[sts9fan]

So I have come apoun a issue with my system that I am not sure how to avoind in the future. I am running a SB3 straight into a Odyssey kartago extreme which has been working great. I just build a bottlehead Foreplay III not for a permenat fixture but to get a feel for tubes. I have been getting a huge amount of tube rush with my ~92db speakers. Now is this an amp sensitivity issue? Does anyone knoe the sesitivity of my amp? Now if I know the sesitivity of my amp and the sesitivity of my speakers how do I pick a pre?

Thanks

[mgalusha]

The Odyssey amps have a gain of 30dB, this is fairly high but not excessively so. How much gain does the foreplay have, though it shouldn't really matter when the volume is down. You might just have a noisy tube, hard to say. Running the SB directly into the amp you have enough signal from the SB (assuming stock) to push the amp to full power, you don't need any additional gain from the preamp in this situation. With a different source this can certainly change.

[sts9fan]

is gain and sesitivity the same? Someone told me the amp is 75mV for 1Watt output does that sound right?  I have changed all the tubes they are new. this rush/background noise is coming from the tubes beacuse even when set to infinite resistance the tubes are still in the loop and I get the rush.

[mgalusha]

is gain and sesitivity the same? Someone told me the amp is 75mV for 1Watt output does that sound right? 

Gain and sensitivity are related but not quite the same. Gain is the ratio of voltage in to voltage out. With amplifiers people often list sensitivity as the amount of input required to reach full power. With a voltage gain of 31.6 (approximately) the Odyssey needs about 1.2V to reach full power. Note that is for a Stratos, the Kartago has a somewhat lower output power so will require slightly less. It could be said that it's sensitivity is 1.2V for full power.

75mV for 1W of output into 8R works out to 31.53dB of gain. For the Odyssey you would need about 89mV for 1W (2.83V)  into 8R.

Hope this helps.

mike

[sts9fan]

ok so how do we match these up to obtain a system that plays well together?

[mgalusha]

I've used several tube line stages with my Odysseys and have never had a problem with tube rush. Since the Odysseys have quite a bit of gain I would choose a line stage with only 6 or 9dB of gain. That should give you more than enough. If you're happy with the sound of the SB directly into the amp(s) it likely won't get any better by adding a line stage. At least in my system it didn't and I have what I consider to be a very nice sounding tube preamp.

[dado5]

Something is up with the Foreplay.

Could be a noisy volume pot. Could be a grid resistor value that is too high. Could be the bias is off and grid current is flowing. Could be a duff tube.

It's probably best to ask this at the bottlehead forum:

http://www.audioasylum.com/forums/bottlehead/bbs.html

[Parnelli777]

Ask George about his AU-1500 preamp. Gain is adjustable from 0-30dB. Seems to me it'd play nicely with just about any amp.

http://www.wright-sound.com/scripts/prodView.asp?idproduct=8

[Kevin Haskins]

I'd look at John Chapmans TAP or the TAP-X if your on a budget.   

www.bentaudio.com

[sts9fan]

Honestly I will most likely use no pre at all or wait for a used Candela to come up because I seem to like a bit of the tubes. My question is how to avoid this in the future. What is a formula people use to match sensitivity of their gear? Does anyone know anything about these 12db RCA attenuators? How do they work?

[rollo]

Sounds like somethings up with the tubed pre.If your source has an output of 2V or more the Promitheus TVC will solve your problem.Now if the input sensitivity of your amp is low [less than 1V] it could be bliss.A $600 or less price for the TVC is quite tempting.I ordered one     Happy Listening     rollo

[toobluvr]

...... Does anyone know anything about these 12db RCA attenuators? How do they work?

You referring to these?

http://www.rothwellaudioproducts.co.uk/html/attenuators.html

Couple of reviews here....opinions seem mixed over sonic degradation, or not:

http://www.tnt-audio.com/accessories/rothwell_e.html

http://www.enjoythemusic.com/magazine/equipment/0803/rothwell.htm

Google on "rothwell attenuators" to get lots more info.

[avahifi]

Using a high gain amplifier with very sensitive speakers does result in a poorer signal to noise ratio than normal.

The noise level from the preamp is fixed (it is after the volume control).  The signal level varies with the volume control setting.  With a high gain amp and highly efficient speakers, you use very little of the volume control to get a loud signal out.  The ratio of signal to noise is low.  The tube rush noise you hear is not surprising to me.  You will have 5 - 10 dB more noise from the amplifier, and another 5 -10 dB more noise from the sensitive speakers and that can make for trouble.

What is ideal (assuming the preamp line circuit is linear when driven harder) is a low sensitive amp and speakers because then the signal to the amp will need to be much higher than the fixed noise from the preamp (and amplifier).

We build a "Low Gain" function in some of our preamps to allow a better signal to noise ratio in situations like yours.  Perhaps your preamp can tolerate a voltage divider at the output to lower its output level.  This will lower the noise level at the same time.  Check with the manufacturer about this.

Regards,

Frank Van Alstine