Adding old tube amp to modern multi-zone setup

[stevland]

Note: *indicates that a photo is attached

I have an old Philips satellite speaker with an integrated tube amplifier* from the late '50s in excellent condition. (It has been discussed previously on this forum).

To keep things simple I will refer to it as "the Philips cabinet" going forward.

It has a single RCA input.

It uses high impedance 800 ohm drivers which I understand were rare in the tube days, and became completely obsolete in the era of solid state.

Long story short, it was originally meant to connect directly to one channel of a stereo turntable without a preamp.

As you might expect, when I plug in a typical audio source (let's say, an MP3 player) I have to keep the volume between 0% to 20% or else distortion will quickly ensue.

In other words, it would be extremely easy to blow the drivers and practically impossible to replace them. More on this later.

Like a crazy person, I want to join the Philips cabinet to my multi-zone home audio empire for occasional use.

I'm seeking suggestions or advice on my plan, which at the moment looks something like this:

1. Turntable > preamp
2. PC > DAC
3. Bluetooth > DAC

↓

Behringer 5-channel mixer > Main Out

↓

Powered 1 in to 8 out AV splitter*

↓

1. Mini class D amp > zone 1 speakers
2. Mini class D amp > zone 2 speakers
3. Mini class D amp > zone 3 speakers
4. Mini class D amp > zone 4 speakers
5. Mini class D amp > zone 5 speakers
6. RCA stereo potentiometer* >
RCA to Cat5e balun* (send) >
50' Cat5e cable >
Cat5e to RCA balun (receive) >
RCA splitter* (used in reverse) >
single RCA cable >
Philips cabinet
7. unused
8. unused


Questions

Looking at #6 above, is there a more elegant way of achieving this? (I'm open to all suggestions, but fairly committed to using Cat5e cable as it is already in place and very difficult to replace. And a weak signal is not a concern in this application. Quite the opposite, actually.)

I have read conflicting reports on whether or not it is advisable to use a simple RCA splitter to combine stereo signals and feed the resulting signal into one mono device. Obviously my top concern is to avoid damaging any of the Philips cabinet components!

If using such a splitter is a bad idea, what is the best way to mix an analog stereo signal down to mono?

In case it isn't self evident, the potentiometer is intended to lower the signal strength and minimize the chances of damaging the Philips cabinet.

One might ask, why not put the potentiometer on the other end? I'm hoping to set it and forget it. I'm also concerned that, if it is in zone 6, someone might come across it and — not knowing what it's for — play with it.

But it also occurs to me that I might get better quality sound if I send the full signal and then use the potentiometer to reduce the signal right before it enters the Philips cabinet. I lack the knowledge to determine whether it would make a difference or not. I'd love some feedback.

Another question might be: why not combine the signals into mono before sending the signal over the Cat5e cable? A: I may as well have stereo on the other end for unforeseen future use.

Thanks for reading.

 

[stevland]

Update: Further research has revealed that, what I probably need for this application is something like this:



Unfortunately these devices seem to fall into one of two categories:

1. Devices with a similar form factor, but different brand names, which all sell for around $30 USD. The sellers of these devices all claim it "utilizes independent 1:1 ratio audio transformers", whereas some astute customers have disassembled them and found no transformers, only cheap resisters in their place. I don't know enough about audio and electronics to know if this matters or not for purposes, but deceit obviously makes me nervous.

2. Or this device, which seems to be higher quality. But its is harder to find in stock and (with shipping) costs 3x to 4x as much, which puts me way over budget.



Any other suggestions out there?

 

[stevland]

Update 2: This seems like the best option I've found so far, but $96 USD (including shipping to Canada) is a bit of a hard pill.

Where is everyone? I would have thought Sunday would be a good time to post. *crickets*

 

[zackthedog]

Update 2: This seems like the best option I've found so far, but $96 USD (including shipping to Canada) is a bit of a hard pill.

Where is everyone? I would have thought Sunday would be a good time to post. *crickets*

I know that I'm scratching my head a bit over how to do what you want on a very small budget. I don't see any way around spending some bucks. The stereo-to-mono converter is necessary to join the channels properly and not introduce cancellation. Add to that a small wi-fi dac/receiver and you're looking at around $300.

 

[cademan]

Look up, "how to build a stereo to mono adapter". If you just simply "Y" the stereo output to the mono input, it makes the stereo output mono. This is why resistors are needed.

Amazon (no affiliation) sells these boxes either with or without transformers and resistor networks. They do work.

This is just speculation on my part but maybe the Phillips amp was designed for a stereo magnetic cartridge hence the distorted sound. If so, it would need to be brought down to line level.

 

[stevland]

The stereo-to-mono converter is necessary to join the channels properly and not introduce cancellation. Add to that a small wi-fi dac/receiver and you're looking at around $300.

Thanks for your feedback.

Do you feel that the cheap stereo to mono converters that use only resistors (without transformers) are adequate?

I'm not sure why you mentioned the WiFi DAC / receiver. Are you suggesting that instead of the Cat5e cable?

 

[stevland]

Amazon (no affiliation) sells these boxes either with or without transformers and resistor networks. They do work.

...maybe the Phillips amp was designed for a stereo magnetic cartridge hence the distorted sound. If so, it would need to be brought down to line level.

