Phono stage 'hiss'

[sprint]

How much 'hiss' would you expect to get from a phono stage?

I have M.F. Nu-Vista integrated amp and using a A.N.T. Kora 3T Ltd MC phono amp. With the phono amp not connected and the vol level at the 12 o'clock position there is some audible hiss with you ear close to the speakers but not at the listening position, approx 3m away. At the 3 o'clock position, don't normally play above the 12 position, there is audible hiss from the listening position.

With the phono amp connected I now get significant audible hiss at the 12 o'clock position and also between the 10 and 12 o'clock which is normal listening level.

I think the gain on the phono amp is about 62db so there will be an increase in noise level (I get no hum whether the arm earth leads are fitted or no) but would you expect to have audible hiss at the normal listening vol level between the 9 and 12 o'clock?

 

[sergeauckland]

Stating volume levels as "9 or 12 o'clock" isn't really very helpful, but I accept that in the absence of any measuring equipment, it'sdifficult to do much else.

The problem is one of gain structure. The Phono stage has an amount of gain, and with it an amount of noise. The pre-amp also has gain and an amount of noise, some of which is before the volume control and so attenuated, some after and so not attenuated. The power amp has some gain and noise, so if the gain structure is wrong, too much noise ends up in the power amp and it can be heard, especially if the loudspeakers are particularly sensitive.

There should ne no noise audible at the listening position, and your description that your normal listening level is between 10 & 12 o'clock, indicates too much gain in the system.

I really don't know why so many audiophiles like the volume control to be below 12 o'clock for normal listening. My view is that it should be something like 2 -3 o'clock as that still allows some extra gain available for "enthusiastic" listening or a quiet recording, but doesn't put too much gain in the system that the volume control has to knock back.

S.

 

[sprint]

Stating volume levels as "9 or 12 o'clock" isn't really very helpful, but I accept that in the absence of any measuring equipment, it'sdifficult to do much else.

The problem is one of gain structure. The Phono stage has an amount of gain, and with it an amount of noise. The pre-amp also has gain and an amount of noise, some of which is before the volume control and so attenuated, some after and so not attenuated. The power amp has some gain and noise, so if the gain structure is wrong, too much noise ends up in the power amp and it can be heard, especially if the loudspeakers are particularly sensitive.

There should ne no noise audible at the listening position, and your description that your normal listening level is between 10 & 12 o'clock, indicates too much gain in the system.

I really don't know why so many audiophiles like the volume control to be below 12 o'clock for normal listening. My view is that it should be something like 2 -3 o'clock as that still allows some extra gain available for "enthusiastic" listening or a quiet recording, but doesn't put too much gain in the system that the volume control has to knock back.

S.

Thanks for the reply.

You indicate that that one should not be experiencing audible noise/hiss at the vol and listening positions indicated and it is likely to be a gain issue?

The amp is rated at 250W/ch but I do not know what that relates to in terms of power or db output relative to the specific vol positions?

Is there anything that can be done to reduce the level of hiss or is it an inherent system issue that will have to be lived with?

Believe me at a vol position between 2 and 3 you would be deafened. Speakers used are KEF 205's

 

[MJS]

How long is a piece of string? Does the volume go up to 11 or further?

MC phono stages have to have a shedload of gain and that comes with noise a lot of the time. As pointed out above add the preamp and amp gain and it becomes quite audible. Naim's MC boards are quite noisy even at 12 o'clock (btw that roughly 0dB gain in the pre-amp if anyone's interested) but that doesn't detract from its quality.

Mark.

 

[sergeauckland]

Thanks for the reply.

You indicate that that one should not be experiencing audible noise/hiss at the vol and listening positions indicated and it is likely to be a gain issue?

The amp is rated at 250W/ch but I do not know what that relates to in terms of power or db output relative to the specific vol positions?

Is there anything that can be done to reduce the level of hiss or is it an inherent system issue that will have to be lived with?

Believe me at a vol position between 2 and 3 you would be deafened. Speakers used are KEF 205's

That's what I mean about the gain structure being wrong!

KEF 205/2s have a sensitivity of 90dB, so I'm assuming your 205s are similar.

With 250W pc that would give an output of around 115dB SPL. Assuming you listen at an average level of around 85-90dB SPL, that gives you some 25-30dB of headroom, or putting it another way, of extra noise. Nevertheless, this should still not be audible. You've confirmed that by saying that hiss is only audible close to the 'speaker, which is what I would expect.

From what I've read of the ANT phono stage, it's pretty quiet, so I would be surprised if that was the cause, unless it's faulty. Is the noise the same in both channels?

If nothing is actually faulty, suspect that it's a combination of a high-power amplifier, sensitive 'speakers and normal phono amplifier noise which is audible in a quiet room even at the listening position. Without making some measurements of signal and noise levels, I can't be any more helpful than this, but I suspect it's something to live with. Can you still hear any hiss once the cartridge is playing an LP? I suspect that the noise you're aware of is still below the background noise of the LP, in which case there's not a lot you can do about it.

