input sensitivity question

[IndianEars]

@IndianEars

Also, if you don't mind my asking, what does the volume control on the pre-amp do ? Increase the voltage ? If so, is the 2 volts at a lower volume position and the 70 volts at it's highest point ? Which is better, keeping the preamp volume higher or amplifier gain higher ? Thanks.

As an over view, If you consider the Pre-Amp as a Box receiving an input signal from a CD Player or DAC etc, the Volume Control on a Pre Amp increases or decreases the signal at the output of the Pre amp.

The Preamp has a specified Max Gain (usually a Max Gain of 400% to 1000%). This is the Gain available from the unit with its Volume control turned to Max.

Let us take some practical examples and Signal levels:

OUTPUT FROM A CD PLAYER
Music sources such as a CD Player or DAC typically provide a Max signal of 1 Volt or 2 Volts. Let us consider a CD Player Specified with with a 1 Volt output.

The CD Player will provide this 1 Volt Output only when the CD has a signal recorded on it at Full Output. Recording Engineers will almost Never record a Full Signal on a CD Player because if the music goes even slightly louder after that, there will be Gross distortion in the recording.
Hence, during typical music played on a CD, the Output Signal from the CD player will probably be around 0.2 Volts to 0.5 Volts.

The Pre amp receives this 0.2 V to 0.5 Volts.
The Volume Control on the Pre amp enables the User to adjust the Sound Level from the speakers to his liking.
Sometimes, the listener may not want any sound at all (eg when he gets a phone call) and will want to turn the Volume to Zero, ie No Output Signal from the Pre amp. The Pre amp then Reduces / Cuts the signal to the extent required.

At the other extreme, the user may want very loud music for a party. The Volume Control on the Pre amp will then increase the incoming signal ( Provide GAIN) as required.

Hence the Pre amp may receive a signal of 0.2 Volts and the Volume control will decrease or increase this signal (as required by the user) from 0 Volts to approx 1 Volt)

The Pre amp Volume Control ATTENUATES (Decrease) or AMPLIFIES (Increase) the incoming Signal.

Usually, at mid position of the Volume control, the Pre amp Output signal level is the same as the input signal level, and is said you be its "UNITY GAIN POSITION."

Does this answer your query ? Or did I not understand it correctly ?


Wishing All A HAPPY DIVALI !

 

[ssf]

Does this answer your query ?

Very nicely explained. Thank you so much. Looking forward to part II.

Wishing all a happy Deepavali.

 

[IndianEars]

@IndianEars

Also, if you don't mind my asking, what does the volume control on the pre-amp do ? Increase the voltage ? If so, is the 2 volts at a lower volume position and the 70 volts at it's highest point ? Which is better, keeping the preamp volume higher or amplifier gain higher ? Thanks.

Responding to your query:
".....70 volts at it's highest point ? Which is better, keeping the preamp volume higher or amplifier gain higher ?"

Replying in 2 parts:

"Which is better, keeping the preamp volume higher or (power) amplifier gain higher ?"

Not many Power Amplifiers have a user adjustable Gain setting.

If your Power Amp Does provide a User Gain setting, try out the options and listen for the best sound ..... Usually Power Amps sound best at their Highest Gain Setting, and Pre amps sound best around their Unity Gain setting. Listen to your combo for what sounds best to you.


".....70 volts at it's highest point ?"

As mentioned in my earlier post, Preamps usually offer Gains of 400% to 1000% ( ie 4x or 10x )

My Preamp offers a Gain of 4x.

To get an output signal of 70 Volts I will need to feed it a signal of atleast 70/4 = 17.5 Volts with the Volts with the Volume on Full!
It is most unlikely that any source will provide me such a large input signal.

So the 70 Volts output will never be utilised. It will never be necessary either.
Its just to have such a large Output capability, to be reassured the the Pre amp, under normal conditions will ever be "overdriven" into distortion.

 

[ssf]

If your Power Amp Does provide a User Gain setting, try out the options and listen for the best sound ..... Usually Power Amps sound best at their Highest Gain Setting, and Pre amps sound best around their Unity Gain setting. Listen to your combo for what sounds best to you.

Unity gain made a lot of things clear. For me, power amp gain at 2 O'Clock and preamp volume at around Unity Gain works perfectly. Thanks again.

Part-2 for Input & Output Impedance.

Looking forward to it.

 

[Doomster69]

@IndianEars : Informative write up! Thanks for putting out the detailed explanation.

I had a somewhat related query, about impedances on pairings. Im trying to understand the relation and the upsides/ downsides of high output impedance from my dac outputting 2 Vrms at 625 ohms from RCA out (more specs below) to my stereo power amp having input impedance of 200 mV/20 kΩ/kohms (specs below) for RCA input.

AMP SPECS
Damping factor (8 Ω/ohms load, 40 Hz – 20 kHz) :100
Input sensitivity/Input impedance: 200 mV/20 kΩ/kohms
S/N (IHF-A, 8 Ω/ohms load) on RCA: 103 dB (2 V input, Rated output)
RMS Power output: 40 W x 2 (8 Ω/ohms load)

DAC Specs
Analog Output

• RCA at 2.0Vrms, 625 Ω
• XLR at 4.0Vrms, 1250 Ω

THD+N: 0.004%
S/N Ratio: 115dB
Dynamic Range: >119d

Going by my limited understanding, sharing some queries. Please confirm or better inform on the below:

a) The Amp requires 200 mv input signal at 20 Ohms to put out its rated power output of 40W pc to speakers?
b) At 2V input voltage and at the rated output, distortion will become apparent beyond 103 dB?
c) Relating to the dac output of 2 VRms, the amp can produce 40W output with SNR of 103 dB?
d) How does the 625 ohms output impedance from dac impact the input sensitivity for the amp?
e) Does the high impedance of DAC output lead to lower voltage signal for the amp?or to put it other way round- Would this then mean the dac's output strength to the amp would be lowered, thereby requiring more from power amp to reach reasonable listening levels (85db - 90db)?
f) Would it be better to interconnect the dac and amps with higher impedance RCA 'cable' (100 Ohms instead of 75 Ohms RCA)?
g) How does the SNR of both DAC and AMP interact here - i.e. good or bad?

