@shock
Du hast mir doch damals die 3 oder 4 Covoxe gemacht, oder irre ich da?

@all
I tested one of my existing covox against dreamblasters-version. I used my 286/12,5 and the EGA Megademo and a LPT 2way switch and a stereo-cinch,
so I could switch between both covox on the fly. This worked quite good. One Covox left, the other right and then I changed between the resistors of dreamblasters-covox.

1. My existing Covox is louder has more bass, more highs, but therefore sounds more "rough", a bit like "loudness"
2. Dreamblasters-Covox is about 35% quieter, sounds without "loudness", more "clean" and natural. I changed the resitors on the fly. My vote goes for the 100. It sounded best to me.

I hope this helps.

Thx
Doc

hi Doc
thanks !!
--> maybe if you could bridge the resistor in the CVX output stage ? It may sound a bit louder then, with 100k

MobyGamer wrote:

keropi wrote:

^ maybe you can send one to dreamblaster to analyze that custom resistor pack

I meant send the CVX-1 and CVX-2 prototypes. The Speech Things stay with me 😀 now that we've confirmed the ladder uses R=100K and 2R=200k; there's no need to destroy the resistor pack.

If there is sufficient measurements and evidence to determine the ladder uses R=100K and 2R=200k, why does the 100k version of the CVX-1 sound so different, and louder? Is its ladder not constructed in the same way?

The R2R ladder itself should be the same, but the CVX-1 output is not attenuated by an extra resistor, while on the original Covox ladder there should be a resistor between output and ground.

Oh! Okay, I'll attempt measuring that (if it exists) tonight and reply back.

dr.zeissler wrote:

@shock
Du hast mir doch damals die 3 oder 4 Covoxe gemacht, oder irre ich da?

Might be ... must have been quite a while already - at least 5+ years.

about 4-5 years could be right. thx!

MobyGamer wrote:

Oh! Okay, I'll attempt measuring that (if it exists) tonight and reply back.

Oh no, don't do anything, it's something you already measured, it is within the resistor array package.

I've determined theoretical resistor values that match within 0.8% of your youtube video readings.

So in the R2R ladder, R=100kohm and 2R=200kohm obviously.
And in the resistor ladder, there is correctly a 2R from LSB 2R node to ground (omitted in patent drawings). It's one of the reasons you can't measure R or 2R directly because there are two paths between any two pins (OK so except measuring MSB and output gives you 2R directly).

And the most important part, there is 25 kohm resistor between output and ground, so that's the other path between any two pins. (In the patent drawings, this was 15k.) Conveniently, 25kohm is R/4, or 2R/8

I think I need to draw and prove this for future reference, but I just had to post my results before I go to sleep.

Another thing is, we don't know if there is a cap inside the resistor network or not.
It is possible that just with the relatively weak 24.3kohm source impedance, the actual RC filter cutoff frequency just is determined by cable length (capacitance) and where you connect it (amplifier input capacitance, impedance).

They did say about capacitance in the patent though, did the Covox come with it's own amplifier box?

Jepael wrote:

Oh no, don't do anything, it's something you already measured, it is within the resistor array package.

Okay, cool. (Slightly relieved, since I didn't make the video with OBS, I had to use two cell phones, 🤣)

And the most important part, there is 25 kohm resistor between output and ground, so that's the other path between any two pins. (In the patent drawings, this was 15k.) Conveniently, 25kohm is R/4, or 2R/8

Nice sleuthing! I wonder how much of an audible difference that makes.

Another thing is, we don't know if there is a cap inside the resistor network or not.

It would have to be a VERY small cap; the resistor SIP looks fairly uniform and even, with nothing bulging.

They did say about capacitance in the patent though, did the Covox come with it's own amplifier box?

Yes; you can see what it looks like here: https://www.youtube.com/watch?v=M0kxcc67l0w The amplifier box has a noticable lowpass filter effect; I didn't measure it, but it felt like the cutoff was 8KHz. (Just to clarify, all of my recordings were direct cable; the amplifier box was never part of the signal chain.)

MobyGamer wrote:

And the most important part, there is 25 kohm resistor between output and ground, so that's the other path between any two pins. (In the patent drawings, this was 15k.) Conveniently, 25kohm is R/4, or 2R/8

Nice sleuthing! I wonder how much of an audible difference that makes.

Thanks - I had to look up what sleuthing means though 😁

The audible difference could be huge.
The CVX-1 with 100k resistors, R2R network itself has 100k output impedance, so unloaded, driven by 5V parallel port, it should output 5Vpp audio signal. That's pretty high amplitude for an audio signal, but as every amplifier input has some load impedance, it will bring the amplitude down.
Real Covox R2R network output has also 100k output impedance, but with the extra 25k load resistor, it divides the voltage down to 1/5, so driven by 5V parallel port, it should output 1Vpp audio signal - much better amplitude for audio. And in fact, I must correct myself that the output impedance of Covox is actually exactly 20k (because when it's connected to PC, there is 100k impedance from R2R network in parallel with the 25k, even when disconnected from PC it calculates and measures as 24.3k).

If you use DEBUG.EXE you can set all data pins high and measure the voltage with multimeter on both devices yourself - that should also verify something 😀
Making a program to toggle the pins could also help with the capacitance measurement, but it's no use without oscilloscope. With a PC recording the output, we can just verify the Covoxes are similar when connected to PC in identical way.
Hmm, would anyone benefit from a program to generate test tones and patterns to covox plugs?
Few waveforms and frequencies and some user triggerable steps to aid with multimeter measurements?
I wish I still had my covox clone somewhere around..

