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Old 10th July 2003, 05:50 PM   #1
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Default Distortion at about 6-8K.

I have built Rod Elliots project 72 gainclone, but have substituted a opa544 for the LM1875. After a fue teething troubles everything seemed ok.
However after extended listening there appears to be heavy distortion at about 6-8K (my guess). This sounds like typical cone breakup distortion, but seems to clean up above and below the distortion band. I checked this on three different sets of speakers (all single driver fullrange) and the result is consistant.
The only major departure I made from the original design spec is to use a 2.2ohm resistor in place of the 1ohm resistor in the Zobel. I find it hard to believe that it is this though as a single fullrange driver is a very easy load to drive.
The circuit has high frequency input filtering and the zobel so it should be well protected against ocillation. The opa544 shows higher THD figures than the LM1875, but I would imagine that most of this is harmless second order as the design has a FET input stage.
Could it be a difference in input impedance between the two chips.

I am fishing here guys so help me out.

Shoog
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Old 10th July 2003, 06:32 PM   #2
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are you talking about a clipping in the amp happening? or is it oscillating?

what are you using for powering the "child of uncertain parentage">??..maybe its running out of steam!!

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Old 10th July 2003, 08:06 PM   #3
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It has a large EI transformer (at least 180VA), which is showing no signs of strain.
It certainly doesn't sound like the sort of clipping I heard with the LM1875, so I would say it isn't clipping.
The heatsink on the chips is not even above ambient.
Oscillating would be my guess, but until I can present it to a scope again I don't know for certain.

Shoog
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Old 11th July 2003, 11:09 AM   #4
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I tracked down the source of the problem. It was output stage instability. I have put a 10ohm resistor in series with the output, and hay presto the distortion has completely disappeared.
The sound is no great. Detailed, tight bass, silky smooth. I would tend to concur with Pete Daniels when he says that he cant understand why he went to all the trouble of building Alephs when the gainclone does it just as well.

Shoog
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Old 11th July 2003, 01:55 PM   #5
gfiandy is offline gfiandy  United Kingdom
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Putting a 10R resitor in series with the output will degrade you sound quality, better to use a small heavy guage wire inductor often put in parallel with a 10R resistor (or even wound round it if it is s non ferrous resistor). This will normally have the same stabalising effect without ruining the damping factor of the power amplifer in the audio band.

Andrew.
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Old 11th July 2003, 05:34 PM   #6
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I may just try that as it was the original design sujested by Rod Elliott for his Lm3875 project.

Thanks

Shoog
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Old 11th July 2003, 08:26 PM   #7
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I tried that and unfortunately the distortion returned.
It could just be that my speakers which are vintage valve design just dont like tight control. Putting in a resistor to make the amp behave more like a valve amp might be just what they need.
It turns out that I designed all my speakers when I had a valve Leak stereo 20 and so they are all tuned for valve amps. I even had to tune my Zen V3 to sound more like a valve amp, to match my speakers.

Maybe that is all the distortion is, but I have my doubts. It could be that the Zobel components are super critical and I absolutely must use the 1ohm resistor as specified. At the moment the result is good enough and I will probably leave it.

I might try using my last remaining OPA544 to build one of the ultra striped down gianclones I have seen mentioned here, and try using it with a pair of more conventional speakers.

it has however sujested a fix for a pair of speakers (vintage alnico siemens 4inch drivers in Buscchorn MkII) which I am tuning for a friend who will be using a musical fidelity B1 Mosfet amp. I will try putting a 4.7ohm resistor in series to fool the speakers into thinking they are looking at the valve amp they were made for.

Thanks for the help again

Shoog
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Old 14th July 2003, 11:33 PM   #8
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I took out the resistor from the output and got a 1ohm resistor for the zobel. The distortion returned. I am going to take it back to my friends to look at a scope on it and find out what the distortion looks like.

Shoog
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Old 15th July 2003, 04:26 PM   #9
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Default Re: Distortion at about 6-8K.

Quote:
Originally posted by Shoog

The circuit has high frequency input filtering and the zobel so it should be well protected against ocillation.
Shoog
I wouldn't guarantee that.
I just hope you have the PSU caps as near as possible to the chip's supply pins.
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Old 15th July 2003, 05:38 PM   #10
SY is offline SY  United States
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Try a few different loads- you may be having an issue with the protection circuitry.
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