Technics Turntable Output Muting Incompatibility with some Phono Preamps

I wanted to post this as an FYI to those that own Technics turntables that have the output muting feature and are searching for a preamp. I recently bought a Cambridge Audio CP1 to go with my Technics SL-1700MK2 turntable and noticed every time the tonearm lifts or sets down I get a loud pop through the speakers. I'd never noticed this before on my previous preamp. That's exactly the sort of thing the output muting feature was designed to prevent.

After doing some research I found that the output muting feature operates by shorting out the output of the cartridge every time the tonearm is lifted. If you purchase a preamp that has a DC bias voltage applied to the input (like the CP1), this voltage will be shorted out through the output muting feature making a pop. This was confirmed with Cambridge's tech support who determined the turntable and pre-amp are "incompatible".

I just thought I would post this as a PSA for other Technics owners. Before you tear your turntable apart thinking the output muting switch is defective, it may just be that the pre-amp is incompatible with that feature. I don't know how many models of turntables had that feature??

 

Lots of Technics tables had this feature. Thanks for posting this. I have an old Technics SL-5200 with a Cambridge Audio 551P. Same popping sound you are experiencing. At first it was intermittent, not sure why. But gradually it happened on every lift up of the tonearm. I actually did tear apart my turntable, sprayed the switch with contact cleaner, re-greased the lifting mechanism. All this with no effect. I assumed the switch was defective. It was made in the 80's after all, not brand new. My next step was going to be to rewire from the tonearm, bypassing the muting switch altogether. Maybe I''ll reconsider this now.

What was your previous preamp by the way?

 

Seems like a preamp that is defective by design.

The cartridge generates current only when the magnet moves within coils. It does not, and cannot, make DC or extremely low frequencies. A DC offset voltage at the RCA inputs would mean that the voice coils of the cartridge are getting power from the preamp, which is a bad thing. It is even recommended to not use just any old multimeter to check the impedance of a cartridge, because the small voltage from the meter can mean big problems for the coils.

Perhaps one could modify the mute circuit, putting an inductor between the switch and ground, so that the pop is lessened.

 

How do you determine if a preamp has a DC bias voltage applied to the input by looking at a product's spec sheet? If there is any DC leakage at the RCA inputs, can you tell this without auditioning them? I'm not an engineer, I would like to know what to look for in a compatible replacement phono stage.

 

If a preamp is "incompatible" with Technics classic or current turntables, the preamp is defective.

 

There are a lot of nice sounding phono preamps with bipolar transistor input stages, and they all have some leakage current unless AC coupled, which hardly anyone would consider doing. Most designers moved to JFET input stages long ago to minimize the leakage, but that doesn't make the BJT stages defective, just not compatible with turntables that have a silly muting relay on the output, at least in my opinion

But yea, as Hardy says, you wouldn't want very much leakage current with a moving coil cartridge, but the CP1 is strictly for MM cartridges. Still a little surprised it has that much leakage, though, but it does have a discrete transistor input stage.