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Old 29th March 2003, 01:32 PM   #21
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Quote:
Originally posted by trwh
Hi I have built my LM3886 PA amp though, here's a pic.
wow man.... i jast love your AMP-PIC......
how much it is cost to u to build this amp...?
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Old 29th March 2003, 06:37 PM   #22
trwh is offline trwh  United Kingdom
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Hi DJ.

The amp didn't cost much to build - I used spare / salvaged parts wherever I could. The main construction costs were: the heatsink (from RS), the PCBs from Rod, the toroidal transformer (very cheap from Rapid Electronics), the reservoir caps (budget types from Rapid again), and the chips themselves (from RS). A lot of the components, like the bridge rectifier, connectors, limiter circuit, etc. were taken from my failed Velleman kit.

All in all, if I were to make the amp again, it would probably cost me around £120-130UK.

Cheers,
Tim.
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Old 29th March 2003, 07:22 PM   #23
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The output RC network will want to be something more like 5 ohms @ 3W and .1uF. 100nF is probably too low.
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Old 29th March 2003, 07:32 PM   #24
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Default The Nanofarad....

Quote:
The output RC network will want to be something more like 5 ohms @ 3W and .1uF. 100nF is probably too low
0,1uF = 100 nF .... (1uF =1000 nF)

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Old 26th June 2008, 11:15 PM   #25
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trwh




you said you were never able to get the velleman kit stable. Is this even when connected to the speaker?





I suppose being surrounded by valve amps has brought me to expect needing a speaker load. Personally I have found amp designs that were marginally stable to sound better with flatter frequency response.
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Old 26th June 2008, 11:35 PM   #26
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Default Re: Running amps unloaded

Quote:
Originally posted by trwh
Why should there be any problems running a solid state amplifier with no load on the output? I know tube amps need loading at all times, I think to prevent voltage spikes being fed into the output stage due to ringing in the transformer.

Some people may remember reading about my difficulties with the Velleman K4005 amp - I have since been told that the amp kept overheating because I was conducting my tests with the unit unloaded. Is this poor design or a common phenomenon? I run my P3A amp in “mute” mode (i.e. no speakers connected) for hours on end, and it doesn’t bat an eyelid. Can someone explain this more fully to me?

Thanks,
Tim.
Sounds to me like a bias setting problem.
I can get my MOSFET amp to run hot if I turn up the bias.
This increases the standing current throught output transistors even if there is no load.
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Old 27th June 2008, 03:30 AM   #27
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My parents had a very old very low quality turntable (bettor) that broke each time it ran unloaded.

The effect was fast and we had several repairs when one of the channels did a bad contact after cleaning it. The last time it failed the guy at the technical service said he could not repair it because he could not find spare transistors, so they probably were germaniums and the amp perhaps was trasformer-coupled push-pull. I doubt it was class A as it did not get very hot.

In class A single ended the dissipation on the active transistor can double if the amp is unloaded.
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Old 27th June 2008, 06:57 AM   #28
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Quote:
In class A single ended the dissipation on the active transistor can double if the amp is unloaded.
How should this be? Even if there's a load connected, there's no current through it (with no input signal) - so effectively (without input signal) the currents with and without load are the same.

Otherwise you would burn dc through the speakers.

Or are you talking about transformer coupled designs?

All the best, Hannes
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Old 27th June 2008, 07:21 AM   #29
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Quote:
Originally posted by h_a


How should this be? Even if there's a load connected, there's no current through it (with no input signal) - so effectively (without input signal) the currents with and without load are the same.

Otherwise you would burn dc through the speakers.

Or are you talking about transformer coupled designs?

All the best, Hannes
When i refered to class A single ended (on ccs) amps i was talking about amps with input signal. I don't remember whether the turntable needed to have any input to burn, but i remember it was very fast.
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Old 27th June 2008, 08:48 AM   #30
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Ah, I see.

But with constant current source the active part will see also with input signal only the standing current - except you're talking about the Alephs

But don't worry, I understand what you mean!

All the best, Hannes
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