Preout mod

[Mooly]

You need to specify where you are measuring the voltages. Not sure if your output voltages are on the speaker terminals or the points marked on circuit.

The points I did mark on the circuit are fed directly from an opamp output so there will be no problem feeding your active speakers from here. The output impedance at this point is very low << 1 ohm and will fully drive loads from around 1K upwards, so no problem. A follower isn't needed. You need a feed as I described earlier with a small isolation resistor (to ensure stability of any capacitive loading due to cable and input circuit of speakers) and a cap to ensure 0.00 volts DC at all times (that's just good practice) and a 100 k to tie that cap to ground (again good practice).

If you do test power amp output voltages then, as you know the amp is OK there is no real point testing with dummy loads. The open circuit voltage will be essentially the same.

[Photony]

Hi,
This sounds great! If I understood correctly from your response Mooly, the ratio "input impedance of the speaker / output impedance of the amplifier", should be large. The Genelecs input impedance was 10Kohm so this sounds good if already 1Kohm is sufficient.

About my earlier message. I made the measurements between the points you marked on the circuit and ground. Should've pointed that out.

I guess I'm all set to go. Thanks again. I'll let you know how everything goes.


-Tony

[Mooly]

The opamp will have no trouble with 10K. Most audio opamps will drive loads as low as 600 ohms (and that's to full output voltage swing).

I can't see any problems... just take it slowly and check your work as you go.

[Photony]

Hi,
I just finished with the preout mod. Yeah =) It seems to work, but only when I have an analog source. I get sound from digital sources, but only when using speaker terminals. Not from the points Mooly pointed out at the pcb in his attached picture.

At this point it's hard to say about the sound quality, since the digital signal is decoded in tv and fed through RCA cable to the amplifier.

So, any ideas what is causing that there is no signal when using a digital source? And how to fix it? Thanks for any help,

Tony

[Mooly]

From what you describe it sounds as though the digital audio in some way bypasses the opamp points I marked on the circuit...

Not having a paper manual where it can all be spread out makes tracing the ausio path difficult.

What I would do is first confirm by scope that there is no audio at the points I marked when listening to a digital source (you mean like SPDIF input ?) I find it strange if there is not audio there on a digital input as that would point to two "volume controls", one operating in the digital domain and one in analogue.

Depending on that result I would start at the power amps for left and right front main channels (or headphone output could be a possibility too) and trace the audio back physically using scope and circuit to refer too and see where that leads you.

[Photony]

Hi Mooly,
Thanks for the tips. I haven't had the change to test those yet.

I'm wondering could this be some kind of resistance or grounding issue. My guess is based on that I discovered that there had to be a resistor in the speaker terminals to get a signal to the oscilloscope from the points you marked. There, however, is a signal also without the resistor if I have the active speakers connected to those points. I would say the reason is that the active speakers generate enough resistance for the circuit to activate.

So, could this be a similar phenomenon: When the analog input is active, the circuit gets ground or resistance while there is something different when the signal is from the dac?

-Tony

[Mooly]

That sounds strange tbh.

You need your scope ground connected to the "audio ground" in the amp which is pin 8 of CP451 in the picture in post #8

I would expect to then see audio on pins 1 and 7 of IC 503 that varies with the volume setting. You need to try that and with a variety of inputs.

[Photony]

Hi,
Still haven't got the change to do measurements. Unfortunately, I don't have my own test equipment.
I recall that I have tested the ground I have used against the pin 8 Mooly pointed out. I got zero, but then again, there was analog signal input when I tested it. So, there is a slight possibility that the grounding point makes a difference.

I'm now considering to take the signal out just before the power amplifiers. How this sounds? At least the signals are merged before that point plus all the possible adjustments to the signal are made before that stage. Is there some obvious disadvantages?

There are three two channel (class H+) power amplifiers. In the schematics
http://koti.mbnet.fi/photony/technics_sa-da10.pdf
they are on the page 113 IC601, IC602 and IC651. One of the six channels is unused. In the schematics, there are also boxes containing text L.P.F and H.P.F. I believe these are low and high pass filters. I probably should figure out if I should or should I not to use the filters.


-Tony

[Mooly]

Can't get your pdf to download correctly for some reason.

To take the signal from the power amp inputs for the main speakers is the correct place signal content wise, and that was what I was trying to trace back on the other circuit in previous links. The correct ground only really matters in ensuring that there is no interference or hum and so on. Complex circuits like this have several grounds... they all read "zero ohms" one to another but use the wrong one and the audio will suffer, maybe audibly.

You can't beat a paper manual for things like this where you can spread it all out in front of you

[Speedskater]

The "pdf" is a 149 page service manual and it downloaded OK this morning.