[vortrex]

My integrated amp input sensitivity is abnormally high and I've been told by someone knowledgeable that this is causing the noise issues I am hearing between it and my phono stage.  Can someone explain to me in simple terms what is happening here?  With no table connected to the phono pre I get an unusually high amount of noise.  If I connect a CD player it is dead quiet.  I've been told how to change the input sensitivity, but before I go there I want to make sure that is really the issue.

Integrated amp:

Input sensitivity 210mV
Input impedance 100k ohms

Phono pre:

Output impedance 10K ohms
Output level -10 dB Vu
Gain MC input 65dB
Noise - 75dBu

[wakibaki]

OK.

A resistance produces noise. The higher the resistance the more the noise. This is called thermal or Johnson noise.

It's common for an input to have a resistance to ground between the signal input and the ground input. This resistor may serve to define the input impedance or set the output voltage of the stage. Even if there's no resistor the input itself (opamp, transistor, whatever) will have a resistance to ground.

The noise produced by the input resistance can be amplified. The more gain the amplifier has, the greater the likelihood that the noise (hiss) will be audible.

The size of the resistance is critical in terms of the source that can be connected to it. If it's too small it can load down the source.

What do we mean by 'load down'?

Every source has an output resistance. A battery, a power supply, a CD player, all have an output resistance. The output voltage splits between the source resistance and the sink (input) resistance. If the source resistance is higher than the sink resistance more of the output voltage appears across the source resistance than across the input resistance. We say that the sink has 'loaded down' the source. This is a 'waste' of voltage, and an undesirable state of affairs. It worsens the signal-to-noise ratio, which is what you're complaining about. In audio (it's different in radio), we like the input resistance to be ~10* the source resistance.

A typical (solid state) audio input resistance is 5~10k. With tubes it's often higher. The value chosen is a compromise based on a number of factors, but it will generally not be so low as to load down any likely source.

A modern source such as a CD player frequently has an output resistance < 600 ohms.

When a modern (or any) source is connected to an input, its resistance is connected in parallel (shunt) with the input. Resistances in parallel combine to produce a resistance lower than the lowest of the two. So the amplifier 'sees' a lower resistance. Whereas before it might have seen 10k, it might now see 500R. This can result in a difference in the perceived hiss at the output.

The noise you are hearing is almost certainly generated at the input to the phono stage and in the phono stage itself.

I would suggest that you simply don't listen to the system without a source connected and particularly without a table connected to the phono pre. The overall noise in a system is much more related to the input (absolute front end) noise and the gain in the stage following than it is to gain in subsequent stages. This is why a satellite dish employs a low-noise amplifier right at the dish, because it is impossible to improve the SNR of the system significantly by fiddling with the gain in the set-top box.

The one thing you can do is spend more money on (or build) a quieter phono stage.

w

[vortrex]

thanks for all that, but, the noise is the same whether there is a source connected or nothing connected or the inputs shorted on the phono stage.  there is no difference at all, which is why I would like to fix this since I hear the noise while listening to quiet passages of music.  also, as I mentioned, this is known to be a very quiet phono stage.  the phono stage is the not the issue.  I've tried another well regarded phono stage and had the exact same problem.  what I am trying to find out is what is it about the specs of my amp and phono stage(s) that causes this problem?

[wakibaki]

the noise is the same whether there is a source connected or nothing connected or the inputs shorted on the phono stage.

If I connect a CD player it is dead quiet.

Which is it?

The design of phono stages is problematic. This is one of the reasons why CDs have enjoyed such success.

The fact that you are dealing with the low-level phono signal almost demands you learn to tune out some low-level noise as you do with vinyl surface noise and just "enjoy the music", as they say.

[vortrex]

it's actually my Oppo BDP-80 DVD player, I don't use it for audio.  I understand phono stages may be a tad more noisy, but I have had perfectly quiet phono stages compared to the current setup.  definitely some sort of mismatch happening.

[wakibaki]

Reread my first post or read this:- http://en.wikipedia.org/wiki/Friis_formulas_for_noise

The SNR of a multistage amplifier is close to independent of the stages at the back end of the system. The problem, if there is one, is in your phono stage. If you reduce the gain of the integrated amp the overall sound will be quieter, (and the noise will obviously be quieter) or you will turn up the volume and you will be back to where you started. You can't fix this problem by messing with the gain of the integrated amplifier, which is the question you were asking.

