Digital to analog - help

[caspercody]

Hello

I am trying to make a ADA MP1 pedal of the tube/solid state circuits. I have them almost built, but I am having issues with the (I think) digital chip TC9176.

This TC9176 is used as a pot with inputs from push buttons. Then I think the output of the TC9176 is digital to some IC circuits that convert to analog to drive a VTL5C4/2 Vactrol that is adjusting the OD1 and OD2 volumes.

These circuits are around U5, U9, and U14 for the OD1 volume. For OD2 they are U3, U2, and U14. Also OD2 is used to adjust the compression circuit when in solid state mode (going into U6).

I was also wondering if I could create a circuit with a pot to vary the LED portion of the VTL5C4/2's? If this would work I could eliminate U2, U9, and U14.

Looking at the TC9176 circuits I do not understand the STB, DATA, and CLOCK inputs. I see these points are connected to a 74LS174 (U27) chip, then to a 74LS138 (U39) chip, then to the main board Z80 (U29). The push button membrane come to the main board through other chips. I know this circuit is very old, and there is probably an easier way to get this done. I just do not know how, and I do not know enough looking through Google.

I have tried just using a pot in place of the TC9176, it worked but turning the pot had no consistency



.

Thanks
Rob

[ElectricDruid]

Hi Rob,

The TC9176 is expecting serial communications to set the pot wiper positions. The TC9176 is a low resolution digipot (8 steps on two channels, 5 steps on the others). It's tied into various bits of the circuit, and I don't think it's only setting levels, so it's used to provide programmability for various parameters (drive, maybe? compression settings, maybe?).

The vactrols you mention look like they're providing gain control. There seems to be a envelope follower sidechain, so one or another might be acting as a compressor, but it's a bit difficult to tease apart on first look. The one on the left could be acting as gain control for that overdrive stage. If I had to guess why they controlled the vactrol with the digipot instead of using the digipot directly, I'd say that it was to avoid zipper noise when changing settings - the digipot will switch instantly and make a click, whereas the vactrols will change relatively slowly and no click will be heard.

The datasheet gives the equivalent resistance of the TC9176's resistor chain as 120K for the 8-step ones, and 14K for the 5-step ones. You should be able to replace it with pots of a similar value and it should work the same. That's the "minimum changes" way to get the circuit working without a processor. Since the steps are in dBs, the pots should be log pots.
Driving the vactrols more directly somehow else would involve bigger alterations, since there's quite a bit of of stuff between the digipot and the vactrol on the LHS.
The serial comms protocol Toshiba used to control the chip is not any standard thing and would have to be bit-banged. It wouldn't be hard to get going on an Arduino or similar. Certainly easier than the Z80 assembly code the original probably used!

[caspercody]

Thanks!!!

So putting a pot around 134K in place of the TC9176 pot connection would be equivalent?

I was also looking at TC9153 (but only 50K and 20K) or DS619-100 (100K) chip in place of the TC9176. Looks like these chips would only require push buttons to go up or down.

[ElectricDruid]

So putting a pot around 134K in place of the TC9176 pot connection would be equivalent?

No. There's two sets of Left and Right pots in the chip, so four pots total. One pair are 120K and 8 steps. The other pair are 14K and 5 steps. All the pots can be controlled independently, which is what's going on here.

100K Log and 20K log pots wouldn't be a bad place to start.

The full datasheet is here, if you haven't got it:

https://pdf1.alldatasheet.com/datasheet-pdf/download/114286/TOSHIBA/TC9176P.html

I was also looking at TC9153 (but only 50K and 20K) or DS619-100 (100K) chip in place of the TC9176. Looks like these chips would only require push buttons to go up or down.

I don't see any particular advantage to using that solution over a analog pot, frankly. Unless you particularly like the idea of a push-button interface?

[caspercody]

I would prefer analog.

I am just trying to understand how the digital part works.

Referring to the TC9176, I do see how the +15vdc comes into the high point of the 14K pot, then its wiper goes to the high point of the 100K pot, then its wiper goes to the respected circuit. Both low points of pots go to ground.
But what type of signal is this feeding the circuit's?
Is this just a different level of voltage? Or resistance?
I am guessing that the input is +15vdc that all the pots are doing is varying the voltage to the respected circuit?
I am curious how a digital signal is sent in what type of electrical form?

Could I use just one pot say a 200KA?

I do really appreciate you helping!!!

[caspercody]

I hooked up the two pots as I described in my previous post. There was no adjustment in sound level. It was either on or off.

I removed both of them and tried just one 200KA pot. No sound until I turned it up to 3/4 (about 3 oclock). Then I started to hear sound. It had a lot of white noise, but if i turned it a little more, white noise went away. As I turn it to max the gain increased.

[ElectricDruid]

Referring to the TC9176, I do see how the +15vdc comes into the high point of the 14K pot, then its wiper goes to the high point of the 100K pot, then its wiper goes to the respected circuit. Both low points of pots go to ground.
But what type of signal is this feeding the circuit's?
Is this just a different level of voltage? Or resistance?
I am guessing that the input is +15vdc that all the pots are doing is varying the voltage to the respected circuit?

