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
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
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
An externally hosted image should be here but it was not working when we last tested it.
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
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
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
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
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
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
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
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
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
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
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
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
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
Unfortunatly the Transformer looks like this:
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
An externally hosted image should be here but it was not working when we last tested it.
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
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
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
Back to back led`s
That is two led`s in paralell, one in reversed direction.
It clips the signal when it exeeds the Vf of the led, usually 1.7V with a red led, one clips the negative going signal and the other the positive.
It is often used in distortion/overdrive stomp boxes. Different diodes/led`s gives different sound.
Led`s has more headroom than silicon diodes because of the much higher Vf.
Here is a link explaining clipping diodes:
http://www.generalguitargadgets.com/richardo/distortion/
Regards,
Peter
That is two led`s in paralell, one in reversed direction.
It clips the signal when it exeeds the Vf of the led, usually 1.7V with a red led, one clips the negative going signal and the other the positive.
It is often used in distortion/overdrive stomp boxes. Different diodes/led`s gives different sound.
Led`s has more headroom than silicon diodes because of the much higher Vf.
Here is a link explaining clipping diodes:
http://www.generalguitargadgets.com/richardo/distortion/
Regards,
Peter
OK, I've read it now and i think I understand how LED back to back works in therory.
If I want some soft clipping LEDs mounted in this amp, how and where do I connect them?? Any aditional components lik caps, resistors???
Roger talks about an pot right after the LEDs, how about mounting them right before the distortionchannels mastervol???
Kind regards
Andreas
If I want some soft clipping LEDs mounted in this amp, how and where do I connect them?? Any aditional components lik caps, resistors???
Roger talks about an pot right after the LEDs, how about mounting them right before the distortionchannels mastervol???
Kind regards
Andreas
I know Marshall had diode clipping in some of their tube-amps, they used a combination of led`s and silicon diodes as in the Silver Jubilee Series 2550.
You can find the shematics here:
http://www.schematicheaven.com/
And here:
http://www.diyguitaramp.com/tech.html
Other interesting stuff:
http://www.lynx.bc.ca/~jc/marshallAmps.html
http://users.chariot.net.au/~gmarts/ampovdrv.htm
Regards,
Peter
You can find the shematics here:
http://www.schematicheaven.com/
And here:
http://www.diyguitaramp.com/tech.html
Other interesting stuff:
http://www.lynx.bc.ca/~jc/marshallAmps.html
http://users.chariot.net.au/~gmarts/ampovdrv.htm
Regards,
Peter
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