I just did a few recordings with the new setup, but... they're not good. Technically I mean. Nothing worth posting. The sensitivity of the line input in the new setup is too low, even with the volume cranked up to the max it's still barely audible. I even tried it with the CVX-1 which was giving me decent output levels in the previous setup. There's also quite a bit of line noise.

Still searching for a better capture solution.

I wanna start building a CVX-3 but I am confused atm 😵 , what value resistors did we settle as being the "best" so I can build the first one with that choice ?

I have a selection of :
- 10K 1%
- 22K 1%
- 33K 1%
- 47K 1%
- 68K 1%
- 82K 1%
- 100K 1%

I want to say it's 100K 🤣

I think the 100K ones were deemed the best choice, because they put an adequate load on the data lines (and this is also what the real Covox uses).
Combined with a 25K load resistor to bring the output to ~1v.

^ gracias 😁

edit:

had a closer look at the CVX-3 - it seems a different beast all together made for experimentation:

so just to be sure 🤣 , populate R1->R26 but bypass R19 and put a 22K resistor instead of C6?
I want to build a "reference" board and experiment with the others

Keep C6; I ended up placing a 25k in C28, and a 4.7nF in C2, can select either by jumpers.

Approximately right:
- Populate R1..R26, all 100k, and bypass R19 with a short
- Put 25k resistor (22k is not correct but of course you can use it) to either R27 or R28 position or C1..C5 position and make sure to jumper it to ground to match original Covox plug (it uses 1 volt audio, if you omit this you get 5 volt audio, and it might be too much). Note that if you put four 100k resistors in parallel, it makes one 25k resistor.
- C6 is not necessary and original plug does not have it because it has its own amplifier box. But it is better to have it so you don't put DC voltages to line inputs. If you don't want it there is a jumper to bypass it so it does nothing and output is DC coupled.
- You can put a filter cap such as 4.7nF there but the original does not have it on the plug. Perhaps it was in the amplifier box..

But frankly, unless you manually select the 1% resistors, using 1% resistors will make it effectively only a 7 or 6 bit DAC, and my guess is it won't be as good as the original. You are better off using 0.1% resistors for the R-2R ladder, or a resistor network.

allright, I have built a CVX3 with the specs as detailed by Jepael in the post above - used a 25K resistor.

The volume output was low, I needed to amplify via Audacity but here is the recording: https://soundcloud.com/keropi666/serdaco-cvx3 … -crystal-dreams

I installed a 100uF electrolytic and a 4.7nf ceramic cap. Thoughts? 😀

keropi wrote:

I installed a 100uF electrolytic and a 4.7nf ceramic cap. Thoughts? 😀

Sounds good, but the capacitor is rounding off the high end a little. But, that may be what you want (it's a lowpass filter). This is a matter of personal preference, but for the record, the original Covox did not use one.

^ are you talking about the 4.7nf one or the 100uf electrolytic?
The 4.7nf can be completely bypassed with the jumpers and produces this result: https://soundcloud.com/keropi666/serdaco-cvx3 … -crystal-dreams

Pinball Fantasies main theme & Stones n'Bones table music: https://soundcloud.com/keropi666/serdaco-cvx3 … nball-fantasies

edit:
the only issue I have atm is the very low output volume of the CVX-3, maybe it needs a smaller resistor at R27? I use the line-in on my x-fi for recording and it's just really really low volume - volumes way up in soundcard control panel. Connecting directly to speakers is OK though...

There's more info previously in this thread. but the original covox was designed with 100k resistors to put less load on the parallel port pins, and was designed to be connected to an amplifier or amplified speakers. As testing in this thread shows, you can use other resistor values to make it louder, but this manifests in distortion (you lose the high-end frequencies).

The intended use of a covox is connected to amplified speakers, so I personally choose the correct resistor values for better audio quality every time (I can always turn my speakers up if it isn't loud enough).

Connecting directly to speakers is fine - connecting to the AUX or LINE IN of my Auzentech X-Fi is another matter , even with no pull-down resistor at all the volume is low. Maybe the card expects professional-level line-in or something... I don't remember having this issue when connecting a SB for example... at any rate I have made some more recordings with various filter caps in case anyone is interested in them:

100pF : https://soundcloud.com/keropi666/serdaco-cvx3 … 100pf-capacitor
1nF : https://soundcloud.com/keropi666/serdaco-cvx3 … f-1nf-capacitor
2.2nF : https://soundcloud.com/keropi666/serdaco-cvx3 … -22nf-capacitor
10nF : https://soundcloud.com/keropi666/serdaco-cvx3 … -10nf-capacitor

My personal preference are the first two; the 2.2nF and 10nF round off the signal too much and it sounds muffled to my ears.

