No, I wouldn't do that. The previous stage - the opamp directly after the DAC - doesn't just do balanced to unbalanced conversion, it also does the LP filter to remove ultrasonic conversion noise. Changing the feedback there will affect that filter.
Will this parallel resistor with mute transistor reduce hiss? Would 6db reduction in gain be adequte to reduce hiss or do I need something like 12dB? Could somebody tell me in words where the transistor is located and how the resistor should be installed. Just solder it to transistor's legs? What resistors are good for audio? So many questions, try to bare 
That depends entirely on where the hiss is coming from. I don't notice much hiss on my system. I'm running balanced to all my amps, though. In the unbalanced configuration, the DCX adds, I believe, 6dB more gain to the output.
Regardless, though, I think you'll get more noise reduction by reducing the 2.32K resistors in the next stage than by shunting the muting transistors.
To reduce the 2.32K resistors, you'd need to order some 1/10th inch 1/8th Watt (IIRC) SMD resistors. 1% metal films would be ideal. You'd also need to be at least a bit comfortable doing surface mount soldering, to remove the original 2.32K resistors. Using 499 Ohm in place of the 2.32K Ohm will drop the output gain by about 6dB, and will also reduce by 6dB any noise sourced between the preamp volume control (if you're using a preamp) and the DAC's LP filter in the DCX.
I suppose, in a pinch, you could also solder, say, a 650 Ohm 1% metal film resistor in parallel with each of the six 2.32K resistors, to give an equivalent of 507 Ohms.
Regardless, though, I think you'll get more noise reduction by reducing the 2.32K resistors in the next stage than by shunting the muting transistors.
To reduce the 2.32K resistors, you'd need to order some 1/10th inch 1/8th Watt (IIRC) SMD resistors. 1% metal films would be ideal. You'd also need to be at least a bit comfortable doing surface mount soldering, to remove the original 2.32K resistors. Using 499 Ohm in place of the 2.32K Ohm will drop the output gain by about 6dB, and will also reduce by 6dB any noise sourced between the preamp volume control (if you're using a preamp) and the DAC's LP filter in the DCX.
I suppose, in a pinch, you could also solder, say, a 650 Ohm 1% metal film resistor in parallel with each of the six 2.32K resistors, to give an equivalent of 507 Ohms.
Thanks SQL!
I am too running balanced signal to all my amps. The signal coming from the preamp to DCX is unbalanced though. The hiss is very small but audible, just that kind of hiss what you get when the gain is too high. My amps and preamp are almost inaudible even when listened straight from tweeter, with DCX the hiss can be heard from 1 metre if the room is totally silent. Small thing but worth putting right
Do you remember the codes on the board for those resistors that should be replaced? There are quite a few of them in I/O board. I could have them replaced same time with my DACs when I got to the guy that replaces them. I'll start googling those 1% film resistors.
EDIT: Is this the right kind of a resistor (SMD 0805 type). http://parts.digikey.com/1/parts/817126-res-499-ohm-1-8w-1-0805-smd-crcw0805499rfkea.html
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DCX EQ power
I have to correct your statement. DCX offers up to 10 EQ filters (parametric or low or high shelving) for each output but limited by the processing power of its processor ie crossover and EQ together. Depending of the crossover slopes uo to 10 filters per output. You can do quite a lot of EQ with the DCX. I used both but discarded the DEQ when I started to use DRC correction. I made the basic equalization with the DCX and DRC makes the rest with very good results. My DCX is fully modified, ie input and output boards, clock and analogue supply for DACs.
Chris
I have to correct your statement. DCX offers up to 10 EQ filters (parametric or low or high shelving) for each output but limited by the processing power of its processor ie crossover and EQ together. Depending of the crossover slopes uo to 10 filters per output. You can do quite a lot of EQ with the DCX. I used both but discarded the DEQ when I started to use DRC correction. I made the basic equalization with the DCX and DRC makes the rest with very good results. My DCX is fully modified, ie input and output boards, clock and analogue supply for DACs.
Chris
This Op amp has 2 499R resistors in series on its -ve input
So Input resistance is 998R
Replacing the 2K32 feedback resistor with a 1K0 would reduce the gain by 7.3db
(see post 2233)
So using a 499R on the feedback would reduce the gain by another 6db making 13db in total.
