DIY Audio Projects Forum

I spent the day Saturday closely examining the power supply, basically to see if it was "ripple-free". Despite the claims made, the design does not do much for ripple. I measured about 50mV 60hz ripple from the supply, using an external tranny. As much as I liked the slo-turn on aspect, it did not provide a clean DC. I then set about designing my own DC with slo-turn on using an LM317. Success! And, I achieved my goals of slo-turn on and ripple free. The original design employed by Oatley used a cascaded voltage doubler - why Paul felt the need for 50+ volts is unknown to me as the "discrete regulator" circuit didn't require that much. I will be using a small form factor transformer of same type as Oatley's feeding a normal voltage doubler. That voltage (~40VDC) is fed in to an LM317 with slo-turn on support with the output set to 30VDC MAX. With my scope set to the lowest volt sensing (20mv range), I could discern absolutely no ripple at its output.


The pre-amp was tested with this design and the output signals were clean even at full quiet.


It's late, and I am way past my normal bed time. This was a good day.

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

Les: What no smoke flow? Good work. I've not see the type of PS before.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |

It works well, but I made an error in my ripple measurements. There is a small amount of ripple of the type exhibited by fast switching rectifiers (spiky). It's less than 5mv p-p, however.

Completed PCB build up of new PS and made a couple changes along the way. R2 & R3 have been replaced by a single 56k resistor. Also, the value for the resistor R4 is 10K (had not established the value at time of posting). The turn on circuit had called for a 50K, but not having one I went with a 39K + 10K (49K). Today I managed to happen up on a 56K. Since the resistors make up the RC tank, a slightly larger resistor only slowed time a very slight bit. In case you're wondering why 2 10uF in parallel? That's what I had one hand that was smaller than 100uF, which took way too long to come up. The time for the 20uF to come up appealed to me at about 30 seconds.

On the pre-amp PCB, all components save for the last 2 capacitors on the power feed have been removed. I added a 10nF snubber to the new DC input which kills the HF noise from the Hi intensity blue LED (power indicator).



I did that today.
Before putting the line voltage to the completed PCB, I was going through what I call "staging". Basically, I check the DC section using a DC lab supply. Then I do a low voltage section test off of the xformer secondary, using a LV AC source, then I'll "put the pedal to the metal" with the mains. Well, I managed to short out my LV AC source and blew its fuse. Not that there is a problem with the design, mind you. The problem was me I let the clip leads get too close and **SPAT**, no LV AC, power indicator out - DANG! Blown fuse. The design is borrowed from the LM317T application notes with a little "personalization".

Pictures of the new PS:



As you may have noticed, there is yet no mains connected. Until I re-fuse my AC LV bench supply and complete AC stage test, it won't be tested on mains.

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

What is the frequency of the "spiky" ripple? The LM317 is not a regenerative regulator, it's purely linear so you should not be seeing this unless it truly is a 120Hz phenomenon. Otherwise it would indicate an instability in the supporting circuitry. Curious.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

Right on the head, Matt. It is 120Hz spikes. I've tried suppression with 100nF and 10nF (all that I have on hand in that range) at the regulator input and at the output with no apparent effect. I have a couple different parallel line filters (choke) that I will try in line from the PS to the amp. I've been able to employ those in the past with some success.

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

I'm in a holding pattern while I await some hardware for completing my project. I hope to have everything in time to finish this up this weekend and post some initial listening reports.

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

I did some more testing on my PS design. Scrap the SK3841. Apparently this transistor is not suited to this type of usage as it became unstable and started dropping the voltage. I took the TIP117 from the original design from Oatley and inserted it in to the circuit. The TIP117 is a Darlington pair type and is better rated for this type of duty. Another problem has crept in in the form of an under rated power transformer. Time for "plan B". I've got to either redesign and use the cascaded voltage divider, get a bigger transformer (higher current capability) of the 12.6 - 18VAC variety, or redesign the PS in to a traditional full wave type with a 30VAC transformer. Ah the joys of PS design......

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

There is no doubt a good amp is based on a good PS. Probably worth going to the effort. That way you will know what you can get out of the amp design.

_________________
Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |

I was able to preserve the design. I had a box of wall warts laying about, including some AC out types. I had one rated 12VAC @ 500mA - PERFECT! I connected that to my power supply and all is good. In fact, it's better. The ripple has gone, so that may have been a result of the previous transformer or possibly was being injected by that SK3841 transistor that went south. I didn't check for ripple after replacing that transistor.


Movin' on! I've got the layout and began working on the case. I'm using a Bud Ind. 7x12x3 aluminum case. It's unpainted and is economy style. Some exposed seams will get a coating of epoxy to cover and smooth, then the case will be primered and then finished in a gloss black enamel. I picked up a nice illuminated toggle switch with hi-bright blue LED to use as my off-on switch. A fused & filtered IEC snap-in type receptacle will be used for mains. The afore mentioned wall wart has been dissected for the transformer and has already been remounted to a PCB to be mounted in the case. It was to big for the power supply PCB, so it got its own PCB. To help limit the tendency of the 6418s to ring, I have installed rubber grommets to that PCB. I have some hi-density foam padding that will also be used to damper the tubes. I've test this type of foam with the 6418 base headphone amp and it works really well. Further more, the case will receive some nice felt feet.

- I'll try to remember to snap some pics this evening.

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

Take photos of the seam work. Sounds interesting.
Cheers

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