How to use class D power amps with tube preamps?

[Steelwitch]

Hey everyone! I would really like some more info on how to incorporate using class D power amps in tube amp designs. For example, I’m looking into possible building a pedal steel amp based off of a twin reverb but with a class D power amp. I’ve been looking at ICE? I know these are popular all over the board and work well, but does it come down to finding one with the specs to support what a PT would be doing in its place? I hope that makes sense. I would imagine that it would come in after the phase inverter. If anyone has any schematics that might be helpful to check out please link them! Thanks everyone!

[Mick Bailey]

A self-contained class D amp doesn't use a phase inverter and just needs an unbalanced feed from your tube preamp section. If you look at tube amp schematics you only need to go as far as where the signal couples to the PI. You will need to ensure that the signal output to the power amp is line level and of the correct impedance. I've had great results linking a tube preamp to a 100W class D power amp.

A class D amp behaves differently to a tube power amp section and is just clean power just like most other solid-state power amps (other than those that use mixed-mode feedback to get closer to a tube amp sound). What you mean when you say it comes down to finding one with the specs to support what a PT would be doing in its place - can you clarify?

[Steelwitch]

A self-contained class D amp doesn't use a phase inverter and just needs an unbalanced feed from your tube preamp section. If you look at tube amp schematics you only need to go as far as where the signal couples to the PI. You will need to ensure that the signal output to the power amp is line level and of the correct impedance. I've had great results linking a tube preamp to a 100W class D power amp.

A class D amp behaves differently to a tube power amp section and is just clean power just like most other solid-state power amps (other than those that use mixed-mode feedback to get closer to a tube amp sound). What you mean when you say it comes down to finding one with the specs to support what a PT would be doing in its place - can you clarify?

so, I have seen some amps using a class D power amp that is also supplying things like 6.3v heater voltage (possibly DC) and DC V and I. Basically with a power supply built in. Is this accurate? The amp I’m thinking of is an orange tiny terror. No PT to speak of at all.

[glebert]

If you are planning on using something like an Icepower that has the integrated power supply, you may get some power rails, but they aren't necessarily tube friendly. If you look at the 200ASC (https://www.parts-express.com/ICEpow...x-200W-326-208) you get +/-12V and +/-47V rails. The 12V rails can do 400mA, so maybe enough to do a few preamp tube heaters (using both + and - )?? You still need to come up with plate voltage, and it seems like the 47V rails wouldn't be enough.

I have looked at how Ampeg did the plate and heater voltages on the SVT7 Pro amps using SMPS and it was dumb, and they had some real problems with them. I think it would make more sense to tap off the mains with a little transformer and rectifier to get the voltage(s) you would need).

[eschertron]

My thought also. The preamp can benefit from a smallish PT, saves trying to jump through hoops to make an SMPS power supply do those things. So a 'vintage' linear PS for the preamp, and a SMPS for the class D power amp. Gets my vote

[Mick Bailey]

When looking at commercial class D guitar amps they mainly use switched mode power supplies, The high frequency at which these PSUs operate means greater efficiency, hence the very small power transformer. It's also very easy to arrange for multiple voltage tappings. All that's designed into the amp and the transformer wound to a particular spec. What you're looking at is buying a commercial power amp module and building the rest to suit, which needs a different approach.

I've built a few amps where I've needed a HT supply from a low voltage transformer output. I've used two methods, both have which worked well. One is to use a 12v-0v-12v transformer and use one half to power the preamp tubes at 12v and use the 24v AC to feed a voltage multiplier to provide the HT. The 24v is also rectified/regulated for the power amp (I used a 24v module)

Another way is to use the same transformer and run the tubes at 12v, but rectify/regulate the 24v for the power amp and also to run a DC-DC converter for the HT. I've built my own using a home-made inductor, but also I have a really cheap and effective PCB that does 400v off a low-voltage DC supply.

The easiest method is perhaps to buy a complete PSU board (I've had a couple of really good 5A Chinese 24v SMPS boards that cost $7 each) and a regular PT - maybe a Hammond - to power the preamp tubes. You only need something small because anything from a Champ upwards will provide enough HT current, you just need to work out the total heater current depending on how many tubes you end up using.

[J M Fahey]

You are using an off the shelf prebuilt Class D amplifier so the built in power supply nwill cater to its own needs, period.
Forget anything tube filament and +V related.
Ice power supplies "auxiliary" +/-25V (maybe one supplies +/-17V) to feed an SS PREAMP, period.

You build your Tube preamp and its dedicated own supply.

Tube preamps CAN supply up to 90V RMS which will DESTROY any SS input, doubly so if Class D , so you mut both attenuate Tube Preamp out and provide some clamping, such as back to back 4.7V Zeners across Class D amp input.

[Steelwitch]

Thanks y’all! I have a much better idea of the approach I need to take to actually pull this off. To be honest I’m a bit hesitant because I do see designs like this fail constantly and become garbage, but I see companies that are making pretty reliable products using class D power amps. Knowing that I don’t need the integrated PSU is helpful.