S.

 

[Darth Vader]

A good phono stage shouldn't hiss.

An MC phono will have a lot of gain and yours has 62dB which in layman terms is over x1.500,000. Hum is the usual problem with an MC phono due to this very high gain.

Which version do you have the 'special' or 'limited'.

PM Dave Cawley here on PFM and ask him to check it out.

FWIW my own phono stage has no hiss but I do get a little hum and I know why - its in close proximity of 4 yes four 1.2KVA mains transformers!

Cheers,

DV

 

[Paul R]

Without the MC amp connected really there should never be audible hiss, the implication is that the amp has a very noisy line level stage before the volume control, so this might be worth investigating.

With the MC amp connected it is reasonable to expect a very modest amount of hiss at normal listening level, but it shouldn't be obvious. This even with a noisy Naim stage...

So I think your MF amp has a problem and it might be worth getting it checked out.

Paul

 

[YNWOAN]

I started to write a response to this question that you have also asked on another forum - my response used the terms "it's all a question of gain structure" and "how long is a piece of string" - which entertainingly (for me) I see two other contributors have also used . In the end, I just thought "yes, probably" would suffice.

 

[Darth Vader]

I have just run a test on my system.

I have a Brinkmann Fein phono stage directly connected to my bridged amps that can deliver 1000W into 8 Ohms. I have set the volume on the Fein to half way (12 o'clock). My speakers are Shahinian Obelisk 2 that have stuck at the top two tweeters and 4 super tweeters. I stick my lughole right against these tweeters and I hear....silence. Way below is the woofer from which a get a tiny amount of hum thats been inducted from the 4 1.2KVA Xformers in my amps due to its proximity.

Your amp requires 300mV for the full 250W into 8 Ohms. To get 250W RMS into a resistive load requires 44V RMS across the load approx. However since speakers are a reactive load the current will often be out of phase with the voltage which means that to get a true 250W RMS will need a higher voltage lets say 60V. Doing the sums this works out at a gain of 23dB. The PFM beloved Naim amps have a gain of 29dB so your amp (and my sums) are within standard values.

I calculate that my amps require 425mV to get 250W so the sensitivity of my amps are 2/3 of yours which in audio terms is not significant.

So I point my finger at the phono stage.

The A.N.T. Audio has only three transistors per channel. Now open up a quality phono stage say a 323/523 Naim boards and you'll see five identical transistors in parallel. You know why? Its to cancel out transistor noise (hissssssss). It works because the noise is random and if you have a number of identical trannies in parallel the noise will cancel out. Well thats the theory.

Now that is not to say that there is anything wrong with the design of your phono stage as it may be innovative but without that noise cancellation and with that huge 1,500,000x gain it may only need a small change in summat and that hissssssssss will be amplified.

BTW my Fein uses the same parallel transistor technique and has if I remember correctly 7 trannies in parallel.

Ask Dave Cawley for some advice/input.

Cheers,

DV

PS link to the NA 323 circuit. Note Q1 has x5 by the transistor type. Its the very first stage and receives the most amplification so any hiss here will be greatly amplified.

http://www.neilmcbride.co.uk/mcboard.pdf

Just spotted this gem:-

"Pretty soon I discovered that the circuit can be precisely tuned for the best sound with a particular cartridge by changing a DC operating point of the FETs in a certain range by adjusting a simple rear panel pot."

Now what if that pot has been incorrectly set and the bias is way off beam? May be worth a play......

 

[Werner]

with that huge 1,500,000x gain

62dB is 1258x, a normal figure for an MC stage.

BTW, I have a DIY stage using a single 2sk369 as front-end, and it is dead-quiet. It really depends on which transistor is selected for what job.


A pertinent question for the OP is: did you do the test with a cartridge connected or not, and if so,
which cartridge?

 

[Darth Vader]

62dB is 1258x, a normal figure for an MC stage.

BTW, I have a DIY stage using a single 2sk369 as front-end, and it is dead-quiet. It really depends on which transistor is selected for what job.


A pertinent question for the OP is: did you do the test with a cartridge connected or not, and if so,
which cartridge?

Arrrrgh my old brain confused power vs voltage!!!! Next time I'll have to revise.

Cheers,

DV

PS At least someone took the effort to check my sums.

 

[sergeauckland]

I have just run a test on my system.

I have a Brinkmann Fein phono stage directly connected to my bridged amps that can deliver 1000W into 8 Ohms. I have set the volume on the Fein to half way (12 o'clock). My speakers are Shahinian Obelisk 2 that have stuck at the top two tweeters and 4 super tweeters. I stick my lughole right against these tweeters and I hear....silence. Way below is the woofer from which a get a tiny amount of hum thats been inducted from the 4 1.2KVA Xformers in my amps due to its proximity.

Your amp requires 300mV for the full 250W into 8 Ohms. To get 250W RMS into a resistive load requires 44V RMS across the load approx. However since speakers are a reactive load the current will often be out of phase with the voltage which means that to get a true 250W RMS will need a higher voltage lets say 60V. Doing the sums this works out at a gain of 23dB. The PFM beloved Naim amps have a gain of 29dB so your amp (and my sums) are within standard values.

