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Old 28th September 2005, 06:28 AM   #1
gongken is offline gongken  Sweden
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Default Class-D poweramp hooked on a guitar tube preamp, questions

Hi!

I´m about to build myself a guitaramp. The philosophy is to use a tube preamp (for good distortion) and hook a Class-d power amp onto that, say for an example an UcD-180...

The preamp schematic looks someting like this

Click the image to open in full size.
How would a class-d amplifier reakt to being hooked onto a preamp like this?

What about the output impedance of the preamp?

Any general tips or hints on the schematic?

I´m not really good at this so i'm thankful for all support, and I
would humbly ask ypu to keep a modest technical level of your answears so I can understand you folks

Kind regards

Andreas
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Old 28th September 2005, 06:14 PM   #2
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Default Tube preamp

The preamp looks fine except for the following. The 470k res right after the clean vol will cut treble considerably. Is this something you wanted? If not they can be left out or replaced with 10k. Also the final grid needs a 1meg res to ground so that the switch won't cause loud pops. If you want to extend the bass response you can replace all the 25uf cathode bypass caps with 470uf @ 16v and the 47nf output cap should be changed to 1uf.
The output impedance is basically limited by the 100k plate load resistors and is a bit marginal for driving the UcD so expect some signal loss and higher distortion. I don’t think this will be a problem.
What can be a problem is the power supply. This supply must be well behaved and well filtered or the low frequency junk may shut down the amp. You don’t need 360VDC, 250 would probably work a lot better to get overdrive. Use a 1000uf @ 400V final filter cap in this supply and follow it with a 330k 2 watt resistor to ground to discharge it after power is off. This slight load will also help regulation.
Aside from this remember amp turn on last after the pre is warmed up and first off before the pre is shut down. A safer way would be to have an output shorting/mute switch so you could shut off the output before you turn on or off power.
Roger
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Old 28th September 2005, 08:56 PM   #3
gongken is offline gongken  Sweden
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Thank you really much!!!

So how would i get 250VDC??? My transformer is an 230VAC step-up to 275VAC... This would be 390VDC... Is it possible to use this transformer
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Old 28th September 2005, 10:34 PM   #4
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Default power supply question

Does the filament power come off of this same transformer? Are you going to have a separate DC supply for them or a dc supply using a separate winding from this transformer in question?
All these questions have relevance as my proposal is to reverse the primary and secondary and bridge rectify the result. This would give you about 275V at the first cap and after the first RC filter should be about right on 250v. This resistor between the 2 caps would be around 10k @ 2w. First cap of 220uf following the bridge and the second one the 1000uf I mentioned earlier. Problem is the filament supply voltage would now be way low if you are using AC direct. If you use schotky diodes in a bridge and a 2200uf @ 25v to generate a dc you can follow this with a 7812 3 terminal regulator and then a 470uf @ 16v final cap. Make the filament supply completely disconnected from ground and connect each of the tubes filaments to the chassis with a .1uf film cap. I would also recommend you make a voltage divider of 200k 2w and 50k 1/2w. Connect the 200k to the 250v plate supply and the 50k to ground using the center point where they connect as a source of 50V to bias the filament supply by connecting it to the filament supply negative output terminal. Connect a 100uf @ 63v cap across the 50k res.
This should all work out ok except the 7812 won’t have much headroom. Use a scope and check the output for ripple. If there is any to speak of you will have to go to a low dropout regulator. At this point I think I would look into one of the 3 terminal fixed switching regulators. The switching regulator shouldn’t require a heatsink but the others will, at least a small one.
Roger
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Old 29th September 2005, 12:36 AM   #5
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Default Good distortion...

Hi,

In my opinion, the good distortion also comes from the clipping (tube)outputstage, not just from the pre-amp.

I have never heard a distorted class-D outputstage, maybe it will sound fantastic!

I have been told that the tube pre-amp distortion is mainly lower order harmonics, and will not give you the same "bite" as the distorted outputstage.

I play the guitar myself, and i have never got the good distortion that i want from the pre-amp alone.

Check out www.amptone.com

Regards,
Peter
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Old 29th September 2005, 01:19 AM   #6
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If you extend the bass responce too much, the result is muddy sound. Smaller caps=tighter sound. Too large coupling and cathode bypass capacitors may also cause blocking distortion, not something you want...

What tubes are you going to use, 12AX7 I guess... If you replace the one in the clean channel (and output)with a 12AU7. You will get lower gain, still enough for clean sound, and you get lower distortion and lower output-impedance. But you have to change the anode resistor to about 31k.

Peter
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Old 29th September 2005, 01:32 AM   #7
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You also might want to add something to limit the outputvoltage to the ucd, to make sure you dont overload the Ucd input too much.

A couple of led`s back to back from the output to ground (after the capacitor) might do the trick, and also introduce some hard distortion when played loud!

Peter
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Old 29th September 2005, 05:21 AM   #8
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Default Distortion

Having a lower plate supply will improve the hard clipping but may not be enough to do much good. The back to back led’s level will still overdrive the amps input. This led solution can work out very nicely if they are followed by a level control so the amp isn’t driven into clipping, that’s an awful lot of power. With the level control you can set a reasonable loudness and still have the overdrive distortion.
Roger
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Old 29th September 2005, 06:45 AM   #9
gongken is offline gongken  Sweden
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Unfortunatly the Transformer looks like this:

Click the image to open in full size.

Peter M, I just want to try the Class-D outputstage, if it doesn't work I will replace it with a tube outputstage, like 4 EL84:s...
I have a ICE-power amp at home and I would simply like to hook my preamp onto that amp and see how it all coems out..

Therefore I would like the amp in this stage to be prepared for that kind of change...

Andreas
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Old 30th September 2005, 01:50 PM   #10
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A 5H choke input filter after the rectifier should take the voltage down to something closer to 250V, and also reduce ripple.

This voltages can kill you in an instant! If you are unfamilear with high voltages and haven`t allready read the the "safety practices" thread at the tube forum, I suggest you do that a couple of times before you start assembling this amp.

The level control Roger suggested after the back to back leds is a good idea. But it will increase the output impedance..

Regards,
Peter
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