Croft Super Micro IV - hum

[Kvaal]

Hi everybody!

I am a lucky owner of a Croft Super Micro IV (octal) with a hum problem. It only occurs at the phono-input. When i touch the right cannel plug it hums in the left channel. I have opened the unit, power on, connected to a power-amp and tried to provoke the hum with a plastic stick touching the inside parts - with no tt connected. No hum. Connected to the tt, it hums in the left channel when i touch the plastic on the right channel plug. I am using a din to rca adapter. The tt is a Dual 1219, but i also have a Lenco L75. Same problem using any of the tt`s.

Anyone with the same experience, with a good advice?

[Macca]

If I am reading you right then it doesn't hum unless you touch the plug?

It's a ground loop, not uncommon if you are using a turntable but can happen with other equipment too.

Can be very hard to track down the cause. It doesn't necessarily mean that anything is faulty.

Do you use a grounding wire from turntable to amplifier? Most TTs have one and most amps with phono stages with have a connecter to hook it up to. If you are try disconnecting it, if not then try connecting it, see if the problem persists.

If it really only happens when you touch the plug then I would just avoid touching the plug, and not worry about it.

[Lawrence001]

I'd rather have an amp that only hummed when I touched it than an amp that only hummed when I didn't touch it. I'd struggle to enjoy listening to the latter if I had to stand next to it all the time to touch it. So you can consider yourself lucky [emoji6]

Sent from my PCT-L29 using Tapatalk

[Barry]

Plastic is an insulator, so I don't understand how you can create a ground loop. Nor do I understand how, when touching the right channel plug, it hums on the left channel.

Make sure the RCA plugs are pushed fully home. If the outer part doesn't make good contact you can often get a problem with hum, because the earth return is being shared between the two channels.

[The Black Adder]

Double check that you have the speakers connected correctly left and right. If not, it could be the reason the other channel hums when you touch the plugs on your pre.

Sent from my BBC Microcomputer 32K

[Kvaal]

Thank you for your replies!

@Macca and Lawrence: Yes you`re right. Better aviod touching it. But i`m still curious about it.
@Barry: contacts cleaned, so it shall be ok.
@The Black Adder: Done, and connections is ok.

[Barry]

Is the problem now solved Johan?

[Kvaal]

Yes. I got inspired to find an adapter (din/rca) with seperate grounding. The Croft sounds great now!

[Barry]

Well done - happy listening! :)

[The Black Adder]

Cool... Hum is very annoying. Enjoy the music

Sent from my BBC Microcomputer 32K

[Kvaal]

Thank you!

My Croft is rare ( i believe). I has octal tubes, and was upgraded in 2011 by Glenn Croft himself. Not many of these i think.

Sounds very good, and it reveals the noise floor on the records and turntable setup. Maybe i need a belt drive?

[Macca]

What's your turntable?

[Lawrence001]

I had a Super Micro IV octal. IIRC it used 12SL7 (or 12SN7?) rather than the usual 6SL7 (or 6SN7). Very nice pre indeed, I regretted selling it but I had to much gear at the time.

Sent from my PCT-L29 using Tapatalk

[Kvaal]

# Macca: Dual 1219, Lenco L75 and a Sansui SR 222 on a shelf somewhere .

[Macca]

well your SR222 is a belt drive.

Was always fond of that deck although never had one myself.

[vinyl]

Thank you!

My Croft is rare ( i believe). I has octal tubes, and was upgraded in 2011 by Glenn Croft himself. Not many of these i think.

Sounds very good, and it reveals the noise floor on the records and turntable setup. Maybe i need a belt drive?

Rare, indeed, here is the one Glenn built me from scratch, which I sold on during a hiatus:

https://theartofsound.net/forum/showthread.php?63166-For-Sale-Croft-Charisma-X-Preamplifier