advertisement


Radford STA25 Mk III -- a lot of questions.

mandryka said:
The comparison with the Krell I can’t answer with any confidence, and you have to bear in mind that they’re in different systems (Krell/Spendor SP1, Radford/JR149) So my next comment is very provisional.

The magic of the Krell is somehow that, in some recordings, it communicates a sense of place, the atmosphere of the place. The magic of my brief fling with the Radford was that, on one recording at least (Salomon Quartet, Haydn op50) it was really exciting! Like the instruments sounded tight, firm, fast and had punch.

What I can say is that the Radford at first sounded much more coloured than I’ve ever heard the Krell being, not in a bad way but certainly at first I said to myself that now I can hear what people mean by the warmth of valve sound. Though after a short time the impact of that wore off and it felt natural.

Oh, and I’ll mention another thought that went through my head, though this is very silly probably. At one time I thought to myself that It was as if the Radford sound was alive, produced by something living, a human sound. The Krell is produced by . . . a Krell.

(I’m listening now to the 149s with the Quad 306, and it sound really good! What do I know? . . . )
Click to expand...

Whilst many months apart I've had both in my system and with Spendor speakers similar to your own and if I'd been able to keep just one of the amps it would have been a very difficult choice!! I didn't find the Radford notably coloured but it was slightly warmer sounding than the Krell. Not that the Krell was a cold sounding amp! Anyone's guess as to which was more "correct" on that score....

Both were punchy and dynamic but the Krell wins that one and ultimately wins on transparency. The Radford was very close in all this and pipped the Krell for a sense of well, it's not easy to describe... but a sense of the music existing separately from the hi fi... if that makes much sense... more like the music was just happening and less like it was being created by a hi fi.

Two of the very best amps I've heard and a tough choice! They say you should never meet your heroes, the legends, but I was very pleased that both fully met their exalted reputations!
 
G T Audio said:
Looks a nice one too... Unspoilt bar several component parts, so should sound good.
Click to expand...

That's CliveG on AoS, he spent a fair chunk getting that amp serviced and is only selling as he got a mint STA100.

I heard it with his Tannoy GRFs and was very impressed.
 
thebiglebowski said:
That's CliveG on AoS, he spent a fair chunk getting that amp serviced and is only selling as he got a mint STA100.

I heard it with his Tannoy GRFs and was very impressed.
Click to expand...

I'm sure. The STA 15 will be a much nicer sounding amp than the STA100. A good STA15 will give about 18 watts/channel. It was always a toss-up as to what was the better sound amp, the STA15 or the STA12. The Monos were always much rarer than the stereo amps...
 
Here is my own personal Radford STA15. If you have a copy of HiFi World (November 1993) then this amp is featured. I used this amp to drive a pair of 89dB Posselt Albatross loudspeakers at several HiFi shows. It always provided a lovely sound...

L1464077.jpg


L1464080.jpg
 
I'd like to try an STA100! It has an even more advanced circuit than the 25 with ECC88 in cascode as first stage and then EF184's in the phase splitter. The main issue though is that it has KT88 output valves and with 100WPC available from one pair per channel it would likely be a bit dodgy with modern KT88's...

Little is ever mentioned about the TT100.... a hybrid with KT88's in the standard valve output stage but all else solid state...
 
Arkless Electronics said:
The main issue though is that it has KT88 output valves and with 100WPC available from one pair per channel it would likely be a bit dodgy with modern KT88's...
Click to expand...

Yes, you are dead right about that! It's almost impossible today to ensure good reliability unless original GEC KT88s are used...
 
Arkless Electronics said:
I'd like to try an STA100! It has an even more advanced circuit than the 25 with ECC88 in cascode as first stage and then EF184's in the phase splitter. The main issue though is that it has KT88 output valves and with 100WPC available from one pair per channel it would likely be a bit dodgy with modern KT88's...

Little is ever mentioned about the TT100.... a hybrid with KT88's in the standard valve output stage but all else solid state...
Click to expand...

A well sorted STA100 can be very good indeed, they get a bad rep due to a few design issues in their stock configuration but once corrected work a treat. The amp referred to earlier in the thread was one I restored. The original circuit is used but uses output transformers of my own design which can be configured for 4, 8, or 16 ohms and are of wider bandwidth than the originals.

The TT100 is also a good amplifier using an unconventional feedback arrangement in which high frequency feedback is taken from the output transformer primary and low frequency feedback taken from the secondary. The feedback networks are aligned in such a way that the resulting frequency response is flat.

Here are some pics of said STA100!

frontsidecoveron2.jpg.jpg


toponbiasside2.jpg.jpg

frontcoveron2.jpg.jpg


bottom2small.jpg.jpg
 
Radford Revival said:
A well sorted STA100 can be very good indeed, they get a bad rep due to a few design issues in their stock configuration but once corrected work a treat. The amp referred to earlier in the thread was one I restored. The original circuit is used but uses output transformers of my own design which can be configured for 4, 8, or 16 ohms and are of wider bandwidth than the originals.

The TT100 is also a good amplifier using an unconventional feedback arrangement in which high frequency feedback is taken from the output transformer primary and low frequency feedback taken from the secondary. The feedback networks are aligned in such a way that the resulting frequency response is flat.

