why mute transitors veil the sound ?

plep

Member
2004-11-14 11:15 am
brussels
Can someone explain why mute transistors veil the sound?
This is a classic trick, but I don’t understand how it works. My spice sims show almost no degradation as long as you pull the gates (I choose a fet version) hard enough to negative.
In advance thank you.
Philippe.
 

plep

Member
2004-11-14 11:15 am
brussels
Dear AndrewT and zener_diode, what you describe seems to me to be a highly non linear phenomenon, why doesn't it appears in as simple linear distortion as Circlotron suggested?
Well, that shows that I don't understand what the "leakage current..." does.

I once thought that the effect of the muting transistor could be some kind of high-frequency crosstalk through the control circuit but it doesn’t seem to be the case.

Relay or nothing at all if the player doesn’t "click" is the diy audiophile solution that I use, but it’s a cure not a cause.

Best regards and thank you for your help.
Philippe.
 
AndrewT said:
With digital where there may be a permanent voltage bias on the muted line then reversal will not apply.
On analogue where the line to be muted is AC then the muting semiconductor is repeatedly suffering reverse polarity.

A relay avoids this problem but substitutes a bit of capacitance which must be designed for.
Reversed polarity is not a problem as long as the voltage is below the breakdown voltage which is 5-7 V up to 15 V depending of transistor type.

My anwser to this is that the mute transistor has a very little influence on the sound. 100 ohms plus a couple of pF isn't very harmful in a low impedance circuit.
 
peranders said:

Reversed polarity is not a problem as long as the voltage is below the breakdown voltage which is 5-7 V up to 15 V depending of transistor type.

My anwser to this is that the mute transistor has a very little influence on the sound. 100 ohms plus a couple of pF isn't very harmful in a low impedance circuit.
Hi Peranders,
my electronics knowledge is struggling to cope with this.
Talk me through the physics as the signal starts to reverse polarity, for the conditions with the muted and unmuted transistor in place.
 
Conrad Hoffman said:
The other possibility is that this is another audio myth, where one hears the effect just because the seed of the idea has been planted. A properly designed mute circuit shouldn't affect the signal to any significant degree, and it can certainly be done with transistors or FETs.


I agree.

I get very suspicious of claims to remember very subtle changes in audio after removing the equipment, stripping ot down, desoldering, reassembling etc etc.

One is no longer the same person, in the same position, at the same temperature or in the same mood. Expectations, however, are high ....
 
AndrewT said:

Hi Peranders,
my electronics knowledge is struggling to cope with this.
Talk me through the physics as the signal starts to reverse polarity, for the conditions with the muted and unmuted transistor in place.
I think also that this is rather weird but the best thing for you to understand is to test it live.

The thing is that the base current is flowing into the base but he collector current is flowing in the opposite direction.
 

poynton

Member
2005-03-10 11:57 pm
UK
dhaen said:
As the junction is reverse biased there will be a varactor capacitance effect, though my feeling is that the amount will not be significant.

edit: typo corrected

As the effect of having muting transistors is akin to having variable (small) capacitors on the output, any changes to the sound are going to be in the region above most peoples hearing!

Being now of an age where my hearing is going slightly, although not by a lot, I cannot say that I could detect the subtle changes that are said to occur.

This is not to say that they do not occur, just that I may not be able to detect them.

Andy
 
The effect also depends on the particular implementation. If you use a series resistor to a mute transistor, you in effect have created a non-linear attenuator. It depends on the series R value, and the non-linearities of the transistor (the non-linear cap has been mentioned), as well as how the 'off' state is switched (also mentioned). All this determines the magnitude of the non-linear effect so you can't say that always this is audible or not. It depends. It IS often measureable.

Jan Didden
 

tedr

Member
2006-07-14 1:29 am
not convinced either way

I don't think either argument has been conclusive yet. Presumably we are talking about the outputs of CD players where the maximum signal amplitude is controlled to a few Volts maximum.

There is a tendency for folk to feel that anything, from DC to daylight, that touches the audio signal path is a source of audible problems. I find that improbable, getting 20Hz to 20kHz clean and quiet is normally satisfactory for accurate reproduction of sound. Acoustic sources are not usually quiet below about 60 or 70dB below FS. Electronic sources may be quieter but the playback room ambient noise level has to be considered.

Is there disagreement about Per's point that 100 Ohms in series with a few pF, presumably shunt to ground, is harmless?

The circuits I have seen use a build out resistor to ensure a large muting action. Together with the stray R and C in the mute transistor this forms a low pass filter with a high corner frequency, higher than that caused by the tens or hundreds of pF shunt to ground in the interconnect to the next stage.

It seems to me the question is not is there an effect, there may be a very small one, but at what level is it in the audio band?