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Technical Topics => The RF Workbench => Topic started by: netsmo62 on January 01, 2017, 2339 UTC
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Hi all, HI, I have made a Corsair II transmitter for MW operation (see pic attached) with a VFO DDS variant and power out is no problem, but modulation is always dirty and distorted at every input AF level, expecially when its transmitting music. Problem I think is in the LM386-TIP31C-BS170 section, because I have tried back xtal oscillator instead VFO DDS drive with always same result: power strenght is ok but modulation is distorted and coarse. I have tried to modify LM386 gain and/or to indrease R3 to 1,2 Kohm with no appreciated results. Someone that tried to built same transmitter could you give me some clue?
Have a good 2017. F.
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I am really rusty on this stuff, but it looks like RV2 sets the bias on Q3. Have you tried adjusting it? The schematic shows it should be adjusted for 1.7 to 2.5 VDC at TP.
The next thing I would try is disabling Q3 (remove the 12 V) and listening to the signal without the benefit of the final amplifier.
Again, really rusty on this stuff, hopefully someone more knowledgeable will comment.
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A Scope would help?.
Not a good design however.
Str.
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Same here. I tried several Corsair schematics and all three had by problems usually terrible mud audio or extremely low level. I futzed with every flavor of 386 replaced each individual component I couldn't test the value on.....Stick to the grenade for a tried and true works first time box.
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Thank you Moof, so you confirm to me that anyone of three Corsair version appearing on Dave Martin's blog is really good working in modulation stage, neither that one use a transistor driver instead latest version with BS170 mosfet. Yes I have tried to change BIAS settings in all way possible, but only with voltage set very very low, modulation quality seems acceptable, so I have tried too to isolate AF LM386 stage before and LM 386+TIP31C stage from rest of transmitter and AF tone until there is regular. Well, what can I say? I will tried "last resort" to consult some experienced HAM in my neighborhood but better will be soon that faulty schemes disappear, 'cause we don't need garbage not really well tested and working that makes you waste time and money.
Thank you.
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I was member of the homebrew group that the Corsair initially sprang from. I recall that during the tweaks on the design, the LM-386 was dropped in favor of another device. As my email box for the group was eaten in the the Great Yahoo Security Breaches, I no longer have access to that info at hand or can remember why the LM-386 was dropped.
There are more than few guys on this board who were members of the group. Perhaps they'll pop up with the info you need?
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I was member of the homebrew group that the Corsair initially sprang from. I recall that during the tweaks on the design, the LM-386 was dropped in favor of another device. As my email box for the group was eaten in the the Great Yahoo Security Breaches, I no longer have access to that info at hand or can remember why the LM-386 was dropped.
There are more than few guys on this board who were members of the group. Perhaps they'll pop up with the info you need?
You can probably get in contact with Dave Martin via the Gary Stevens Pirate Radio Forum. He is on there from time to time
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Ok, I have tweakening schematic doing many many tests and finally I think to have found what was wrong. In schematic AF output of LM386+TIP31C pass through a 17 uH inductance (L2 in schematic) in parallel with 560 Ohm resistor (R5) to BS170 drain. I have eliminate the resistor and substitute the 17 uH 1/2W "resistor-like" inductance I had placed before with a red toroid (T50/2) with 59 turns of 0.3 mm (AWG 28) copper wire.
But the real problem to solve was to separate input AF to LM386 input, so I have added a 1:1 600 ohm audio transformer, with one side directly connected to one RCA input, isolated from ground and other side connect to input of transmitter.
Now modulation is good (see short video linked) normal BIAS regulation without rustles and whistles, IRF530N power mosfet runs from warm to moderately hot depends from BIAS voltage, reach nominal 10W output, 1-1,3 A absorption.
https://youtu.be/DN_Q6t4k530
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I love the recycled PSU heatsink ;D
I still prefer high level modulation of the final, but I'm glad you got it running. Someday when I tire of high power (not real likely) I'll start building rigs like this just for fun.
+-RH
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Oh, this was an old prototype when I was thinking final Mosfet become hot like hell and in the back of heatsink put a PC fan ;D Now is a simple heatsink in the back of metallic box...
