Good option, thanks @yogiboy
Potentiometer would achieve the same results as an attenuator, but does get around my issue of which -dB value to choose.
Over-Loading Pre-Amp Stage??? Prima Luna Floyd
I have two inputs to the Prima Luna Floyd Hybrid integrated amp, an Auralic Vega DAC and a Denon FM Radio.
The DAC outputs 4.4v and the Prima Luna has an input sensitivity of 415mv. The FM Radio plays at approximately the same volume at any given position of the volume knob, so assumption is this is outputting ~4v.
Herman from Prima Luna has advised that the amp needs 415mv to be able to output its full power. I will be getting some Rothwell attenuators to achieve the recommended -20dB (although these are also available in -10cB and -15dB so unsure whether either of those would have advantages).
My question is, could the ~4v inputs be responsible for damaging the pre-amp tubes?
In the first 12 months one of the PL tubes started an intermittant high pitched whining. Swapping tubes one by one from left to right channels isolated this, and replacing the tube fixed the issue.
Since then I have been using military grade Brimar tubes, including this one CV4003 12AU7WA BRIMAR NOS VALVE TUBE (T14B) - Langrex and one of these has recently had the same behaviour, and this would be less than 12 months use.
It is worth mentioning that the system is used almost every day for 12+ hours, however it would still take longer than this to reach the 5-10,000 hours that a 12AU7 should last in a pre-amp.
Until the attenuators arrive I can set the DAC to 60% volume to achieve the same -20dB but there is nothing I can do with the FM Tuner as this is a fixed output.
Keen for all thoughts on the use of attenduators in general and if the issues I am having are related. The other option Herman suggested was hard-wiring a resistor into the amp, but I am not capable or keen to do that when the DAC is likely to be upgraded at some point.
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NO, absolutely not. Your tubes are not getting damaged by input level. In these cases either the tubes are going noisy on their own, or the amp design is very sensitive to noisy tubes. So the Prima Luna’s sensitivity of 415mv indicates a gain of (calculating...) 37 dB!? Holy cow!! I’ve NEVER heard of an amp that high. No wonder. That means the pair of 12AU7 for "input" slots are under a sonic electron-microscope (so to speak), and need to be EXTREMELY low noise - and stay that way, over time (that’s the bugaboo). (EDIT: I forgot the 37db is from adding "line stage" gain to power stage gain - it's OK then, but still reasonably high) If you see tubes graded "MC phono grade", that’s what you need here. Frankly, it’s almost baffling for an amp to have this much gain. The most I’ve ever had is 33dB and even that seems crazy at times. Adding attenuators before the Prima Luna (adding after is a big no-no) will NOT help your case here. The amp’s volume control should be positioned BEFORE the input tubes (as in 99.9% of all preamps), which protects them from overload by input signals - BUT the downside of this is that you get to hear the amps’ "full gain" at all times. That means tube noise issues are made readily apparent, and if your speakers are sensitive you’ll hear lots of hiss noise floor too. If you have channel imbalance issues from the potentiometer tracking at low levels, then the attenuators can help with that - but not with the noise. So if an integrated amp did place the volume control after input tubes, then your noise issues would be a whole lot better - BUT now you could easily overload the input stage (especially with modern DACs), and that will make your sound have lots and lots of crunchy distortion. |
Ah OK thank-you for this @mulveling So after your edit above I am reading this as the gain is still pretty high as a result of the amp design, and this would mean I should be looking for 'low noise' tubes rather than 'extremely low noise'? I thought the CV4003 would be a good choice, but am open to other suggestions. I have also been told the the middle two tubes are the most critical, and am fairly sure these are the gain tubes which would track. Overall though I think I may just have had bad luck on the tube front. The PL aren't supposed to be terrible, but I am disappointed in the NOS Brimar. The CV4003 that I JUST replaced was giving a different kind of whining on day 1. Oddly enough, a small tap with a pencil fixed that...I won't even start. I'm going to press ahead with the attenuators as this will make the volume knob travel more user friendly. As it is right now the fine level of control required is annoying. Using the remote it is hard not to go from too quiet to too loud. Using the DAC volume at 60% makes a good listening volume available at 12 o'clock instead of 8-9 o'clock and helps greatly. And thanks for the input on the channel imbalance too, the left channel is audibly louder at very low levels so hopefully the attenuators will address that too. Fun stuff this audio game. Interesting, madenning, too expensive, but fun! |
Also have a set of these to try as they were pretty well priced and I read a review somewhere that they went well with PL gear...what could possibly go wrong!!! |
That is most likely it, combined with your high-usage patterns. As you said: "the system is used almost every day for 12+ hours" - which makes me wonder, do you turn the amp on/off each day? Since these are preamp tubes w/ SS output devices, you might be far better off leaving the amp on 24/7! I’m just thinking of many, many heating & cooling cycles loosening up those mica spacers over time (whining/whistling noises and microphonics). My favorite amp maker (VAC) basically advises leaving preamp tubes on more often then not unless you’re using sparsely. If ever there was a scenario for 24/7 ON tubes, yours is it! Anyways, getting the attenuators for better volume control travel positioning makes sense. Try those, some new tubes (selected for low noise), think about leaving the amp on, and good luck :) If you think vintage 12AU7 going noisy is bad, be thankful you’re not dealing with old 1940s 6SN7GT or 6922/7308/6DJ8 tubes. THOSE are the most frustrating tubes in my experience! |
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