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Onkyo TX 8511 Stereo Receiver.

Manufacturer: Onkyo USA, 18 Park Way, Upper Saddle River, New Jersey 07458; 201/785-2600; www.us.onkyo.com Price: $300 Source: Manufacturer Loan Reviewer: David Arthur Rich

Features:

5 line-level audio inputs 3 line-level record outputs 1 RIAA equalized phono input Built-in AM/FM tuner FM can be tuned directly using numeric keypad 9 individual radio preset buttons on front panel Switching between 2 component video signals

While most of the mainstream industry is dedicated to 7.1 A/V products, many of the mainline manufacturers of audio equipment still offer stereo receivers. The Onkyo stereo receiver under test here looks to be a bargain with respect to its power rating relative to its price. Opening the unit reveals it is almost all power amp. One finds the same size transformer, heat sink (an aluminum extruded unit) and primary filter capacitor (10,000uFd) as you would find in similarly priced AV receivers, but in the Onkyo, these beefy parts are serving only two channels. The output transistors are 2SA1943/ 2SC5200 complementary pairs. They have 50% higher maximum current sinking capability and 50% higher maximum breakdown voltage than the typical transistor found in an AV receiver at $300 price point. Obviously, a conservative power amplifier designer never utilizes a transistor near its maximum ratings but instead builds in a factor of safety. In most cases you have to spin the price dial pasta $1,000 to find a 7.1 AV receiver with better-performing transistors than the 2SA1943/ 2SC5200. Clearly, big cost savings are to be had when only two power amp channels are required instead of seven.

The back of the unit indicates that the maximum average power it will draw is 310 watts. This indicates the unit can easily meet its 100-wpc continuous power output specification into 8 ohms with both channels driven from 20 Hz to 20 kHz even after taking into account the inefficiency of a class AB power amplifier. That all channels would be driven is something we have always expected in a stereo receiver but it is a lot harder to do with seven channels.

Try a thought experiment. With four of these receivers, you could form an eight-channel amp that would by definition let you drive all eight channels at the same time to 100 watts. To do this, you would have 78 pounds of metal on the table (each TX-8511 weighs 19.5 pounds) and you would be pulling 11 amps from the wall. Obviously some of the weight could be reduced if it were all packaged in a single chassis, but I hope this visualization helps you to understand why AV receivers with the ability to drive all channels continuously are so big, heavy, and expensive. The little exercise also tells you that all the design and production engineering knowledge that has been developed to produce these monster AV receiver at prices the average audiophile can afford has been applied in reverse to allow the TX -8511 to sell for just $300.

Compared to a modern 7.1 AV receiver, the preamp section of the TX-8511 is microscopic. No video signal processing, ADCs, DACs, or DSPs in sight. Those of you who own old stereo equipment or just purchased something old off eBay know the biggest service problem is with the mechanical switches. In modern designs the such as the TX-8511, the switches are replaced by transistors. A Toshiba CMOS analog switch array (TC-9273N) selects the desired input and drives the three record output jacks. In more expensive designs (again, well over $1,000 for AV receivers) each input is buffered by an opamp before it is connected to the CMOS switch. This reduces the potential distortion the switch can introduce by ensuring that the switch is being driven by a low output impedance. Active opamp buffers may also be placed at the output of the CMOS switch driving the tape outs. This prevents the load of the tape recorder from affecting the signal at the output of the switch. Given the five inputs and three outputs the TX 8511 has, we would need 16 additional high-quality opamps, which is clearly impossible for a unit at this price point. On the other hand, some audiophiles might find it advantageous that an active buffer has been removed from the signal path. The CMOS switches are high quality and get substantial [+ of -]12V supply rails to keep on resistance of the switch low. All opamps also get the [+ or -] 12V rails. AV receivers in this price range typically use [+ or -] 5V rails.

Once we are past the solid-state switches we are in pure analog land. The volume and balance controls are analog pots. The volume control is motorized for remote applications. You get to watch it spin when you hit the remote. The tone controls are also analog, but they cannot be changed by the remote. No bypass switch is provided for the balance or tone controls. Instead, a fourth lead comes off these pots, which is to be connected to ground. This lead grounds the center of the pot's resistive element, ensuring that the detent is placed at a point that removes the potentiometer from the circuit.

