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Old 18th September 2005, 10:07 AM   #21
guido is offline guido  Netherlands
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Quote:
Originally posted by ash_dac
Hi Guido,

So in the black box: -6.2 -15 = 8.8 divide by 2500 = 3.52ma

So lower output current (good) but the voltage -6.2V is quite high for that pin input(bad). Is this correct ?

There is a 1K resistor to ground on the dac output listed in the service manual which is confusing me because it seems way too high for tda1541a. Maybe reading the schematic wrong or an error!

Ashley.
Think the output current can be anything within spec, but it could depend on the analog circuits behind the dac.

As for the voltages. -6.2 seems to be too much indeed. About the calculations above, it's just my theory and i would be interested to know if indeed the output current increases when -6.2 is decreased.

As for the 1k, there must be something in the schematics that is not shown. Normally an opamp is used to keep the output pen at virtual gnd. Expect something similar here, but it seems to be transitors?
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Old 18th September 2005, 10:54 AM   #22
Ruach is offline Ruach  Singapore
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Hi guido,

I can send a copy of the service manual if you have the time to go through it.

Just drop me an email at ruach@starhub.net.sg
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Old 18th September 2005, 11:00 AM   #23
Ruach is offline Ruach  Singapore
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A picture of a modified Black Box

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Old 18th September 2005, 11:02 AM   #24
ash_dac is offline ash_dac  United Kingdom
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Hi,

Yes there is a differential input stage with transistors so I guess that is the explanation.

I think I will lower the supply from -6.2v to -5V. The datasheet states an absolute maximum of -7V!

Ruach: How does it sound ?


Kind regards,


Ashley.
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Old 18th September 2005, 02:42 PM   #25
Ruach is offline Ruach  Singapore
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The Black Box is not mine. It belongs to a forumer at Audio Asylum.
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Old 19th September 2005, 02:17 PM   #26
Ruach is offline Ruach  Singapore
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The Arcam Black Box's audio stage from the service manual:

The audio output filter/amplifer is in two stages. The first stage consisting of Q1-Q7 forms a long-tail pair with Q5 as the output amplifying stage. Q3/4 and Q6/7 are two current sources. The feedback network from the collector of Q5 back to the base of Q1 determines the amplifier gain. R10 sets the gain in the audio band while C4 progressively reduces it above 20khz, this being the first stage of filtering. A network consisting of R13, C5, C6 can be switched in to provide a de-emphasis rolloff in the audio band for discs with pre-emphasis. FET Q12 is used as a switched for the de-emphasis circuit.

The second stage is a two-transistor pair, Q8/9 with current source Q10/11. Audio band gain is set by R19/20. Outside the audio bank the stage acts as a 2 pole filter set by R15/16 and C7/8. This filter, combined with the first stage roll off forms a 3 pole Bessel characteristic linear phase filter, -3dB point set at 42 khz.

IC1 with C10 and C11 forms an integrator with high d.c. gain and acts as a D.C. servo so that the D.C. offset on the output stage is very low. For this reason IC1 is an LF411 op-amp with very good offset specification.
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Old 2nd October 2005, 04:45 AM   #27
Ruach is offline Ruach  Singapore
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Old 27th November 2005, 04:29 PM   #28
ash_dac is offline ash_dac  United Kingdom
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Default problems....

Hi,

Finally I got around to look at the black box schematics...


Here is the list of the main 'problems' as I see them:-

1. Relay coil is enegised from the -15V supply that is also shared with the tda1541a. This helps to explain the filtering on the -15V dac supply pin. Also this might explain the brightness.

see: -http://www.diyaudio.com/forums/showt...806#post341806

2. In the analogue supply the +11v / -11V rails are decoupled to ground, and also to each other.

3. Tda1541a is overdriven at 6.2V instead of 5V. Jean-paul said that's why Arcam's , old Cambridge Audio players die. I see loads on ebay that say "...distortion in one channel like a scratching sound..".

4. Mine is the earlier version so it only works with certain transports (works with 1 out of 3).

Would it be possible to dump the DC servo section , and add a dc blocking cap at R23?

Would adding a ceramic cap, and oscon across the relay help?



Kind regards,

Ashley.
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Old 29th November 2005, 02:01 PM   #29
Ruach is offline Ruach  Singapore
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Hi Ash,

Will it be ok if the relay coil is removed completely?
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Old 29th November 2005, 06:24 PM   #30
ash_dac is offline ash_dac  United Kingdom
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Quote:
Originally posted by Ruach
Hi Ash,

Will it be ok if the relay coil is removed completely?

"The audio output is shorted by a relay , RLA1, until the audio stages have stabilised on switch on. This delay period is set by a time delay in the mute circuit."

From this... I guess the dc servo takes x seconds to start working so the relay stops your speakers from being destroyed by dc.

From my reading of the datasheet the relay is decoupled with a 10uf elec' cap' so maybe the decoupling scheme could be improved there.

I wouldn't remove it! But that's just my reading of it....



Kind regards,

Ashley.
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