the t.amp S75 bias modification

hering · 16th December 2007, 09:06 PM

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

gfle · 7th April 2008, 09:21 AM

It does!
Thanks for the idea!

weissi · 7th April 2008, 01:59 PM

Servus hering,

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

Thanks,

Markus

gfle · 8th April 2008, 06:56 AM

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

weissi · 8th April 2008, 09:56 PM

hmm, this ones height is 1HE...
I think for an amp to mod, a E400 or E600 would be the better start.

gfle · 9th April 2008, 11:34 AM

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.

joe · 30th October 2009, 10:47 PM

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

gfle · 11th November 2009, 06:19 AM

Yes joe, bias increase does improve sound. Heat also goes up a bit.
No schematics sorry.

16th December 2007, 09:06 PM	#1
hering diyAudio Member Join Date: Nov 2006	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

7th April 2008, 01:59 PM	#3
weissi diyAudio Member Join Date: Sep 2002 Location: Austria	Servus hering, Can you tell us more of the internal? What power transistors are in there, etc.... Pictures would be great too! Thanks, Markus __________________ regards / servus

8th April 2008, 09:56 PM	#5
weissi diyAudio Member Join Date: Sep 2002 Location: Austria	hmm, this ones height is 1HE... I think for an amp to mod, a E400 or E600 would be the better start. __________________ regards / servus

30th October 2009, 10:47 PM	#7
joe diyAudio Member Join Date: Oct 2001 Location: Hamburg	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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7th April 2008, 09:21 AM	#2
gfle diyAudio Member Join Date: Mar 2001 Location: Athens, GREECE	It does! Thanks for the idea!

9th April 2008, 11:34 AM	#6
gfle diyAudio Member Join Date: Mar 2001 Location: Athens, GREECE	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.

11th November 2009, 06:19 AM	#8
gfle diyAudio Member Join Date: Mar 2001 Location: Athens, GREECE	Yes joe, bias increase does improve sound. Heat also goes up a bit. No schematics sorry.

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