AMP + PRE-AMP choices - Trying scientific approach

[Falt4rm]

Hello there,

Long time reader, first time Poster. Mainly due to my slow learning curve in this domain.
I am in the current situation : first audio setup.

Already purchased :
- DAC : Topping D10
- Passive Loudspeakers : Elac Debut B6.2 (x2)

Since I've not bought yet AMP + PRE-AMP I would be grateful if you can correct my case.
- AMP : AUDIOPHONICS PA-S125NC (based on Hypex NC122MP)
- PRE-AMP : Schiit Sys

1 - Setup Summary :

| PC | --> | DAC | --> | PREAMP | --> | AMP | --> | P. SPEAKERS |

DAC

OUT Vrms : 2V​

OUT impedance : 100 Ohms​

PREAMP

In : 10K Ohms​

Out : 5K Ohms (max)​

AMP (NC122MP based)

Continuous output power : 25 W​

Input impedance : 47K Ohms​

Voltage Gain : 25.5 dB​

Full technical information page can be downloaded at : https://www.hypex.nl/product/nc122mp-oem/90
THD+N - 4 Ohms (extract from Hypex pdf)

THD+N - 8 Ohms (extract from Hypex pdf)


What I can see from the graphs above :

4 Ohms ~ 70W before Clipping​

8 Ohms ~ 30-40W before Clipping​

SPEAKER

Input sensitivity :[email protected]/1M (1W)​

Nominal impedance (in) : 6 ohms​

Peak Power = 120 W​

Sine wave : RMS = Peak Power / root(2)​

According to : https://en.wikipedia.org/wiki/Root_mean_square​

RMS ~ 85W​

2 - Approach

a. Can I use nc122MP amp with Elac B6.2 ?

@RayDunzl A big thanks for all the contributions you've posted - It really helped me a lot.
https://www.audiosciencereview.com/...-for-4-ohm-elac-uni-fi-ub5s.5340/#post-118221

Assumption : ~40W from AMP at 6 Ohms (to ensure no clipping)
Question : How much dbSPL will be produced with 40W output from the amp ?

I told myself Great! You have solved the AMP part.
Now you have to find out how to lower volume so it could match data from previous thread :
Sound from TV :
- 68.2dB averaged and 86.3dB peak Spl
- -58.6 dbFS

So I figured out I should the same and use digital attenuation.

BUT PROBLEM :

Input RMS voltage for 40w 6 Ohms (according Formula from Hypex) is Vrms = 0.831 V​

DAC Vrms = 2 V - I will never be able to use it directly into the Amp :'(​

And I will need more likely lower Amp voltage to get lower dBSPL from the loudspeaker.​

I went backward and tried to figured out How to pair those two (DAC + Power AMP).
This is the time where I learned Voltage bridging concept.

SOLUTION : I need Impedance Attenuator --> I need passive pre-amp --> Schiit SYS

b. PRE-AMP : Max/Low Impedance

First requirement Voltage Bridging (DAC - PREAMP) : OK

New Question : Can I match PREAMP + AMP ?

(I am not 100% sure about this part)

From the specs : Out Impedance = 5k (Max)
Does it means At Lower Volume ? ~0 ohms at full volume, right?

Regarding graph from passive behaviour of Schiit Saga (which it should be more likely same).
It outputs :

3.7 KOhms at full volume​

~0 Ohms at min volume​

Forum's thread link : https://www.audiosciencereview.com/...easurements-of-schiit-saga-tube-pre-amp.6520/

I thought it would be similar mechanism. In my case :

~ 5KOhms at full volume​

~ 0 0hms at min volume​

​

First requirement Voltage Bridging (PREAMP- AMP) at worst case scenario (aka Full Volume) : ~OK ~1:10?

Second requirment : PREAMP Vrms OUT ~< AMP Vrms IN
At worst case scenario (aka Full volume)

Found this formula : http://www.sengpielaudio.com/calculator-voltagedivider.htm

Therefore I shouldn't be able to endanger the amp. Is it correct ? (0.66V < 0.831V)

Does my approach and these choices make sense to you?

Best regards,
Falt

 

[RayDunzl]

@RayDunzl A big thanks for all the contributions you've posted - It really helped me a lot.

You're welcome.

I used a 10K passive potentiometer between a CD player and an Acurus A250 for a long time.

Simple home made thing.

I wouldn't be concerned about using it again.

From the specs : Out Impedance = 5k (Max)
Does it means At Lower Volume ? ~0 ohms at full volume, right?

Yes.

The knob slides a contact wiper across, and dividing, the resistance in the pot.

The output voltage changes with the ratio of the divided resistance.

 

[Blumlein 88]

Your maximum output impedance will occur at -6 db. Higher and it is less (yes approaching zero at full volume). And lower it is less eventually also approaching zero. Now if using a 10 kohm pot the maximum output impedance will be 2500 ohms. Not 5000 ohms.

At a point where you have 5000 ohms to ground and 5000 ohms back to the output ( -6 db) those are effectively in parallel and output impedance is 2500 ohms. Any other setting it will be less. Even at 2500 ohms it will be good to keep the cable between volume control and power amp short. You can safely use a longer cable from source to volume control.

Now even with a pot you'll be able to apply nearly full voltage from the source. If it is like most volume pots halfway will be -6db and your point of most output impedance. Which will be half voltage or 1 v for a 2 v source.

 

[Ron Texas]

I don't use a preamp. Some DAC's like the topping D30 have a digital volume control which tracks Windows system volume. Or, use the volume control in your playback software. It's likely an NC122MP does not need more than 2V to drive it to clipping.

 

[Falt4rm]

@Blumlein 88 Thanks for clarifying impedance max output.

Now even with a pot you'll be able to apply nearly full voltage from the source. If it is like most volume pots halfway will be -6db and your point of most output impedance. Which will be half voltage or 1 v for a 2 v source.

Will 0.30m be short enough ? (couldn't find shorter to buy)

I don't use a preamp. Some DAC's like the topping D30 have a digital volume control which tracks Windows system volume. Or, use the volume control in your playback software. It's likely an NC122MP does not need more than 2V to drive it to clipping.

I will mix volume control from software + pre-amp.
My plan was to not rely completely on Software part.

 

[Blumlein 88]

@Blumlein 88 Thanks for clarifying impedance max output.



Will 0.30m be short enough ? (couldn't find shorter to buy)



I will mix volume control from software + pre-amp.
My plan was to not rely completely on Software part.

Yes, one meter is probably fine, and 1/2 meter or less is going to be okay. It is better if it is low in parallel capacitance, but keep it under a half meter and no worries.