Extended FP III with 60 Hz buzz

[mingles]

I have an Extended Foreplay III with 6SN7. I've had it since 2009. The previous owner said it was built by Bottlehead as a demo for a show. It's had a 60 Hz buzz since the day I got it. It's been in storage for many years. I just pulled it out. The buzz doesn't change with volume. It's the same at volume 0 as it is at 10. The buzz disappears while music is playing, but it's obvious between songs. Swapping tubes and changing sources does nothing. This isn't a ground loop problem. Other preamps are dead silent in its place. The Foreplay sounds absolutely spectacular otherwise. I'd really like to fix the problem. I have decent troubleshooting skills. Unfortunately, I don't have voltage check instructions for the Extended version. Searching through the forums, I found someone with a similar problem that was resolved by replacing the 220uF power supply caps:

https://forum.bottlehead.com/index.php?topic=2249.msg17356#msg17356

If the solution to my problem is this easy, can someone point me to the correct 220uF 250V caps that are drop in replacements?

Thank you in advance for your help!

Mark

 

[Paul Birkeland]

The first thing you want to do is to listen to a 60Hz tone and a 120Hz tone and let us know which you hear.

The other thread you found is for a stock Foreplay III without the extended upgrade, and what's being gone after in that preamp isn't present in yours.

Is this noise in both channels or just one channel?

 

[Doc B]

Just a minor point - a buzz is 120Hz. 60Hz noise will typically be a soft tone associated with the radiated magnetic field of nearby transformers. I don't see anything in your description that completely rules out a ground loop problem. A simple test is to use a cheater plug on the FPIII and see if it changes anything. It's possible that 13+ year old electrolytic caps are causing the problem, or bad tube pin connections, or oxidized switch contacts, or oxidized attenuator contacts...

So my suggestion is eliminate the easy to test stuff first. Along with the cheater plug try cleaning tube pins, sockets and switch contacts as well as the rca jacks. Only after all that consider replacing components.

 

[mingles]

I cleaned the tube pins with Deoxit and installed a cheater plug, but that didn't change anything. The buzz is in both channels. It's present when the volume is at zero. It increases slightly with the volume knob. The buzz is the same when the source is switched. I can't tell if it's 60 or 120 Hz. It sounds similar to a phono ground problem. Here is a recording that I made:

https://drive.google.com/file/d/196wV_eVJ03fYCKJf-jN2_lFUrdS64ujJ/view?usp=sharing

Thank you again for your help.

Mark

 

[Doc B]

The extended FPIII has a lot of gain and will tend to boost the noise floor more than other preamps. That 120hz buzz sounds like it could be an open ground or a ground loop. Try disconnecting all the interconnects of your source components from the input jacks on the FPIII, turn the volume control all the way down and see if that changes the noise level.

 

[mingles]

Hi Doc, I only have one source (CD player) connected to the Foreplay. The buzz is the same when the volume is at zero.

 

[Paul Birkeland]

What sits after the Foreplay? (amps and speakers)

 

[mingles]

Hi Paul, the Foreplay connects to a Parasound A21 and then into Ohm speakers. I have several other preamps and none have this buzz.

 

[Paul Birkeland]

Do you have some clip leads handy?  If so post a picture of the bottom of your Foreplay and I can show you some places to connect them to try to isolate where the noise is entering. 

The A21 does have 30dB of gain, which is a ton, but the Ohm speakers aren't super sensitive, so I wouldn't expect tremendous issues from that.  Still, you'd likely find the range of the controls on the Foreplay would work a bit better with Harrison Labs -12dB attenuators plugged into the amp.

 

[Doc B]

Hi Doc, I only have one source (CD player) connected to the Foreplay. The buzz is the same when the volume is at zero.

OK, I think PB is heading you in the best direction. Either there is a ground that is not 100% that needs to be addressed or you simply have an amp and a preamp with so much gain that the signal needs to be attenuated a bit. Is the volume getting loud at low knob settings, i.e., not much adjustment range/hair trigger control?

 

[mingles]

Hi Paul,

I have a multimeter. I'm not sure what picture to share, so I'm attaching a few.

According to this archived page, the FP3 has 10 dB gain:
https://archives.bottlehead.com/wp-content/uploads/2013/11/Foreplay-Line-Stage-kit.pdf

For what it's worth, I have three other preamps with 12dB, 17dB and 20dB gain that don't produce a buzz when connected to the same amp and speakers. The A21 amp has gain control which allows me to reduce the gain to get better volume control.

 

[Paul Birkeland]

Try putting clip leads as shown by the blue lines to see if this changes the nature of the noise coming out of the preamp.

The center board of your EFP3 needs two transistors swapped out at some point down the road, or you can add small heatsinks to the MJE350s to keep them a bit cooler.  Based on the condition of that center PC board, this preamp has been used very, very little.

 

[mingles]

Hi Paul, I don't know what blue lines you are referring to.

Can you point me to the replacement transistors you recommend?

Thank you!
Mark

 

[Paul Birkeland]

It looks like the upload didn't stick the first time.

 

[mingles]

I see it now. I'll try this tonight and will report back. I sincerely appreciate your time and help!

Mark

 

[mingles]

Hi Paul, I added the jumpers, but it didn't seem to change the buzz.

 

[Paul Birkeland]

How about these two?  They go between coupling cap black leads and the grounds you were using for the last test.