Just because the specs of the phono stage look good, it doesn't mean that it is performing up to specification.

w

[vortrex]

I appreciate the advice, but I think you are kind of missing the point in some ways.  there's no way this would be one of the most highly regarded phono stages available today if it had noise like this in everyone's system.  it's obviously a mismatch, which is what I am trying to find out in one or two simple sentences.  I'm not too smart.  now you might say there's something wrong with my exact unit, but I've had two of them and they were identical in noise.  I've also had a 4th phono stage that had less gain and was quiet with my amp.  so, it seems to someone like me who knows next to nothing that the gain is the issue, but why?  that maybe tells me it goes back to the high input sensitivity I mentioned initially.  now with the specs I posted for my amp, what sort of max phono gain am I limited to?  if you look at the gain calculator it'll tell me I need somewhere around 62db with my cart, which is close to what I am at (65db), yet I have this noise issue.  if I drop the gain of the phono pre way down to rid myself of the noise am I sacrificing the performance of the cart at that point?  can I change the input sensitivity of my amp (not the gain) and do no harm?  so many questions...

[wakibaki]

I appreciate the advice, but I think you are kind of missing the point in some ways.  there's no way this would be one of the most highly regarded phono stages available today if it had noise like this in everyone's system.

The noise it produces is the noise it produces is the noise it produces. You can't make it quieter by pairing it with a different integrated amp. The noise comes from the preamp, not the integrated amp. The integrated amp amplifies the signal + the noise, if you decrease the sensitivity of the integrated amp then the noise will be quieter but so will the signal.

it's obviously a mismatch, which is what I am trying to find out in one or two simple sentences.

Sometimes one or two simple sentences don't hack it. If you could explain how to design an amplifier in one or two simple sentences, everybody would design their own. As an amplifier designer, I'm telling you it's not a mismatch.

you might say there's something wrong with my exact unit, but I've had two of them and they were identical in noise. 

Either the phono amp is not performing to spec, or it is performing to spec and your expectations are unrealistic.

I've also had a 4th phono stage that had less gain and was quiet with my amp...   ...the gain is the issue, but why?

No. The fact that it had less gain is coincidental, either the signal-to-noise ratio was better, or you were simply listening at lower volume.

  that maybe tells me it goes back to the high input sensitivity I mentioned initially.  now with the specs I posted for my amp, what sort of max phono gain am I limited to?  if you look at the gain calculator it'll tell me I need somewhere around 62db with my cart, which is close to what I am at (65db), yet I have this noise issue.  if I drop the gain of the phono pre way down to rid myself of the noise am I sacrificing the performance of the cart at that point?  can I change the input sensitivity of my amp (not the gain) and do no harm?  so many questions...

If you hold the output power constant then there are 2 ways of lowering the sensitivity. You can change the gain, or you can attenuate the input signal. If you attenuate the input signal you will make the noise worse, because the noise figure of an attenuator is equal to its attenuation, and if you look at Friis' noise equation, you will see that putting an attenuator at the input of an amplifier has absolutely the worst impact on its noise.

You've got some problems standing in the way of dealing with the situation.

1) you know next to nothing

2) you've already decided what the problem is and what to do about it, even though you know next to nothing.

3) you don't know expert advice when you see it.

No amount of 'it must work like this' or 'it can't work like that' or 'there's no way' on your part will change the way it works. I've told you how it works, but you don't want to accept it.

Why do you think CDs have largely replaced vinyl? Amongst other reasons, it's because no matter how hard you try, or how much money you spend, or how much you believe that it's better, the signal-to-noise ratio is always worse. It's about 60dB at best for vinyl and about 120dB for CD with appropriate dither. It's 96dB for CD without dithering.

I suggest that you modify the gain of the integrated amp and once you've convinced yourself that it hasn't improved things, come back and we'll start over. Either that, or you'll convince yourself you've fixed it and you won't come back.

[Speedskater]

It's most likely a Gain Structure (Stage Gain) problem. Here is one paper that I quickly found, a search should find others (maybe even better papers).

[vortrex]

I'm relaying the info here that was given to me when I spoke on the phone to the designer of my phono pre.  he's the guy who designed and built a phono stage that has received nothing but praise from every professional reviewer and end user.  unfortunately because I am not as technical as you I cannot relay everything he said, but he did say it was because of the high input sensitivity of my amp.

"Either the phono amp is not performing to spec, or it is performing to spec and your expectations are unrealistic."

definitely not true in either case.  I know what to expect.

"No. The fact that it had less gain is coincidental, either the signal-to-noise ratio was better, or you were simply listening at lower volume."

incorrect, I was doing the same exact test at the same volume.  as far as the signal-to-noise ratio I cannot comment, since that spec is not posted.

"If you attenuate the input signal you will make the noise worse, because the noise figure of an attenuator is equal to its attenuation, and if you look at Friis' noise equation, you will see that putting an attenuator at the input of an amplifier has absolutely the worst impact on its noise"

that's interesting, because I can put some inline 12db attenuators on and the noise factor is better for me.  the attenuators do add some coloration that I don't like though.

