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Expanded κDCX input and output gain configurations

kappaDCX

Expanded κDCX input and output gain configurations

Postby amb » October 24th, 2016, 2:45 am

I have expanded the range of recommended input and output gain settings, and updated the κDCX website parts list to reflect this. Basically, you can now set each input or output channel to one of the following maximum voltages (at 0dBFS):

  • 1Vrms (+2.2dBu)
  • 2Vrms (+8.2dBu)
  • 4Vrms (+14.3dBu)
  • 8Vrms (+20.3dBu)*
Each one is a 6dB increment, and provides enough range to support just about any source or power amp combination in pro audio or consumer audio applications, and everything in between.

* The 8Vrms configuration is for input only. The current design will achieve only up to about 5Vrms output before the output muting circuit begins to "clip" and give rise to high distortion.

FYI.
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Re: Expanded κDCX input and output gain configurations

Postby franjam » January 4th, 2018, 5:41 am

Hi all,
I'm new to this forum and have just purchased kDCX pcb with accessories.
I would first like to say thanks to all concerned for the work and research done to enable this project and I am very excited as I await the pcbs and list up the parts on Mouser.
That said I would like to make it clear that I learnt to solder electronics about 1 year ago and am no audio engineer ; I love music, HiFi and building stuff so I'm a good technician but I do not pretend to understand most of what I'm what I'm putting together. I successfully built the Elekit TU8500 pre-amp kit which I think is awesome and I'm very proud of it but this project is a step up from that I feel, especially in the tiny soldering department.

To get to the point I'm a little lost in choosing the correct resistors for the input /output configs.
INPUT:
For the Elekit TU8500 pre-amp I have Max.output voltage: 22V rms (1kHz) for the line stage and Max.output voltage: 7.4V rms (1kHz) for the equalizer amp-stage, which I suspect is the phono section which I do use alot.
OUTPUT:
I currently have 1x15k resistor in series and a 6.7k resistor in parallel in each output cable plug on the DCX for an attenuation of 12db into each A60AP power amp with an input impedance of 20kohms.I found the calculation on an app on a french site which I use. I only use 4 output channels with a crossover at 2000hz.

Basically I have no idea how to correctly configure the correct resistors on the new pcb and complete my order list on Mouser. Any help and explanations would be greatly appreciated but as I have said I am no mathemetician just a half decent handyman.

Thankyou in advance for any help.
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Re: Expanded κDCX input and output gain configurations

Postby amb » January 4th, 2018, 9:52 pm

The preamp is tube-based, so it's not surprising that its maximum output voltage is so high (due to high supply voltage). THat said, the typical output voltage under normal use conditions is most likely to be a lot lower. Otherwise it's going to cause clipping in the next stage (whether it's a power amp, or in your case, a DCX2496).

You said that you're currently attenuating the preamp output by 12dB before going into the DCX2496. Does it cause at least 2-3 LEDs on the DCX2496's input bargraph meter to turn on at moderate listening levels? Also, are you using XLR balanced interconnection between preamp and DCX2496, and from DCX2496 to power amps?

This will help determine what the optimal input and output gain should be.
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Re: Expanded κDCX input and output gain configurations

Postby franjam » January 5th, 2018, 5:14 am

Hi Ti,
Thanks for the prompt reply, I think you may have read my post a little quickly :) the pre-amp output goes straight into the DCX unattenuated ; it is the output from the DCX to the power amps which are attenuated in the XLR plugs. I have about 2 leds light up on the onput bars at 50% vol, which is quite loud. I have no clipping problems. My horns are quite efficient so my volume control is rarely beyond 12 o'clock and i do like it loud sometimes :-)....and it feels nice to have that reserve....
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Re: Expanded κDCX input and output gain configurations

Postby franjam » January 5th, 2018, 5:34 am

Forgot to add that all connections are unbalanced ; RCA to XLR and XLR to RCA
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Re: Expanded κDCX input and output gain configurations

Postby amb » January 5th, 2018, 10:01 am

Ok. In your case it doesn’t matter whether the fixed attenuator is before or after the stock DCX2496, as far as clipping is concerned because the signal levels don’t get close to the clipping threshold. On the modified DCX2496 with κDCX, you can set the output gain lower so that you won’t need the attenuator any more. What power amps are you using and their input sensitivity specs?
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Re: Expanded κDCX input and output gain configurations

Postby franjam » January 5th, 2018, 4:09 pm

I'm currently using 2 A&R Cambridge A60AP power amps substantially modded/re-built which I will be replacing when I'm done with the DCX , but all in good time ; their input impedance is 20k and I find that 12db attenuation suits me with the actual DCX, I don't want too much attenuation as the phono output is somewhat lower than the line output, (CD for example) especially with my Denon DL103 mc cartridge.
Last edited by franjam on January 5th, 2018, 4:22 pm, edited 1 time in total.
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Re: Expanded κDCX input and output gain configurations

Postby franjam » January 5th, 2018, 4:20 pm

With the volume control at 12 o'clock on phono there is only 1 led lighting up on the input bars.
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Re: Expanded κDCX input and output gain configurations

Postby amb » January 5th, 2018, 4:28 pm

The A&R Cambridge A60AP is an integrated amp. Are you connecting the DCX2496 to one of its line inputs (i.e., Aux, CD, Tape, etc) or is there a way to connect directly to the input of the internal power amp? Many 70's integrated amps have pre-main jumper bars on the rear panel that allows you to separate the preamp from the power amp sections and use them independently.
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Re: Expanded κDCX input and output gain configurations

Postby franjam » January 6th, 2018, 1:23 am

Alow me to correct you, the A60 is the intergrated amp ; the AP version splits the pre and power sections by using one of the rear din inputs with a small additional pcb connected to the pre-power bridge on the main pcb. I managed to get one of these cards, which are quite rare, for one amp and modded the other by building another small card. The pre and power sections are quite seperate and I am only using the power sections of each amplifier.
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