Magnavox bi-amp experiments

[electra225]

As many of you know, I have been working with the two Magnavox stereos I have that are equipped with the split-frequency or bi-amped amplifiers. I have been trying to find the cause for the strangled bass or strangled fidelity in these amplifiers generally, and when used with the tape input specifically. These findings herein shared are not scientific by any means. I won't use 75-cent words (since I don't know any) or use audiophile terminology or nonsense. These findings are specific to the two instruments I own. They do not speak to this type of instrument generally. This was done to satisfy my curiosity. My comments will make no recommendations and will likely be shot down by those with greater knowledge than I have. Fine, so be it. I feel better knowing what I have learned messing with this topic. I now know that Magnavox did not design these bi-amps stupidly nor haphazardly. They are well-built and were arguably the best there was in their day. That day, for purposes of this discussion, is 1960. I will make my comments in several installments, locking the topic between installments so my comments will make as much sense as possible. I hope what I have discovered will help others who experience similar performance from their Magnavox bi-amps. I'll unlock the topic when I'm done and will invite your comments, criticism, suggestions and questions like always....

A 1960 Magnavox Concert Grand console stereo is a big record player, period. Magnavox included an AM-FM radio tuner, with stereo-compatible FM and remote control, 44 tubes, all for $995. The Imperial was also a big record player with stereo-compatible AM-FM radio tuner, optional remote control, 26 tubes for $695. Both housed in beautiful hardwood cabinets in mahogany or cherry finish. Both instruments also included hookup for tape input and tape recording. Were these big record players worth the money they cost? Apparently, Magnavox sold several of them. They have Collaro 4-speed changers with four-pole motors and ceramic phono pickup cartridges. Electro-Voice and Euphonics typically were the manufactures of their stereo cartridges. Arguably the best record player built in 1960....

Fast forward 64 years to the present day. Digital FM stereo, compact discs, and streaming audio are some of the modern media our 1960 record players are being asked to reproduce, and reproduce it like a 2024 model device would. Like having a 1960 Buick and expecting it to have the performance, safety features and tech included in a 2024 model Buick. The only similarity between a 1960 Buick and a 2024 Buick is how GM spells "Buick".

Current audio production is accomplished either on a cellphone or earbuds, each with 1/4" speakers. You can buy "speakers" which are plastic boxes with flashing lights and a couple 6" "woofers" that uses streaming audio. They are loud, but they are not stereo. They do not reproduce any frequency with any type of precision. Streaming devices have relatively low signal output, so a line amplifier will be needed to get any performance out of the tape input on a console stereo. Even then, the fidelity is not enhanced, only the volume. When using such a device with the CG, the output was louder, but the fidelity was not better. With the Imperial, I didn't see much difference in volume, but the fidelity was a bit better as well. We'll get to the reason for this in a moment.

Another theory I had was that when you connect two stereos to, say, one cassette deck, the audio is not as loud. When the audio is attenuated, fidelity can suffer, usually starting with the bass. I connected two devices, bridged to mono, to one stereo. A cassette deck in one channel, a CD changer to the other channel. While I had two different programs in the speakers, it increased the volume and the fidelity in the CG, not as noticeable in the Imperial. This leads me to believe that since there are four separate amplifiers in a bi-amp stereo, the input signal is being attenuated, since it is being fed to more than one device at a time. "One device" in this instance being a stereo channel with both bass and treble amplified by one channel of the stereo amplifier. I believe that since the wiring is the same in the tape input on bi-amps and regular stereo amps that the signal is being attenuated by having a double set of stereo amps. I don't see any other reason for it.

I mentioned that the volume was not enhanced by the line amp on the Imperial as much as it was on the CG. This because the Imperial has been completely rebuilt down to the component level, while the CG still needs to have that done. Before I can swear to what I have found, the CG will need the same level of care and concern when it is being rebuilt as the Imperial received. I believe this to be key to satisfactory bi=amp performance. If a component by component restoration of the CG makes as big of a difference as it did in the Imperial, I will shout it from the rooftops. Referring back to the 1960 Buick for a moment, how many 1960 Buicks are running around with original tune-up parts. Points, plugs, wires, belts, hoses, tires, battery that type of things. My 1960 CG still has many of it's original "tune up" parts still in use. They are 64 years old.....

