Solid State amp question

[DinoBob]

Forgive a newbie to the page for this question, if it is elementary.

In a solid state amp, what stage of the amp is an interrupting headphone jack usually located? Is it in the power amp stage, with a pad to reduce output? Or is it in the preamp stage?

Thanks.

[52 Bill]

Welcome DB!

Due to the need to drive low impedence headphones, normally all amps have the headphone jacks located at the output stage. They are often hooked up to a switching jack that will disconnect the speaker and in the case of tube amps add a load resistor in place of the speaker load. Then there will be a small limiter resistor (pad) placed in series with the headphones.

Some amps will actually add a small amplifier section to the amp, just to drive the headphone jack, and will not involve the amp's internal power amp. These are usually small IC chip amp circuits like an LM386.

[DinoBob]

Well, thanks for the welcome and thank you very very much for your reply. I have had a difficult time getting any attention in a couple of other foirums and it is nice to find a place where I get a welcome AND some assistance.

If you will indulge me long enough so that I can explain my situation...I am handy but not really knowledgeable about the finer points of electronics (though I would love to learn more).

I was given a very very nice Yamaha CVP-7 Clavinova a week or so ago. It is twenty years old but in great shape and works perfectly. Being that old, though, the FM-synthesized piano sounds are less than stellar. I'd like to use the MIDI capabilities of the Clav to drive an external Kurzweil Micropiano module, which sounds excellent.

Now, what I absolutely love about the Clavinova is that it is a self-contained unit. It is mounted on a set of real legs, it has a full set of real sustain/soft pedals, and there's a 30wx2 amp onboard that powers a nice set of component drivers in an attached enclosure. You see, I want to basically sit down, turn on the piano, and play. Not set up a stand, keyboard amp, sustain pedal, yadda yadda. I sometimes have 10 minutes to play and by the time I set that all up, forget it.

The Kurzweil has a front-mounted power switch and volume control, and 1/4 inch stereo outs. Also, I can send program changes to the Kurzweil from the Clav (I love MIDI). So, in my little world, having the Kurzweil means only turning on one more power switch and I am in piano Nirvana. So, I wanted to run the audio outs from the Kurzweil through the power amp of the Clavinova, and keep my sound output self-contained.

But (dramatic music)...my Clavinova does not have aux or line in jacks.

What I wanted to do was add a stereo line in to my Clavinova. Ideally, connecting inputs to this line in would disconnect the output from the Clavinova itself (which I presume could be accomplished with a switching jack). Additionally, this would allow me to use the headphone jack on the Clav even when I am using the Kurzweil module. That's what prompted my original question.

But getting those line ins to work is proving to be more challenging than I thought. I have been experimenting carefully. Here's what I have tried so far:

The Clav has (RCA) Aux Out jacks, which are on a small separate PC board. The preamp section of the Clav sends output to these via a small 4-pin cable. It is the same type of cable/connector that is used for the power amp input. So, I thought, here's a chance to test. I can disconnect the cable that runs from the preamp to these jacks, re-route it to the power amp input, and they will become input jacks. Well, there was a very loud hum and very low input volume.

Then, I built a small mono 1/4 inch test jack, with a standard jack, some wire, and two pins. I hooked this up to the Kurzweil's output and used the pins to tap in to the power amp input. I got some input from the Kurzweil, but it seemed cllipped, and there was an oscillation which manifested itself in a regular "click" through the amp. I think I may have needed to disconnect the internal input from the Clav's preamp when I did this, but I did not. I just tapped into the two pins for one of the channels with my homemade test jack.

In any case, I'd really be grateful for some assistance, and I hope all of this makes sense. It seems to me that someone with even a little more knowledge than I have could accomplish this task easily. I'd sure appreciate a little guidance. I am not afraid to do the work myself if pointed in the right direction.

In a nutshell: Help me add a set of line ins to my Clavinova's 30wx2 internal power amp!

[DinoBob]

<Edited due to accidental double-post>.

[52 Bill]

DB:
What you propose, sounds logical and should be quite simple to accomplish. What would probably help you is a copy of the schematic to find out exactly where the power amp inputs are.

Maybe someone here has a copy that they can send you, or you can get one from Yamaha. I know that they used to have a great website for older products. If you don't get any leads here, maybe repost on the Music Electronics part of the board.

[DinoBob]

I have ordered a service manual, which I am sure will have the schematic.

I have traced the power amp inputs. They are very clearly marked on the board. I guess I do not know why I had input issues when I tapped into them with my test jack.

But I do agree that a schematic would help.

[Enzo]

Yamaha service manuals are about the best there are. Schematics, board layouts, mechanical blowups, service procedures, how to remove/install keys, MIDI implementation chart, and most anything else you might want. Well worth what they charge.

[DinoBob]

I did accomplish this by making a shielded cable/jack setup and bringing this into the preamp in point of the circuit. It worked out perfectly.

Thanks everyone.