I don't know if reducing the gain of this stage below unity gain is practical or not?
As to if this reduces the noise floor depends on the gain structure of your system. If you are attenuating the DCX outputs into the amplifiers then reducing the voltage drive probably is not going to make much difference.
So Input resistance is 998R
Replacing the 2K32 feedback resistor with a 1K0 would reduce the gain by 7.3db
(see post 2233)
So using a 499R on the feedback would reduce the gain by another 6db making 13db in total.
I don't know if reducing the gain of this stage below unity gain is practical or not?
As to if this reduces the noise floor depends on the gain structure of your system. If you are attenuating the DCX outputs into the amplifiers then reducing the voltage drive probably is not going to make much difference.
Thanks for pointing this out. The default (apparently) is 1 filter per output, and, until I just went now to re-read the manual, I didn't know that NR stood for number of filters allowed.
What's DRC?
Also, how many filters can you practically use before running out of CPU?
Thanks,
Paul
Ooops. Good point. The dual 499's made me overlook the second one.
Overall, I would say that at the amps is the best place to attenuate because you'll reduce everything, including any noise induced into interconnects. But that didn't seem to be exactly the question asked.
DCX EQ power
In my experience and with the use of 24 L-R crossover slopes, I'm able to set 6 to 7 bands per output before the CPU gives up ;-)
DRC stands for Digital Room Correction which is a filter convolved digitaly to the music buy the player plugin in the computer or in a hardware device. It takes care of amplitude (toward a taget curve), phase and time alignment of the speakers. See the link. A whole new world but not easy to use I must say.
Main Page - DRC
Chris
In my experience and with the use of 24 L-R crossover slopes, I'm able to set 6 to 7 bands per output before the CPU gives up ;-)
DRC stands for Digital Room Correction which is a filter convolved digitaly to the music buy the player plugin in the computer or in a hardware device. It takes care of amplitude (toward a taget curve), phase and time alignment of the speakers. See the link. A whole new world but not easy to use I must say.
Main Page - DRC
Chris
I'm trying to reduce the "basic hiss" even thought it is small. This little hiss is present even outputs at -15dB but not when DCX is not connected. I thought that too big gain in DCX would be causing this, at least some of it. Maybe it it the PSU and DSP that are causing it? I can live with the hiss though, but if perfection can be achieved in this regard, I will gladly try to achieve it
If i try reducing the gain I will try it with 1Kohm resistor...
This is mentioned earlier in this intolerably long thread, but just to reiterate......if you're using XLR/RCA adapters on the DCX outputs you need to perform surgery on those and break the pin 1-3 connection. This lowers the gain of the output stage by 6db and will reduce hiss by that amount and force a 6db higher signal level within the DCX gain structure. Both of which are good things.
Cheers,
Dave.
Cheers,
Dave.
Hi Davey,
Legis actually mentioned that he was running balanced to his amps. So, he's either going to need to add an input voltage divider at the amps, or to reduce the output gain as discussed here.
Legis, I assume you've already verified that shutting off the DCX eliminates the hiss? Just to make sure the noise is coming from/through the DCX. If so, one other attenuation option would be to add series and load resistors in the headshell of the amp-end of the interconnects. That's much easier than modding either the DCX or the amps and again should reduce even noise being induced in the cable.
I'd try something like 4.7KOhm series (between signal wire and XLR signal pin) and load (between XLR signal pin and XLR ground pin), for both the + and - signals. That will give you 6dB attenuation at the amp itself. See if that does the trick. If you need more attenuation, you can increase the series resistance or (up to a point) decrease the load resistance.
Cheers,
Paul
Legis actually mentioned that he was running balanced to his amps. So, he's either going to need to add an input voltage divider at the amps, or to reduce the output gain as discussed here.
Legis, I assume you've already verified that shutting off the DCX eliminates the hiss? Just to make sure the noise is coming from/through the DCX. If so, one other attenuation option would be to add series and load resistors in the headshell of the amp-end of the interconnects. That's much easier than modding either the DCX or the amps and again should reduce even noise being induced in the cable.
I'd try something like 4.7KOhm series (between signal wire and XLR signal pin) and load (between XLR signal pin and XLR ground pin), for both the + and - signals. That will give you 6dB attenuation at the amp itself. See if that does the trick. If you need more attenuation, you can increase the series resistance or (up to a point) decrease the load resistance.