Here are some pics of said STA100!

frontsidecoveron2.jpg.jpg


toponbiasside2.jpg.jpg

frontcoveron2.jpg.jpg


bottom2small.jpg.jpg
Click to expand...

Very nice Will!

Indeed the lack of 4, 8 and 16R taps on the STA100 struck me as rather a compromise...
 
Arkless Electronics said:
Very nice Will!

Indeed the lack of 4, 8 and 16R taps on the STA100 struck me as rather a compromise...
Click to expand...

Thankyou!

It's certainly a bit of a pain because if you work out what the actual optimal loading is based on the winding sheet of the OPT design it comes out as something odd like 11 ohms. This was the main reason I went with different OPTs, just to cover all bases with speakers people actually have!

Due to the lower leakage inductance of the new OPTs there's no "sound" penalty from moving to different ones, if anything there should be an improvement.
 
Radford Revival said:
A well sorted STA100 can be very good indeed, they get a bad rep due to a few design issues in their stock configuration but once corrected work a treat. The amp referred to earlier in the thread was one I restored. The original circuit is used but uses output transformers of my own design which can be configured for 4, 8, or 16 ohms and are of wider bandwidth than the originals.

The TT100 is also a good amplifier using an unconventional feedback arrangement in which high frequency feedback is taken from the output transformer primary and low frequency feedback taken from the secondary. The feedback networks are aligned in such a way that the resulting frequency response is flat.

Here are some pics of said STA100!

frontsidecoveron2.jpg.jpg


toponbiasside2.jpg.jpg

frontcoveron2.jpg.jpg


bottom2small.jpg.jpg
Click to expand...

Yes, very nice too!
 
Radford Revival said:
Thankyou!

It's certainly a bit of a pain because if you work out what the actual optimal loading is based on the winding sheet of the OPT design it comes out as something odd like 11 ohms. This was the main reason I went with different OPTs, just to cover all bases with speakers people actually have!

Due to the lower leakage inductance of the new OPTs there's no "sound" penalty from moving to different ones, if anything there should be an improvement.
Click to expand...

My guess is that with so much power on hand they weren't that bothered about optimum matching...

Did you optimise the feedback for the three impedances?
 
Arkless Electronics said:
My guess is that with so much power on hand they weren't that bothered about optimum matching...

Did you optimise the feedback for the three impedances?
Click to expand...

The Radford STA100 was designed as a PA workhorse c/w 600 ohm balanced line inputs.

4 were used at the Covent Garden Opera House driving Acoustic Research AR LST loudspeakers.
 
Darmok said:
The Radford STA100 was designed as a PA workhorse c/w 600 ohm balanced line inputs.

4 were used at the Covent Garden Opera House driving Acoustic Research AR LST loudspeakers.
Click to expand...

You could order a version with transformer balanced inputs and it had 100V/Line output IIRC but although no doubt intended more for pro use than in home hi fi I wouldn't call it a "PA workhorse"!

Interesting about the opera house! I knew they used AR LST's as AR put it in an ad at the time but interesting that they also used Radford amps!
 
stevec67 said:
That poster is mistaken. No cap in a circuit will remain significantly charged for more than an hour or so after being unplugged.
Click to expand...

For completeness, this is not true and you can get a pretty hefty bolt from them depending on design, not all draw current when switched off - my DIY SE valve hybrids gave me a nasty bolt more that 24 hours after switching off...well, the first one did. High wattage resistor and a digital voltmeter - bleed it off until the meter is down below 50v :)

Similar to my old TV repair days - high voltage shocks are horrible at the time, but you feel great for a few hours after initial recovery, it’s like being on a high
 
Interesting, thanks Dowser. I didn't realise that valve designs can hang on to charge like that, just like EHT tubes.

Probably just as well that I stick to SS designs. PSU electrolytics are all done in an hour.
 
stevec67 said:
Interesting, thanks Dowser. I didn't realise that valve designs can hang on to charge like that, just like EHT tubes.

Probably just as well that I stick to SS designs. PSU electrolytics are all done in an hour.
Click to expand...

Any electrolytic capacitor will hold on to its charge. It doesn't matter whether it's used in a valve amplifier or a solid-state amplifier. The only thing is a valve amplifier generally runs at a higher voltage, so the holding charge can be higher. However, if you discharge an electrolytic capacitor to zero volts and then monitor its 2 terminals you will see a charge start to appear and rise across the terminals. This will rise to a certain level. If you have a big solid-state amplifier (say 200watts) the power supply capacitors will be very large and these will hold a very large charge, which will still be capable of harming someone if contact was made with the terminals. Obviously good design and the fitment of bleed capacitors across all large electrolytic capacitors in the power supply will reduce any charge to zero over a set period of time, so this should not be an issue, whether it's a valve amplifier or solid-state amplifier.
 
Radford Revival said:
Indeed - the amp was set for 8 ohms and feedback network tweaked accordingly.
Click to expand...

So it doesn't have an individual feedback network for each impedance, switched with the speaker tap, as the STA25 has? Does it configure the secondary windings to use all of them simultaneously as the STA25 does?
 
You must log in or register to reply here.


advertisement


Back
Top