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Old 11th January 2013, 07:31 AM   #2671
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Quote:
Originally Posted by TimA View Post
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.
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
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Old 11th January 2013, 07:54 AM   #2672
mikelm is offline mikelm  England
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Quote:
Originally Posted by philpoole View Post
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).
You might like to try small ( 1uF ) film caps across the fuses - it might sound a little cleaner
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Old 11th January 2013, 11:48 AM   #2673
AndrewT is offline AndrewT  Scotland
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Originally Posted by mikelm View Post
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.
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.


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Last edited by AndrewT; 11th January 2013 at 12:04 PM.
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Old 11th January 2013, 11:57 AM   #2674
mikelm is offline mikelm  England
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I wasn't thinking of resistively damped - just a 1uF film cap across the fuse
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Old 13th January 2013, 03:20 PM   #2675
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Quote:
Originally Posted by deduikertjes View Post
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
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.
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File Type: gif jlhcapmultfig1.gif (8.4 KB, 657 views)
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Old 14th January 2013, 08:10 AM   #2676
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Quote:
Originally Posted by TimA View Post
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.
Nice! Thank you, MArco
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Old 25th January 2013, 10:29 AM   #2677
Puffin is offline Puffin  United Kingdom
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My completed JLH with speaker protection.

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Old 29th January 2013, 07:46 AM   #2678
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Default 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
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Old 29th January 2013, 07:57 AM   #2679
Mooly is offline Mooly  United Kingdom
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The gain is set by the feedback factor of R6 and R8 in the circuit in your link (the transistors have no influence). Altering the gain could/would alter the whole character of the amp, its stability margins and DC conditions.
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Old 29th January 2013, 08:18 AM   #2680
AndrewT is offline AndrewT  Scotland
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If a low powered system is not loud enough then it is wrong to look at increasing gain or increasing input signal to raise the signal sent to the speaker.

It is far better to look at increasing the efficiency of the speaker to give the sound output available from the low power amplifier.
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