MobyGamer wrote:

Another thing is, we don't know if there is a cap inside the resistor network or not.

It would have to be a VERY small cap; the resistor SIP looks fairly uniform and even, with nothing bulging.

I am positive there is no discrete SMD cap, it looks like it's just substrate and thin-film resistor material deposited on it - but it's possible to make small capacitances on a substrate, it's just plates of conductive material with isolating material in between. Maybe some of the filtering is because of stray capacitance from the structures on substrate, even if it is not specifically designed there.

MobyGamer wrote:

They did say about capacitance in the patent though, did the Covox come with it's own amplifier box?

Yes; you can see what it looks like here: https://www.youtube.com/watch?v=M0kxcc67l0w The amplifier box has a noticable lowpass filter effect; I didn't measure it, but it felt like the cutoff was 8KHz. (Just to clarify, all of my recordings were direct cable; the amplifier box was never part of the signal chain.)

Thanks I've never seen the amplifier/speaker box before. 8 KHz means about 1nF capacitance with the original Covox source impedance. That much could come from cabling and PC input. There is usually some RC filter on sound card input as well, but with these modern sigma-delta ADCs, that frequency is usually set to kick in at 20kHz or so with normal source impedances.
Yeah, can't really say much without really measuring the Covox plug with an oscilloscope.

Jepael wrote:

Real Covox R2R network output has also 100k output impedance, but with the extra 25k load resistor, it divides the voltage down to 1/5, so driven by 5V parallel port, it should output 1Vpp audio signal - much better amplitude for audio.

Just for curiousity, I decided to do just that (I had a 22K lying around) but it didn't seem to make much difference. Warning: The chance that I don't know what I'm doing is very high.

If you use DEBUG.EXE you can set all data pins high and measure the voltage with multimeter on both devices yourself

Ah, so something like this:

1  mov dx,378h
2  mov al,FFh
3doit:
4  out dx,al
5  jmp doit

...would work to measure the maximum voltage? I can do that; I'll report back with the CVX-2 (which I haven't molested) and an actual covox.

Hmm, would anyone benefit from a program to generate test tones and patterns to covox plugs?

The article I linked to previously had a quick qbasic program that did just that, I thought...

Well, that was educational:

CVX-1 Prototype: 5.04 vDC
CVX-2 Prototype: 4.80 vDC
10-pin Covox: 1.024 vDC
11-pin Covox: 1.025 vDC

MobyGamer wrote:

Well, that was educational: […]
Show full quote

Well, that was educational:

CVX-1 Prototype: 5.04 vDC
CVX-2 Prototype: 4.80 vDC
10-pin Covox: 1.024 vDC
11-pin Covox: 1.025 vDC

I think Jepael is on to something! Great sleuthing indeed 😀
(I know what a sleuth is, I have the entire collection of Columbo episodes on DVD 😀)

MobyGamer wrote:

Well, that was educational: […]
Show full quote

Well, that was educational:

CVX-1 Prototype: 5.04 vDC
CVX-2 Prototype: 4.80 vDC
10-pin Covox: 1.024 vDC
11-pin Covox: 1.025 vDC

Wow, that's spot on with the theory, amazing!

I don't remember which one you had with 100k/200k R2R network and which one with 7.5k/15k network, but here's where the 25k should be:

On CVX-2, the R19 needs to be a short (zero ohms), and if it had 100k/200k R2R network, it needs the 25k resistor in parallel with (or in place of) the 4.7nF cap C1.

On CVX-1, R10 needs to be a short (zero ohms), and if it had 100k/200k R2R network, 25k in parallel or in place of the 4.7nF cap C1.

Pictures of these were on page 7 of this thread.

Well, heck, with such exact directions, I feel obligated to make the changes and see how it goes 😀 All I have are 22K and 27K resistors on-hand; I'll use 27K and then report back.

Jepael wrote:

...it needs the 25k resistor in parallel with (or in place of) the 4.7nF cap C1.

I would suggest the 'in place of' option, firstly because it's easier to do, secondly, because there seems to be no sign of such cap on the real Covox, so not having one would be the most similar circuit.

My CVX-2s have 7.5K resistors, what is the correct value for the resistor in C1's place?

Jepael wrote:

On CVX-1, R10 needs to be a short (zero ohms), and if it had 100k/200k R2R network, 25k in parallel or in place of the 4.7nF cap C1.

I did that: https://goo.gl/photos/L1ykmzuJrgxB6ZSC9

...but I'm still getting 5.06 Vdc measurement from the device...?

MobyGamer wrote:

Jepael wrote:

On CVX-1, R10 needs to be a short (zero ohms), and if it had 100k/200k R2R network, 25k in parallel or in place of the 4.7nF cap C1.

I did that: https://goo.gl/photos/L1ykmzuJrgxB6ZSC9

...but I'm still getting 5.06 Vdc measurement from the device...?

You put the resistor in place of C2, not C1 (C1 would go where the socket is).
You need to put a shunt in place of C2 I believe, if you remove it.

Scali beat me, exactly.
If you measure at output connector, replace C2 with short so it does not remove DC.

Whoops! Okay, made the changes, and 1.083 Vdc now achieved! I'll make some new recordings.