Bottlehead Foreplay Hiss

[jaydacus]

I just purchased this strange looking(compared to other Bottleshead equipment I have seen) preamp. It sounds really good, but the gain level is way too high for my ST-120 tube amp and as a result I'm getting a hiss that I can hear from across the room even with the volume all of the way down. From what I have read I need something called a "snubber" and or I need to add more padding to the gain circuit. Having not built this unit and being somewhat of a novice, the information I have been able to gather thus far has not been very helpful. I don't know how to read schematics, but I do know how to solder and source parts. The kind of information that would be helpful to me would be specific information on what I should change eg. "The resister connected to pin X of the attenuator/  tube socket should be changed to value X".

I checked locally to see what someone would charge me to just make the changes and the estimate range went to almost $400. For that kind of money I could just sell the Forplay and start over with something from Dynaco. I would rather just make this preamp work, but I need some clear information on how to make it work like I want it to.

From what I can tell this particular Foreplay has the "anticipation" mod and a single 34 step attenuator that I cannot find any reference to on the internet.

 

[Doc B]

That's not our kit, at least not our chassis or layout. Go to Parts Express and order a couple of inline attenuators. That will solve the issue with your Dynaco being too sensitive.

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=266-244

 

[Paul Joppa]

The problem is most likely to be that the optimum input to the power amp is much smaller than the optimum output from the preamp.

In this statement, I mean "optimum" in terms of signal level - an optimum balance between overload if the signal level is too high, and noise if the signal level is too low. In this case, I think the signal level in the preamp is too low because you have to turn the attenuator too far down; the sensitive power amp then amplifies both signal and noise floor.

(There is a possibility that you just have a noisy preamp tube. The easy check is to see if there is equal noise in both channels, it's unlikely that two tubes will be equally bad. More reliable but more expensive and difficult is to try another pair of tubes.)

The best solution is to attenuate the preamp output to bring the operating signal levels closer to optimum, in both the preamp and the power amp. The best location for that attenuation is at the power amp input. On the Bottlehead Community page, I have a white paper on signals and noise which goes into much more detail if you are interested.

An input attenuator can be built into the power amp; it requires four resistors (two per channel). If you don't want to modify the amp, you can make an interconnect cable with the attenuator at the amplifier end. There are also in-line attenuators; be sure to get one with at least 10K impedance. The question is, how much attenuation is appropriate? To calculate that, you must know the amplifier sensitivity and the speaker sensitivity, as well as the preamp gain - clearly yours is a custom build, so it is not clear what the sensitivity might actually be. From your description I would guess that 12 to 20dB attenuation is needed - but that's a guess.

The snubber is a circuit modification that reduces buzz, a kind of sharp-sounding hum. This does not seem to be your problem - hiss is also called "tube rush" and is a different phenomenon.

This problem, while not common, still comes up reliably a few times every year on the forum here. A search will turn up several variations on the above comments. I'm sorry that we do not have detailed instructions for the various solutions, but many have done this and will help you do it too.

 

[Grainger49]

BTW, you are not alone with this combination of ST-70 and Bottlehead Foreplay.  There are a number of Bottleheads, customers who have this combination.  If you search you will probably find a few threads about this.  I think you want 12dB of attenuation.  But check first.

 

[jaydacus]

It sounds like the easiest solution by far would be the line level attenuators. Literally plug and play, requiring no modification at all, and the price is right. I really didn't want to modify the Foreplay since it already sounds really good and I may end up pairing it with an amp that has a more "normal" input gain later.

The preamp is definitely all Bottlehead stuff inside. The custom case was a huge selling point for me. I never took the Bottlehead stuff seriously because of the everything on top design and I was really missing out.

 

[Grainger49]

I think I remember this unit.  Pictures were posted on the old site.  The long skinny red LEDs are different than mine.  My FP 2 is newer, I guess.  

Here are a few pictures from poster Wardsweb:

http://wardsweb.org/audio/foreplay.html

But I would swear that I have seen this one before.  Nice construction, I would have twisted the wires coming out of the transformer.  That would help if there is some residual hum.