I think that problem in the end is the impedance mismatch from AF input through LM386+TIP31C to intermediate Mosfet BS170 and then to final mosfet. When I was testing, with an higher impedance signal injector square wave input signal (without transformer) audio was clear and no distorted. Could be instead use a 1:1 audio transformer same result can be achieved with a little one-transistor preamplifier in front of LM386 input...
Ciao F.
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Hi there!
But the real problem to solve was to separate input AF to LM386 input, so I have added a 1:1 600 ohm audio transformer, with one side directly connected to one RCA input, isolated from ground and other side connect to input of transmitter.
Now modulation is good (see short video linked) normal BIAS regulation without rustles and whistles, IRF530N power mosfet runs from warm to moderately hot depends from BIAS voltage, reach nominal 10W output, 1-1,3 A absorption.
Late me is late. Good you found a workaround. Feels god to get stuff done, hm? :)
Anyway, may I add my opinion on that? It may be a bit tech heavy, so feel free to ask if I'm confusing you
I've to agree, that the modulation circuit around the LM386 is prone to cause issues. With a distorted modulation I assume that the amplifier goes into clipping (is overdriven). The design is kinda ugly and one would be better off with a circuit that feeds the signal from the Emitter back to the OpAmp/Driver input. Along with a potentiometer to adjust the gain (and modulation grade). Makes it easy to run with input levels from - say - 100mVss to 800mVss.
I've got two theories why the transformer solved the issue:
- 1) You have a ground loop problem. You can check this by feeding an Audio signal to the input (without transformer) by using a entirly battery driven device (that has no other connections than the one to the transmitter).
- 2) The input signal is too high and overdrives the amp. The transformer solves it because it's input of 600R is too low for a regular audio source and causes the signal to drop on a way smaler level (which doesn't overdrive the Amp).
Kind regards,
Zazzle.
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Interesting that ground loop question. I will try battery operated, however (I don't know if it count) audio source come all from battery operated device (CD Player, Smartphone MP3 player).
Thanks, F.
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Hmmm... CD players are notorious for having problems with RF, I've had them go "phht" using quality insulated audio cable wrapped around a ferrite. The things innards just aren't robust enough to handle the RF field around the transmitter, IMO. And I'm talking about frying them exclusively with a transmitter that outputted around 14 W of carrier. Not much power at all. If they'll fry at 14 W, they'll fry at 10. Not enough difference to matter.
If you can still find audio cassettes to record on, nothing beats a simple cassette deck. I've never had a problem with RF at these power levels with an MP3 player either.
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Hey,
(...) IMO. And I'm talking about frying them exclusively with a transmitter that outputted around 14 W of carrier. (...)
Actually, yes. Very possible. I haven't thought about that because, on my wordesk, I always use a dummy loads and short wires. So no high levels of RF escape. But yes, now that I remember the Video with "a lot of open wires"...
Happened to me outdoors when testing the 30W CW-Beacon. I was using a Laboratory Power Supply. The distance between Antenna and PSU was about 5M. But depending on where I stood the µC in PSU went mad and locked up in emergency shutdown mode.
Kind greetings,
Zazzle
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Believe me Zazzle, it's no fun to go out in prime transmitting season, this time of year, 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.
You don't need a radio to transmit then. People can hear you cussing for kilometers as you pack up and tromp out in the dark, hoping your flashlight doesn't die.
The things I used to go through to put on shows for this bunch.
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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.
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Walking up on bears coming the other way, but that's another story.
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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.
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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
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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 :)
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Perhaps a picture or two showing this marvellous design in the flesh!
Perhaps not . ...
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A Scope would help?.
It's pretty infrequent but I actually find myself agreeing with Stretchy's comment from years ago. Miracles do happen.
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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.
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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
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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".
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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
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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.
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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.
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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
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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.
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4W is with 14V applied to the PA. Not really class E, as all I did was put a resonant tank in the drain of the fet tuned to the carrier frequency. More like class C operation, although I did get north of 80% efficiency out of it.
