Thorens TTA-2000 DC offset

[zoldi211]

Hi, I bought Thorens TTA-2000, but I am affraid connect speakers because DC offset between the +/- terminal are 2,5V!!! This is normal value on this "A" class amplifier or is this faulty?

[R_J]

Seems a bit high, does the amp have a DC offset adjustment inside? Are both channels the same?

[petehall347]

looks like cap coupled output so will read some dc unloaded ..try a load on it ..

[redwire]

zoldi, can you download the schematic from that forum and post here?

Do both R, L channels have the same offset of 2.5V?

[zoldi211]

Hi, schematic on german forum is incorrect. I found one in korean forum and this have only one error, R11 are before R12,R13. Some values are also incorrect, but there are only minimal differences. I measured input voltage: 71V, voltage after trimmer 53V. Voltage between R1 pins: 0,380mV. Heatsin after 30 min 60°C, but this is normal with this amplifier!

[zoldi211]

Yes, both channels are same. DC adjustment maybe not...

[petehall347]

if you haven't got a resistor to load the output use a crappy speaker .. that dc will go away .

[zoldi211]

I haven't tried load on it, because I don't want to destroy the speakers...

[petehall347]

its capacitor coupled output so unless the caps are leaking its normal to see some dc without speaker loads .

[zoldi211]

so do you think, i can use it normally with speakers?

[petehall347]

you should test it with speakers you don't care about first . or a suitable resistor .

[zoldi211]

OK, thanks. What type of resistor is suitable for this test (value in ohms, wattage)?

[petehall347]

8 or 16 ohms or around there or even higher ..there won't be any signal therefore little current as vol will be turned down . i guess anything over and including 1 watt should do it .. saying that i believe its class A so idle current will be high . just be quick . you just need to see it drop to 0v .

[R_J]

2.5vdc is not that much voltage, but you can place about a 1 amp fuse in series with your speaker just to be safe. Do not have a source connected and playing music etc. as that would blow the fuse for sure. just use this setup to check the dc offset

[petehall347]

one thing it isnt dc offset on this amp .. its called dc balance .. one output is at rail and the other about half that . that's why it has a capacitor to block the dc ..

[redwire]

As others have mentioned, this amplifier is not DC-coupled to the loudspeaker. It uses a large coupling capacitor C1 10,000uF
So your DCV reading is likely due to small leakage currents in the big cap. In pictures this amp has a speaker protector relay board? Does it have one?

I did blow up an amp Dynaco ST-120 which also uses huge output-coupling caps. You have to be careful.
When you switch it off, the cap is still charged for a long time, and I was changing speaker wires and shorted them or something heard a thump and it blew the amplifier- even though it was turned OFF.

[R_J]

Now that I look closer at the schematic in post 5 I see the output cap. so I guess there is no _____ adjustment.
I don't think the stock unit has speaker protection.

https://old-fidelity-forum.de/thread-41267.html

RIP Meatloaf

[petehall347]

adjustment for dc balance is done with a scope and set for equal clipping .
one note is when adjusting the idle current can change the dc balance and visa versa vi best to have 2 meters hooked up along with the scope and sine wave input then sort of trade off idle current and dc balance till its right .

[redwire]

I think you need a speaker relay to stop the huge thump until the output caps charge up. Not for (shorted output transistor) protection.
There are pics of TTA-2000 with and without one. It might be a mod or add-on. The old Dynaco ST-120 used to make a loud thump with woofer cone lurching out on power up, and slowly sinking back in until output cap charged up.
I don't know what the trimpots are for.

RIP Bat Out of hell, among my first rock LP's bought.

[zoldi211]

The test is done. With old speakers and 1A fuse, because I dont have suitable resistor. After turning on, the current disappeared. I measured only 0,04 or 0 mA.

A have a version with protection board with relay (1. photo in post #19).