 

[jaydacus]

I tried the inline attenuators. They greatly reduced the hiss. They also greatly reduced the sound quality. The open, airy, grainy sound is replaced by a flat boring sound that I could never live with. It makes me wonder if any attempt to reduce the gain of this preamp would have a similar effect. I even tried connecting the Foreplay to a SS amp this I know to have much lower input gain. The Foreplay hisses like a rattle snake through that one as well. Sounds great with music playing though.

 

[Grainger49]

Jay,

Do you really mean gain?  If it is a Bottlehead circuit there are many out there who have paired this generation FP with a ST-70 and been very happy.  I was one of those.  

It is possible the tubes are at fault for the hiss you hear.  Some of them are just noisy or become noisy as they age.  

Did you try a new pair of tubes?

What speakers are you using?  What is the sensitivity of them?

Do you have a Tubed ST-70 or the Transistor ST-120?  It is easy to spot the tubes in the ST-70.

By the way, the snubber you mention in your first post is located on this site:

http://www.siteswithstyle.com/VoltSecond/index.html

The snubber removes noise caused by reverse leakage of standard diodes, not hiss.

I bet it is the tubes.  Try another pair of 12AU7s.  If you have other tubes in there the circuit has been changed.  (there are a lot of numbers that are equal to 12AU7s so post if they don't say 12AU7)

 

[Paul Joppa]

What inline attenuators did you get? Any idea what the input impedance is?

I ask because I have seen some that had very low impedance (less than 1000 ohms) as well as the more usual 10K. Foreplay should be able to drive 10K pretty well, but it will strain mightily to drive a tenth of that.

The ones I have were acquired long ago, and I don't know what's available today. You can measure the impedance at the input, center conductor to case - it's just resistors in there.

 

[jaydacus]

I actually have a tubed ST-120. It's exactly like the ST-70 tube amp, but has larger transformers and uses KT-88 tubes. I tried several sets of tubes in the Foreplay including Psvane and GE Hammond which are both very quiet tubes. I like the sound with the RCA clear tops. The level of hiss is identical with any of the 3 sets.

I purchased the attenuators that the mod linked me to after my first post

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=266-244

I'm getting a reading of close to 9000 Ohms.

 

[Grainger49]

Jay,

The reason for my question is that I sold Dynaco.  There was a ST-120 which was a compact 60WPC transistor amp.  

Again, are you using very sensitive speakers?  The combination of high gain and super sensitive speakers exacerbates the tube rush problem.

Looking at the link of the attenuators it doesn't say what the input impedance is.  9000 ohms is close enough.

So not the tubes, not the attenuator, I guess the problem is with the construction of the unit you bought.  I am running a FP 2 that has fed a Krell KST250-S, ST-70, GainClone and Paramours.  There has never been this problem.  But I put it together.

If not a problem in the construction of the amp it might be contact in the tube sockets.  I know it sounds stupid but take out the tubes and re-insert them about 6 times.  This cleans both the tubes and the socket.

See question #2 on this page:

http://www.bottlehead.com/smf/index.php/topic,2408.msg19060.html#msg19060

 

[Doc B]

My hunch is that it might sound better if an attenuator was built into the amp inputs using better resistors than whatever may be inside the inline attenuators. The ultimate would be to put adjustable trims at each input so you could find a balance between reducing the noise and keeping the dynamics.

 

[jaydacus]

This would technically be a Dynaco clone.

http://www.tubes4hifi.com/bob.htm

I know of the SS ST-120 that you are talking about. From what I've heard it was kind of the Windows Vista of amplifiers in terms of sound quality. No remembered fondly by anyone that owned one.

I'm using CW3 speakers which are of coarse very sensitive. Even my amp had a little hiss, but I would have to put my ear right against the tweeter horn to hear it. The amount of hiss was about 1/100th the amount I'm hearing with Foreplay connected.

 

[Grainger49]

I'm using CW3 speakers which are of course very sensitive.

Jay, I don't know CW3 speakers but Googled it.  My friend (moniker Coytee) answered a question you asked about bi-amplifying over at AA.  Good guy, his name is Richard.

Sooooo... Klipsch... Ok we may have found part of the problem.  60W with HIGH sensitivity speakers... 

Where are you? I should just bring over my Paramours to show you what SET sounds like.  That is what you should be playing with your speakers.

Yeah, yeah, I get it, you don't want to buy a new amp.  But it would make your speakers SING ! ! !