cd input too loud can it be attenuated?

[homertooties]

Hi everyone can i attenuate the signal to the inputs on my Sony Taf 540 e amp,the volume control goes around fully twice,however it is still too loud i know you can attenuate speakers with resistors in series and across loads,vcan this be done with line level signals,if so any resistor values would be great

Thanks in advance

 

[Spev]

yes you can - I've attenuated different inputs to different levels on my amp - one has 10k series/10k to ground, the other has 20k series/10k ground which I believe give approx -6db and -10db. It was easy enough on mine as the rca sockets have flying leads to the pcb, rather than being attached directly to pcb...

 

[homertooties]

attenuation

Hi spev many thanks for reply,i am also going to look at the volume more closely also as you can quietly hear the sound when on minimum,i have a few rca sockets lying around so ill try 10k series 10k to ground and 20k series 10k to ground,

When this particular amp is working it is very impressive in detail and absolute power due to design,but the volume when turned down has a little bit of play ,gonna get me parts boards out a try your suggested values.cheers

 

[AndrewT]

Hi,
using a source resistance as high as 10k or 20k will kill all the treble, if your amp has the usual RF filter at the input.

Give us more details and we may be able to help.

 

[sreten]

Hi,

sounds drastic but the ratio you need is more like 10:1.

I built 8:1 attenuators into new phono plugs on the amp
end of the leads for the CD. 6.8kohm and 1Kohm resistors.

/sreten.

 

[homertooties]

attenuation

Hi members many thanks for replys i shall remove the volume pot which is at the rear surrounded by metal(sheiding)with a shaft to the front and remove the input board to see whether any filters are around the rca,s ill keep you posted ,,cheers

 

[homertooties]

Follow up to cd attenuation

Hi all

This is some more info about cd input too loud

Firstly the volume control was faulty,the end stop iside was broke,drilled four rives took the pot apart(Alps 139c)8 pins but only six used swapped the inner brown box sections of this pot around so now the pot stops and starts where it should success,the pot is supposed to be 120 k x2 but actually is 105 and 108 l and r channel.

All inputs on the board have a 1 meg resistor going from input live to ground that is all .

Yesterday i built two attenuators ie 10 k series 4.7 k to ground,this did nothing to the volume ,but the 10 k series 3.3k to ground had very good results enabling me to swing the volume a lot more gradually instead of the normal move a bit and in your face .

Is it possible that the pot is buggered and needs to be replaced
Dimensions are 18 mm wide 20mm tall 18mm deep i can not find out any info from Alps site or anywhere,

Asimple solution would be a blue velvet but too big as the pot is mounted on a horizontal board which plugs into the vertical input board,also the pot shaft is not completely circular it has a chamfred face and fits into a coupler which connects to a long aluminum shaft.

I am happy with the latter attenuator i built however it has been noted that this could affect the top end

Any info would be greatly considered

Thanks in advance

 

[sreten]

Re: Follow up to cd attenuation

Yesterday i built two attenuators ie 10 k series 4.7 k to ground,this did nothing to the volume ,but the 10 k series 3.3k to ground had very good results enabling me to swing the volume a lot more gradually instead of the normal move a bit and in your face .

Is it possible that the pot is buggered and needs to be replaced
Dimensions are 18 mm wide 20mm tall 18mm deep i can not find out any info from Alps site or anywhere,


Thanks in advance

Hi,

The first attenuator should have done somethng to the volume.

If your pot is fine on other sources there is nothing wrong with it.

You need 10K series, 2k to 1k to ground for the CD input.

Treble will not roll-off if done at the amplifier end of the lead.

/sreten.

 

[AndrewT]

Treble will roll off if the source impedance rises into the RF filter on the front end of the amplifier.

 

[sreten]

Treble will roll off if the source impedance rises into the RF filter on the front end of the amplifier.

with a maximum of ~ 2k to ~1k with suggested values it won't, /sreten.

 

[Spev]

I'm a little confused here (sorry to hijack the thread).

This is from a post on another forum in the context of a 27k series, 15k to grnd config:

It's simply a voltage divider, so using Ohms law you can work out the attenuation, in terms of voltage.

The output voltage will be: -

Vout = Vin * (R2/(R1+R2)) [Where R1=27k, R2=15k]

If we plug in 1V as an input, you can see that the output voltage would be: -

Vout = 1 * (15000 / (27000+15000))
Vout = 0.357V

To convert this to dB, we can calculate the dB difference between the two voltages: -

dB = 20*log10(Vout / Vin)

dB = 20*log10(0.357 / 1) = -8.9 dB

The combined series impedance of the pair dictates the load the source sees - currently 42k, change the resistors to 2k7 and 1k5, which gives the same attenauation, but now the source sees a harder load (2700+1500 = 4200ohms).

But the preamp sees a lower driving impedance, or in the case of a cable, it would raise the frequency rolloff higher in frequency, if the attenuator was placed at the source end.

There are conflicting requirements therefore in terms of impedances and loads.

If one goes a step further to situations where you want a power transfer between items (rather than simply a voltage) it's important to maintain impedance matching at both ends, which would use a calculated combination of resistors in a 'pi' or 'T' configuration.

where to strike the balance?

 

[sreten]

Hi, the values suggested, 5K to 10k series, 2k to 1k parallel, /sreten.

 

[AndrewT]

Hi Spev,
assuming you want attenuation and not maximum power transfer then the standard rule of receiver about ten times source works for the passive attenuator.

Take a 50k input impedance power amp, divide by ten and use a 5k pot now divide by ten again and use a source impedance about 500r.
The 5k pot will show a source impedance to the power amp of 0r upto (5k+500r)/4=1375r. The RF filter inside the front end of the power amp now sees it's own series resistor + this variable source as the R part of the RC filter. If it was made up from 1k0series R and 1nF grounding cap then the turnover frequency varies from 156kHz (Rs=0) down to 67kHz. At -6db the RF filter has become a mild treble cut filter.

At the source end the DC blocking capacitor forming a high pass filter will see the load as 5k//50k~= 4k5 (maximum volume = -0db)and upto 5k at maximum attenuation (not much variation here).
But 5k or so needs a large film cap of about 10uF or even an electrolytic//smaller film cap for extended bass performance.

Using ten times on both sides of the attenuator is workable but needs careful selection of the component values either side to get a wide passband that is relatively unaffected by changes in attenuation.
Fixed attenuation avoids the variable part but still requires value selection.
If the power amp has 10K input impedance, then all the above resistance values need to be factored down by a ratio of five and capacitors adjusted accordingly.

 

[Spev]

thanks both, I'll give this some pondering. In the meantime, I've quickly swapped my 10k series 10k para for 1.2k para - which has massively upped attenuation from -6 db to -20db, which is clearly noticeable on the vol pot!

will listen further to see if I can hear any difference in frequency response

 

[homertooties]

attenuation

Hi there gosh some serious food for thought,will the 1 meg resstors have any effect on the divider network to be worked out yet,many thanks for input so far.

Is a 50 k pot gonna upset the power amp when it was originally loaded with a 120k pot...............chers