I'll start a new thread for it when the video is posted.
+-RH
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Just depends on how you drive it, squarewave = class E, sine =C, sure there's more to it as the o/p has to be bandpass but still the quicker the switching the greater the eff% pure and simples.
Str.
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It's not the best design and have no idea why it gets so much attention.
The reason why it gets some much attention is that many have been built and used in the past. I've found out so much about the designs asking the right people. The parts are easy to get hold of and if your into home construction it can cost under 20 UK pounds to build depending on the version.
I've built Corsette (LM386), Corsair (LM386) and Commando (Transformer) that's on Dave's website and in my opinion they all work well.
Never used these class D ampilifiers but now I've got a few builds under my belt this will be my next project.
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So a design that uses less parts (still commonly available) and is more efficient etc gets less attention? Doesn't make sense, apart from the fact it's got some kind of web/utoob kudos. I guess it's the internet generation....
There's so many designs for stuff like this around and 99% would never work without decent layout etc and was the only reason I ever designed a PCB
The class D mod amps are great, bought 100 a few months ago direct. Shame they no longer have the space for the regulator and the front end has a really dumb response, most strange, easily sorted with a few simple component changes.
Details are available..
Str.
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So a design that uses less parts (still commonly available) and is more efficient etc gets less attention? Doesn't make sense, apart from the fact it's got some kind of web/utoob kudos. I guess it's the internet generation....
Str,
I'm not saying it should or will get less attention. For me it was, I've had prebuilt Corsair from Dave back in 2007 and it worked I stupidly sold it. I wanted another one but this time I had to build it, pretty much had all the components for this design in my junk box and yes details of the Corsair are on the net with many people building them and had some excellent support from a HFU member.
Been guided towards the LuLu design by the person that helped me and it will be my next project. If you are willing to provide support I will value you experience and knowledge :)
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With any decent design using a PCB is by far the best method and sure they used to be expensive but now not so. Shame as I've just given away 20+ LuLu PCBs to a UK chap and don't have any left.
Also the main 'problem' is to how you're going to generate the frequency you want in the first place.
So may I ask, what method do you intend to use?
Str.
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Just check "Simplest Tx" post on here.
To modulate just use the class D amps mentioned.
Super simple!
Str.
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Just depends on how you drive it, squarewave = class E, sine =C, sure there's more to it as the o/p has to be bandpass but still the quicker the switching the greater the eff% pure and simples.
Str.
Umm yeah, there is more to it.
Like, amplifier class is not defined by how it is driven. It is defined by the shape of the drain/collector waveforms.
Also sinewave drive can also be Class A or B, in addition to C, it depends upon the associated drain waveform. Squarewave drive can associated with class D,E,F, S or J.
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The reason why it gets some much attention is that many have been built and used in the past. I've found out so much about the designs asking the right people. The parts are easy to get hold of and if your into home construction it can cost under 20 UK pounds to build depending on the version.
I've built Corsette (LM386), Corsair (LM386) and Commando (Transformer) that's on Dave's website and in my opinion they all work well.
Shhh. Quiet. You're destroying Stretchy's bullshit narrative that everyone MUST.THINK.LIKE.HE.DOES and no one could be happy and content without doing what he wants. There's probably been more Corsette, Corsairs (I&II) and Commandos built than he has ever sold and that gets under his skin.
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How wrong you are...
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How wrong you are...
Well, you do try to lord all over a lot of people here so...
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This latest exchange is a good reason that all companies should have a public relations department. PR serves up the platitudes, and it's fake sincerity, but those words, saying nothing, seem to soothe people.
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the LM 386 runs hot because of the 12 volts you need to set up a 8 volt regulator circuit for the LM386
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(https://www.linkpicture.com/q/crystal-controlled-am-transmitter-with-100-modulation-v0-jqalvyrwig491.jpg)
This is another solution who wish to do it the old fashioned way. It outputs 40mW of carrier with 160mW PEP. It can drive an IRF510 for 1W out with 4W peaks at 100% modulation. Audio sounds great. Remove the mic and feed audio directly to pin 3 of lm386 via 100n capacitor or C8. A variable 10K pot fed by a 10u capacitor can also be added to pin 3 to set the modulation depth.
