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Expanded κDCX input and output gain configurations

kappaDCX

Re: Expanded κDCX input and output gain configurations

Postby amb » January 6th, 2018, 1:27 am

OK, I'm interested in knowing what the input sensitivity of the power amp section is. I.e., how many volts RMS input will cause the amp to reach its maximum output? This info will be helpful to arrive at a good output gain structure for the DCX2496.
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Re: Expanded κDCX input and output gain configurations

Postby franjam » January 6th, 2018, 3:43 am

Ok this is what I have from the .pdf manual for the A60, (which is easily findeable on the net),
Hope this is enough :

PowerAmp A60AP only :

Sensitivity 600mV
Input impedance 20Kohms
Noise less than -100db
Gain to loudspeaker terminals 29db, phase non inverting.

For information and just in case it helps these are the A60AP pre-amp specs ;
Nominal output level 600mv
Maximum output before clipping 1.8V RMS
Noise less than-80db
Distortion at nominal output level less than 0.03% at 20Hz-20kHz
output impedance 500mV

This is all I can find on the A60AP....
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Expanded κDCX input and output gain configurations

Postby amb » January 6th, 2018, 4:43 am

Ok, that helps a lot.

The amp is supposed to have 40W output into 8Ω. So let’s work the numbers. 40W into 8Ω is 17.89V. If 600mV input is enough to make 17.89V output, then the gain is 29.81x, or 29.49dB. That’s pretty close to the specified gain of 29dB.

So basically 600mV to the power amp input will reach max power output. You probably want a bit more than 600mV maximum output from the DCX2496 in case the source level is too low. But not so high as to make the volume control range on your preamp unusable. We also want to be mindful to keep as many LEDs on the DCX2496 lit as possible to maximize bit precision.

The stock analog board is essentially unity gain. Your preamp has much higher output than 600mV to make you add a 12dB attenuator. 12dB is a factor of 4. So a nominal maximum output from your preamp is 600mV * 4 = 2.4V.

I think you’ll do well to set the κDCX input gain to 4Vrms so that it will never clip (4.4dB headroom). The κDCX output gain can be set to 1Vrms so that it can drive your power amp to full output without clipping (another 4.4dB headroom).

4V input for 1V output just happens to be a 12dB attenuation, so you will no longer need the resistors for an optimal gain structure.

EDIT: I forgot to take into consideration the 6dB loss at the κDCX output from unbalanced interconnection. So you'll want to add 6dB of output gain to make up for it. Thus, set your κDCX output gain so that it will output 2Vrms maximum, which will really be 1Vrms into an unbalanced load.

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Re: Expanded κDCX input and output gain configurations

Postby franjam » January 6th, 2018, 6:50 am

That's great Ti, sounds good to me, thanks for taking the time to explain, I'll get on and finish my Mouser list :D
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Re: Expanded κDCX input and output gain configurations

Postby franjam » February 7th, 2018, 5:20 am

Ok Ti , now that the kDCX is up and running I think that there is room for improvement on the input/output gain configuration.

The good news is that that annoying hiss through the tweeters has dissapeared :)

May I remind you that I'm using the kDCX in two-way configuration, LHLHLH, with ouputs 5 and 6 muted. The kDCX is currently set at 4V input for 2V output as you suggested below.

However I mostly listen with the volume control between 11 and 12 o'clock ; at this volume I have two bars on inputs 1 and 2 and zero to one bars on outputs 1 to 4 ; this can go up by one bar, (on the the outputs), depending on the recording, but I feel it is still too low to be getting the best from the DAC. (the ouput is in general 1 bar higher on the mid/bass ouputs 1 and 3 than the tweeter ouputs 2 and 4, the crossover is at 2000Hz ; I presume this is normal).

Should I increase both inputs and outputs by one volt each to 5Vrms and 3Vrms?

This is listening with my CD player, with my MC cartridge the gain will be even lower,(when I get it back ;) ).
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Re: Expanded κDCX input and output gain configurations

Postby amb » February 7th, 2018, 6:08 am

To get more LEDs to light up, you want to decrease both input and output gain, not increase them. But if you set them too low, the power amp may not be able to be driven to full power even if the volume control is maxed. It may or may not be an issue to you, as long as you could get the maximum SPLs you want.

EDIT: instead of calling them "input and output gain", it's technically more correct to say "maximum input and output voltage", as they are shown at the κDCX website.

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Re: Expanded κDCX input and output gain configurations

Postby franjam » February 7th, 2018, 6:53 am

Ok I will see what I get by dropping the input to 2V by changing R7 A-C to 2.87K, simply because that is the simplest change to make first ; dropping the output is going to be more extensive...si I'l see what this gives first...
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Re: Expanded κDCX input and output gain configurations

Postby franjam » February 11th, 2018, 9:16 am

Quick question regarding the above which came to me. At the moment max input voltage is 4Vrms with R7 at 13.3k. To drop it to 2Vrms I need to replace R7 with 2.87k.

Can I experiment with any value between these two for R7 (ABC) ? ( I have some 6.8K)

Also does (13.3k - 2.87k)/2 = 5.32k = 3Vrms?
Last edited by franjam on February 11th, 2018, 9:33 am, edited 1 time in total.
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Re: Expanded κDCX input and output gain configurations

Postby amb » February 11th, 2018, 9:26 am

Yes, you can use a value in between, and the max input voltage will also be somewhere in between. The formula is not so simple, though.
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Re: Expanded κDCX input and output gain configurations

Postby franjam » February 11th, 2018, 9:36 am

I have just found some 2.7K would this be close enough to the 2.83K to work?
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