Need help with "hiss" with Ayre V5Xe amp and Audible Illusions Modulus 3 pre-amp
KDas -- Tue, 11/10/2009 - 17:37
I just bought an Ayre V5Xe amp and I notice a hiss when I connect it with my system. I talked To Ayre and they suggested it's because of the pre-amp. I don't quite understand why that is the case and would appreciate any help in understanding and solving this problem. Here are the details -
1) I hear the hiss when the pre-amp and amp are on and the pre-amp is "unmuted". I have used an SPL meter to measure the hiss - it's about 55 dB right next to the speakers (Maggie 1.6). The magnitude does not change if I change the volume using the pre-amp volume controls.
2) The pre-amp I have in the system is an Audible Illusions Modulus 3. It'a a tube pre-amp.
3) The Ayre has both balanced and unbalanced inputs. Since my pre-amp is unbalanced, I am using the unbalanced inputs. There's a switch in the back that toggles between balanced/unbalanced. I have the switch set to "unbalanced". If I set that switch to "balanced" the hiss increases.
4) If I take the pre-amp out of the signal path and connect the amp directly to a D/A converter (Benchmark DAC 1 USB) the hiss goes away. So the hiss is purely because of the pre-amp.
Previously, I had used a hybrid amp, the AMC CVT2100A. I never heard any hiss from that system. The Ayre amp has higher gain. Is that the problem? The pre-amp has a residual noise and I hear it because this amp has a higher gain? And is the only solution getting a new pre-amp?
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Have you tried replacing the tubes?
I haven't tried that. Thanks for the suggestion. Even if the hiss is because of the tubes being old, do you have any theories why the old amp doesn't pick up the hiss but the new one does?
Could be the Ayre amp has a higher, (a low voltage number) input sensitivity than your previous. Compare those specs on both amps and let me know.
You are right. I couldn't find the input sensitivity for the Ayre, but the input impedance of the Ayre V5xe is 100 kilo-ohms/phase. The input impedance of the old amp, the AMC CVT2100 is 500 kilo-ohms/470 pico-farad. I think the ratio of voltages is the same as the ratio of impedances. Therefore, the Ayre amp has 1/5th the input voltage of the older amp.
Do you think that the tube hum is due to the tubes aging and the new amp picks that up because of higher input sensitivity?
You are right. I couldn't find the input sensitivity for the Ayre, but the input impedance of the Ayre V5xe is 100 kilo-ohms/phase. The input impedance of the old amp, the AMC CVT2100 is 500 kilo-ohms/470 pico-farad. I think the ratio of voltages is the same as the ratio of impedances. Therefore, the Ayre amp has 1/5th the input voltage of the older amp.
Do you think that the tube hum is due to the tubes aging and the new amp picks that up because of higher input sensitivity?
So is it hum, or hiss????????? If in fact the Ayre amp is that much more sensitive, then that is the reason why you are hearing the hiss. If it is "hum" then you definitely have a tube problem. At any rate, I suggest shopping for NOS tubes that are graded for low noise.
Mark
Probably more of a hum than a hiss. I think changing tubes is a good idea. I will try that and let you know the result. Thank you very much for your help!
If what you are hearing is a hum, you may have a grounding problem.
Read the original post!
KDas, I use to own an AI preamp. I am 100% certain that the noise you are hearing is typical tube hiss. As the tubes age in a preamp, hiss goes up.
Hiss in tubed electronics is a fact of life, it comes with the territory, however, excessive hiss (heard from the listening chair) is a problem.
There is no correlation between input impedance and amplifier gain. My guess is that your new amp probably has higher gain or better high-frequency response than the previous amp.
The AI Modulus is particularly sensitive to tube quality and tube noise, because its line stage has zero, nada negative feedback. It also runs the tubes hard, which wears them out pretty fast. Worn-out tubes show at least one of three symptoms: too much hiss, lack of bass punch and lack of dynamics.
95% of the tube preamps have the volume control ahead of the tubes, so hiss pretty much stays constant, no matter the volume setting.
You may want to try a Manley Shrimp, it places the volume control after the first pair of tubes, so it is much quieter than AI preamps.
By the way, if you want to shop around for tube preamps, avoid 12ax7-based tube line stages. The 12ax7 is a high-gain tube, it is awfully difficult and expensive to buy quiet 12ax7s.
Good luck