I would be happier if the level and tone controls were also done by solid-state switches, as this would reduce the inter-channel crosstalk that occurs when using stacked potentiometers. It would also enhance reliability.

I would also think it would be a cheaper approach to use digital switching. Look at a $100 executive system. All the functions are done in solid-state and the knobs, if any, are just rotational position sensors. So it looks like the use of the mechanical pots in the Onkyo is more expensive and the engineers have chosen them for sonic issues. Clearly we are in the land of high-end audio. All yours for just $300.

A single active line-amp stage is all that is used in the TX-8511. The opamp for the stage is the NJM2068L, which is as good as low cost opamps get. Its performance is not far off from the NE5532 opamp that is used in all but the highest-end equipment.

The TX 8511 has a button marked "Selective Tone" on the front panel. While it sounds as if it might be a tone bypass, it is actually the loudness contour button. This function is implemented with CMOS switches. The transistors and the passive components associated with the function look out of place given the design philosophy that has been applied to the rest of the line amplifier. Many of the extra components that are used to implement Selective Tone are connected to the negative input of the opamp, adding stray capacitance to that critical point in the opamp's feedback network. It is hard to understand why the same engineers who worked so hard to get create a simple analog preamp added the Selective Tone function.

Let's summarize the signal flow with the tone controls set to center detent. We go through one CMOS switch, two analog pots, one opamp, then on to the power amp. The signal also "sees" four electrolytic caps for coupling in the forward path or DC blocking in the feedback loop. The so-called "direct" paths in an AV receiver are not nearly so direct and in some cases the number of capacitors, opamps and switches could increase by three times. Of course if you are driving an SPDIF signal into an expensive AV receiver with top-of-the-line DACs and sample rate conversion, then the signal going into the power amp will have less noise and distortion than the one coming off the analog outputs of the low-priced CD player that would typically be connected to the TX 8511. (Finding a low-cost CD player with high-quality DACs is something I will try to do in a future issue of T$S.)

Moving on to the power amp internals, we find that Onkyo rates this unit to have at worst-case distortion of 0.08% at full power across the frequency band. To achieve that distortion, which is an order of magnitude below what is seen in the specification sheet of many AV receivers below $1,000, the power amp design must be more complex. This is confirmed in the schematic of the TX-8511 (It is for some reason getting harder to get service manuals out of companies. My thanks go out to the press reps that work hard to get me the schematics including Gordon Sell, the Onkyo press rep). The differential pair is biased by a pair of transistors that forma current source. The second voltage gain stage of the power amp is fully complementary. The unit thus has twice the transistors that would be used in the least-costly implementation of a power amplifier front end. On the other hand, the power amp front end is not complex enough to achieve the 0.02% distortion numbers of the best AV receivers and separate power amps.

Of course, this is a $300 receiver, so only one emitter follower buffers the second voltage gain stage from the output device. The dynamic power rating supplied by Onkyo (no distortion limit or duration specified) indicates the unit can sink a peak current of 13 amps into a 2-ohm load. This is less than would be achieved with the addition of another emitter follower buffer and doubled-up output transistors that one would find in a separate power amp, still it is likely that the Onkyo, will supply more current than you will need to drive low-impedance speakers to loud levels. I must note, though, that I am disappointed that Onkyo does not supply an FTC rating for the power output of the unit at 4 ohms or at least 6 ohms.

The primary voltage rails are 58V (unloaded) in normal operation but these can be reduced to 44V with a switch on the back panel. The switch position is to be changed when the receiver is connected to low-impedance loads. The theory goes that under low impedance loads, the signal swing at the speaker will be limited by the current sinking capabilities of the power supply and output transistors rather than by the voltage available to the output transistor. (Remember, power is voltage times current.) To reduce sag on the power supply line, the tap on the transformer is changed to reduce the effective resistance seen looking into the power supply secondary. In practice, it is more likely just to reduce the heat from the unit when driving low impedance loads. Because loudspeaker loads are complex and it is very rare for the real part of the impedance of a speaker to go below 4 ohms over its complete frequency range, I recommend that you set the switch for the higher primary supply rails. If you are using a weird speaker with a very low impedance and low efficiency you can always try the lower voltage setting and see if things sound better if that weird speaker drives the Onkyo near clipping. Again, with normal speakers the SPLs will be higher than OSHA would declare safe before this unit clips.

