Trying to add pre out/main in to integrated amplifier. Am I thinking right?

[Coconuts 500]

Hi!

I was thinking about adding main in/pre out jumpers to an old Denon integrated amp. I haven't done this before but I think I might be on the right track. I have looked at its schematic and have made notes where I think I should cut the signal. I've enclosed the schematic here and have made green lines below R57 and R58. Am I thinking right?

Schematic (gif image)

If this will indeed work, will I need to do something else as well? I've read other threads on the subject and it seems further modification might be involved as well. Any insight would be very appreciated.

Thanks for your time.

[ingenieus]

The volume control is the right place to tap the pre-amp. Might be a good idea to add voltage follower op-amps to buffer things.

[wahab]

The volume pot is part of the loudness circuit, you cant use
its input/output as separating point.

This should be inserted before R54 .
Just cut the circuit at this place, with R54 being the main input.

[ingenieus]

It usually works. But then there are gotchas like this.

[Coconuts 500]

Originally Posted by wahab

The volume pot is part of the loudness circuit, you cant use
its input/output as separating point.

This should be inserted before R54 .
Just cut the circuit at this place, with R54 being the main input.

Thank you for your help, guys.

I tried this, and unfortunately, all it did was bypassing the tone control. The amp worked as normal, except that the tone control did nothing. Putting in the jumper re-enabled it.

This amp only has two knobs. One called "tone enhancer", which was what I now disabled. It basically works by removing detail, and adding some bass. I don't really understand its purpose, I never use it. And finally the volume control.

EDIT: By the way, the "tone enhancer" is VR1 on the schematic.

[Michael Bean]

The place you should break into the circuit is after the selector switch and before the 1.8k resistors (R51 and R52) . It also may be necessary to add DC blocking caps.

Mike

[Coconuts 500]

Thank you for your reply.

How will I know if that is necessary? And what kind of caps can I use? Will Polypropylene 1uF be okay? I have tons of those on hand.

And finally, how do I "add" them?

Sorry for these questions, I am a total newbie at this, it is a lot of fun to learn!

[Michael Bean]

Here:

PRE-OUT MAIN IN.JPG

It's probably safest to go ahead and add the caps.
If you notice a loss of lower frequencies, you can parallel additional 1uF caps to compensate.

Mike

[Coconuts 500]

Fantastic. Thanks a lot for all your help and effort. Made my day.

Just one thought though, it looks like I will lose the volume control by doing this. I mean, if I connect the pre section to a different power amp.

Because the idea of this modification is that I would like to use it as a pre-amp.

[Michael Bean]

Whoops, you're right, my thinking cap fell off. Instead, just put the RCA jacks between the balance control (VR3) and the 22uF caps. And since there's already a cap in place, you don't need the resistor and cap I had in the diagram, just put in the jacks.

Mike