Input to IcePower module

[marcelomd]

Hi,

I'm playing with the idea of getting an IcePower power amp module (such as the 50asx2) and using it with a variety of preamps. The Idea is to get a small clean poweramp I can use with both bass and guitar at home or at our rehearsal space. Ideally I'll have two instrument inputs and an aux input.

To use the module like this I need a small adapter board. Can I get a quick check on my design?

This is what I came up with:

This is based on the module's designer manual and other amps' service manuals.

- The module provides unregulated +-25V, which I'll step down to +-12V or 15V in a daughterboard (upper left);
- On the main circuit, I have a high impedance input for two instruments (simple passive mixer), an active mixer for the aux input;
- A differential output stage. Needed to drive the module in BTL mode.
- The module has 3 status pins. Thermal and overcurrent protection are floating and are pulled to 0V in case of fault. The Enable pin is pulled to 5V in normal operation and to 0 in case of faul. It can also be pulled low to disable the module. I'm using this to light an error LED (lower right. The connectors are supposed to be a 4N25).

Questions:
- Does any of you guys have any experience with these things?
- I intend to drive it with a preamp pedal. Do I need to have some gain at the input?
- I've never worked with dual supplies. Am I missing any decoupling caps?

Thanks a lot!

[PRR]

I suspect you need gain to get from Instrument to ICE at full roar. Leave some pads for gain-setting parts.

[marcelomd]

I suspect you need gain to get from Instrument to ICE at full roar. Leave some pads for gain-setting parts.

Yeah. My idea is to use a preamp pedal in front of this. Like a Sansamp, or even a Boss eq. Would that be enough gain to get this thing moving?

[anotherjim]

Line level input ought to be enough - so a DI type preamp like Sansamp would be best. I think an EQ pedal designed for guitar signal chain isn't going to be hot enough. The input impedances seem on the low side (8k) so the preamp needs a proper low output impedance line driver so the coupling caps don't lose bass.

By the way, I think a class D with integrated mains power supply is a wonderful idea. How much do those things cost?

[PRR]

The input to the ICE seems to be 1Vrms. It is probably silly to have a Volume control right before a 1V input-- if you "use" the volume control then you have to deliver several or many Volts to it. (And an Audio taper pot implies that you will sometimes "use" the pot at high losses.)

[marcelomd]

By the way, I think a class D with integrated mains power supply is a wonderful idea. How much do those things cost?

A quick search on Ebay: between US$100 for the 50asx2 (dual 2x50W or 1x200W) to US$125 for the 125asx2(2x125W or 1x500W). Sounds reasonable.

[marcelomd]

I based this design on other amps which use ths module (GK MBx00 series, Fender Rumble, etc.).
The general architecture is: Preamp (buffer+eq) -> send -> return (simple buffer) -> mixer (for aux input) and the balanced driver. Some have the master volume before the send/return and other after the mixer, so I THINK the headphone output of a phone might be enough to drive it. Maybe I'll move the volume knob to a gain knob on the first buffer.

[marcelomd]

Necropost!!!

I finally got my Icepower module, so I can finally put this in motion. Its an ice125asx2, more or less 500W into 8 ohms =)

The input to the ICE seems to be 1Vrms. It is probably silly to have a Volume control right before a 1V input-- if you "use" the volume control then you have to deliver several or many Volts to it. (And an Audio taper pot implies that you will sometimes "use" the pot at high losses.)

I'm not sure I got what you were trying to say here. Why is it silly?

My main bass can output 3V according to the manufacturer. Pretty close to the maximum 3.3V of the module inputs. Even if not hitting hard, any preamp with any boost will bump into the limits.


Just for fun, some numbers:

The ice125asx2 has a voltage gain of 30.7db and is capable of 500W in BTL mode. The equation to get the input signal, in volts, for a given power and load is inputRms = sqrt(power * load) / (10^(gain/20)), correct?

>>> (sqrt(500 * 8 )) / (10 ** (30.7 / 20))
1.8451428543095267

To get the peak volts: peak = inputRms * sqrt(2)

>>> 1.8451428543095267 * sqrt(2)
2.6094260490803367

I modified a bit the input

[PRR]

Cute. Have you tried it yet?

My instinct says low-V Zeners are very soft and cost more than plain diodes. If the clipper is not well matched to the ICE you either don't get all the power you paid for or you get blatting.

A scheme like this uses cheap diodes to limit from 1.2V peak to >10V.

[marcelomd]

Not yet. Waiting for a bunch of parts to arrive, most important a soldering iron =)

LTSpice says it will clip at 3.2V. But, as we know...theory and practice are closer in theory than in practice...

Nice. I'll try that limiter too.

Thanks!

[R.G.]

It's not just a limiter.The universal complaint about low distortion power amps is that they sound too clean or even "sterile". You can apply a soft clipper in front of the power amp so the clean power amp cleanly reproduces the clipper's distortion to the speakers. It's a neat trick, and works well.

This trick has a lot of side variants. If your clipper circuit has a variable output, you're effectively dialing down the power amp's power by turning down the clipping level. You could design the clipper to max out at the power amp's maximum input for full output power, with soft clipping. As you dial down the clipper's output, the power amp's output decreases, but it still has the same tonal quality, just at a lower power output level. It's a kind of master volume that really does dial down the output power without changing the clipping qualities.

[marcelomd]

The idea of an ultra clean power amp with a clipping preamp is nice. I actually do it with my guitar rig. A Mesa V Twin tube preamp into a cheap TPA3118 amp module. Sounds awesome.

My main objective with this input thingy (and it's associated clipper/limiter) is just not to fry the inputs of the expensive IcePower module. A preamp with distortion and eq is in the works =)

I'll simulate the suggested clipper as soon as I can.

Thanks

[marcelomd]

A scheme like this uses cheap diodes to limit from 1.2V peak to >10V.

Ooook... I'm sure I did something wrong here... not the shape I expected.