Who designed the Radford ZD22 preamp?

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Befor Richard Hay left he was working alongside Arthur Radford designing successors for ageing valve amplifiers, or so some claim. He probably left just before SS Radfords were introduced ? John Widgery after Radford continued with valve designs didn't he? Woodside??
 
Yes John Wigdery created Woodside when Radford retired but he was apprenticed at Radford when younger and is credited as being heavily involved in creating some of the early / classic designs at Radford , unfortunately i can't dig up any thing more specific.
 
Radford Revisited:

"Curiously, although Radford is well remembered, Bailey seems to have been overshadowed and largely forgotten. Yet the four HFN articles were credited to Bailey and Radford, with Bailey’s name given priority. If you read the articles the first few paragraphs make plain that Bailey’s contribution was vital in identifying the problems, and in devising the solution in the form of the pentode-triode phase splitter stage. This becomes even clearer when you read the WW article by Bailey.

A similar fate seems to have happened to another design for which Radford has remained well known. Many people recall that Radford patented a design for the ‘transmission line’ form of loudspeaker. Yet if you look at Wireless World, then again you find an article by Bailey[6] describing the loudspeaker, mentioning it is patented, etc. Bailey also developed the early Radford solid-state amplifier designs – with details appearing in Wireless World – but these failed to gain the reputations of the earlier valve designs."
 
The name of the Hedley Radford designer is Jens Langvad from Denmark. He was also the designer of the Hedley Radford LDO3 ultra low distortion generator. This oscillator had distortion below -120 dB at 1 kHz. K2 -124 dB and K3 -126 dB typically. -110 dB at 20 Hz and -100 to -106 dB at 20 kHz sample dependent. K4 and up < -135 dB down. As seen on the HP3580A spectrum analyser connected to the distortion out of the DMS3. The oscillator used the sample and hold technique to provide almost bounce free settling after changing frequency. With a Vactrol, MCD521, as the active gain control element. Sweeping the frequency dial up and down showed barely any visible change in amplitude. I met Jens Langvad and Artur Radford at the AES convention in London spring 1975.

I bought a LDO 3 and DMS 3, Distortion Measuring Set, on the spot as nothing available then had such low inherent distortion. Having used the HP334A for a couple of years with its typically 0.008 % (<-80 dB) inherent limit at 1 kHz and getting the Radford being another - 40 dB down was nice. I still have the hand drawn schematics of the LDO 3 oscillator signed by Jens Langvad. Göran Finnberg Agent for Hedley Radford Instruments in Sweden from 1975 to 1980.

Douglas Self:
>He is unknown to Google.
Not so. Using Google and the correct name Jens languid provides this:
FEEDBACK (Oct. 1987)

Göran Finnberg Sweden
 
Jens Langvad can also be found in ”Letters” Wireless World , March 1980 p. 61:

Langvad, J. ”Rumble Cancellation Filter”

Douglas Self quotes the above in his book Electronics for Vinyl, Subsonic Filtering, p. 247 while fig. 12.54 same page shows an op amp implementation of the Langvad crossfeed filter arrangement.

Self also quotes Langvad in Linear Audio Volume 11 p. 78.

The subject in those two books is the ”Devinyliser” a Self refinement of the Langvad Crossfeed rumble filtering arrangement.
 
The name of the Hedley Radford designer is Jens Langvad from Denmark. He was also the designer of the Hedley Radford LDO3 ultra low distortion generator. This oscillator had distortion below -120 dB at 1 kHz. K2 -124 dB and K3 -126 dB typically. -110 dB at 20 Hz and -100 to -106 dB at 20 kHz sample dependent. K4 and up < -135 dB down....

Hi ! i am very interested in this topic. Looking at the schematic do you think that a portion of the circuit can be used as a high performance line stage preamp ? or even just as buffer for a good quality attenuator ? I am obsessed by "minimalistic" line preamps ... for me it is the center of the audio system. The good ones are very expensive i understand.

