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Old 19th January 2011, 11:54 PM   #1
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Default DAC to go with CDA-254 Kit

Hi,

I am in the process of setting up my CDA-254 Kit from Class D Audio.

I don't actually know a lot about electronics so I am trying to educate myself while doing this as well lol. I have a few numbered questions, trying to keep it simple for now. I don't know enough to make any great decisions on parts lol, so I come to ask the knowledgeable.

1) I am looking for a DAC kit or BIY or something that I can run into this kit. Need SPDIF output + coaxial (tv + computer). Trying to keep it under say $70. Any suggestions? eBay is cool.

2)I'm also looking for a passive volume control either a double gang pot 10K or stepped attenuator 10k. Can anyone recommend a good one under $25

3) Seeing as I might want to add an EQ into the setup, would it be more beneficial for me to run the DAC to RCA jacks at the back of the box, so I can run it to the EQ or use a small rca cable to plug it straight into the rca input when not using a EQ. If I can find a nice rack mount 30 band stereo EQ for cheap on eBay I might build it into the box. This option would make question 4 obsolete as there would only be 1 rca input....

4) I will be running from both the DAC and a set of RCA into the amp, through the pot/stepped attenuator do I need to have a source selector at the analog stage here, or just ensure I don't run 2 sources into it at once?

My whole setup is going to be a stereo amp 250watts/chan @ 4 ohm. Can input pretty much anything into it. Easy to move around. Going to be used to power 2 floorstanders with high efficiency (105db/1watt/1m).

All the best,

Mitch
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Old 20th January 2011, 01:06 PM   #2
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Mitch,
CDA 254 has a low input impedence of 7k and will need some kind of preamplifier with a gain of 5. I have tried to just use pots and have not been pleased with the results. I suggest to start, use a opamp based gain stage with 10k pot in the front. The bipolar power can be provided by the extra winding of 18v in the transformer ( Iam assuming you bought the kit). You will need a DPDT switch if you plan to run two sources.
You should read through this long thread on AC.
$175 Class D amp--120 wpc
Hope this helps

Dinesh
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Old 20th January 2011, 03:29 PM   #3
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Hi thanks for the response. I did indeed purchase the kit. I also had the gain adjusted upwards a bit to compensate for the constant use of sources such as computer soundcards and iPhones/pods which dint historically have high voltage outs. I dont have the most audiophile minded housemates.... I did ask Tom from class d audio and he recommended a dual gang 10k impedance pot for the amp. I am also seriously considering just making the amp with a simple volume control stand alone. Then workin on a DAC at a later date.

I will read that thread in morning when I have some quiet time. And research opamps to find what they do and how. I think I will read up about impedance in source components a bit more. I understand it in speakers but not in this context so well.

Thanks
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Old 21st January 2011, 09:25 AM   #4
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Nice amp.

The above is good advice, and you appear to be well on with your attenuation solution.

Think about adding an impedance matching device between the dac and amp at some stage in the future. A Nelson Pass B1 Buffer is simple, and good value, and about as good as anyone would need. Read: Nelson Pass B-1 preamp kit

DAC: The small Gigaworks DAC is about as cheap as you want to go. Anything much cheaper would be a toy.

High Grade 24bit/192khz DAC with fully assembled kits on eBay (end time 26-Jan-11 09:48:28 GMT)

Read: New Small DIY Gigawork Dac?

Traditional equalizers are not normally regarded as audiophile, as they input a lot of signal degredation into the line. Have a look at the Behringer DEQ2496, you can use this after the dac, but it is much better if you put it in front. Take your source's digital output and connect to digi in on the DEQ2496, then digi out to your dac. This way the signal is not DA converted, and AD converted twice. It uses XLR connections, but do not be put off as you can convert to RCA easily enough. They are fairly cheap second hand on ebay. This is an amazingly versitile machine. You would not be dissapointed.
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Michael
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Old 22nd January 2011, 09:45 AM   #5
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Ok, please excuse this really really really nub question, I'm quite new to electronics, nearly all my experience so far has been reading forums for planning a speaker build + now working on this amp.

Impedance matching? What am i going to notice from this? As far as I can work out by reading around, what I'm looking at with the Nelson Pass B1 is that I will be making any source come out the other side at 10k impedance which matches with the amp. I just don't understand the benefit. What are the consequences of having too high or too low an impedance?

All the threads I have read, talk about it but don't define it

I have had people mention that I could possibly look at running a crown xti 2000 or some such, and going mono + sub, which would give me an amp, dsp and cross overs all in one. Is there anything like this that has more than 2 channels preferably 3 or am I going to have to custom something up? I would use this to run 2 tops and a sub. 2.1 is enough aural bliss for me. I just want stereo sound, good for both music and running the tv.

Thanks
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Old 22nd January 2011, 03:05 PM   #6
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I do not understand the full need for impedance matching either, but basically: A device(component) like an amplifier or a DAC will have a specific input impedance, and a specific output impedance. Usually a component will perform better if the impedance of its neighbouring component is similar to its own. This is why we have preamps between power amps and the source (an integrated amp has its own preamp stage). A preamp can also be replaced by a buffer stage of some kind.

You can get way without a preamp/buffer or any impedance matching sometimes, if the components have matching impedance, or if your components function well with mismatched impedances.

