Audio Innovations Series 500 Amplifier

Grzegorz Makarewicz, This email address is being protected from spambots. You need JavaScript enabled to view it.
(Description published on the TRIODA-Grzegorz Makarewicz website on January 8, 2018)

The Audio Innovation Series 500 is a 25W Class A integrated tube amplifier. As the name suggests, it was manufactured by Audio Innovations, based in Brighton, UK. Audio Innovations was founded by Peter Qvortrup and Erik Andersson in 1984 and manufactured tube amplifiers from 1984 to 1996. In 1991, the company was acquired by Audio Components, and in 1996, the brand was acquired by Audio Partnership. The 500 Series amplifiers were developed during Audio Innovations' early days in the market, back in the 1980s.

A general view of the amplifier is shown in Fig. 1.

Fig. 1.

 The basic parameters of the amplifier are presented in the table below.

The amplifier described here differs from the specifications provided in terms of the tubes used. The primary, visible difference is the use of 5881/6L6 tubes instead of EL34s. This "substitution" is very common among owners of this model. Even a cursory internet search shows that versions with the original EL34 tube configuration are in the minority.

Design-wise, the amplifier combines a truly classic enclosure shape (reminding me of the Lorelei amplifier, a well-known design among Polish audiophiles) with an unconventional addition: a transparent decorative panel mounted at the top. Besides its purely artistic function, it also serves an important informational function, as it displays the input sockets and switches.

Fig. 2.

 In Fig. 3 you can clearly see the white descriptions printed on the plastic cover.

Fig. 3.

Here's a side view. An impressive set of RCA input sockets and a banana socket for connecting the amplifier's ground wire. Imagine operating the amplifier without a transparent cover. A real nightmare. I experienced this firsthand. Not owning the amplifier and therefore not having any hardwired reflexes for its operation, I spent fifteen minutes trying in vain to correlate the signal sockets with the switch settings. I was determined to do this without wasting time going to the other room and fetching the cover I'd carelessly left there. And what? – and it didn't work. Unhappy as I was, I finally went to get the cover and only with it did I power up the amplifier.

Fig. 4.

The only side of the enclosure where the manufacturer has chosen to display any information is the rear of the amplifier (Fig. 5). Here, you'll find a set of speaker terminals. There are three load impedance configurations available: 4, 8, and 16 ohms. The terminals may be a bit unassuming in appearance, but they are very solid and securely attached to the chassis.

Fig. 5.

The amplifiers from the final period of their production (probably after Audio Innovations was purchased by Audio Components) had only two load resistances to choose from – there was no 16-ohm tap, as shown in Fig. 6. Unfortunately, this modification affected not only the number of taps but also the quality of the output transformers used. Owners of the 16-ohm version are doubly happy.

Fig. 6.

After a few photos showcasing the amplifier's exterior, it's time to show what's inside. But first, for the most inquisitive visitors, here are a few schematics. Fig. 7 shows the amplifier's basic circuit.

Fig. 7.

 In Fig. 8 we see a dual input switch arrangement.

Fig. 8.

 The amplifier is equipped with a dedicated "phono" preamplifier shown in Fig. 9.

Fig. 9.

 And finally, the power supply (Fig. 10). An often underestimated component of the system—but in fact, incredibly important.

Fig. 10.

So, with the schematics out of the way, let's take a look at the amplifier's interior. Unfortunately, the most common reason for this is a device failure, which sends it to the workbench (Fig. 11).

Fig. 11.

After removing the tubes to prevent damage during inspection (the amplifier weighs about 17kg) and unscrewing the mounting screws, we eagerly grab the cover to peek inside, and a surprise awaits us. The amplifier cannot be opened, only slightly tilted (Fig. 12).

Fig. 12.

The mounting method used makes it practically impossible to access the interior. Accessing the section containing the transformers and power supply is VERY difficult. Fig. 13 shows the widest opening I could achieve with the enclosure covers extended.

Fig. 13.

Since I gained slightly better access to the interior after some time fiddling with wooden wedges and spacers, it would be a shame not to capture this fact in a few photographs. Fig. 14 shows two impressively sized output transformers.

Fig. 14.

Fig. 15 shows the output transformer and the anode power supply components, consisting of two large electrolytic capacitors (220uF/400V) attached to the chassis with clamps and soldered directly to the capacitor terminals: a bridge rectifier and a high-power resistor (47ohms/11W). The resistor and capacitors constitute the first stage of the anode voltage filtering circuit shown in the diagram in Fig. 10.

Fig. 15.

 Fig. 16 shows the anode voltage rectifier and the top of the mains transformer - as impressive as the loudspeaker transformers.

Fig. 16.

 And now (Fig. 17) an enlarged view of the previously described elements of the anode power supply.

Fig. 17.

What should you do if you need to replace one of the anode power supply capacitors, and disassembly and reassembly requires a laparoscopic approach? If the access slot is equal in width to the capacitor diameter (as ensured by the amplifier's designer), this can be accomplished using a properly prepared "replacement" of the original component. This is shown in all its glory in Fig. 18.

Fig. 18.

To be objective, I can't constantly complain about the complete lack of access to the amplifier's interior. The situation is slightly better when it comes to the printed circuit boards. At the bottom of the housing is a large window protected by a sheet metal cover with holes (Fig. 19).

Fig. 19.

We unscrew a few screws and there you go - here I am, the amplifier interior (Fig. 20).

Fig. 20.

And now the same view from a slightly different angle. As you can see, the mounting boards are perfectly visible. Phew, what a relief – repair and service work can be performed. However, it's not as easy as the photo might suggest. The wires leading to the boards (and there are quite a few, as you can see in the photos below) are very short. So, the damaged component is clearly visible, but desoldering and replacing it requires nimble hands, a lot of prestidigitation skills, and a lot of patience.

Fig. 21.

And here's the printed circuit board assembly. Even a cursory glance at the interior reveals that the amplifier has unfortunately been modified. I say "unfortunately" because I'm a staunch opponent of making unnecessary changes to devices manufactured many years ago and aspiring to "iconic" status. It's like someone adding additions to a portrait of the Mona Lisa based on new trends in contemporary painting. However, my orthodox approach (probably stemming from the fact that I collect retro tube gear) isn't "blind," and I accept technical interventions when they involve, for example, replacing a faulty component.

Fig. 22.

Below are a few photos showing the amplifier's assembly details. In Fig. 23, the foreground shows the phono preamplifier board, with two distinctively sized 0.47uF/400V MKT capacitors, which connect both preamplifier channels to the amplifier's input stage. On the tube socket adjacent to the board, you can see a rather unattractive "mushroom"—a thickly tinned laminate ring whose appearance suggests it's not the original (one of the "modifications" the amplifier has undergone).

Fig. 23.

Fig. 24.

Fig. 25.

Fig. 26.

Fig. 27.

Fig. 28.

Fig. 29.

Fig. 30.

Fig. 31.

Finally, a view of the amplifier in an unusual position on the workbench during test measurements.

Fig. 32.

Typically, every review of an audio device ends with an opinion on its listening qualities. Such reviews are characterized by an exceptional abundance of metaphors, through which the author attempts to convey to others what they experience subjectively—in other words, what is essentially impossible to convey. Without wishing to go down that route, I'll simply say that in my opinion, the amplifier sounds really good. On the "Radford" scale, I'd give it an 8 out of 10.

Author: Grzegorz "gsmok" Makarewicz, (www.mytriode.com, www.grzegorz-makarewicz.pl)