An excellent power amp from france from the 70's

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.

taj

diyAudio Member
Joined 2005
Two things strike me about the Radford amp:

The documentation is incredibly detailed. That speaks volumes about the company behind the product. I would be proud to own one of these.

Also, the asymmetrical internal layout of the chassis components is obviously focused on technical ideals rather than visually pleasing symmetry. This is something us amp builders could take lessons from.

..Todd
 
Hi,
The purpose of Marcel Vaissaire, the designer and owner of Audiotec, was to get linearity by using two stages, each with a lot of negative feedback.

The first one gives a lot of voltage gain at low impedance to feed the inverting input of the second stage.

The second stage delivers the power to the output and has a voltage gain of about 2 only.

Note that R14 / 100 kOhm provides mainly a DC negative feedback to T6 (input of the second stage) while R35 / 3.3 kOhm provides the main AC feedback and encompasses the output capacitor C15.

About the same configuration appears in the first stage around R11, R12 and C6.

More documentation (in french) about Audiotec, with some schematics :

http://haute.fidelite.com.online.fr/audiotec/page1.html
 
Gaetan,
thanks alot for raising awareness on these old amps.
The radford hd250 was the first amp I ever built. Still have the copies of the service manual you posted but barely holding together. I remember the nights spent with transfer strips to make pcbs. I built the whole thing as an integrated amp. The sound was great with a garrard 301 that a friend sold me for 20 bucks...little did he know.

The JBL T as in triple emitter transfer is something I would actually love to build. Is the schematic 100% correct? Do you have any idea of what the PS looks like?

All the Best,
p.
 
grataku said:
Gaetan,
thanks alot for raising awareness on these old amps.
The radford hd250 was the first amp I ever built. Still have the copies of the service manual you posted but barely holding together. I remember the nights spent with transfer strips to make pcbs. I built the whole thing as an integrated amp. The sound was great with a garrard 301 that a friend sold me for 20 bucks...little did he know.

The JBL T as in triple emitter transfer is something I would actually love to build. Is the schematic 100% correct? Do you have any idea of what the PS looks like?

All the Best,
p.

Hello

About the JBL amp, I never try it, remember that the rail voltages on that schematic are wrong, the outputs transistors section should have a rail of 41.5 volt, the drivers section should have a rail of 46.5 volts, and the input /Vas section should have a rail of 52 volt.

But anyway, check that schematic before doing it.

Bye

Gaetan
 
This is another "stone age amp with single-voltage supply and output capacitor":
http://www.geocities.com/quhno/black-devil/
I heard it in a fully active Manger system and it was tonally much better than most "modern" solid state amps. After tuning it with a ton of mica it was even able to fully exploit the impulse response capabilities of the Manger.
 

Attachments

  • diy_scheu_1999_3_gr.jpg
    diy_scheu_1999_3_gr.jpg
    83 KB · Views: 441
Thank you very much Gaetan. This morning I paid 12 dollars and got the manual online that included the pre-amp circuit.
However, I was disappointed to find that the transistors were not listed in the schematic.
It would be nice as a point of reference to get the approxiamate type.
I see that you include transistors in your earlier schematic are those good enough?
There are a couple of small additions to the schematic you posted like diodes for protection at the output.
Does anyone else out there have more info?
 
Hi,

More info about the JBL SA660 :

http://www.jblproservice.com/pdf/Vintage JBL-UREI Electronics/JBL-SA600~SA660.pdf

http://perso.wanadoo.fr/francis.audio2/C17_JBL_SA660.gif

Simulation (in the middle of the doc)
http://perso.wanadoo.fr/francis.audio2/Concours_Conception_Ampli_part2.doc

I also have an article by Bart Locanthi himself which was published in Electronics World (not the british magazine which was called "Wireless Word" at that time), january 1967, as well as some pictures. It was a very nice looking amplifer.

Send me an email to get them.
 
Forr,
thanks for that link, the simulation document is great! There are a number of pretty cool designs. That's where Gaetan found the first schematic he posted. Now I am a little worried about instability on capacitive loads for the JBL schematic.

PMA
I have seen that done with a couple of small rectifier bridges with form factor for heatsink mount. Those can be bolted to the heatsink for good thermal coupling but I would also put a potentiometer in series. I have heard, from an un-reliable source, that this type of biasing adjustment sounds better that the usual other scheme. ;)
 
Last edited:
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.