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Testing a custom high power amplifier.
Testing a custom high power amplifier.
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Old 23rd October 2018, 07:23 PM   #1
edbarx is offline edbarx  Malta
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Default Testing a custom high power amplifier.

Since I am about to build a high power amplifier using a +/-85V DC power supply, I would like to explore what can be done, so that, the critical phase of first time testing is non-destructive and successful. If I remember well, Mooly instructed me to use a filament lamp in series with the toroidal primary. This in the hope of limiting the current the transformer secondaries, thereby, protecting the amplifier's electronics.

So, the first step is to use an incandescent bulb in series with the primary and check whether the output is correctly at 0V and whether the rail voltages are approximately at the levels they are expected to be. One problem that hits my mind, is even with this precaution, the large smoothing capacitors will still be charged at +/-85V, which mean, in the event of a malfunction, a temporary, but sufficiently destructive current can damage components. So, smoothing capacitors have to be scaled down to prevent this from happening.

Any more ideas, I do not want to destroy expensive powerful speakers.

P.S. I tested a prototype pre-driver circuit with the output for several days and no damage to speakers was done.
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Old 23rd October 2018, 07:33 PM   #2
bullittstang is offline bullittstang  United States
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The incandescent light is only to test the amplifier (no speakers should be attached). Once you verify that nothing smokes or blew up, then you start up the amplifier without the lamp and test the bias and verify there is only minimal offset voltage on the speaker terminals (like 10mV or close to it) - still no speakers are attached to the amp.
Only after you have sufficiently verified the amplifier is stable and not producing any bad behaviors, do you ever plug in speakers. Even then, use cheap ones the first time.
- I would also add, I tend to error on safety and include a speaker protector in the case.
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Old 24th October 2018, 11:43 AM   #3
DF96 is offline DF96  England
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The best way to start testing a custom high power amplifier is to first build, test and debug a custom low power amplifier using a similar topology.
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Old 24th October 2018, 12:25 PM   #4
Kay Pirinha is offline Kay Pirinha  Germany
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Agreed with DF96!
Besides of this, in case of any error that causes excessive current demands, due to the lamp's relatively high impedance your power supply filter caps won't be charged to their nominal voltage. So no need to be worried about!
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Old 24th October 2018, 12:33 PM   #5
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Another pre-test you can do is to put lamps as load for the amp. Having 85Vpeak, 110V lamps will perform pretty fine. I use 3 of them in star when testing industrial frequency inverters, from 220/380 mains. A gate problem in final IGBT stage is easily seen in the lamp sequence/bright, preventin from use large current spikes with inductive motor) loads.
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Old 25th October 2018, 12:08 AM   #6
mbz is offline mbz  Australia
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I would use a variac and gradually wind up the voltage while monitoring key voltages. The lamp/dim bulb tester in series would also help. If you don't have a variac then use any larger spare transformer you may have, applying 30-50V might be less destructive than full mains..
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Old 25th October 2018, 12:10 AM   #7
rayma is offline rayma  United States
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Quote:
Originally Posted by mbz View Post
I would use a variac and gradually wind up the voltage while monitoring key voltages.
Make sure that the Variac is properly fused on the output, to prevent its failure. Most amplifiers are not designed for continuous full power operation, since music doesn't require that and the cost is much reduced, so test only briefly near full power. Proper load resistors for high power testing can also be a problem. Some use water heater elements, in water for dissipation.

Last edited by rayma; 25th October 2018 at 12:23 AM.
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Old 25th October 2018, 12:19 AM   #8
nigelwright7557 is offline nigelwright7557  United Kingdom
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I design quite a few amps so I built my own test rig. The transformer will only supply a couple of amps. All connections are made by flying leads with crocodile clips. On the output is a speaker protector module in its box with its own power supply.

With no speaker connected, I power up the amp with a scope on output. If the output voltage hits power rail or the transformer grumbles I know there is a problem and turn off quick.
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Old 25th October 2018, 12:20 AM   #9
avtech23 is offline avtech23  Australia
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Testing a custom high power amplifier.
Quote:
Originally Posted by edbarx View Post
..Mooly instructed me to use a filament lamp in series with the toroidal primary. This in the hope of limiting the current the transformer secondaries, thereby, protecting the amplifier's electronics.
+1 for the DBT for initial fault testing.

For very little cash, I built one from hardware store items:

Batten mount light fitting, double power switch (main power and lamp bypass), switch mounting block, power extension cable (cut in half), insulated connector, small piece of MDF to mount, scrap metal shield for lamp.

It's all safely insulated and allows remote operation from the DUT so I don't even need to touch it.

Build once in under half hour and it is always there whenever you need it.
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Old 25th October 2018, 12:21 AM   #10
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by avtech23 View Post
+1 for the DBT for initial fault testing.
t.
I just get the wife to turn it on while I stand well back !
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