Cheers,
Paul
Thanks for this tip as I will soon connect an amplifier that has only rca input. But right now my both sub amps and mains amps have XLR inputs so hiss can not be cured that way with them. What would be the best way to reduce the hiss? Will that already mentioned resistor mod/gain reduction help at all? Reducing outputs' gain from DCX does not take the hiss to "reference level" (= without DCX), even though it reduses it somewhat.
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I have 3 feet long XLR cables from Monoprice (97,5% shield coverage), I can hardly believe they would introduce hiss since I have tried same cables between my E-MU 0404 and integrated amplifier's analog in without any hiss.
But I will do that cable mod if I need extra attenuation, I quess this is the same thing than replacing the resistor inside DCX?
I am going to change the op-amps to LM4562s, maybe this will reduce hiss somewhat since new op-amps have less noise and better CMRR/PSRR... I'm also going to change the DAC to AK4396, but this isn't propably going to help since it same nearly identical specs to AK4393. But it will enhance sound otherwise.
Maybe I'm just too picky about this little hiss? It really is not that bad, about the same than with almost every active monitors but since I'm accustomed to virtually totally siltent system I would like to reduce the hiss to lowest possible level.
Maybe linear PSU would help, or would it? What about changing 5V+ voltage regulators? Any ideas are welcome
But I will do that cable mod if I need extra attenuation, I quess this is the same thing than replacing the resistor inside DCX?
I am going to change the op-amps to LM4562s, maybe this will reduce hiss somewhat since new op-amps have less noise and better CMRR/PSRR... I'm also going to change the DAC to AK4396, but this isn't propably going to help since it same nearly identical specs to AK4393. But it will enhance sound otherwise.
Maybe I'm just too picky about this little hiss? It really is not that bad, about the same than with almost every active monitors but since I'm accustomed to virtually totally siltent system I would like to reduce the hiss to lowest possible level.
Maybe linear PSU would help, or would it? What about changing 5V+ voltage regulators? Any ideas are welcome
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I agree with some of the earlier posters. Reducing the value of the 2.32k resistor is the most logical place to reduce the gain (and hiss) of the DCX. (Internally.) However, adding pads externally as outlined by Paul will also work well.
I doubt changing out the op-amps with LM4562's will reduce hiss, but you could try. I think venturing into most any modification like this begs the question.....why not just change out the whole freakin' board with something like one of Jan's designs?
The DCX is a lot of fun, isn't it?
Cheers,
Dave.
I doubt changing out the op-amps with LM4562's will reduce hiss, but you could try. I think venturing into most any modification like this begs the question.....why not just change out the whole freakin' board with something like one of Jan's designs?
The DCX is a lot of fun, isn't it?
Cheers,
Dave.
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So Legis, your amps have no volume control at the front end? Most pro crossovers, like the DCX, are made to drive pro amps that have a volume control. Usually 1 per channel.
By not running the volume all the way open, you don't get the hiss from whatever is upstream of the amp.
In normal practice the amp volume is set to a level that will just allow clipping if the crossover is just clipping. It can also be set lower, if the systems gets too loud with the crossover running at clipping. But reducing the volume at the amp even further, you may be able to hit the hit max SPL you need with DCX running a strong signal.
Most power amps have too much gain. The gain amplifies the hiss (noise floor) of whatever is upstream from the amp. Be reducing the gain, the hiss gets pushed down low enough that you don't hear it.
By not running the volume all the way open, you don't get the hiss from whatever is upstream of the amp.
In normal practice the amp volume is set to a level that will just allow clipping if the crossover is just clipping. It can also be set lower, if the systems gets too loud with the crossover running at clipping. But reducing the volume at the amp even further, you may be able to hit the hit max SPL you need with DCX running a strong signal.
Most power amps have too much gain. The gain amplifies the hiss (noise floor) of whatever is upstream from the amp. Be reducing the gain, the hiss gets pushed down low enough that you don't hear it.
The DCX is a lot of fun, isn't it?
Cheers,
Dave.
Definitely, it's quite a piece of equipment for the price, huge value for the money. The only sound detorating effect of it is that little hiss (I think some modder/mod sellers tend over exaggerate the "faults" of stock DCX). Still I will mod my DCXs to somewhat better sounding machines, they even might enhance the original sound after the modding, you never know