Question on the AR amplifier

[ar_pro]

I bought the original AR amplifier in 1972 (anyone remember "Boston Audio"?), and used it with the 2ax, and then the 3a. I was very much pleased by its ability to drive both of these low-impedance, low-efficiency systems reliably, and with apparent ease. At the time, many audio salesman-types dismissed the AR amp as a glorified Dynaco ST-120 design. Now, having dramatically blown up a 120 with the 3a's, I never believed that the two amplifiers had that much in common - other than their rated 60 watts/channel.

Years later (about 1990), I had the opportunity to completely rebuild an AR amplifier, and took note of what appeared to be a pair of output transformers in the audio circuit. To the best of my knowledge, McIntosh was/is the only manufacturer utilizing coupling transformers in their solid-state designs. Are there any experts out there who can describe the function of the transformers in the AR amp?

[tysontom]

>

>Years later (about 1990), I had the opportunity to completely

>rebuild an AR amplifier, and took note of what appeared to be

>a pair of output transformers in the audio circuit. To the

>best of my knowledge, McIntosh was/is the only manufacturer

>utilizing coupling transformers in their solid-state designs.

>Are there any experts out there who can describe the function

>of the transformers in the AR amp?

The AR Amplifier did use transformers, but they were coupling transformers used *between* the left and right Driver circuits and the left and right power amplifier sections. These transformers are not on the output side of the power amplifier as in the case of the bifilar-wound autotransformers used by MacIntosh, and were therefore not impedance-matching transformers as in the Mac. In addition, you will never measure any dc-offset voltage on the output terminals of a Mac amp with autotransformers, whereas the AR amp's offset voltage (usually small but occasionally large) is always an issue due to bias issues. A rare type of failure in an AR amp caused by a breakdown of the bias circuit can send full rail voltage in dc into the output sections. In the event of such failure, AR supplied fuses in the output section to protect woofer voice coils. This type of failure is impossible in the output-transformer coupled MacIntosh; such failure results in zero voltage on the output terminals.

Nevertheless, a properly working AR Amp is still an excellent amplifier, primarily because of high power and low distortion into low impedances, but also because of its innovative preamp controls. The tone controls are superb.

--Tom Tyson

[ar_pro]

Thanks for the info, Tom...I just assumed that they were output transformers. That's still an unusual design, wouldn't you say?

[soundminded]

I have had both the AR and the Dynaco Stereo 120. Both were excellent amplifiers. But in their designs they have little or nothing in common. The Dynaco uses as I recall, a Darlington pair for the pre driver/driver stage. It has a unique biasing circuit that has and needs no adjustment. The output was AC coupled through large electrolytic capacitors. The AR used interstage driver transformers as Tom said and was DC coupled at the output stage. My Dynaco amplifier was built as a kit and gave very reliable service for 23 years until it blew up. I never repaired it but replaced it instead with another kit, Mosfet Stereo 120 which was a Klaus/Peterson design which I bought from Stereo Cost Cutters (Sound Valves) for $200. My only regret is that I did not buy several more of them when I had the chance. It has been successfully driving my AR9s all this time. (The Dynaco Stereo 120 blew up driving the AR9s.) The Dynaco power amp circuit was made available without a regulated power supply as the Stereo 80 and the Stereo 80 was also sold as an integrated amplifier with the PAT4 (I have 2) preamp circuit as the SCA 80 and SCA80Q (I also have 2.) The main shortcoming of the PAT4 circuit is it's very poor bass control which has a tremendous boost or cut with even the slightest turn, no detent, and no defeat switch. This was corrected in the PAT 5. Its tone controls use the conventional hinged response where the AR amplifier uses a sliding inflection point which not only increases the degree of boost and cut with increasing rotation but brings the hinge point from the extremes closer to the midrange as well. A truely innovative and excellent design which made a loudness compensation switch unnecessary. Mine had to be repaired once at Midtown Audio many years ago and has worked perfectly ever since. It was also an excellent value at about $250 manufacturer's suggested retail price in 1968. I think I got it at a discount for about $180.

The AR amplifier was Acoustic Research's way to make an affordable highly reliable amplifier of excellent performance available to its customers to drive even their least efficient speakers AR3 and AR3a without compromise. IMO, at least from a technical point of view, it was an extraordinarily successful product. A matching tuner was also offered and both were offered on one chasis as the AR receiver. They are often available on the used market at rediculously low prices by people who don't know what they are about. For someone building a 2 channel system on a budget, it is an excellent way to start.

