ash_dac said:
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?
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
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
A picture of a modified Black Box
An externally hosted image should be here but it was not working when we last tested it.
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.
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.
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/showthread.php?postid=341806#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.
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/showthread.php?postid=341806#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.
Ruach said:
"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.
Update..
Hi,
I 'corrected' the digital input pcb as laid out in the manual (previously an inverted signal came out of IC301). Now I can use with my Marantz cd63, and Philips cd723 transport.
Good sound still a little bright though. Nice mid range!
Alarmingly it still pops on turn on / off so will put on the back burner.
...and Guido you were right that the comparator requires a power supply 'above' -6V for proper operation.
Therefore drives the tda1541a at 6.2v which isn't good. I don't think running a comparator, and a tda1541a on the same supply line is ideal.
I think the power supply will need rebuilding to correct this problem.
Kind regards,
Ashley.
Hi,
I 'corrected' the digital input pcb as laid out in the manual (previously an inverted signal came out of IC301). Now I can use with my Marantz cd63, and Philips cd723 transport.
Good sound still a little bright though. Nice mid range!
Alarmingly it still pops on turn on / off so will put on the back burner.
...and Guido you were right that the comparator requires a power supply 'above' -6V for proper operation.
Therefore drives the tda1541a at 6.2v which isn't good. I don't think running a comparator, and a tda1541a on the same supply line is ideal.
I think the power supply will need rebuilding to correct this problem.
Kind regards,
Ashley.
Luke said:
Hi,
All that I had to had was a UF4002. Measured the voltage, and got 2.1V !
So I went back to the wire link (6.2V).
Note that while I had the board out I replaced the digital IC decoupling cap's with oscon's. The highs are now much cleaner, and the soundstage has been improved. I'm very happy with the upgrade.
Kind regards,
Ashley.
PLL: - Replacing the HCt4046 VCO with 74HCT9046
Hi,
Replacing the HCT4046 VCO with HCT9046...
74HCT9046 seems very close to a drop in replacement as the in the Arcam the phase detector is in the separate custom chip.
Jocko said this is a BB part but not sure which one so I have no idea of it's performance.
http://www.lavryengineering.com/white_papers/jitter.pdf
Kind regards,
Ashley.
Hi,
Replacing the HCT4046 VCO with HCT9046...
74HCT9046 seems very close to a drop in replacement as the in the Arcam the phase detector is in the separate custom chip.
Jocko said this is a BB part but not sure which one so I have no idea of it's performance.
http://www.lavryengineering.com/white_papers/jitter.pdf
Kind regards,
Ashley.
No.....not the phase detector thingie......the SPDIF RX chip........this was before the CS8412 came out.
No, I did not say "BB". I said that it was a Sony part with a TI logo on the bottom side of the chip, where no one could see it.
Unless you took it out...................which I did.
Jocko
No, I did not say "BB". I said that it was a Sony part with a TI logo on the bottom side of the chip, where no one could see it.
Unless you took it out...................which I did.
Jocko