4W AF Amplifier

Hi Lionel,
FAR will withdraw my project from their site. I have no objection to people using my projects for non-profit and for knowledge sharing.
If anyone wants to make money from any of my work then they should have an agreement with me. The "usual" agreement is perhaps 1$ commission per board sold, and I will also give support for the customers. In this case, Fred just copied and sold without asking.

Over the years I have employed lawyers in the USA if a companies uses my designs for profit. They usually settle out of court with just one "official" letter. I have earned a few 100 dollars on that over the years ;-)

I have been thinking about starting my own PCB service for my own projects. I used to do that, but after the first house moving I suddenly had no time. But I believe that will change in the next few weeks. But then again, pigs may fly   We could have peace on Earth   I could loose some weight    Anything is possible.

As regards the 4-Watt amplifier, it was designed to be as simple as possible and easy to build. The standing current in the output devices is very small but the use of negative feedback to control the gain cleans up any crossover distortion.

It worked really well as a modulator for AM. I only need to add a bit of capacitance across the output.

I think it is about time to add the Zobel network to that project and add a couple of comments/mods about HF decoupling and stability. If you have had problems then you will not be the last.

Really interesting to read your reply and thank you for your comments.

Very best regards from Harry - SM0VPO

You have some interesting applications, especially VLF, though too bad about the Klipsch; been there.   FAR has been around for a long time and having a PCB made chasing the parasitics less confusing.  I hope you and FAR work it out.

A discussion of your amp is on the Drake group.io site and that is what spurred me on to build one.   I built three because of price and shipping on the PCB's.  The R4C audio design, class A. is panned by many as inefficient, noisy, and unnecessarily complex. Sherwood has a replacement amp but I will use your design. 

I matched the 2 557's and 547's as well as the TIP41.  I used TIP41C's from Mouser. In reality,  all devices beta matched with 5% and all 3 amps perform the same so matching was an academic exercise.  The TIP41C's have the same spec as TIP41's except for max collector voltage.  The DC voltages are as expected. It comes down to the speaker.

The MS4 coil measures 38uH using a Chinese inductance meter ar 500khz.  Measuring the phase difference in uS on a Siglent 1202 gave me:

Z(1000) = 3.9 +j0.9 (149uH)   Z(2000) = 4.5 + j1,7 (132uH) at Z(10000) = 9.5 + j5.6 (89uH)

Other than the R4C I will use the other two for a homebrew set of powered speakers that could be better than any reasonably priced commercial variety.

I didn't check above 100kHz but perhaps an audio LP filter at 20kHz or so may make the speaker irrelevant. I did the calcs for a Zobel network though it seems to me these are speaker characteristic dependent. 

I'm enjoying working with these.   Neat design. 

tnx

Lionel, N5LB

Hello Lionel,

Your post was extremely interesting. Thank you for the tip about FAR PCBs. To be honest, I did not know that they were selling stuff that I had designed. I have a policy of giving away my designs for non-profit. If someone wishes to use them for commercial gain then they should have an agreement with me.

But as regards the AF amplifier. I have built that many times for different purposes, but never had any RF instability. I have used it as the final stage of a VLF transmitter. In view of the stability I decided not to bother with the Zobel network, I though with just 4 Watts I would get away with it. But a lot depends on the speaker. They all have very different properties when it comes to inductance.

I DID have a problem using it with a modified 230 V to 12-6-0 V transformer. I took out the E and I laminations and re-assembled without interleving them and with a piece of 0.1mm thick paper between them, so the transformer could also carry DC without saturating. I had to use a Zobel network and reverse the polarity of the 2700uf output capacitor with that project.

In the article I wrote "There was no trace of instability throughout the AF range and up to 150KHz so I thought it unncecssary to include a Zobel network."

The project was a 10-Watt MW / SW Amplitude Modulation transmitter using a modulation transformer, the old-fashioned way.

I thought the parasitic instability was due to the presence of a lot of RF in the same tin box. Perhaps I should do some tests in that design as there is is no HF decoupling at all. That was deliberate so I could also use it for frequencies over 40kHz, up to about 150kHz. It DOES go a lot higher

I first used the manual "Dalo" PCB version (2 off) with a pair of Klipsch speakers Dual bass/midrange, HF unit and 15" passive bass reflex unit at the rear. In my little room the two 4-Watt amps gave me more volume than I needed. I lost the speakers in a nasty divorce   One of my biggest regrets.

But thank you very much for your feedback, and it is very nice to see you posting so soon after registering. It also shows to me that one annoying bug in the forum configuration has been cured - new uses had to wait a week before positing a new topic. But thanks to Ivan I got that sorted out.

Very best regards from Harry Lythall - SM0VPO

I've built 3 using the FAR circuits PCB. Two have the gain set at 48 and measured at 40-45 into a 4W of 3 ohm carbon resistor. The third one's gain is set at 11 and measures the same.  In all three units, no parasitics appear.  On scope, the sine wave output is clean all the way to 100kHz.  Power out is near 4W before clipping.

This is the schematic.  The FAR CB matches the schematic.

When connected to a Drake MS4 speaker (3 ohms) distortion and parasitics (well into Mhz) appear.  Before using a Zobel network I shunted the amp output with a 10-ohm 2W carbon and all became clean, output power to the speaker is down but more than adequate.

Bench power is an  Astron 8A 13.6V (V measured at amp) PS.

So, they are working but the oscillations without the shunt when connected to the MS4 is unexpected. 

Has anyone noted this using an MS4 or any other speaker? I also recall seeing a post somewhere with a mod to Harry's original circuit but can't find it again, if it exists.

BTW I built three for fun and maybe will use one in my R4C at some point.

tnx