I still think having a simple op-amp on the Covox would be a good addition. It can deliver a buffered signal at line level, making it easier to connect to external equipment. It could also be used for unpowered speakers or headphones.
I think it could be an 'optional' component: Eg, you could just split the output from the DAC ladder to the input of an op-amp, and a direct passive output.
Then if the op-amp is not powered, you can still use the passive output like we do now. And if you connect a power supply (could make a simple cable to get +5v from a serial port for example, or just a separate transformer), the op-amp will be powered and the amplified output can be used.

Scali wrote:

I still think having a simple op-amp on the Covox would be a good addition. It can deliver a buffered signal at line level, making it easier to connect to external equipment. It could also be used for unpowered speakers or headphones.
I think it could be an 'optional' component: Eg, you could just split the output from the DAC ladder to the input of an op-amp, and a direct passive output.
Then if the op-amp is not powered, you can still use the passive output like we do now. And if you connect a power supply (could make a simple cable to get +5v from a serial port for example, or just a separate transformer), the op-amp will be powered and the amplified output can be used.

True, it would be nice. If I had a Covox that has power available, I'd also be very tempted to put other stuff there as well - like real DAC - and the feature creep never stops, just a second ago I thought 8 channels output, pr volume control to play mods without software mixing, or an audio codec to get SPDIF output..

There's just one catch, I think you can't feed the DAC output to an unpowered op-amp input without the signal getting distorted, so when you use it unpowered, you'd have to disconnect the op-amp somehow. There's audio jacks with switches, or, maybe even a real switch.

Perhaps the requisite supply voltage could be leached from the signal lines prior to the ladder pack? The additional current required would have to be minuscule, but I think it's possible.

Thanks Keropi for your tests and valuable feedback, we're getting there.
CVX4 will have dip switches to allow easy tuning !

What about Covox (LPT-Dac) vs. fast P4/newer PCI-Slot-only Motherboards.
I am currently checking out the SCENIC-T Mini-PC. It's a very fine machine that has the D1321 Motherboard.
Due to the fact that it has only one AGP and three PCI-Slots I did not put a soundcard for dos in it.
It has an AD1885 Soundchip but I am currently not using it.

I connect one of my Lpt-Dac with it and guess what?
glx /oC1 plays very well. The soundoutput is great, BUT! there are some dosgames that should work fine too, but they DON'T!

I clearly hear lots of scratches/cracking-noise while playing PinballDreams2 or PinballFantasies.
Why is that? What causes this. I changed to all possible soundconfigurations, but it does not get any better.

Has someone an advice for me?

Thx!

i would entirly stay away from the r2r ladder and instead use a cheap chip dac..
there are very good ones with single supply for just 1$.. and integrated op-amp

I clearly hear lots of scratches/cracking-noise while playing PinballDreams2 or PinballFantasies. Why is that? What causes this. […]
Show full quote

I clearly hear lots of scratches/cracking-noise while playing PinballDreams2 or PinballFantasies.
Why is that? What causes this. I changed to all possible soundconfigurations, but it does not get any better.

I clearly hear lots of scratches/cracking-noise while playing PinballDreams2 or PinballFantasies.
Why is that? What causes this. I changed to all possible soundconfigurations, but it does not get any better.

Seems like you have some weird signals from other sources on your lpt..
i also can hear harddisk access on the covox.
A Chip DAC will also perform better here 😀

This one maybe ?
http://www.analog.com/media/en/technical-docu … eets/AD7224.pdf

Thx, but there is no "ready to go" solution with a chipdac as far as my research.
This does not explain either why glx seems to be fine und the pinball-games
seem to be "oversteered" with cracking-noises.

Ah i didnt notice you didnt have this problems with glx

maybe pinball dreams using a different access method, did you try it with a LPT ISA Card ? is there also cracking noise ?

i already have experimented with DAC0808 and others, maybe we can roll a kit.. but currently i'm limited in time and also have already todo with the ps/2 converter kit.