[RFI] Clean Audio Amplifiers For Amateur Use?

[Grant Saviers]

I think most stereo/5/7/9 channel "receivers" are transformer power 
supplies and AB designs. At least the couple dozen I've owned.  Maybe 
the latest $100 ones at Best Buy are now SMPS.  If the power label says 
100 to 240v or the box is too light, or the distortion isn't sub 0.1% 
then it might be SMPS or class D etc.  The higher end amps usually have 
toroidal transformers.

I think the most common audio RFI problem is pickup by the speaker leads 
and rectification in the output transistors. CM chokes at the amp output 
make a big difference. Then CM chokes on power and low level RCA signal 
leads (good cables help a lot).  Balanced shielded low level XLR 
connections usually make small signal RFI not a concern on high end 
equipment.

There have been a few class A power amps, one was the Perreaux PMF 3150 
from ZL, I could fry an egg on mine @ 150w/ch.  Awesome sound tho.  Now 
on ebay for more than I paid in 1994.  I see they are back in business 
with A/AB MOSFET amps. Other rare bird class A amps on ebay as well.

I haven't seen an "SMPS" in mid/high end other than in the rare Bob 
Carver designs (Carver & Sunfire) which have a signal following power 
rail. Awesome 800w/ch.

The only in production transformer coupled semiconductor amps I know of 
are from fabled McIntosh, the ultimate wizards of transformer design. $$$$$

Most all of the car amps > 20w are switched mode power and class D. 
Awful sound.

Ebay and the local thrift stores are good places to look for older 
receivers w/o the 4k and HDMI bells and whistles and almost always AB 
plus linear power supplies.  At lower power levels (<50w) a complete AB 
amplifier on a single chip. (TI, ST)

Grant KZ1W


On 10/13/2020 09:02, Rob Atkinson wrote:
> The problem with solid state is that if the RF field is strong enough,
> a cheap analog solid state amp will pick up enough RF on the internal
> leads and the transistors will rectify it.  This is especially a
> problem with cheap plastic enclosures.  Tubes are less vulnerable to
> this even if run unshielded.  As someone else pointed out, tube amps
> have output transformers that isolate the speaker leads.   The problem
> is solved whether the problem is the amp generating RFI or being
> vulnerable to it.  You might not get it all, but bypass caps on the
> audio inputs will usually be the death blow.
> 
> 73
> 
> Rob
> K5UJ
> 
> On Tue, Oct 13, 2020 at 8:25 AM Hare, Ed W1RFI <w1rfi at arrl.org> wrote:
>>
>> Many of the old solid-state amplifiers also have analog supplies.  The key is -- weight. If it's got iron, it's analog.
>>
> _______________________________________________
> RFI mailing list
> RFI at contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
>