[mrdb]

The AR Amplifier was an interesting design. It used interstage driver/matching transformers feeding a push-pull output stage consisting of 2N3054 driver transistors that in turn drove 2N3055 output transistors. The circuitry feeding the primary of the driver transformer consisted of a complementary symmetry (NPN/PNP) stage that was capacitor coupled to block DC in the primary. Most amplifiers at that time had done away with driver transformers and used the quasi-comp circuitry to drive the outputs in a direct-coupled manner. The factory service manual (which I have, along with the amp, two receivers and the tuner) shows a bias circuit modification to eliminate runaway problems. The original design of the amp (and early receivers) used wirewound pots as part of the bias divider and the pots would become intermittent and the output stage could overheat and fry transistors, swing to one of the rails and blow the supply or output fuses. The mod consisted of replacing the pots with fixed resistors that were selected for a bias current that gave a certain IM distortion spec and that also gave an output offset DC voltage of 100mv or less, if I remember correctly. The transistor mounting hardware to the heatsinks were also to be checked for proper tightness. There were also some other mods to minimize RF susceptibility in the phone preamp and various clicks when changing switch settings.

The preamp circuitry was pretty much what other manufacturers were doing at the time. The bass tone control was designed to have characteristics that allowed loudness compensation that would match the Fletcher-Munson curves at differing output SPLs. The phono circuit was a simple two-stage design little different from most other circuitry of the time. The power amp seemed less elegant than other good manufacturers were doing back then . Marantz, for instance, had fully complementary power amplifiers with no driver transformers and direct coupled outputs. The AR, on the other hand, had no output protection circuitry, relying on fuses for protection. This could have some sonic benefits, as some have maintained that protection circuit activation during normal listening can affect the sonics.

The tuner is quite a nice design, with simple circuitry and good sound quality. I picked one up last year on e-bay and am in the process of restoring it.

The receiver is the amp and tuner packaged on the same chassis with some changes and simplifications in a few areas. For instance, the tuner has much better multiplex carrier rejection circuitry than the receiver.

I am not positive, but I believe that much of the design work was done by Robert Grodinsky, who has a number of patents in audio.

Bob

[roundhome]

Hello

I just picked up a ar amp, model au.

I gave 20 bucks for it but he said that it had weak output and that it needed caps then he was pocking around and it smoked. so to start with I need a service man and diagram for it. I cant down load the stuff on the amp from the forum stuff. any help would be great I would like to drive my all original AR3s with this.

the face is near perfict with all knobs and the logo looks great I have not opened it yet.

Thank you

Jim

[dynaco_dan]

The AR Amplifier was an interesting design. It used interstage driver/matching transformers feeding a push-pull output stage consisting of 2N3054 driver transistors that in turn drove 2N3055 output transistors. The circuitry feeding the primary of the driver transformer consisted of a complementary symmetry (NPN/PNP) stage that was capacitor coupled to block DC in the primary. Most amplifiers at that time had done away with driver transformers and used the quasi-comp circuitry to drive the outputs in a direct-coupled manner. The factory service manual (which I have, along with the amp, two receivers and the tuner) shows a bias circuit modification to eliminate runaway problems. The original design of the amp (and early receivers) used wirewound pots as part of the bias divider and the pots would become intermittent and the output stage could overheat and fry transistors, swing to one of the rails and blow the supply or output fuses. The mod consisted of replacing the pots with fixed resistors that were selected for a bias current that gave a certain IM distortion spec and that also gave an output offset DC voltage of 100mv or less, if I remember correctly. The transistor mounting hardware to the heatsinks were also to be checked for proper tightness. There were also some other mods to minimize RF susceptibility in the phone preamp and various clicks when changing switch settings.

The preamp circuitry was pretty much what other manufacturers were doing at the time. The bass tone control was designed to have characteristics that allowed loudness compensation that would match the Fletcher-Munson curves at differing output SPLs. The phono circuit was a simple two-stage design little different from most other circuitry of the time. The power amp seemed less elegant than other good manufacturers were doing back then . Marantz, for instance, had fully complementary power amplifiers with no driver transformers and direct coupled outputs. The AR, on the other hand, had no output protection circuitry, relying on fuses for protection. This could have some sonic benefits, as some have maintained that protection circuit activation during normal listening can affect the sonics.

The tuner is quite a nice design, with simple circuitry and good sound quality. I picked one up last year on e-bay and am in the process of restoring it.

The receiver is the amp and tuner packaged on the same chassis with some changes and simplifications in a few areas. For instance, the tuner has much better multiplex carrier rejection circuitry than the receiver.

I am not positive, but I believe that much of the design work was done by Robert Grodinsky, who has a number of patents in audio.

Bob

Hi Bob;

I was so pleased with this older write-up, I thought I would add to it, with the newly found 4 schematics of the AR bias pot modifications.

I have also copied 2 of the bias pages from the AR service manual.

For those wondering about where they were found, deep, real deep, in my computer under the CSP file name.

This topic was deep too, on page 48, of the AR forum.

Enjoy!