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Old 15th October 2008, 11:22 PM   #1
baald is offline baald  United States
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Join Date: Feb 2005
Location: cali
Default Using class D amp and x-former for HV supply

Using class D amp and tranny for HV supply

The project:
building a little amp for jazz guitar. single channel tripath output (using a single supply, stereo 41HZ module); tube preamp.

The objectives:
keep it simple; able to run off a 12VDC SLA.

The main problem:
where do i get my B+?

The Big Idea:
use the spare channel from the t-amp to amplify a signal (since wave) that can then be stepped up via a transformer to appropriate tube B+ voltage and rectified.

My back-of-napkin calculations go like this:
B+ needs to be 250VDC, so i need about 180VAC. If i use a regular 220VAC:6VAC transformer in reverse, (37:1 turns ratio, 1350:1 z ratio), i'll need to drive it at roughly 5volts. sourcing 5ma off the B+ means it should look approximately like a 37 ohm load to the amp and therefore using under a watt of power.

The questions:
(For this discussion, I'll call the channel of the amp driving the speaker ch A, and the one driving the B+ supply ch B. )

1. will a t-amp be ok driving a transformer>>rectifier>>capacitor as a load? I'm thinking it would, but i might have to add some compensation C/R/L on one or both sides of the tranny. How do I ascertain this?

2. one worry is that the signal I use on hte input of ch B could leak through to the Ch A. this got me to thinking about how i want to drive the rectifiers...

from what i know, using a higher than 60Hz sine wave should net the following improvements: easier to filter the ripple, smaller transformer can be used. so, i plan on using a higher frequency, however what are the limits on how high i go? i know that crosstalk starts increasing with higher frequency, and i also don't feel the need to get into ultrasonics, radio transmission, etc or anything. if i can look at a crosstalk graph for the amp, i should probably aim for a frequency that is in a sweetspot of the plot.

however, it seems to me that if *some* crosstalk from ch B to ch A is inevitable, what would i want that x-talk to sound like? personally i'd much rather have a bit of rush than a steady tone. which leads to my next question...

3. is it feasible to use other signals besides a sine wave to drive rectifiers for a DC power supply? specifically, bandwidth limited noise? what are the issues inherent in doing that?

4. what should I keep in mind when picking a step-up transformer for the b-channel? are there reasons why i should aim for a certain frequency band for my AC source due to suitability of certain transformer types over others?

Thanks for all responses, and please forgive any naivete or blind spots demonstrated in my questions.

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Old 16th October 2008, 01:35 AM   #2
star882 is offline star882  United States
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Join Date: Mar 2007
Much easier to just build a DC/DC converter. In fact, just buy a cheap power inverter and rewire the output of the high frequency transformer into a doubler.
"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
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