Would be great to understand these aspects better!

Tx

 

[IndianEars]

Will answer all your queries individually, but before that, a reassuring statement:
Your DAC and Amplifier will interface very well, even though the Volume control on your amplifier will need to be set pretty low, around the 9 O'clock position. You have Nothing to worry about.

A brief overview of your situation, first: The hand drawn block diagram below shows your DAC feeding your amplifier.



Every device has a input Impedance (think of it as a resistance, for simplicity) at its input and
an Output Impedance at its output.

Ideally, Input impedances need to be high & Output impedances need to be low so that all the Voltage being fed out gets delivered to the next input.

You have a Very good situation ("match") if the Output impedance of your source / sending equipment is atleast 10 Times LESS Than the Input Impedance of the next / receiving stage.

Based on the specs that you have shared,

1. The Output impedance of your DAC is a (nice and Low) 625 Ohms

2. The Input Impedance of the amplifier is 20K Ohms ( ie 20,000 Ohms , K=1000 as in 1 Kg = 1,000 Grams)

3. The DAC is sending out 2 Volts which gets divided between the 625 Ohms & 20,000 Ohms resistors acting as a simple voltage divider. The 20,000 ohm of the amplifier will receive 1.93 Volts out of the total of 2 Volts sent out by the DAC. Hence the Amp is receiving almost all the voltage sent out by the DAC, very little is wasted.

4. The amplifier needs ATLEAST 200 mV (milli Volts ie 0.2 Volts ) at its input, to provide full power.

As we have seen in point 3 above, the amp will receive (more than enough) 1.93 Volts.

5. The amplifier Damping Factor is 100 into an 8 Ohms speaker (specification mentioned by you), so its output impedance is 8/100 = 0.08 Ohms which I have shown in the block diagram above.

6. Now some Info on S/N (Signal To Noise) specification

i. The amplifier's S/N 103 dB means the Noise is 103 dB below (ie 141,254 Times smaller than !) the 40 Watt output! its not likely to bother you
ii. The DAC's S/N 115 dB means the Noise is 115 dB below (ie 562,341 Times smaller than !) the DAC output! It will certainly not bother you

iii. The DAC (115 dB ) S/N feeding the Amp (103 dB) will result in a combined S/N ratio of 103 dB... a very good figure under most circumstances.

-----------------------------------------

Now to respond to each of your queries individually:

a) The Amp requires 200 mv input signal at 20 Ohms to put out its rated power output of 40W pc to speakers?
Amp requires atleast 200 mV input signal at 20, THOUSAND Ohms, which is delivered easily, as explained in point 3 above

b) At 2V input voltage and at the rated output, distortion will become apparent beyond 103 dB?
103 dB is the Signal To Noise (S/N) ratio of your amp, Not Distortion. Distortion will be provided as a separate specification, most likely to be around 80 to 95 dB.

c) Relating to the dac output of 2 VRms, the amp can produce 40W output with SNR of 103 dB?
Yes, as explained in Point 6 iii above.

d) How does the 625 ohms output impedance from dac impact the input sensitivity for the amp?
Since 625 ohms is atleast 10 Times smaller than the input impedance of 20,000 ohms, its will have almost NO Impact on the sensitivity of the amp, as explained in point 3 above.

e) Does the high impedance of DAC output lead to lower voltage signal for the amp? Or to put it other way round- Would this then mean the dac's output strength to the amp would be lowered, thereby requiring more from power amp to reach reasonable listening levels (85db - 90db)?
As explained in your question d above, the DAC's 'strength' (signal sent) is not lowered significantly.
Even after the small loss from 2 Volts to 1.93 Volts, its still Much much larger than the 0.2 Volts required by the amplifier.


f) Would it be better to interconnect the dac and amps with higher impedance RCA 'cable' (100 Ohms instead of 75 Ohms RCA)?
You have a pretty ideal situation, you dont need to change anything.
... and Incidentally, Analog RCA Cables do not have any well defined / specified impedance, but that is another story all together.
75 Ohms is the specified impedance for Digital Cables carrying Digital signals.

g) How does the SNR of both DAC and AMP interact here - i.e. good or bad?
If 2 pieces of Cascaded equipment have S/N more than 6 dB apart ( in your case 103 dB & 115 dB, then the Lower S/N ratio is thge effective combined S/N ratio. I have mentioned this in Point 6 iii above.
103 dB S/N is Very good for your application.

 

[Doomster69]

@IndianEars Its responses like these that makes this hobby so enriching and insightful.. And thank you for being ever so grateful in responding with such a well written reply, easing out the nervosa and making one more knowledgeable!!

As i was reading this, alot of 'aha', 'oh ofcourse' type revelationary moments occured..

Many Thanks!!

 

[IndianEars]

Thank you Doomster69. Its VERY thoughtful of you to have responded with a separate post. REALLY Nice of you.

A single "ThumbsUp" would suffice