Someone had Corsair II AM TX experience?

[Zazzle]

Hi,

(...) walk in to your tx site in temps well below freezing, be tuned up and ready to go and have one of the things fry on you.

Aren't we here for the adventure? At least a little bit? :)
And well, what pisses us off today becomes a funny story to share in a few years.

But yeah, I feel with you. I had it in Summer 2016. Just the other way around. I was installing the setup for the 30W Beacon. In a night with 36°C. On a black tar roof - all hot from the day - doing acrobatics. Eventually, after hours of sweating, everything was in place. I plugged the PSU in and.... dead. It had been working 5 minutes before. I literally kicked that damn Toshiba Notebook PSU from the roof and half across the complex yard.

I guess it's what brings the fun. Having a small adeventure during night hours. Collecting memories. :)

Kind greetings,
Zazzle.

[Pigmeat]

Walking up on bears coming the other way, but that's another story.

[netsmo62]

In the end altough I have made so much tweakening and modification, modulation quality was not better than average-mediocre. So I have decided to change completely the modulation type from "low level" to a more classic "high level" directly on the final mosfet, using a simple serial modulator built between a little SMD TDA2030 AF Module (you find very easily on Ebay or Ali or Bang for pennies, like this: http://www.ebay.it/itm/TDA2030A-Audio-Amplificatore-18W-6-12V-Amplifier-Arduino-Module-Board-/252397102253?hash=item3ac40a24ad:g:9lMAAOSwq5pXP19y) and a good 2N3055 TO-3 transistor with a radiator. This was taken out from the open source book "Low Power AM Handbook". This design involving you raise the power supply for TDA2030 AF module, 2N3055 transistor and RF final IRF530N mosfet to 24V and almost 3A, so you need to put a 7812 linear regulator (with radiator too) to supply DDS module, BF245 and BS170 stage. Now modulation is very clear and somewhat more RF power out! To adapt a normal potentiometer to AF module I have literally cut out its 10K trimmer leaving some mm of old connectors to solder up new connection for the new 47 Kohm linear panel potentiometer.

Hi F.

[radiorob]

i know it is an out topic yes,
I'll tell you anyway I will try this too!
if only looking for fun with a TDA2030 and also with the TDA2050.
I'm going to try it on SW and on MW
I accidentally got some Darlington transistors RCA9228D to see what you can bake with that!
 
greetings from the Netherlands!
RadioRob

[Intruder]

Sorry to resurrect a old thread!

A few years ago I had a Dave Martin built Corsair that I purchased off someone from Facebook and remember it having poor audio, I don't know if the guy I bought it from had played around with the insides but it sounded distorted on air so I sold it on!

Recently I came across some pictures I took of the insides of the one a sold and seeing a post on another forum of someone who recently built one I decided to order the bits and reproduce my own.

I built the experimental gate modulated version to roughly the same size and layout as Dave's versions with images I had and found on the other forum. This did have the same issue with distorted audio and they only way I could improve it slighty was putting a 10nf capacitor across the 1uf capacitor but wasn't great.

Today I finished a 2nd version of the experimental gate modulated version but the layout is completely different and I can say it noticeably better with no distortion to the ear that I hear.

So can confirm the experimental version works ok 

[Stretchyman]

Perhaps a picture or two showing this marvellous design in the flesh!

Perhaps not . ...

[Charlie_Dont_Surf]

A Scope would help?.

It's pretty infrequent but I actually find myself agreeing with Stretchy's comment from years ago. Miracles do happen.

[Charlie_Dont_Surf]

So I have decided to change completely the modulation type from "low level" to a more classic "high level" directly on the final mosfet, using a simple serial modulator

This is a positive step.

I may have missed it in this thread but I don't think we ever got a clear definition of what was termed "distorted audio". ("Distorted" is somewhat imprecise because it doesn't relate much about the nature of the problem.) This is why an oscilloscope image would have been very helpful.

In any case, one of the issues with the original design is that the modulated audio was probably getting into the oscillator and "pulling it". Modulated audio was driving the oscillator buffer (as the low-level modulator) and that buffer is - like it or not - part of the oscillator circuit. Its parasitics will affect the oscillator stage before it, changing the oscillator frequency, and the amount of oscillator shift will vary with the modulation. This would produce frequency or phase modulation along with the amplitude modulation and result in distorted audio. (I can't be sure that this is what is happening because I never saw oscilloscope images or heard recorded audio, but it is a safe assumption given the schematic.)

There are ways to fix this and my favorite way is to add an additional layer of oscillator buffering to better isolate the oscillator from the modulation. In effect this is what you have done by changing to a high-level modulator. That is, by modulating the final amplifier directly.

Good work. Congratulations.

[redhat]

You could also replace the series modulator with a $11 class d audio amp board from amazon.  Efficiency would be MUCH higher, and it makes more sense to go this route since you already have a 24V supply at your disposal.

+-RH

[Charlie_Dont_Surf]

You could also replace the series modulator with a $11 class d audio amp board from amazon.  Efficiency would be MUCH higher, and it makes more sense to go this route since you already have a 24V supply at your disposal.

+-RH

Not disagreeing with you (you're absolutely right about the efficiency) but some day I should write up my experiences trying to use a cheap Class-D amp as a series modulator, requiring the cheap amp to supply all the bias for the RF stage.

The cheap class-D amps that I am aware of don't like to drive loads below 1-2 Ohms, which is exactly what you would be asking it to do by becoming a series modulator. It gets "unhappy" when you do this and I damaged more than one by asking it to deliver x Amps continuously as DC bias for the RF final, in addition to modulated bias. Sometimes the damage was not catastrophic, sometimes it was catastrophic. (It's a good thing that the amplifiers are "cheap".) My proposed solution (which I never got around to trying) is to add 2 Ohms in series between the class-D amp and the RF amp. That way the Class-D amp is never seeing less than 2 Ohms and is less likely to be "unhappy". Of course, this is wasteful and might eliminate the efficiency gains you made by using a Class-D amp. 

Well, I guess I have now written up my experiences on this, so today is "some day".

[redhat]

I've used this method with several different PA's over the last few years and have not noted any failures or issues.  Load impedance in my case was around 9 ohms, so no problem there.  Not sure what happened in your case.

+-RH

[Charlie_Dont_Surf]

Sorry, I was confusing my memory of this with something else when I wrote the above and in my case the load was probably around 3-4 Ohms but the peaks probably presented the difficulty for my Class-D amp. (My guess.) Either that or the amplifier was not what the manufacturer claimed.

In your case, 12 V/9 Ohms = 1.33 A. (12 Volts ^2)/9 Ohms = 16 Watts (average). (24 V ^2)/9 Ohms = 64 Watts (peak).
Your power was/is probably close to that of the Corsair.

[Stretchyman]

It's not the best design and have no idea why it gets so much attention.

The LuLu PA stage is class E and this is AB and is just going to generate more heat and be less efficient. It's also way more components and you've got to set up the bias correctly etc.

Very old design...

Class E is hardly new either and has been around since 1977, best to use it as it's by far the best method for simple narrow band transmitter stages.

Str.

[redhat]

I will be posting a YT video soon outlining a single device low parts count 4W transmitter driven by one of these class D amps.

+-RH

[Stretchyman]

Run from 12/14V 4W us about right with 6V on the  RF stage at carrier. Good if you could show the drain and gate waveforms. Folk moan about class E as you really need a scope to see the drain waveform but in reality you can tune it with your finger as when it's cool it's working the best!

I've published the values for normal class E operation on here in an older post so very simple to replicate.

Str.