Foam Inside BHK Preamp

Just wondering if this should be there. It’s behind the board that’s behind the tube. Kind of pink in color. Thanks!

My BHK preamp has the foam in place. The boards originally were a push fit then at some point in time they were soldered in place and now with current models they are back to a push fit… Don’t know the reason for the foam.? Just guessing, possibly the foam is there to stop movement and to put pressure on the contacts by stopping the vertical board moving hence the reason for placing the foam between the vertical board and the heat-sinks… Just have to hope the foam is heat retardant or over time they will disintegrate.!


My first thought the foam is acting like an insulator preventing the rear of the board from shorting out to the heat sink.

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Possible… But, if that is the main reason, it isn’t a very well thought out design…is it.? I think it is to put slight pressure on the vertical board because early model BHK’s had the boards falling off/bad connections in transit ( this happened with my first bought BHK preamp… ) hence why they were soldered in place for a while…but have reverted back to the original design… I am sure Paul will enlighten us.


Indeed, hopefully the heatsinks stay cool, Heat and foam (plastic based foam), not a good idea.
It would be very poor electrical insulation, for that you need defined:

  • dielectric resistance
  • thickness
  • comparative tracking index (CTI)
  • creepage distance
  • clearance

None of which is well defined with the foam applied as shown in the pictures.


Hope you’re enjoying the pre! The foam is in there predominately for shipping purposes. Depending how rough the shipper is with it, it can be jostled loose. If you’d like, feel free to take them out, or leave them in. Don’t matter either way.


Ok makes sense. In order to decide whether it can remain their or not two questions should be answered.

  1. What is the continuous operating temperature rating of the foam?
  2. How hot can it get in the position the foam is being applied under maximum rated power and load of the those particular circuits in the pre amp?

That’s the beauty of a fantastic engineering team. They aren’t going to allow foam inserts to stay in a unit that they think even has an ounce of possibly of causing harm to it.


Thanks for everyone’s replies. I’ve been running it like this for over 400hrs. Only noticed them because I got a pair of Tungsram 7DJ8s and put them in yesterday. I’ll remove them next time I open it up.

You might use them as coasters for your coffee cup.

Really? Is that a joke? You trust PSA to make the dac, but, not sure if they know the implications of putting a damping material inside?

In case the data and measurements match I have no doubt it will work. But if it’s foam, that is manufactured for packaging relevant data is hard to get. If a manufacturer seeks certification of compliance to safety standards, that manufacturer will have to have that data available. Just facts and evidence.
Nothing more.

I didn’t have the foam on those daughter boards in my BHK. That said I adhere to the tried and true adage ‘if it ain’t broke, don’t fix it’. Packaging foams are actually designed to withstand environmental extremes way in excess of what those pads will ever see. Speaking from my experience in Aerospace where we evaluate commercial packaging foams, pads, etcetera to crazy environmental standards. Those pads will never come in contact with the tubes. You’re fine.