I use a stepped attenuator in one of my rigs, instead of a preamp or buffer. It sounds good, but may be even better with a buffer???

Many people complain of a dull sound from their system when using just a stepped attenuator, others think that they are fantastic. It will depend on your system.

Nelson Pass explains it better in his paper on the B1: http://www.passdiy.com/pdf/B1%20Buffer%20Preamp.pdf
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Old 23rd January 2011, 02:43 AM   #7
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Dublin, the DAC you listed has a 75ohm output impedance. This would then need a large gain to match the 10k impedance recommended for my amp? And if this impedance isn't matched the most likely effect is the music coming out weak + dull sounding because the amp is never being driven fully? Just for reference, My amp is set for a gain of 35. Looked up my e-mails from talking to Tom @ Class D.

So a Nelson Pass B1 would act as a mini amp (? i think this is how it works), buffering the 75ohm impedance of the dac output to match the 10k impedance recommended for the CDA-254.

Is is possible to make an adjustable low pass cross over, that provides a preout for the sub and passes the rest through to the amp? Seems like the simplest solution for providing a sub output seeing as I would probably have a plate amp, or 2nd amp for the sub. Would only need to be adjustable between 70-120 Hz. You see things like this in plate amps, but I want to make sure none of this low signal is being passed to my tops and stressing them out. Otherwise I could add in a HPF into the passive xovers in the speakers themselves. But that makes them unflexable then.

I know going for a DSP would do this but they all seem to only provide 2 outs, seeing as most PA is done mono. Where as I am after L/R + Sub. I want a Crown XTi 2000 with 3 channels lol

So I'd go DAC ->................................................. ..............Tops Pot -> Amp
....................... DPDT Switch- > Buffer -> Adjustable Split ->
....Analog Input ->................................................. ........... Sub Pot -> Amp


And obvisouly I need to work out some kind of equalizing or DSP into this as well.

Kind of the ultimate 2.1 system I guess. Just starting to wonder whether I shouldn't just buy an AV receiver lol. But I bet I will get much better sound out of my own creation and it's more fun. Nearly everything I use will run through the SPDIF or coax inputs, but I don't want to skip the EQ for analog input either. Makes it damn hard.

BTW reason I am going 2.1: A lot of people have recommended it as being superior for recorded music playback and movies etc. That extra dig down to 30-20Hz helps a lot. I was planning on building 2 floorstanders that provided 105db SPL down to bout 30 Hz but then had a massive drop off. But instead looking at 2 Short Line Arrays + a folded horn sub: more extension with fairly similar SPL and very good natural horizontal dispersion to broaden the sweet spot a bit.

Last edited by Mitch311; 23rd January 2011 at 02:49 AM.
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Old 23rd January 2011, 12:49 PM   #8
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Hi Mitch.

Read Nelson Pass Paper on B1 again. I think that you are on the right lines but are using the wrong terminology. Gain usually means a boost in signal strength measured in volts. The B1 buffer has zero gain, but has input and output impedances that make its neighbours very happy.

As I said before, you may be able to get away with a simple stepped attenuator.

I strongly advise that you ask for advice (as to whether you need a buffer) at both the Classdaudio thread over on Audio Circle, and on the Small Gigaworks thread in the Digital Line Level section here at Diyaudio (these are both linked above). These two modules are so popluar that I am sure that someone has attempted this combination before. Just ask.

I have thought about doing line level High Pass Filters (prior to power amp) in my system, but I have not bothered, as I have moved away from fullrange drivers. In My Opinion, there is not much to gain if you are using a well designed floorstander. Just feed the subwoofer plate amp the whole signal and let it use its own filter. If you are using a single fullrange driver for your main speakers then there would be a good improvement to be had. As you say, there are ready made units available that can do this well. It is not impossible, but also not straightforward to diy this method. Many people have though. Look up active/electronic filters.

Adding a passive HPF to an existing speaker is bad news apparently. This will give you some useful info: why

Get some advice from the other forums/threads, and report back.

Good luck.
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Michael

Last edited by dublin78; 23rd January 2011 at 12:52 PM.
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Old 23rd January 2011, 02:50 PM   #9
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So feeding all the signal for the lows that are been taken care of by the sub to the tops isn't going to seriously degrade their SQ or send too much power through them? They have a fairly sharp drop off below 80Hz but I don't want to stress them by trying to incidentally force them to reproduce sound below this level. Power handling should be well above 250watts though.

This might be a load of crock, I just don't know enough

Thanks for all the replies dub, I am posting over at AudioCirc now.
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Old 23rd January 2011, 03:10 PM   #10
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This is quite a complicated subject because there are so many things to consider, and an equally large amount of ways to achieve what you are aiming for.

What I was trying to say (badly probably) is that if your loudspeakers were designed to accept a fullrange signal, then they will not mind handling the low frequencies as well, so the simple approach is to let your subwoofer add to the existing low end.

You are correct in thinking that the removal of the low frequency work will make your main speakers happier, but this can be complicated. It is best handled at line level (prior to amplification), as anything post amp may affect the loudspeakers' own crossovers. As I said above, there are lots of ways to do this. Ask over on the Loudspeaker Multi Driver forum here on diyaudio. For now, I suggest that you keep it simple. The beauty of diy using modules is that you can add DSP, or line level filters, buffers/preamps in the future, and enjoy the continual improvements to your system.

Keep us posted on progress.
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