DAC + headphone amplifier based on Marantz DR4050

[peterpl]

I am still thinking why sound from external source is weaker in headphones than sound from CD. I thought that maybe CD sound is more amplified.
Now I start to suspect that external sound can be muted by killing transistors 7410 and 7411.
Picture shows CDR board and exit from op amp 7408.
Can you tell me how to disable those transistors ?

 

[peterpl]

Just a small addendum to a post above, as you see current coming to transistor base goes from "kill" circuit (attached). What is a purpose of that circuit and what would happen if I cut voltage supplying it ?

 

[rfbrw]

You could try going from the input of 7401 to the analogue output on 7407. The switch is controlled by a single pin.

 

[peterpl]

Hi, glad you're back.
You mean probably input from 7701 pin 59 sel_hp_out coming to pin 9 of 7407.


What I am going to do is to disable switch 7407 by joining pins 5to3 and 12to13. That way I will not care if signal from CD is switched to heaphones or not.
Another thing is to cut otput from "KILL" circuit shown in post #22 by cutting a wire just after point F804 so not allowing for current flow to transistors 7410 and 7411 and others of course.

 

[peterpl]

I have failed. Disabling "kill" circuit gave nothing. Sound was still silent the same.
So I decided to connect CDR analog in directly to IC7409 which is an opamp LM833. CDR signal after decoding by IC 7406 goes first to IC7409 and then to headphone amplifier IC7408 (there's a mistake in the block schematics). But even that didn't help. Sound was silent and is silent. No I have no clue what to do.

 

[rfbrw]

Have you tried tracing the signal either with an oscilloscope or a small speaker or a pair of headphones ? You can then follow the signal from the external connector to the input of 7401 (A1 and A2). Do the same for 7407. Once you know what goes where you can cut and bridge as necessary.

 

[peterpl]

I have bridged 7401 moving signal from pins 3 and 13 to 7409 pins 3 and 5. Then I have bridged 7407 moving signal from pins 3 to 4 and 13 to 14 so signal goes from 7409 to 7408 pins 5 and 3. I thought that 7409 will amplify the signal and 7408 will amplify it even more. But in fact the level in headphones is the same with 7409 like without 7409 or the difference is minimal.
Signal from CD is also going through opamp similar to 7409 (it has numer 7120) and in parts description it is LM833D while on schematics it is TDA1308T. Maybe that opamp is stronger than LM833D?

 

[rfbrw]

7408 drives the headphones. It has a gain of 1. 7407 is a switch. So the path to the headphone O/P via 7407 and 7408(A&B) has no gain. The path via 7401, 7801, 7405, 7406, 7407 and 7408 has gain but is controlled by the DASP.

 

[peterpl]

I think it would be better to analyze gain on CD part.
that signal path is as follows :
7309 UDA1320ATS gain ???
7120 TDA1308 T gain ???
7407 74HC4053D gain 0 ( switch)
7408 NJM4556AD gain 1


CDR part has following path:
7406 UDA1341TS gain ???
7409 LM833D gain ???
7407 74HC4053D gain 0 ( switch)
7408 NJM4556AD gain 1

So stronger signal on CD must come either from 7120 or 7309

 

[peterpl]

It seems that TDA1308 is the amplifier for headphones (description attached).
Question - how to connect external source to it ?

 

[rfbrw]

The gain of the TDA1308 is fixed. The UDA1320 has a volume control. You can connect before the 22k resistors but you will not have any control.

 

[peterpl]

The sound from CD is not volume controlled it just playing loudly and that's it. What about adding a stereo potentiometer just before headphones socket?

 

[rfbrw]

Best after a switch but before the amp.

 

[peterpl]

Finally I have took out the CDR mechanism as because of broken laser it will not be used. I took out connections as described in post #27 and connected to them new TDA1308 based on schematics in pdf file. That circuit is supplied 5V from power supply unit filtered additionally with 10K uF condenser. Sound is the same as from ther other TDA1308 on CD part ( I just wanted to leave CD part untouched). Now I will try to adapt external DAC and connect it to Marantz's sockets. By removing CDR mechanism there is space when you lift left drawer cover. That place could be good to put a slider potentiometer which would normally be hidden by that cover.