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Re: [RFI] Clean Audio Amplifiers For Amateur Use?

Subject: Re: [RFI] Clean Audio Amplifiers For Amateur Use?
From: Grant Saviers <grants2@pacbell.net>
Date: Tue, 13 Oct 2020 10:07:07 -0700
List-post: <mailto:rfi@contesting.com>
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@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.

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