After I tinkered with streaming audio, I went back to more antique technology to compare results. An Akai M9 open reel tape deck actually gave the best results as to fidelity and bass response. I think to save typing, I will refer to fidelity to include bass response. Another thing I noticed was that all recorded material is not created equal. Some is better than others. Hank Williams never recorded a note in stereo. He used accoustic instruments. His material that has been transferred to LP has been electronically reprocessed so many times that NONE of it has any fidelity at all and has a lot of noise, both surface noise from its MGM records, but also tape hiss. Many of his recordings were done direct to disc. Ray Price, Jim Reeves, Mel Tillis, Carl Smith's stereo sides, Faron Young all had great fidelity on nearly all their material. Factory recordings sound better than homebrew. I found I need to touch up some of the settings in my music processing software. CD's are as good as tape, generally, again depending on the material.

As to streaming audio to an old stereo goes, I can see why people like it. You don't have to store physical media. You don't have to own older reproduction equipment that may need repair before you can use it. You don't have to twiddle with miles of wiring. The supply and selection of material is endless. Nearly everything ever recorded is available online. I had trouble getting my phone to link to the Bluetooth adapter. A young man I work with brought his tablet over so we could experiment a bit. The audio quality available is as variable as that found on physical media. YouTube had the poorest quality overall, YouTube Music was some better, but Spotify had acceptable quality. I didn't try Apple since this young man had a Spotify subscription. For me, for how I manage my music collection, I'll use physical media because that's what I have and I have a lot of it. People put a Bluetooth adapter in an old radio, then sell it for the big bucks because it has Bluetooth. You need the line amp if you use streaming audio with a Magnavox bi-amp. Not so much on the Symphony with the 93-series amp.

This topic is on the tape input on bi-amps. One thing I didn't do when I had the chassis out of the Imperial was to measure how strong the input from the tuner was in volts. I'm betting it is stronger than the voltage from a tape deck or CD player. Plus, in a bi-amp, that voltage gets divided somewhat by having the split-frequency amplifiers. Using a line amp with a tape deck, etc., may result in overloading the amp. Now that I am more aware of what is happening, I can live with somewhat less than desirable audio quality from certain recordings. I know that the instrument is working properly and that the input device is working properly.

Another suggestion I received was to set the balance controls on the amp. Magnavox bi-amps don't have channel-to-channel balance controls, per se. What they use is a phono balance control, located in the hollow shaft of the treble control on the Imperial and located on the tuner chassis in the Concert Grand. Remember, in Magnavox's mind, these instruments were record players foremost. The other features were add-ons. Apparently, there could be output issues with the early stereo phono cartridges, and Magnavox wanted to build THE BEST record player on the planet. So they included a balance control to control the output from the cartridge to the amplifier. This control does nothing about the output of the audio amplifiers like most balance controls typically do. The only way to control the channel-to-channel output balance is by having matched components in the entire audio chain. And to use matched output tubes in both treble and bass amps.

In summation:
* A Maganvox Concert Grand or Imperial was built to be a record player.
* They are analog record players. They may not reproduce digital material as well as modern equipment built for digital.
* Digital devices input to the tape input may result in variable performance from the amplifiers.
* The best balance controls for bi-amps is good condition matched components, including output tubes in both amps.
* Audio fidelity in analog and digital material may be highly variable.
* Bi-amps keep the signal "honest" by revealing sounds both good and bad than might be heard in a traditional stereo amp.
* Magnavox bi-amps are criticized for exhibiting wimpy bass. What is more accurate is that they can exhibit wimpy fidelity. When volume and power drops, bass is the frequency most affected, since it takes more power to reproduce bass. So when the signal is weak, fidelity suffers, bass response is noticed first. Homebrew media can be suspect. Make sure to check the equalization on homebrew media. A little extra volume and perhaps a touch more emphasis on the lower end, say, under 1000 hz. wouldn't hurt, either. I personally feel that the bi-amps split the input voltage, thereby rendering it weaker than what goes into a traditional stereo amp. This contributes to poor fidelity.