[Folsom]

I personally would find it helpful to know the exact products you are using, and used.

However to give the answer you want... Put in a buffer. A buffer can have an output much lower than 10kohm, so your amplifier will be driven easier. I couldn't tell you that this will fix the problem, but it is the answer you are looking to find based on what you have said.

Let me see if I can give an easier to understand explanation... All audio must form a complete circuit, the electrons flow towards the signal origination. Your phono preamp shares the signal to your amplifier via completing the circuit, or the loop, through the amplifier. One big circle, in and out of your amplifier and back to the phono preamp (signal being pulled through). Here is the tricky part... You want it to pull electrons starting at the phono preamp creating signal, pulling through the amplifier, ultimately from the phono preamp (like if you put an RCA cap that connected the outside conductor to the inside, but now pretend the amplifier is the RCA cap). However the higher the output impedance (think of it as resistance to electron flow) of the phono preamp, the more it will pull electrons from the amplifier, and not the phono preamp. If you lower the output impedance on the phono preamp, than it is easier to get more electrons from the phono preamp through the amplifier; hence the signal passing through the amplifier is stronger. When output impedance is high, the phono preamp signal will get more electrons from the amplifier instead, which lowers the signal passing through the amplifier's actual amplification stage because it'll get the electrons just before it; and hence can not "drive" it as well. - That is a technically inappropriate explanation but for someone that knows nothing I think it'll help get the basic concept. It is certainly more complicated, but I think this will help a little.

[vortrex]

Zesto Andros phono stage (55db and 65db)
Line Magnetic 218IA integrated

I also tried K&K Maxxed-Out w/A23 SUT (64db, 200ohms output) and Manley Chinook with and without A23 SUT (71db/60db, I think 50ohms output?) phono stages, same issue.

the Leben phono stage with A23 SUT (49db) is the only one that's quiet so far.

[Folsom]

You are amplifying noise much greater. It could be noise from your RCA cables, from the wall power, of the tubes, a lot of things.

A lighternote pre-amp might be a good way to tone down the signal pre-amplifier (the most colorless option I know about). But I'm concerned the noise is coming from somewhere else, and just lowering the gain may not be the answer. (your 12db attenuators lower the gain, but they are not correcting the noise the Leben did not have?)

[Folsom]

*nm

[vortrex]

I updated my previous post with more info.  but yes, the Leben is about 50db total and the lowest.  so, as I said, we are back to gain again, no?  it doesn't appear to be anything to do with the output impedance since I tried 50ohms up to 10kohms, unless I totally misunderstood what you meant.

[Folsom]

I updated my previous post with more info.  but yes, the Leben is about 50db total and the lowest.  so, as I said, we are back to gain again, no?  it doesn't appear to be anything to do with the output impedance since I tried 50ohms up to 10kohms, unless I totally misunderstood what you meant.

A Lighternote might give you the attenuation in a colorless fashion that you want. But noise appears to be the issue.

[vortrex]

Your figures for gain are different than manufactures.

Sorry my last two post are not the same thing, now that you have different information.

Again a Lighternote might give you the attenuation in a colorless fashion that you want. That is my current best suggestion.

As I originally stated, a buffer would fix impedance issues. This sounds like a gain issue, however.

So the 12db attenuators, do they make it have less noise than the Leben, but with coloration?

how are my figures different?  the A23 SUT is 26db. 

Zesto = 55 or 65
K&K = 38 + 26 = 64
Manley = 45 + 26 = 71 and 60 with no SUT
Leben = 23 + 26 = 49

is it really an impedance issue if a 200ohm pre and a 10kohm pre behave the same?

I only borrowed the Leben for a few min while my dealer was over and didn't do direct comparisons, but my guess is no, the Leben is best of any combo.

[Folsom]

Ohh I see, with a step up transformer.

You don't have an impedance issue worth mentioning. 10kohm into 100kohm is a normal amount. Ask your dealer if they have a buffer to try if you want, but this doesn't sound like the main issue.

You are collecting noise from something far as this reads.... Lowering the gain might help, but isn't the right answer.

[Folsom]

I tried 50ohms up to 10kohms, unless I totally misunderstood what you meant.

If this is true, a buffer would do nothing. Your output impedance is adjustable on the Zesto?

Have you tried any of the lower gain phono units without the SUT?

[vortrex]

No, the output is not not adjustable but the K&K is 200ohm.

I haven't tried any other lower gain units.  It seems difficult to find anything less than the Zesto for MC.  The other problem is I really like the sound of transformers and want to stick with that.