BHK 250 noise issue

I recently bought a BHK 250 amp and I am having a mysterious noise issue that so far I haven’t been able to get a handle on.

  1. With no inputs connected some amount of hiss is audible with the both channels of the 250
  2. If I connect my Directstream DAC, BHK Preamp, or both, noise doubles
  3. With the DS and BHK Pre in series the preamp volume will amplify the noise coming from the DS (presumably it originates at the beginning of the chain, even when no inputs are connected to the DS or BHK Pre)
  4. If I connect the DS, BHK Pre, or both, and turn one off or put the Pre in Standby, all noise goes away (it becomes quieter than BHK 250 with no inputs).

This issue did not exist with my previous amp in the same chain, with the same cables, etc.

I’m using balanced cables, Audioquest Colorado XLR, and Shunyata Alpha NR power chords that are connected either to a Shunyata PS8 or a PS Audio P15.

If anybody has some insights, the would surely help. Thanks.

Thanks for posting this. I think best bet would be to give our service team a ring - you can get a hold of them at 1-800-PSAUDIO. They’ll be able to help you get to the bottom of this in no time :grin:

Sounds like one or both tubes may be not up to snuff. Was the amp new when you got it.


Sounds like your tubes might need changing. If you have some spares give it a go. I was surprised how much hiss disappeared after I replaced mine. Yours may have been faulty from the start.

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I swapped the tubes and unfortunately seems like the problem is elsewhere.

The amp is second hand, but only a few months old.

You may also have a faulty input board like I did. Best to send it back I think. I know it is a big hassle but get PS Audio to check it out.

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The BHK amp with unterminated inputs should have a small amount of hiss if you put your ear up to the speakers. That’s normal. Adding the preamp or DAC, themselves having a slight amount of hiss, will increase the total noise at the speaker because the BHK
amplifier is amplifying that input noise.

If you put the BHK preamp in standby the outputs are disconnected and (if memory serves) the inputs to the amp are now terminated, thus should be lower noise.

So I haven’t looked at the schematic in a while but this seems to make sense to me.

Thank you, Paul. This makes a lot of sense. The question now is how much noise is too much?

I did a test using a somewhat inaccurate decibel meter on my phone 1 1/2" from my tweeter:

With all inputs disconnected on the BHK 250, the noise floor is about 8-10db higher than with DS connected but turned OFF. (effectively zero)

With the DS connected and turned ON the noise floor is about 20db above when it is turned off and 10-12db above the BHK 250 with no connections. Switching on the DAC -20db output pad eliminates the noise.

With BHK Preamp between the DS and 250 I can reduce the noise floor by maxing the DS volume and setting the Preamp to 25, which brings the noise level is close to the 250 solo, but there is a quirk: If I change the setting from 25 down to 24 I gain about 6db of noise. The same thing happens going from 52 to 51. So to get a low-ish, but still audible noise floor I have to find sweet spots on the preamp volume.

Thus, the best I can do with this setup is between 10-16db of added noise.

This might seem trivial, but even at the lowest setting the noise is masking some of the music at my listening volume.

My speakers have 92db efficiency.

With your speakers if the DS is set at 100 and the BHK at 0 how close to the tweeters do you have to get to hear the hiss? With my speakers “spec’ed” at 93 db it is about 6 inches with the exact same gear.

I can hear it at about 20ft and then I hit the wall.

Either you’ve got a really bad set of tubes in the amp or the input board is bad. Have you looked to see what tubes were in the amp when you got it and do you have another set to try?

That’s too much and suggests to me that something is wrong (even if you have very sensitive speakers). With my system I need to really turn the volume up high with no music playing and have my ear right up against the tweeter and mid driver (inches away) to hear the hiss.

From my listening position even at high volumes I don’t hear a thing. Just for comparison my speakers have 90db sensitivity and I’m in a small room where I’m less than 10 feet away from them.

I think he said he tried replacing them already and it didn’t make a difference. That suggests to me it might be a faulty amp.

Yeah I guess he did. Sounds like it is time to box it up.

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After messing around a bit I discovered part of the reason the noise seemed so loud was because I was getting an additional 6db of noise on the left side from a bad preamp tube (This was the side I didn’t initially measure, so it was really loud). Swapping the tube it seems to be much better, although not silent.

Now, with the BHK Preamp set at “1” I can hear the noise clearly at about 6 inches, like you said. With the BHK set at “25” I can hear it at about 5ft, but at 24 I can hear it past 10ft. If I set the Pre to “0” I basically have to put my ear up to the tweeter.

I’m starting to the think the extremely high noise floor using the DS straight into the amp was a bit of a diversion, and it’s possible that the noise I am hearing now is at least partly from the preamp tubes (those Tungsram E88CCs…). I guess the amp itself is not silent, and I have no idea how anyone uses the DS and 250 together at less than club volume without massive noise, but the preamp tube helped a great deal.

Also in the midst all this, one of my Audioquest XLRs completely failed and I have no idea why, so now I need a new cable to continue testing. Maybe it got cooked by the 250.

I had the same issue with BHK 300s. I ended up putting whirlwind 10dB attenuators between my BHK pre and amps. Fixed. Paul told me that with highly efficient speakers, it was normal. My speakers are 95dB efficient. Hiss was always louder with 6/7 volt tubes.

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Like Kyle said you will get less noise with 12 volt tubes in the preamp. I tried over a dozen pair of 6 volt tubes and could never find a pair that was quiet enough for me. The best of the new issues is the Gold Lion but you can still find quiet reasonably priced 12AU7/5814 NOS tubes from people like Andy at Vintage tube services. A safe fairly priced tube from him would be the RCA Cleartop. It’s a much better sounding tube in the preamp than the Tungsrams. though they seem to be O.K. in the amp.

IMO the Gold lion 12au7 are excellent, but, The Telefunken 12au7 and the Mullard 12au7 are equally as good. New production tubes are as good as any of the several NOS tubes I’ve tried.

Never tried any of the new production including the ones shipped with the preamp. I already had about three dozen pairs of NOS tubes so I just stuck to those and went with the tone I preferred. The Mullards were good but don’'t own Tele’s. It comes down to how much he wants to spend.

I love NOS Telefunkens (I use them in some vintage microphones), although I’m not sure I could replace a pair every year or two. I have some Amperex 12AU7s, but they had a bit too much bite.