ST70 VTA board slight hum. Preamp?

I just finished installing a VTA board into my st70, and it sounds great.

However, I am getting more hum than I anticipated.

I bought the low gain version

I am using my thorens turntable, playstation, or ipod into a Marantz pm500 integrated and using the headphone out of the Marantz to get into the st70 which goes to Klipsch Chorus II’s

If I have the unit on and connected to speakers, but with no source, it is dead silent. When I leave the cables into the st70, but short them at the other end, it is still silent.
When I plug the cable into the headphone jack and turn the Marantz on, the slight hum starts.

The hum fades into the background eventually, but at that point it is LOUD. I think I can thank the highly efficient chorus II’s for this

For watching tv or using the playstation, I can only get the gain up to about 8 o'clock for normal listening levels. At 9 o'clock the house is shaking. With the turntable, 9:30 is normal volume. at 8:30 you can hear the turntable, but it is 50/50 hum. at 11 it is almost painful. At about 10 o'clock with the turntable the hum is no longer noticeable, maybe 98/2 or so.


I have recapped most of the Marantz, but maybe I need to look into the headphone section. I have never noticed a hum when using headphones.

I think the problem is that the headphone out is slightly noisy, and that when that is further amplified, the hum becomes noticeable. Sound right?

If I bought an earlier Marantz integrated with a preamp out, could I use that and would that be quieter? I could also open up the pm500 and find the preamp/power amp function and run a line to the rca's on the back...

I cant really afford a tube pre, but maybe in the future I will get a dynaco pas3 and modify it with the other vta boards for it. This will be awhile in the future though…

I know I can adjust the feedback resistors on the vta board, but it seems to me like the problem isn’t the vta board.

Any ideas for solutions to this?

you shouldn't be usiing a headphone output as the input to the amplifier, although this will usually work with something like an iPod, but not recommended for something like the Marantz. Your Marantz should have a pair of RCA output jacks labeled line output.
The headphone output obviously has way too much output, also, it should have a volume control on it, and that should be turned down.
If your Marantz doesn't have a line output it should have a "record output"

Do any of your sources have a grounded chassis (3 wire plug?) If so, this might well be a ground loop if your ST-70 chassis is grounded (a good idea for safety).

this marantz doesnt have preamp outs or a line out.

The only outs on the back are the tape ones, but those are unaffected by the volume control, so I have no volume control.

The headphone jack is the only option I have at the moment.


Does anyone else use one of the 70's marantz integrated amps that have a preamp out?

I can get several of those for less than $100, and $50 or so if it has problems (which are usually the output stage, which I wouldnt need if I was using it as a preamp)

I think that would be my cheapest route until I can get a real preamp

Just to follow up, I built an sp8 and the humming is reduced substantially!