Back on topic i have some doubts about the schematic of the zd22 preamp

radline.gif

I see 3 sections: - input amplifier with pot - phono stage - output amplifier with pot

Why not increase the gain of the phono stage and do with just: - phono stage - output amplifier with pot

Then i have other questions. If excellent THD+noise figures can be achieved with simple/basic/text book design why the need of more complex designs? and moreover ... what is the limit of performance of an already very good simple design ? maybe optimizing voltage and parts selection ?

The zd22 is an old love of mine ... and i will end very probably with using its optimized output stage (lowering its gain) as a base for a line preamp to be put just after a very nice passive attenuator (this is an open issue).

Thanks for all the information and advice. Have a nice day ...
 
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Leaving aside the phono, I see:

- input level control (dual)
- CFP emitter follower buffer
- Baxandall tone control stage terminating at gain stage with CCS giving 5x gain
- volume control (dual)
- another CFP buffer.

I guess these days we would dispense with one of the volume/level controls and use an active gain stage. But if you want a virtuoso exercise in simple discrete design, this line stage board is it. 67V rail, CFPs instead of emitter followers, CCS gain stage. Very low distortion, and one of the best sounding preamps I've ever heard.

In the examples I've seen the 120K lower arm of the voltage divider is replaced with a 6V2 Zener.
 
... if you want a virtuoso exercise in simple discrete design, this line stage board is it. 67V rail, CFPs instead of emitter followers, CCS gain stage. Very low distortion, and one of the best sounding preamps I've ever heard. In the examples I've seen the 120K lower arm of the voltage divider is replaced with a 6V2 Zener

Hi ! are you referring to a specific project ? maybe i understand wrong ... but the CFP provides gain already ... what is the CCS gain stage ? To explain i was thinking at using the output stage only for line preamp duties ... maybe optimizing it in terms of gain and distortion ... (i really do not need V gain of 6 times) Some time ago i tried to sim the output stage but i would like to select the best bjts for the task first, among those now available ... and with their models for sim ... just to play with SW before actually solder I guess modern parts should outperform the old original ones ... a friend of mine went further building a little prototype on perfored board ... he was impressed by how the sound changed with feedback resistor then he moved to tubes ...
 

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Ok ... but the preamp controls the volume ... a very delicate task For instance also on the basis of some reading the weak part of some integrated is indeed the preamp section some have a bridge that allows for the use of only the power amp section and they sound very very good Usually in the audiophile world many use tube preamps ... i really do not know why They mention better midrange and 3d soundstage with tubes
 
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To explain i was thinking at using the output stage only for line preamp duties ... maybe optimizing it in terms of gain and distortion ... (i really do not need V gain of 6 times)
So, what do you need then? (In terms of gain, noise, source impedance, load impedance and levels.) Eventually you'll start running into problems at low gain with this circuit, with noise possibly not being as low as you may like. At some point you start having to add an output pull-down resistor or current source.
Some time ago i tried to sim the output stage but i would like to select the best bjts for the task first, among those now available ... and with their models for sim ...
just to play with SW before actually solder
Good idea.
I guess modern parts should outperform the old original ones ...
As long as they aren't too busy going extinct again, that is.

A key to the good performance of these rather simple circuits is their high supply voltage. It saves you the odd current source (while still allowing high loop gain) and keeps Cob low. The downside is that some desirable parts top out at 45-50 V C-E and may require cascoding.

For some further refinements you can check out my 3 transistor design. It uses some more refined input biasing that gives better PSRR (at the cost of adding one resistor and capacitor per input), and the more sensitive and not very current-hungry input amplification circuitry got its own little RC power filter that doesn't have to feed the output buffer.

One thing I never really got worked out entirely is the matter of turn-on thump. The output necessarily has to rise to about half supply or around 24 V in this case. It works out OK if I use a 2µ2 output capacitor and 10k of pre-loading, but I'm not entirely happy with that. Perhaps going 2nd order would help, otherwise a delay / muting circuit may be required. This really is a drawback of just about any single-supply circuit if you also want some serious low-frequency extension.
 
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