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Add another linear push-pull power amp for 15W carrier and 60W PEP.
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Yeh and with 60W you can drive one of those valve things to generate KW's.
Unbelievable this design being published in 2012!
Str.
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Yeh and with 60W you can drive one of those valve things to generate KW's.
Unbelievable this design being published in 2012!
Str.
Settle down, it's 2023. I bet they've got one that will do 65 watts by now? And it's "yeah" not "yeh". "Yeh" is a noise you make when you're preparing to throw up, followed by "yeeackkk!"
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Nice one Pigster, on form as ever!
Trust you're super fine, all good here.
Whatever you do don't ever mention 6G, a particular non favourite at the moment.
😁
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Unbelievable this design being published in 2012!
Str.
Well my final word on this matter! The reason it's still being published is the fact of being a learning aid (well for me). Yes it's old design but a circuit diagram that was shared and with many people who also tried to build them (some successful, some not) and people shared their experiences with it.
Having never really built anything like this before, it started with a circuit diagram that was published and some people comfirming that it worked and produced a signal that could be used and unlike some circuits I've spent many hours on that were found on the internet, once I had built never worked and were never going to due to design.
I was given details on a easy layout to follow when building it on a copper board that allowed me to change out components to experiment with. I gained a good understanding of the stages and picked up knowledge by speaking to others on many things like, test point to check when I found things weren't working or swapping out components to see if things improved or got worse.
Once I had "what I" believe is a working copper board layout I wanted to see if it was possible to make a PCB version. I have never used PCB software like this or had any prevous experiences so found it a challenge. I was able to produce a PCB with the help of someone and a few boards was made. Yes, at first it wasn't perfect but networking with others and them giving me their opinions and suggestions to try different things to make it sound better. These things made small improvements but again these things didn't fully solve the some problems but after walking away from it for abit and a clearing my head I was able to spot something after looking at all the different variations of circuit designs that somethings were missing.
This latest PCB version I ordered to get made "to me" sounds alot better and produces a good output. No its can't be compared to a commerical broadcast transmitter but for the cost and through hole components that many people still enjoy to use and build with like me works fine. I'm not kitted out for surface mounted tech yet and yes it's the future but how many hobbist start out with SMD off the bat!
The whole time I've been mucking around with the Corsair it has taught me alot about circuits, fault finding, testing, PCB design to getting it manufactured and whats involved. I now appriecate the research & development that goes into projects and I have met some decent people along the way too. It's given me confidence to keep trying even when things fail.
Oh if your wondering here's the image of the Corsair and audio limiter/compressor PCB's. It will never get a true test as I don't have the equipment to carry that out but wanted to state what someone can achieve having very limited knowledge!
https://ibb.co/mtGDnnW (https://ibb.co/mtGDnnW)
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My only point being...
While it's 2012 why not publish a design that is up to (that) date?
Modern designs are usually far simpler and efficient especially in the world of electronic engineering. There's no room for nostalgia using old designs that weren't very good in the first place.
The world of engineering always moves forward, well it does for some and possibly not for others.
Engineering is my, hobby, job, trade and has been for over 40 years. I wouldn't be true to myself unless I tried to use the best design and components available whilst keeping costs reasonable and the size compact.
I think with most things I've made I've achieved that and will continue to do so.
I'm afraid things like through hole components just aren't used anymore in manufacturing. SMT is a piece of cake and Yes, I'm old, probably older than most and just use a magnifier.
Sure if your building one or two it doesn't really matter on the technology however when you build 100+ SMT is the only choice.
I guess the debate will always go on and on.
My fave being the ham chap who seem to be on a mission on my YouTube channel, most amusing..
I don't think I've done anything special but at least I've done something.
All GaN All SMT coming next 100W carrier wlth integrated modulator on a PCB the same size as the one you've shown.
There's loads of really good devices becoming available so no need to use valves, 11N90s or IRF510s anymore!!
Str.
https://imgur.com/a/4XvmiCv