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Amazingly low distortion -- Radford-based design


diyAudio Moderator Emeritus
2004-01-27 8:55 am
There have been a few threads on Radford's unique design but not much about what the Radford amps actually sound like.

I've been playing with my own version with LTspice, using 6SL7 for input, 6AU6+6SL7 as LTP splitter with cascoded BJTs CCS in the tail, 6SN cf drivers and EL34s in full pentode AB1 PP outputs. GNFB is present at about 20dB, with the usual corrections for LF and HF peaking.

This is, admittedly, a long way from what Radford produced, but it seems to work really well and the FFT analysis shows extremely low distortion.

I realize that a sim doesn't really provide a definitive answer on distortion and there's nothing like the real thing, so I was wondering if anyone has heard a real Radford style amp? Are there any pitfalls that my sim doesn't reveal?


2008-11-08 8:25 pm
Devon UK
Many of the performance gains that Arthur Radford made with the STA 2 & 3 were due to the transformer design. How does your sim emulate the output transformer?
I own a Radford STA15 Mk3 which was upgraded with Mk4 PCB's (more or less just adds some more bi-pass caps)
It sat in my front room as my Hi-Fi amp for over 25 years, and kept me and everone else satisified with its performance. Not many amplifiers can do that! ;)


diyAudio Moderator Emeritus
2004-01-27 8:55 am
My sim doesn't really address the OPT's real performance but the model I use is the same one I've used in many other simulations, including Mullard, Williamson and all-differential designs. I'm sure it doesn't proprly reflect reality in all respects, but it does show a very marked difference between distortion (and band-width) of the Radford and the others.
The Radford STA100 (100W) was not one of my workhorse amps; although I used this in the early 1970's with trumpet. It certainly delivered the goods but tube life was attrocious. (fixed bias B+ 600V + at 60mA quies); a stage retrofit with fresh tubes without all the gear was out of the question. A Linear Concord 50W (which was really a beefed up Mullard 20W)was one that was reliable but at slightly lower power.

For hi-fi it the ST100 exhibited a bitter sound..there was something wrong about it....even though thd was quite low..The low thd coupled with poor quality sound is a contradictory mix....neverless it has been proved that amps with less global feedback systems do sound more relaxed. This amp had 36dB and was unconditionally stable.

It was one of the very few amps that used only 3 stages to get 100W output.

Years later doing a bit of research, I discovered the principle fault of poor quality lay in the LTP phasesplitter using EF184's, so during mid 1980's I changed this into a 4 stage Williamson design which greatly improved the sound. The Radford STA25 amp series were very much hi-fi amps, using 6550's, EL34's & KT series. In the 1960's there were alot of vendor amps about in this power class. The STA100 was competitor against newly arriving power solid state stuff which on sheer weight couldn't compete against. Generally the output transformers were excellent quality and haven't come across any around.