I probably forgot something important. My notes are not the neatest and I may have missed something since they "got cold". You guys may have questions and comments that we can discuss further. Thank you for your time and attention. I hope some of this may help you if you own a Magnavox stereo equipped with bi-amps. I hated mine until I took the time to figure out how they worked and how to make them work as well as they could.

A fix for the low signal issue when using the tape input might be to add an adjustable preamp stage. There is a hole in the tuner chassis in the CG and I think the Imperial tuner chassis as well. Use a 12AX7 with variable input level like they use in the MPX and treble amps. You could then vary the amount of signal so the amp wouldn't be overloaded. You wouldn't even have to change any wiring that is currently there.

The topic will be unlocked now so you guys can add your two cents. Thanks for your patience.

[danrclem]

Hi Greg. I read your thread where you reworked your Imperial. It sounds like you were very satisfied with the results after everything was said and done. I read where you put new tubes in it and I may have missed it but after reworking did you switch the old tubes back in to see how they made it sound. I'm just curious about how the new tubes compared to the old tubes.

[William]

Hey Danny, and I am kind of answering for Greg. With the changeover on the forum of the HOST, Greg is temporarily not able to login. Neither is Rex, but for some reason I can. Anyway, the only part of your question that I can answer is Greg left the new tubes in the Imperial as they were all matched sets. As to how things sounded between old and new tubes, he will need to answer that question once he can log back in.

Bill

[danrclem]

Thanks Bill. I read about the problems with the site and hope it gets back to normal soon.

I know it would depend on the condition of the old tubes but I'm just curious as to how much difference a set of new matched tubes vs a good set of old tubes would make. That is with a completely reworked amp like Greg has done.

[William]

Hi Danny! Greg did read you question, as that is all he can do right now is read posts on the forum. He and I have been communicating via email and he asked me if I would respond to your question. Instead of me trying to say what he wrote me, I am just going copy and paste it here.

Danny responded to the Imperial topic, asking if matched output tubes are better than regular tubes. You may answer that, if you care to, by saying that matched output tubes are a MUST if you want your Imperial to be the best it can be. Yours and mine are both ample proof. You might tell him that I made mine as close to yours as I could. And, that I had two sets of "old" tubes in the Imperial before I got it done then put in the new production matched tubes. The original set was so poor I couldn't get good bias readings. I used tubes I had on hand that tested approximately the same and got to take my bias readings. Tube matching places won't match NOS tubes and guarantee them. They will only match new, current production tubes.

I hope this helps, Danny, and also for the rest of you guys. The only Imperial I have heard is mine, and like I have said before, it is the best sounding console stereo I own. And yes, it does have lots of BASS.

Bill

[danrclem]

Thanks for relaying the message for me Bill. It sounds like buying new matched tubes are worth the money if a person wants the best sound that can be had.

Maybe someday I can rehab one of my Imperials and have a great sounding console like you guys do.

[electra225]

Danny, the best way I can illustrate what I mean is this. Let's use a car as an example. I have a 1960 Buick. I give it a good tune-up. Compression check, cylinder leakdown test, check timing chain slack, check for vacuum leaks. Replace points, plugs, condenser, distributor cap, rotor, plug wires, vacuum advance, throw a kit into the old AFB, make sure the centrifugal advance is free. Start it up, set the dwell, set the timing to 36 degrees at 1200 rpm, set the carb to 1.5 percent CO. Take it for a test drive, runs like a scalded dog. Which part or procedure made it run so well? They all did. They each made their contribution to making sure the old 401 would run as well as it possibly could, as efficiently as possible, no stalling, no hard starting.

The parts in my old Imperial were all over 60 years old. They were not that great when they were new. 15% tolerance is too much. Many of the parts I replaced were well within the 15%, yet the stereo sounded very strangled. Keeping tight tolerances I would say was the #2 biggest improvement. The matched output tubes were icing on the cake, the new turbocharger, the rebuilt 4-barrel carburetor. Matched components, channel to channel, was also key.

[danrclem]

I'm glad it all worked out for you Greg. It sounds like you might have a new favorite console. When you get finished with the Concert Grand, it may really knock your socks off.