That is exactly what it was designed for. It was built in 1958 and was one of the first home stereo systems. The TT had a stereo magnetic cartridge, but the radiogram cabinet had the traditional mono amp. To produce stereo separation they ran one half of the cartridge to an RCA output, which then connected to the Philips cabinet (satellite speaker) this post is about.

So you nailed it.

I am an IT guy, but I don't know as much as I wish I did about electronics and especially audio.

I'm not familiar with the term line level, but I will look into it.

My plan to use a potentiometer was intended to address this. But your response seems to imply that it's not what you would consider to be the best option. If you have a minute, please elaborate.

 

[zackthedog]

Thanks for your feedback.

Do you feel that the cheap stereo to mono converters that use only resistors (without transformers) are adequate?

I'm not sure why you mentioned the WiFi DAC / receiver. Are you suggesting that instead of the Cat5e cable?

Oh, I see, you're connecting this via Cat5 to a pair of audio converters. So that's an analogue signal carried over CAT5? I've never encountered that.

If this is a phono-only unit, you're going to have a tough time knocking the gain down to a line-level input.

 

[cademan]

I am an IT guy, but I don't know as much as I wish I did about electronics and especially audio.

I'm not familiar with the term line level, but I will look into it.

I have actually had a few old tube stereos which used separate satellite speaker amps to create stereo. Before they made a stereo amplifier in one cabinet, these were a pain because while they were separated by so many feet, you had to walk across the room between them just to adjust both left and right volume and tone controls.

Line level is the signal put out by tape decks, CD players, tuners, phones, MP3 players etc. They usually put out something like 1/2 a volt to 2 volt to drive an amplifier to full power. A magnetic cartridge puts out smaller amounts of voltages something like 0.5 to 5mV so the amp needs its own input with a certain amount of gain for a magnetic cartridge.

I suspect that extra 6AT6 tube is for the extra gain for magnetic cartridges so anything else used with this satellite amp (tuner, tape deck, phone, etc.) will be distorted at any volume level.

If you know any electronics or anyone that does, this amp can be brought down to accept line level inputs so you would be able to use it on anything else besides a turntable only.

You can find a lot of old tube amps (mono or stereo) which only need 3 or 4 tubes made for line level equipment.

Without a schematic, I am only guessing here but the 6AT6 is not needed in your amplifier to use and accept line level equipment. It only needs the 6CA4 rectifier tube, 6CW5 speaker output tubes, and the 12AX7 triode input tube.

If you knew how to bring this amp down to line level, basically all you would do is eliminate the 6AT6 tube, and run the input straight into pin#3 of the 12AX7 tube. That's it.

 

[jetstream]

Long story short, it was originally meant to connect directly to one channel of a stereo turntable without a preamp.

If I get this right, that Philips unit has a phono preamp in it?

 

[gadget73]

more likely it paired with a ceramic phono cartridge but a schematic would help a lot here.

 

[cademan]

If I get this right, that Philips unit has a phono preamp in it?

It does have an extra tube (6AT6) that is not needed and the OP Stevland says the turntable has a magnetic cartridge but not totally sure without a schematic. Sure would make sense if he used any line level device and they were all distorted at low volumes.

 

[Lavane]

Look up, "how to build a stereo to mono adapter". If you just simply "Y" the stereo output to the mono input, it makes the stereo output mono. This is why resistors are needed.

Amazon (no affiliation) sells these boxes either with or without transformers and resistor networks. They do work.

This is just speculation on my part but maybe the Phillips amp was designed for a stereo magnetic cartridge hence the distorted sound. If so, it would need to be brought down to line level.

Yup, summing boxes are cheap and easy to make. Just do a search for a stereo to mono summing box and schematics will pop up on various sites. All you need are three rca jacks, a few resistors, a bit of wire and something to put it all in.
I built one from stuff I had on hand.

 

[jetstream]

First of all I'd plug a turntable (single channel, left or right) in the Philips to see if the sound is normal, not mp3 player or any line level signal. After op does that and report back, I'll give further ideas, depending on the result.

 

[zackthedog]

It does have an extra tube (6AT6) that is not needed and the OP Stevland says the turntable has a magnetic cartridge but not totally sure without a schematic. Sure would make sense if he used any line level device and they were all distorted at low volumes.

There's a schematic attached to the original thread he linked to.

 

[gadget73]

looks like just an amplifier with a tone control section. No RIAA stage for a mag cartridge. The 6AT6 just makes up for the signal that gets thrown away in the passive tone control circuit after it. The stuff after the tone controls is just a power amp.

Being an OTL design its a bit different than what we normally see but this is actually not that different from an early solid state power amp. The tubes are essentially connected in series across the supply, output from the mid point, cap-coupled to the 800 ohm speaker. Some of these use dissimilar tubes, a 6bq5 on top and a 6cw5 on bottom. Or EL84 and EL86 if you prefer that system. Not a clue why they did that, but thats what they did.

 

[cademan]

So basically a ceramic cartridge which is like a line level input. MP3 player should work but leave it at about 20% and just use the amps volume control.

Can't securly download the schematic on my end so I didn't even try.

 

[gadget73]

if its too much gain, removing the cap across the cathode resistor on the 6AT6 stage will drop it down somewhat without doing any real work.