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Old 16th December 2007, 10:06 PM   #1
hering is offline hering  Austria
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Default the t.amp S75 bias modification

Hi,
I just bought a very cheap amplifier fromr thomann.de, a t.amp S75. Although it seems to be good value for the money at first (torroid transformer, good connectors, etc.), it sounds terrible at low volumes. The level of distortion is unaccpeptable, even having low demands.

The reason for that was quickly identified: An idle current at the output transistors was simple not existing, thus leding to excessive switching distorions. It is not possible to raise the voltage between the driving transitors to a level that would cause an idle current (approx. 4x0.7V) in the original configuration, even with the trimmer full clockwise. So I shunted a 10k resisor in parallel with the existing 2k resistor near the idle current trimmer (both, the 200 Ohm trimmer and the 2k resistor are between base and emiter of the small "virtual zener-diode" that defines the bias voltage between the dirves bases) which increased the bias voltage range about 20 percent.

This allows to set the bias current to a reasonable value. I chose 10mA. The switching distortions disappeart.

Hope, this helps other S75 owners...
Cheers,
Hering
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Old 7th April 2008, 10:21 AM   #2
gfle is offline gfle  Greece
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It does!
Thanks for the idea!
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Old 7th April 2008, 02:59 PM   #3
weissi is offline weissi  Europe
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Servus hering,

Can you tell us more of the internal? What power transistors are in there, etc.... Pictures would be great too!

Thanks,

Markus
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Old 8th April 2008, 07:56 AM   #4
gfle is offline gfle  Greece
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Hello,

Here ....

2SA1013
2sa1015
2sa1837
2SA1942
2sa970
2sc2240
2sc4793
2SC5199

IMHO it sounds horrible but I don't think I could build it for 98euros!! It WILL be modified of course
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Old 8th April 2008, 10:56 PM   #5
weissi is offline weissi  Europe
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hmm, this ones height is 1HE...
I think for an amp to mod, a E400 or E600 would be the better start.
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Old 9th April 2008, 12:34 PM   #6
gfle is offline gfle  Greece
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You are right of course but I wanted 1U due to limited space.
Anyway, the mods will be in electronics arount input mostly so no bulky parts like psu filter caps and xformer will get involved.
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Old 30th October 2009, 11:47 PM   #7
joe is offline joe  Germany
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Default Any news?

Did you try some mods in the mean time? Does the increased bias current improve the sound?

I have the same unit, a lot of parts for the money, good build quality.
I don't hear distortions but the sound is a bit boring and it hums slightly.
Could be interesting for active speakers if modding can solve above issues.

Does somebody have the schematics?

Joe
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Old 11th November 2009, 07:19 AM   #8
gfle is offline gfle  Greece
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Yes joe, bias increase does improve sound. Heat also goes up a bit.
No schematics sorry.
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