Schematic for a *working* PCE rgb amp with 7314?

That amplifier is a modified common-collector amplifier, also known as an emitter-follower or voltage-follower. What it does is buffer a voltage, usually from a high-impedance source, and then allows the signal to drive a low-impedance load (such as a TV, LED, motor, whatever). In other words it "amplifies" current, not voltage. In fact it loses voltage, which is signal information. The other common transistor configuration-- common-emitter, provides voltage amplification but doesn't have the high input impedance, or low output impedance, so it's extremely common for common-collectors to be inserted at the input and output of multi-stage amplifiers, and between the stages, so they don't influence eachother. The common-emitter configured transistors provide the voltage gain/amplification.

Video amplifiers are rather simple in operation (but hard to design). For the most part they take a signal that varies from 0-1V in, and amplifies that same signal to 0-2V out so that it may be divided back to 0-1V by the TV from termination. The transistor amplifier above takes a 0-1V signal in, shifts it up to 2-3V so that it doesn't distort too much, and puts a 1.4-2.4V signal out. Brilliant. This kind of thing doesn't matter for audio which doesn't have strictly specified signal levels, or a low impedance input like video. Basically the amplifier forces the 0-1V voltage on the TV, burning up a lot of current in the process. It works, mostly, but it uses more than *2x* as much current as it should and it doesn't allow you to series terminate the signal (the 75 ohm resistor), which is the whole reason behind a video amplifier's voltage gain.

RE the NJM amps: I don't think there's anything wrong with them, in fact I think they're interesting, but one important thing abuot them is that IIRC their bandwidth is really low, without question you won't get the crispness you would from a THS.

Maybe in practice it doesn't matter with a PCE and/or your monitor, but personally I'd choose amplifiers with ample headroom. This to me means for the best SD performance you'd choose at least DVD quality, which is 6.75 MHz, multiply that by 5 for nice square waves, then multiply that by 2 because video amplifiers amplify 2x and you get ~70 MHz. A 70+ MHz "gain-bandwidth product" amp will deliver SD video extremely well to a display.

The THS is complicated because it would have that bandwidth, but it also has an integrated filter. The combination of the two means that you get a graceful decrease in high frequency information. It's not ideal, but it's not bad either. Basically the frequency content rolls off on a curve, so you still get hints at high frequency information, unlike a really slow amplifier which would flat out just lose it.

 

I thought you'd say something along those lines. Might also explain why I've seen it work on some displays and refuse to give a solid signal for any real length of time on others without any obvious rhyme or reason (probably to due with the circuitry handling video input as well as the tolerance built into it for crap like this).

The amp most of us have been using is ~7mhz IIRC which falls quite short of 70mhz. I've been tempted to check out what Digi-Key has to offer in the 70mhz realm although price is of concern as I doubt people will want to spend extra money on a N64 sporting an amp that they can't tell the difference between it and the former beyond the price tag. Obviously not much can be done to help composite video but when you're talking S-Video and RGB you might as well start with the best available if possible. Unless I'm stuck on an island and a crate of transistors washes ashore I don't see much reason to opt for an inferior circuit that has compatibility problems.

How much extra current are we talking? Enough that a 7805 might struggle with (with obvious caveats involving the rest of the hardware being supplied power)?

 

In a properly designed amp the RGB signals should consume 50 mA together, sync should consume 5 mA, and the amplifier itself should consume less than all the signals put together, unless it's high-end/has its own power supply. The THS has a quiescent current of 16 mA, which is pretty good so the total consumption is around 70 mA. Not insignificant to a voltage regulator, but not horrible. In the transistor amp it's a little more complicated to calculate, but I think it's basically around 180 mA, (not 4x mistake >_<). I'd be curious for someone to measure it and see if that's what it draws, but it should be around there.

7 MHz BTW is DVD quality, and by 7 MHz they probably don't mean a gain-bandwidth product of 7 MHz because that's low. Just sayin' it's possible to see "above" DVD quality video on capable displays, you did it all the time with a VGA monitor. DVD quality is great for natural video, but it's not great for synthetic game video, with very "sharp" unnatural pixels. To get very sharp video you want whatever bandwidth is necessary to deliver 5 x 7 MHz to your monitor, which maybe listed as bandwidth, "bypass bandwidth" (on the super THS chip), or in traditional op amps would mean a GBP of 70 MHz.

 

As always I want to experiment
So Calpis, you think the guy uses 3 10uF caps , one for each color right?
And what is the "3.5M? pullup" ?

as a sidenote, I showed the transistor-amp to someone that has electronics knowledge and said that it should draw 150mA with a quick look ...

edit: we also have a hint that the ebay guy uses a THS 7315 on his amp , I don't know if it makes a difference or not, it does have higher gain and supports AC-Bias input ...

 

Yup one cap for each color.

I thought you already tried the 3.5M pullup.. You didn't? On each RGB input there should be a pullup to 5V, right after the series capacitor. 3.5M is what the datasheet recommends.