Take a look again at the features presented at the beginning of this review. This is a different world from that of the AV receivers. You do not need an 11-year-old kid to set it up for you, because the only thing to set up is the front panel FM presets (it does have an option to give a name to each radio station--if you want that then you may need to find the kid after all).

Note from the picture of the unit how big the buttons are. Each button directly implements a function. No need to press a microscopic multifunction button six times to get the receiver into DTS Neo 6 Music synthesis mode because IT IS A STEREO RECEIVER!

Unlike an AV receiver, the front panel has enough room for nine preset buttons. No need to use a pair of tiny up and down buttons any more. Instead, you push that special button reserved for one of your nine favorite stations. Even better, you can enter the frequency of the station directly into the keypad just like you would a TV channel number. For some reason, the lettering on the preset buttons is a very low-contrast gray, making it hard to know what button does what. All other lettering on the receiver is more distinct ...

Unfortunately, the tuner is not good in stereo except for the strongest local stations. Even with the ultimate indoor antenna (AudioPrism 8500), I could only listen in mono to stations that came in cleanly on my super-tuners. Only strong local stations came in clearly. This is par for the course these days even on the most expensive receivers. Car radios are much more powerful than the FM tuners in home consumer products these days. Clearly, most consumers are not listening critically to radio at home anymore and that leaves the tuner section open for big-time cost-cutting. Go to eBay and look at the type of tuners that were included in receivers made in the 70s. Back then, FM was important.

On the Onkyo, the 75-ohm input wires are attached by spring-loaded connectors as are the 300-ohm connections. I did not have an adapter that would convert coax connectors to bare wire (has anybody ever seen such a thing?), so I went with the 300-ohm connections on the unit and used a 75-ohm to 300-ohm adapter, which you can get at Radio Shack.

The phono stage is for moving-magnet cartridges only. Most annoying is the ground connection at the back of the unit, just a sheet metal screw. For a few extra cents, they could have put in a binding post. The phono stage uses a single active opamp, an NJM 4558, to do the phono equalization. Perhaps this particular opamp was chosen for its high open loop gain at 20Hz. High open loop gain is required at low frequencies if the RIAA frequency response is going to be accurate. In other respects, the NJM2068 opamp used in the line amp is better. The phono preamp in the TX-8511 is good enough for casual listening to vinyl. Most other receivers, including the expensive AV units, have similar circuits--if they have a phono stage at all. To really do phono correctly, you need more than just an opamp and a handful of passive parts.

By the way, in a future issue we will look at two low-cost, state-of-the-art external phono preamps from Parasound ($150) and Rotel ($200). These use two stages of active signal processing, better opamps, and better passive components. Given the low price of the TX-8511, though, you should have enough money left over to purchase one of the external preamps if phono is important to you.

What more can I say? This unit performs as well as AV receivers that cost three or more times its price. If you do not need multi-channel, why pay for stuff you will not use? Add to that the very simple setup and dramatically enhanced front panel ergonomics, and this unit is a winner at its $300 price.

Addendum: The TX-8511 has just been replaced in the Onkyo lineup by the very similar looking TX-8522 Remote control ergonomics have been improved and the ability to control the Onkyo DS-Al iPod docking has been added to the remote as has XM[R] Satellite Radio support through what Onkyo calls an XM Passport[TM] module. They also resolve my principal complaints with the TX-8511. Five-way binding posts replace spring-loaded clips for the speaker wires. The FM antenna input changes from the 300-ohm spring-clip pair to the standard coax plug. I do not know whether the internal circuits have been changed.
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Title Annotation:COMPONENTS
Author:Rich, David Arthur
Publication:Sensible Sound
Date:May 1, 2007
Words:2623
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