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#1 |
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![]() 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?
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#2 |
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![]() Seems a bit high, does the amp have a DC offset adjustment inside? Are both channels the same?
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#3 |
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#5 |
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![]() 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!
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#6 |
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![]() Yes, both channels are same. DC adjustment maybe not...
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#7 |
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![]() if you haven't got a resistor to load the output use a crappy speaker .. that dc will go away .
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#8 |
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![]() I haven't tried load on it, because I don't want to destroy the speakers...
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#9 |
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![]() its capacitor coupled output so unless the caps are leaking its normal to see some dc without speaker loads .
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#10 |
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![]() so do you think, i can use it normally with speakers?
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#11 |
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![]() you should test it with speakers you don't care about first . or a suitable resistor .
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#12 |
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![]() OK, thanks. What type of resistor is suitable for this test (value in ohms, wattage)?
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#13 |
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![]() 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 .
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#14 |
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![]() 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
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#15 |
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![]() 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 ..
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#16 |
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![]() 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. |
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#17 |
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![]() 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 Last edited by R_J; 01-21-2022 at 04:30 PM.. |
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#18 |
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![]() 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 . |
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#19 |
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![]() 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. Last edited by redwire; 01-22-2022 at 12:17 AM.. |
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#20 |
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![]() 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). |
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