50mV is not a problem, I would set it so that it is closest to zero when fully warmed up.
For my peace of mind in my fully DC linked designs I have resorted to a DC sensing circuit that switches off the amp if the averaged DC at the o/p reaches about 1.4V which - even with 4 ohm speakers - will only generate about 1/2W worst case
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On Rod Elliots site you'll find a design that switches of the speakers in case of a DC offset. It prevents turn-on noises as well.
Such a thing protects your speakers when one of the power supply rails die or one of the output transistors or ...
Saved my speakers more than once.
MArco
Yeah - saw that one but in the end I designed one that didn't include switched outputs - because we expect / hope that this will be a very rare event that this circuit will actually switch I elected to simply have the amp turn off which means the relay can switch the mains - a less critical connection in terms of impacting audio quality.
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The trick is to find a relay which doesn't degrade the sound to much when it is connected into the signal path. I tried a Velleman kit with my JLH some years back but it added a hard, metallic quality to the sound. I eventually settled on a circuit which shuts the amp down by reducing the power supply rails to almost nothing via small relays connected in the voltage regulator circuit of the power supply. It works like a dream and doesn't degrade the sound.
Tim.
Tim.
My JLH 96 has no capacitor, so I fitted fuses, assuming it would be a stop gap, and haven't gone back.
After a good year of occasional use, they haven't blown, and I doubt they are really impairing the sound (best amp I've owned!). When warm, the offset is typically around 20mV I guess (they've varied over time as I've tweaked the bias current).
After a good year of occasional use, they haven't blown, and I doubt they are really impairing the sound (best amp I've owned!). When warm, the offset is typically around 20mV I guess (they've varied over time as I've tweaked the bias current).
Would you mind sharing the circuit? Sounds interesting to say the least.
Is the soultion fast enough for protecting the speakers on e.g. a dying rail?
Thanks, MArco
You might like to try small ( 1uF ) film caps across the fuses - it might sound a little cleaner
A resistively damped HF bypass across the milli-ohms of connection resistances.You might like to try small ( 1uF ) film caps across the fuses - it might sound a little cleaner
That's one I have not noticed before, but probably worth exploring.
The resistive damping is inherent in the fuse wire resistance and the fuse connector resistances, plus a little bit of trace resistances.
That's enough from me today.
Taken me two weeks of non posting to calm down and quell the anger that built up.
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Geoff Moss kindly provided me with the circuit. It is based on a standard detector circuit similar to that used in Rod Elliot's project 33 speaker protection circuit. It does not provide any time delay/muting at switch on.
The protection circuit is powered from the +ve rail via a fixed regulator which is fed from the +ve rail just after the reservoir capacitor. During a fault the amplifier is effectively shut down as the supply rail voltages are reduced to close to zero by a double pole relay which simultaneously disconnects the base of Q1 and Q2 (in the attached diagram) from rest of the capacitance multiplier circuit. The protection circuit remains powered until it is 'reset' by manual switch off/on. The circuit 'fails to safe' so Q1 & Q2 are disconnected during power down also.
My capacitance multiplier also includes a CCS voltage regulator which is not shown in the attached but I imagine the disconnection of the base of Q1 & Q2 would have a similar effect with or without this. I believe the circuit will give adequate protection under most scenarios.
Tim.
Attachments
Increase gain
Hi List,
I'm very happy with my JLH1996 which I've build like fig 3 on The Class-A Amplifier Site - JLH Class-A Update with some minor deviations.
I'm using it in combination with a B1 buffer pre amp with no gain. In this combination the input sensitivity of the JLH is too low for my liking. I need some more gain.
Is there a way to increase the gain (enlarge input sensitivity) without changing the design of the amp to much (eg by changing transistors)?
Thanks, MArco
Hi List,
I'm very happy with my JLH1996 which I've build like fig 3 on The Class-A Amplifier Site - JLH Class-A Update with some minor deviations.
I'm using it in combination with a B1 buffer pre amp with no gain. In this combination the input sensitivity of the JLH is too low for my liking. I need some more gain.
Is there a way to increase the gain (enlarge input sensitivity) without changing the design of the amp to much (eg by changing transistors)?
Thanks, MArco
Thank you. My system is not low powered. I've built the high power version with double output transistors.
The problem is that I am not using this power. With the B1 preamp and the volume on max the system is not loud enough. If I use a different preamp with some gain, the system is more than loud enough.
So that is why I'm looking for some gain, but I don't like to ditch the nice sounding B1 or add gain stages to it. Some more gain in the power amp would be great.
MArco