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Experiments with the MC34063 and Audio Ckts
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Author:  BowToEd [ 09 Oct 2018, 15:00 ]
Post subject:  Experiments with the MC34063 and Audio Ckts

I recently purchased a board on Ebay that seems to be for Nixie Tubes. It also seems to work very well for Nixie tubes, although I haven't tried anything with it yet. I do know it works for a small amount of DC power, yet I found that it would be unsuitable for anything related to audio.

The kit uses an MC34063AP in a boost converter circuit. The idea is to have a 12v supply be converted to a high voltage for Nixie tube plates; the output power is 5w from a 12v source, with slightly higher powers available if you can source a higher voltage. A laptop charger would be good for this (it has to do with peak current...higher input voltage means it has to cram less current through the inductor for a given output current at B+). I figured it would be good for a low-powered amplifier or at least a preamp. Sigh...


The block diagram of the controller shows a comparator gating an oscillator which controls a power switch. I figured this means PWM. Nope. I was getting a sub-1 kHz ripple on the output. When I investigated, I found that the output consists of pulse bursts at low power output. The comparator is slower than the oscillator (for some reason) so what ends up happening is the 60 kHz pulses make it to the power switch in bursts. Looking on the scope, you can see the pulses as short bursts among big gaps. Measuring the time between the bursts reveals that they are happening at an audio frequency. I'd expect the controller to adjust the width of each individual pulse instead of gating them.

Why do I care? After all, I was able to get 5w of DC power into a resistive load at 120v. The problem is the all-too-obvious ripple at AF that still needs to be filtered out. I tried this when prototyping an amplifier using 12AU7 and 12ED5. It worked, but there was an obnoxious buzzing/whining noise that couldn't be removed with HF filters. At this point, I didn't think there would be anything but hash above 20 KHz. Because of this, I'd rather choose another chip where I don't have to filter out 60 kHz AND whatever audio frequency this thing decides to spew forth. That makes me just want to buy a 60 Hz transformer and use that. I'd do that if I weren't trying to develop a 12v-to-B+ system so I can have ham radio fun with tubes in the field.

I clipped C3 out of the circuit, thinking that would speed up the comparator in the IC and cause true PWM to happen. That did nothing but make the AF ripple larger (From something like 5VPP to 38VPP. F$^@.)

I also know others have been working on SMPS-technology because it's annoying to spend $35-60 per transformer when all you want is 10-20 mA of high voltage power to make a small preamplifier or something similar.


Author:  M. Gregg [ 10 Oct 2018, 13:05 ]
Post subject:  Re: Experiments with the MC34063 and Audio Ckts


I played with this some time back on a line stage.
I found the answer but its not "cost effective". I used a charge pump and generated voltage 50V higher than needed then regulated with a Maida regulator followed by C/L HF filter.
As always there are hidden issues linked to Heater lift and isolation via B+.
I set myself a spec to build an Aikido line stage powered from a laptop PSU so only DC allowed into the amp no AC.
Yes it is much easier to use AC and step up but that wasn't of interest at the time.
In the Aikido the tubes are SRPP and as such heater off set voltages are required and generated via B+, but the heaters are also powered by the DC powering the B+ so you have an isolation problem, but it was interesting to do.

M. Gregg

Author:  BowToEd [ 10 Oct 2018, 13:14 ]
Post subject:  Re: Experiments with the MC34063 and Audio Ckts

M. Gregg wrote:
I found the answer but its not "cost effective". I used a charge pump and generated voltage 50V higher than needed then regulated with a Maida regulator followed by C/L HF filter.

Right. If I want to make a SMPS at 60 KHz, then I expect only to have to filter out 60 KHz. Part of the convenience of the SMPS is not needing to build a four-stage filter to get it all. This is also for a line-stage amp, where the ripple can be 1 mV or so without serious issues (the peak voltage from a line source is 500 mV). For a phono preamp or anything guitar-related, it's a definite deal-breaker.

I suppose that, should I have a big enough chassis and some high voltage electrolytics available, I could make a large filter to get ripple down in the sub-microvolt range so a phono preamp would be happy. To me, it seems like a better idea to choose a switching IC that doesn't present these problems.

The other part of my intent is to make a portable HF station using tubes. That is, a CW transmitter and receiver with a TRF or Regen detector (or both!). I don't want to put extra work into making a filter that adds bulk and weight. Not too good with cramming 10 pounds of electronic components in a 5 pound bag.

I would want to have dual supplies, though; one supply to make B+ (let's say 250 for this case) and another supply to make 6.3v or 12.6v for filaments. Since the battery is about 12v, what one could do is make a buck-boost supply that inverts your filament voltage, making -6.3 or -12.6v for the filament. Such voltage would be regulated well as the battery discharges, and having a negative voltage can also provide a way to bias your tubes if you need a C- supply.

Oh well. Much discussed. Some disgust. Time to hit the books again. Imma figure this out.


Author:  M. Gregg [ 10 Oct 2018, 13:31 ]
Post subject:  Re: Experiments with the MC34063 and Audio Ckts

When I started researching this,
this was a point of interest for a while, it led on to miniature guitar "valve" amps.

Then to charge pumps, and higher current.
Here is something that may give you ideas:

M. Gregg

Author:  M. Gregg [ 10 Oct 2018, 14:11 ]
Post subject:  Re: Experiments with the MC34063 and Audio Ckts

Here is more to get you thinking,

If I remember correctly there is a very interesting multiplier stage for B+.
I can't find the circuit at the moment it may be in the links.
If I find it I'll post.

M. Gregg

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