VFE Project DC Voltages

[benny_profane]

Am I correct in assuming that, in the case that all components are rated appropriately, the maximum input on VFE projects can be interpreted from the zener value on the switching board? I took a look at the VFE project pages and compared the listed power input values to the zener listed in the switching board document, and they seemed to match up:

• The Scream: 9-18V with 18V zener
• Choral Reef: 9-15V with 16V zener
• Merman: 9-12V with 13V zener

Another question I have is about the 4558 voltage rating: it lists an operating voltage of +/-18V. With caps, I know a little bit of breathing room is important (e.g., using caps rated at 25V when running at 18V), but is this the case with this IC? The Vid (input differential) is listed at +/-30V; does this mean that the voltage potential must not exceed 30V? If that's the case, the 18V supply with a zero ground should be totally safe?

https://www.mouser.com/datasheet/2/294/NJM4558_NJM4559_E-364362.pdf

[HamSandwich]

I'm not familiar with the VFE stuff, but maybe I can help with your 4558 question!

It's not the best practice to run things at their rated specs. There's stuff out there that'll be ok, but you do increase the risk of failure. For 4558, running it at +/- 15V is perfectly fine as 'breathing room'. You could probably run it at +/-18, but at its limits, you increase your risk of failure.

The input differential is the total input voltage between the two inputs of the amplifier, not the power for the IC or the input voltage. The power for the IC is the operating voltage (+/-4V to +/-18V) and then the input (signal voltage) max is +/-15V or less depending on the what voltage your powering the IC from. You'd have a hard time getting +/-15V swing out of your guitar without trying very hard.

Note that the +/- voltages can also be relative to ground as in your 18V and ground comment, so +/-18V could be viewed as being able to run the IC at 36V maximum.

But to answer your question, yeah you'll be fine powering it from +18V. Depends on the circuit, but to gain the benefits of increased voltage, you have to raise your bias voltage up to 9V instead of the typical 4.5V. A schematic of what circuit you're talking about would help. I'm guessing the scream?

[benny_profane]

Thanks!

The explanation of the input differential is very helpful. (I had a feeling I wasn't getting that one right.)

Looking into supply voltages for The Scream is what got me to investigate the zener ratings and the listed supply voltages on the VFE site. Since the site says that The Scream can run at 18V, I wanted to make sure all of my components were within range. I hit a stumbling block with the 4558 datasheet. I know that TS variants can be powered at 18V and that the 4558 chip is standard for the circuit, but I wanted to check about that spec. If it meant that 18V was the ceiling, that would leave the IC vulnerable to damage from a transient or spike rather easily. Thanks for explaining that the +/-18V represents a 36V differential (that definitely makes much more sense) and not +18V to zero ground or -18V to zero ground.

If you're interested in looking into how the bias voltage behaves with the increased supply voltage, I'm sure I'd benefit from learning more!


Cheers

[HamSandwich]

I wasn't sure if you were building from scratch, or talking about a project, but for a circuit that runs properly biased at 9V, it'll be fine at 18V, no worries there. For running the screamer at 18V, just make sure your caps at rates at 25V!

For the Zener, you don't want to go much over the rates value, or (I think) you start creating heat and noise, although the diode and inductor before May help limit that.

[benny_profane]

Caps checked so I should be good to go. Might want to look at some past builds though.

That's a good call on the zener. I think that that value does indicate the max supply of the circuit, though. To illustrate, the scream is fine at 18v, but the BBD of the choral reef and the charge pump of the merman limit the voltages, hence the lower zener.

Does anyone know if across the board that rule applies? Of course ensuring that all components are rated accordingly.

[Aentons]

Caps checked so I should be good to go. Might want to look at some past builds though.

That's a good call on the zener. I think that that value does indicate the max supply of the circuit, though. To illustrate, the scream is fine at 18v, but the BBD of the choral reef and the charge pump of the merman limit the voltages, hence the lower zener.

Does anyone know if across the board that rule applies? Of course ensuring that all components are rated accordingly.

I think the Merman is the only one that is listed on the VFE site as 12v max. Most are 15v and a few are 18v.

I did also notice that the Scream is 18v and the Ice Scream is 15v which seems a little strange.

[benny_profane]

Caps checked so I should be good to go. Might want to look at some past builds though.

That's a good call on the zener. I think that that value does indicate the max supply of the circuit, though. To illustrate, the scream is fine at 18v, but the BBD of the choral reef and the charge pump of the merman limit the voltages, hence the lower zener.

Does anyone know if across the board that rule applies? Of course ensuring that all components are rated accordingly.

I think the Merman is the only one that is listed on the VFE site as 12v max. Most are 15v and a few are 18v.

I did also notice that the Scream is 18v and the Ice Scream is 15v which seems a little strange.

I don't have the schematic for that circuit. Isn't that an adapted TS-style circuit, though? I don't know why that'd be the case, either. Is there anything that you can see?

Still looking for a definitive answer on the zener issue. Although, it seems to be more or less settled that that value does correspond with the maximum input supply voltage.

[Aentons]

 :Ice Scream Schematic is here:
http://vfepedals.com/schematics/icescream.pdf

I think it's a clone of the Ibanez ST9

[Aentons]

Also, that document shows that these are the only 18v projects:

Alpha Dog, Blues King, Distortion3, Fuzz Duo, and The Scream

Plus, all the new format pedals are listed on the site as 18V

[benny_profane]

I don't see why the Ice Scream shouldn't be able to take 18V from that schematic?

It does seem to be that the zener value corresponds with the maximum voltage input, though.

Plus, all the new format pedals are listed on the site as 18V

I haven't really looked into those circuits. I notice that there's an optional charge pump listed on all of them, though.

[Aentons]

I don't see why the Ice Scream shouldn't be able to take 18V from that schematic?

After closer inspection, It looks like the TS has a voltage divider for the vref and the IS does not. Hmm, maybe that's the difference

[HamSandwich]

The Ice Scream runs on a bipolar supply, so it doesn't need an artificial voltage reference, it just uses ground.

[HamSandwich]

Using the Zener rating is probably OK for the VFE stuff because it uses a very particular and what looks to be an extensively thought out power system.

In general, your best bet is to take stock of the active devices and figure out what the max voltage is. For example, if you look at the Ice Cream schematic, the 15V Zener may tell you what the max voltage is, but it won't tell you why. Look up the data sheet for the TC7662 and you'll see the max input voltage is 15V, which would be why. That chip wouldn't be happy at 18V. It is also the chip that supplies the -V for the bipolar supply.