Archived from groups: rec.audio.pro (
More info?)
On Tue, 16 Aug 2005 00:54:18 +1000, Phil Allison <philallison@tpg.com.au>
wrote:
>
> "James Perrett"
>>
>> Thanks for all the replies. Parts of the amp were getting hot when
>> feeding
>> a low impedance load with no input signal. When looking at the output
>> on a
>> 'scope, with a sinewave input the output would appear asymmetric up to a
>> certain level and then go back to being symmetric when the level was
>> pushed up further. My colleague has delved a little further and thinks
>> that one or more of the output devices are faulty.
>
>
> ** Probably not.
>
> Those symptoms are a sure sign of high frequency oscillation - possibly
> at
> up to 10 - 15 MHz. Causes include damaged capacitors or resistors in the
> output Zobel network, bias set too low or faulty earth connections.
>
>
>> This amp was being used in a Sonar system rather than a standard audio
>> system and it was driving a transformer so maybe it wasn't too happy
>> with
>> that - although it has worked OK in the past.
>
>
> ** I have seen what using a mosfet audio amp for SONAR transducer
> testing
> does - it damages the amp.
>
> The Australian Navy bought a couple of German made "Hertz" mosfet amps
> for
> that purpose and were experts at buggering them - either by pouring in
> 100
> kHz sine waves from a signal generator or by shorting the output to the
> input signal ground by using a *jack socket* patch bay for BOTH inputs
> and
> outputs!
>
> Use with an output transformer is asking for trouble too - all
> transformers magnetically saturate at some combination of voltage and
> (low ) frequency. This will blow rail fuses and can easily kill mosfets
> too.
>
>
>
> ......... Phil
>
>
Do you have any recommendations about what we could replace it with that
would be more reliable? We need at least 1000 watts at a frequency of
around 4.5kHz.
Cheers.
James.
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