This isn't quite right! You're confusing Watts with VA. Whilst there is indeed a realationship between phase angle and power, unless you know what the phase angle is, there's no way of telling what the difference between watts and VA actually is. Nor does it matter, as loudspeakers don't work on Watts, they work on volts, and volts don't care about phase angle. What Phase angle affects is the effective impedance the amplifier sees, and the current its asked to drive. However, as this is an MF amplifier, which could do service as a welder, it's a moot point.

Complicated this stuff, isn't it!

S.

 

[sprint]

Thanks for all the replies.

The A.N.T. Kora 3T Ltd MC I believe has 4 fet's, if that make any difference?

The Nu-Vista also has its own independent phono input, in MM mode and with no input it is has more hiss than the tuner input I am using with the A.N.T.

Certainly with the A.N.T. connected more audible hiss is generated than when it is not connected.

I will give Dave Crawley a ring next week to see if can offer any advise.

I am just trying to ascertain what hiss level is normal or to be expected to establish if there may be a problem or not, but the current consensus seems to be fairly split between yes you will expect audible hiss to no there should be none?

 

[RustyB]

I really don't know why so many audiophiles like the volume control to be below 12 o'clock for normal listening.

Simple answer: they don't have a choice!

Especially as some manufacturers (one in particular) behave as if CD was never invented.

 

[Darth Vader]

Thanks for all the replies.

The A.N.T. Kora 3T Ltd MC I believe has 4 fet's, if that make any difference?

The Nu-Vista also has its own independent phono input, in MM mode and with no input it is has more hiss than the tuner input I am using with the A.N.T.

Certainly with the A.N.T. connected more audible hiss is generated than when it is not connected.

I will give Dave Crawley a ring next week to see if can offer any advise.

I am just trying to ascertain what hiss level is normal or to be expected to establish if there may be a problem or not, but the current consensus seems to be fairly split between yes you will expect audible hiss to no there should be none?

From the designer:-

" this simple, only 3 FET per channel circuit easily bettered my best previous efforts."

Hence 3T = three transistors........per channel.

Cheers,

DV

 

[hifi-dog]

try a valve stage instead, the hiss is much more musical ...;-0

 

[eguth]

"...I am just trying to ascertain what hiss level is normal or to be expected to establish if there may be a problem or not, but the current consensus seems to be fairly split between yes you will expect audible hiss to no there should be none?"
Sprint

I don’t think there is anything quite as impossible to ascertain as ‘normal’ for hiss. My own phono stage, a modified Quad 44 into a home -made passive preamp with an active buffer stage (Audio Synthesis), has no audible hiss whatsoever even with your ear an inch from the loudspeaker at the highest volume setting. This may or may not be achievable with your equipment.

 

[sprint]

From the designer:-

" this simple, only 3 FET per channel circuit easily bettered my best previous efforts."

Hence 3T = three transistors........per channel.

Cheers,

DV

From the same article I believe but a bit further down on the page:

The result of this development was the Kora 3T phonostage - produced in two versions the "Special Edition" and the "Limited Edition". Both share exactly the same electrical circuit, the difference made only by component quality and the selection criteria of the FET's used. Later in the year, on request from some of my friends and customers I've started the development of the MC version. In September I've met Dave Cawley at Heathrow Show and he did encouraged me to put more effort in the MC design. So by November 2008 the Kora 3T MC was born, both in SE and LE variants. Higher gain required for MC forced me to add one more FET per channel but I've kept the "3T" in the name even thought "4T" might be more appropriate.

http://www.time-step.com/distribution/ant.html

 

[sprint]

"...I am just trying to ascertain what hiss level is normal or to be expected to establish if there may be a problem or not, but the current consensus seems to be fairly split between yes you will expect audible hiss to no there should be none?"
Sprint

I don’t think there is anything quite as impossible to ascertain as ‘normal’ for hiss. My own phono stage, a modified Quad 44 into a home -made passive preamp with an active buffer stage (Audio Synthesis), has no audible hiss whatsoever even with your ear an inch from the loudspeaker at the highest volume setting. This may or may not be achievable with your equipment.

Well clearly hiss free is achievable as many of you are reporting? This is my first experience with a separate phono stage so I have nothing to compare or judge it with. I was not expecting to hear the level of hiss I do hence asking the forum for some advice.

As to whether the hiss I hear is due to poor components, incompatibility, just simply what one would expect or if it is an indication that the amp, phono amp or another source/component is possibly faulty I do not know?

That some systems are apparently hiss free and others are not is clearly due to certain factors. What I am trying to understand and establish is if you do have a hissy phone stage should one automatically assume that there is a fault?

At this stage there does not seem to be a definitive answer since the indication is that both hiss free and hissy phono stages would appear to be the norm?

 

[demotivated]

Did we establish that the hiss is with a cartridge wired in to the phono amp? This can lower the noise of a non inverting stage.