09:37 AM   #2001
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Hi Bigun,

On my side, I fixed the PIN-1 issues on both monoblocks that was participating to the buzz of the setup. The buzz has reduced a lot but not fully disapeared. I'm doing measurements between chassis, power ground, earth safety ground, signal ground to see how the whole thing behaves. I started rewiring the grounding completely on one mono-block to have an A-B comparison point. The next steps will be to implement some shielding, doing physical layout changes, implementing a ground loop breaker.

Regarding the PSU, I'm contemplating the idea to implement two PSU, one with voltage regulation for the first two stages and one with C-R-C or C-L-C style filtering for the last stage. I've seen this in a class A amp as the needs are quite different between these two parts of the amplifier but don't know yet how this would work here.

The first step will be to do some simulation using SPICE - good activity for summer break rainy days to learn new things

Cheers,
Jean-François.
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Old 10th July 2011, 11:09 AM   #2002
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A few short cables with crocodile clips on either side is great for exploring groundloop issues.
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Old 10th July 2011, 01:53 PM   #2003
Bigun is offline Bigun  Canada
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Yue1970,

....be careful how you split up this amp into stages for different power supplies. Actually, I don't see how the JLH '69 version can be configured with split supplies, it seems to me it will mess it up.

Another thing - be careful about flux leakage from your power trafo, it can inject hum if you have sensitive circuits nearby.

Voltage regulation will certainly clean up the power rails, but it puts quite a burden on the power supply as you need to allow enough voltage drop across the regulators for fluctuations in the mains AC and this means quite a bit of power dissipation in the regulators. Not that you should avoid this though, it will really clean things up. I don't have enough voltage headroom to do this as my power trafo doesn't give me enough headroom to start with.
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Last edited by Bigun; 10th July 2011 at 01:55 PM.
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Old 30th September 2011, 06:47 AM   #2004
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Hi all,

Well after a comedy first attempt, I have revisited my 1996 JLH and have finally had success.
I settled for TIP3055s this time as an experiment, and smaller heatsinks than I wanted to use. They get very hot at 1A, so at the moment, I'm not exceeding that.
My PSU is a little underrated in the voltage department. I'm getting +/- 17v from it - so instead of using a 7815 regulator, I'm currrently using a 7812.
oh, and the PSU is unregulated at the moment too - I'm worried that any regulation or capacitance multiplier would drop the voltage too much (how much voltage drop would a capacitance multiplier cause anyway?).

After a bit of tweaking to get the dc offset right (I'm less pedantic about Iq - about an amp will do for now), it seems pretty stable. Even on a rubbish little speaker I had lying around, it sounds remarkable. And very loud.
I've fitted a 1A fuse for now (is that a sensible value?) and I've hooked it up to a Mission 751 and it just sounds awesome! really detailed. I think the lack of class B-esque crossover distortion must make it seem 'louder' than a class B amp (I recall in on of Mr Hood's books, he describes crossover distortion as making the amp sound 'thin').
I have a lot of hum, I suspect that is possibly an earthing issue I have (I haven't exactly followed the earthing suggestions on Geoff's website - yet), but also the PSU is not regulated, and there's probably not enough capacitance (20,000uF per rail at the moment - to be increased).

So, should I consider regulation - even though my rail voltage could drop to something like 14-15V? Or should I go hunting for a different toroid?
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Old 30th September 2011, 07:01 AM   #2005
Mooly is offline Mooly  United Kingdom
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20,000 uf for 1 amp current draw is plenty. A little hum (as in ears buried inside bass speaker of a floorstander) would be normal but anything more is not.

So I too suspect your hum is down to wiring issues...
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Old 1st October 2011, 04:25 PM   #2006
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Ok, well, maybe its an acceptable level of hum then. I first had a lot when I just used the power rail instead of the regulated supply to the dc offset circuit (I didn't think I had enough headroom for a 7815 at 17V). Then I fitted a 7812 for that and it improved, but I was still a bit paranoid about the hum. I can really only hear it with my head next to the speaker.

Been fitting a bracket to a big heatsink in order to fit some MJ15003s to see how that sounds (and so I can set a bigger Iq).

Don't think I'll investigate regulation at the moment, just increase capacitance. I don't want to drop 2v per rail, and I like the idea of low output impedance for the PSU.

Do any of you worry about speaker protection? I have a 1A fast blow fuse at the moment, but am thinking of implementing relay protection (as in Doug Self's book). At the moment, I'm constantly checking the output DC with a multimeter (that's going to be trickier with stereo and one multimeter), and hoping the fuse works (and vowing to not plug in my LS3/5As - no matter how tempting it seems).

Last edited by philpoole; 1st October 2011 at 04:29 PM.
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Old 1st October 2011, 05:52 PM   #2007
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As to the hum...

The nature of hum is a big clue. Pure deep 50/60hz is normally induced into the inputs etc. Always evaluate hum with the inputs shorted.

If harmonics are present (harsh raspy sound) then it's actually at twice line frequency and down to PSU/wiring/grounding issues.

Some amps just have a relatively poor PSU ripple rejection ratio meaning noise on one or both rails can be heard.

If the hum varies with bias current (as in more curent drawn = greater ripple) then you have another clue to work on. Try each amp (of a stereo pair if they share the PSU) in isolation, that is to say one amp only powered and its input shorted directly at the input to the amp.

DC offset protection... tough one to answer as it's tears at bedtime if a valued speaker goes up.
If the design is good and properly built and not subject to abuse (shorting speaker leads etc) then I would be reasonably happy without. To me its more important that the design is silent at power on/off with a foolproof arrangement. As to speaker relays, I've been converted to solid state... have a read at this (all of it )
Output Relays
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Old 1st October 2011, 08:24 PM   #2008
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Thanks for the advice mooly, much appreciated.
I'm happy with the hum level, it is low really (I'm used to class B amps with virtually no hum at all), but I will investigate. I hadn't thought of lowering the bias (although I was keenly aware that I'd get more ripple when I up Iq).

Now running with MJ15003s on a bigger heatsink. Takes longer to heat up, but still gets rather hot . Can't really tell if it's better (possibly is) than TIP3055, as I'm listening in mono, but I can play with increasing Iq.

After an initial settling time of a couple of minutes, and setting the DC offset to close to zero, the DC offset rose by only about 2 hundredths of a volt - so does seem quite stable (I wonder if it will ever completely settle? I guess so).

Thanks for the link, I shall have a read...
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Old 2nd October 2011, 06:54 AM   #2009
Mooly is offline Mooly  United Kingdom
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That's a very low DC offset for a single ended DC coupled amp (I remember JLH presenting the modified version in Wireless World). It will never be totally drift free or absolutely stable with temperature but it's good enough.

From memory... in the Wireless World article there was an error. The feedback return electroylitic was shown coupled to the negative rail ? rather than ground and that worsened the hum/noise performance. A correction was issued. Wiring is so important... even a cm or so of wire or print develops enough volt drop (depending whats running through it) to cause hum if say a feedback return and input ground share that connection.
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Old 2nd October 2011, 07:04 AM   #2010
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You could use 2 electrolytics back to back,
with center pulled negative by 1 megaohm.
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