Yes, it's a control voltage. Cascading the pots like that (good spot, I hadn't noticed they'd done that) is probably to increase the resolution. Arranged like that, you get a "Coarse" setting which gives the maximum voltage, and then a "Fine" setting which gives you steps up to that maximum voltage.

In which case, the exact pot value shouldn't be very important at all - it's just a divider providing a voltage somewhere between 15V and Gnd. The exact equivalent resistance will vary anyway because of the parallel combination of pots, and in the case of the lower pair, it's explicitally buffered before being used, so again, the pot value shouldn't matter. If you're getting weird problems with that one at least, something else is probably going on. Testing the control voltage would be a simple matter of checking the voltage at the buffer output varies as you expect when you move the pot.

I am curious how a digital signal is sent in what type of electrical form?

You mean to the original digipot?
The details are in the datasheet, but in short it uses a series of clock pulses to clock data bits into an input buffer ("CK", pin . The data bits are whatever the voltage is on the DATA input (pin 9), either high or low. It uses Vdd and Gnd for the logic side, so it's 15V logic in this case, like 4000-series CMOS. It clocks in 20 data bits, and then a pulse on the Store pin ("ST", pin 10) latches them from the buffer into the chip. Those three pins do the whole job.

The protocol is unusual in that it uses the *position* of the bits in the supplied 20 bit number to determine the value. That's the sort of thing you can only do when you're talking about 8 or 5 settings! Imagine if you used the same method for a quad 256-step digipot...you'd have to clock in at least 1024 (256 x 4) bits! Much simpler to use binary in that case and reduce each pot's position to an 8-bit number.
The advantage of of the weird position-based method is that it doesn't need any decoding, so it makes the chip's internals simpler. A 3-bit binary number (for 8 steps) would have needed some decoding, which implies a (slightly!) more complex chip.

Could I use just one pot say a 200KA?

See my comments above. The pot shouldn't matter too much. The upper pair feed a 750K+750K, so very large values would be best avoided, but even 200K is small by comparison. The lower pair feed a buffer which will have an impedance of many megohms, so 200K is even smaller by comparison.


Sorry if some of my earlier remarks are a bit misleading. I haven't dug into this in depth, so I'm only seeing things as you bring them up, and that means that I don't have the full picture yet and am learning it as you're learning it. But I think we're making some progress, right?

[ElectricDruid]

I hooked up the two pots as I described in my previous post. There was no adjustment in sound level. It was either on or off.

I removed both of them and tried just one 200KA pot. No sound until I turned it up to 3/4 (about 3 oclock). Then I started to hear sound. It had a lot of white noise, but if i turned it a little more, white noise went away. As I turn it to max the gain increased.

This is disappointing. If you have the top and bottom of the pot connected to 15V and Gnd, and the wiper going to where the pin 2 (and pin 15 for the second pair) go to, then assuming the rest of the circuit is ok, I don't see why it shouldn't work.
That "assuming the rest of the circuit is ok" is a pretty big assumption though. How sure are you? How much have you tested it? How can you with no working digipot or pot?

[caspercody]

The rest of the circuits seem to work. The buffer (Q7) , distortion stage (U4) , and tube circuit all work. Right now I put a 2K pot where the first Vactrol (U5) is located and a volume pot after the 2K pot going into the distortion stage (U4). This seems to work. Then going into the tube circuit and a master volume going out. I put a 100k pot in the tube section (OD2 output U3) pin one to ground and pins two and three connected together seems to work for the tube gain. It is kinda how the second Vactrol (U3) looks to be wired. Then I connected the OD1 output (U4 pin 1) from the distortion circuit to the upper compression circuit (R58) and this seems to add more distortion, I added a pot for volume from (U8) compression circuit and connected it to the master volume. This looks like what they are doing in the circuit (U13) on the far right of the schematic. The tube circuit output and this compressor output look to be connected in parallel (U13). I added a 4PDT to switch off this compressor/distorted and some tube components to get a clean tube sound. I do not know how the lower compressor circuit input works since it looks to get signal from the TC9176 circuit.

I have the TC9176 circuit separated with just power and the pots. Just looking at the first TC9176/Vactrol (U5, and U14) circuit it is weird. I get negative voltage readings on the IC pins. Pin 1 ranges from -0.27 to 9.03. But does not hit positive until turned over half way then quickly jumpy to 7 volts then to max. Pin 2 is -3.44, pin 3 is zero, pin 4 is -9.10, pin 5 is -7.33, pin 6 is -6.17, pin 7 is -7.81, and pin 8 is 9.33. I am using a One spot 9v into a 1044 chip to get a negative 9 volts. Reading the voltage on the pot it starts at a negative number and as I turn the pot not much happens until half way the voltage goes up to max and then starts to go down. LED positive lead on Vactrol starts at -0.23 and as I turn the pot goes up to 1.96. Have to turn pot 3/4 to get the voltage to start to go up. The resistance of Vactrol pins 4 and 5 have to turn pot 1/4 before starts to read resistance then from zero to infinite. Pins 5 and 6 zero to 1Meg.

Please let me know if there is anything else you want me to test?

Thanks
Rob