I would not use that THS7315, that 14.3 dB = 5.2x voltage gain, WTF, I have no idea what application that is for. You want a gain of 6 dB = 2x voltage gain. If the THS7416 is available inexpensively go for that of course, its filter is much higher up at 36 MHz, otherwise it's the same as the '14.

 

nope, I haven't tried a 3.5M pullup, I still don't understand what is it... :sorrow:
is it a resistor? another cap? can you please explain it in simple terms? (I have the feeling it involves adding more current to the rgb lines?)

 

3x 3.5M Ohm Resistors connected to 5v at one end and the other connected to each of the RGB lines

 

Like this?



is this correct?

also I can't seem to find 3.5M ohm resistors on eBay, are these 0.25W 3.6 M ohm Metal Film Resistors OK?

 

Is the "Io (Typ) (mA)" power usage? In which case the THS7316 seems to be an improvement over the THS7314 as stated by Calpis whilst essentially being the same. Would you need such a good filter though?

 

^ IMHO even if they have 10mA difference it doesn't matter that much since our current amps draw more than double... my system works fine with an everdrive and a Tennokoe2 attached

 

Yeah I'm sure it's not a huge improvement. Was watching a vid on YT earlier and it was mentioning the power draw of the Sony chip I mentioned earlier which was adding about 750mA to the draw of the system! Crazy.

I'd be interested in using a THS7316 now..

 

keropi: yes that's how you connect the pullups.

Don't buy those resistors, I checked the datasheet again and my memory was wrong, it's 5.1 M ohm. This resistor value will also work with the THS7316 if you move to that. Don't buy too many either, large resistors aren't very useful. Maybe you can build your own 5.1 M ohm to test using 5 x common 1M ohm? I can't guarantee it'll solve your problem so I don't want you to waste money.

Segata: yup it's the power usage. The THS7316 isn't really an "improvement", I think they're the same chip internally, they just have different filters for different applications. The THS7316 has a much higher filter cutoff because it's intended for analog 720p/1080i where the THS7314 seems to be intended for composite and S-video. The true bandwidth I suspect is around 150 MHz, but the filters are in place for anti-aliasing (before ADC) and reconstruction (after DAC) purposes.

 

Thanks again Calpis, I'll try it tomorrow, don't have 5.1M resistors atm but I can build a test amp with 1M ones as you mentioned
First I'll try without the pullup to see if I can get any result at all (since the eBay guy sell it with just caps as we can see...) and then with the pullup , I'll post my findings

I am also using c-sync on my current amp, apparently it needs the same amplification too , so I guess I use a 2nd 7314 just for it?
What should I do with the 2 extra inputs/outputs of the 7314? can I leave them n/c or ground them? (I'm gonna check the datasheet again but I wouldn't mind a more knowledgeable confirmation)

 

C-sync does not need the same kind of amplification, don't waste another chip.

Where are you pulling C-sync from?

I believe there must be some place on the PCB where C-sync is provided as a digital signal. What you'd want to do is buffer it with a logic gate (74HC04 would be good) then I'll come up with a resistor circuit for you to divide it to the correct "TV level".

 

I am grabbing c-sync from C22 point at the expansion slot (with the info from here: http://mmmonkey.co.uk/console/pce/rgb.htm ) and currently it just goes to the transistor-based amp, it has 4 subsections all the same (rgb+sync)
TIA Calpis!

 

I have made a very quick n' dirty 7314 amp based on the new info (no pullup atm though) :cocksure:

first of all here are some photos of the OLD transistor-based amp that uses c-sync
please view them fullscreen 1:1 size



As you can *hopefully* see the colors are great, there is no bleeding and because of c-sync there are no patterns/jailbars/checkerboards on solid colors. No complain on picture quality tbh (efficiency is another matter).


... and here is the test monstrosity , :excitement: don't judge the work on it since it's only a quick test...




I used composite for sync on this one until Calpis finds a good way to amplify the c-sync... composite is taken as normal from the Expansion port and it has the side effect of adding a checkerboard on the image, most apparently on sky and the tree that are solid colors. You need to pay real attention to the photos to see it, the camera does not really captured that. Other than that the image quality is pretty much the same at my eyes, no bleeding, no miscoloration and sharp. I assume that once c-sync is used with the 7314 amp then the image quality will be at least the same or exceeding the old amp.



...so we are in a good road I think :victorious:

Given that result Calpis, do you think that the pullup is needed? is there a measurement I can make to help decide that?

edit: I just tried with un-amplified c-sync + 7314 , I get an image , there is no checkerboard pattern but the colors seem more bright/saturated, they are noot very good... when using composite they are way better , like with the old amp.

 

Same and interesting disussion is going on over @nfggames forum: http://nfggames.com/forum2/index.php?topic=4822.msg32753#msg32753

 

@keropi: Looks like progress to me!

@rocco: Thanks for the link - seems progress will be made soon and we'll have the right values for the THS7314 in a PCE!

Also, is the THS7314 levels in the mmmonkey N64 mod correct?

 

thanks Calpis but I can't say I understand this ... :sorrow:



does it mean to connect the digital c-sync signal to A1, A2, A3 and A4 and then take the combined output from Y1, Y2, Y3 and Y4 ?