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Bridgeable floating output transformer help

tizman

Member
2012-02-04 10:51 am
Toronto
Hello All. I'm trying to figure out how a floating (ungrounded) output stage works in a tube amp. I have built a Decware Zen SE84 from both the PCB and in a point to point version. I know that the outputs of these amps are bridgeable in both series and parallel, and I have done so with the point to point one. How exactly does this work, and how can it be applied to any other amplifier that I build? I am hoping to build a couple of single ended EL34 amps that I can bridge to double their output as a set of monoblocks when I want to. The SE84 uses an output transformer that is 8K to 6 Ohms or 9.8K to 6 Ohms, while a typical EL84 uses 5K to 8 Ohms. This gives a SE84 lower power output than a normal EL84 amp, but allows it to put out more than double when bridged. I'm hoping that someone can help me with an explanation of how it works, and how the principal can be applied to other tube amplifiers. I have looked online with no success. Thanks in advance for your help.
 

Koonw

Member
2013-04-09 9:37 pm
Like Jan said normally all you need is to invert the input signal of one of the channel. This can be done if you have a pre-amp with phase inverting output switch, so that Rch out will be (+) and Lch will be (-), you connect the speaker one to Rch+, the other to Lch+. Make sure that your amp is bridgeable.

But if you have floating output, you can have same effect like you say in "series", so that the output of one channel is reversed. Lch and Rch must have must have the same input signal (no inverting required then).
 

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Koonw

Member
2013-04-09 9:37 pm
I know that it works in series because I've done it with the SE84. Will it work with any tube amp that I build as long as I feed it the same signal into both channels, and make sure that I build it with the outputs not grounded (floating)?

For technical point, it should work, but from safety point, better to ground the center point of output taps. Do you have gNFB disable also?
 
I know that it works in series because I've done it with the SE84. Will it work with any tube amp that I build as long as I feed it the same signal into both channels, and make sure that I build it with the outputs not grounded (floating)?

This is a very wide, open-ended question. There is no specific answer that can satisfy the question. The main idea is: yes, you can slap isolation transformers on any old set of monoblock amps and make them feed a common loudspeaker. Performance will vary. Integrating this isolation into the output of the amp between the tubes or transistors and speaker terminals becomes an exercise in electrical engineering design. It can work, but there are trade-offs to be made. Pick 2 of 3: Sound quality, Safety, Economy. Something hi-fi that will work for years ain't gonna be cheap (even if you wind your own transformer).
 
Sorry guys , you don't get it or think OTL SS amps.

What tizman is actually asking is whether you can wire floating OT secondaries in series or parallel.

In series, no big deal; in parallel they must be exactly the same, and signal also exactly the same, although matching is not as critical as in SS amps, because a non NFB tube amp behaves like a constant current (or high internal impedance) source..

I think the word "bridge" is misused, should simply say "connected"

And "safety" is not a concern, we are talking 10 to 30V audio here, plus we do NOT want to reference output to ground for maximum flexibility.
 

tizman

Member
2012-02-04 10:51 am
Toronto
Thanks to everyone for the responses. I would like to build an identical pair of EL34 or 6L6 single ended, triode connected, stereo amplifiers. I will normally use them separately, but I want to be able to bridge them and use one amplifier per channel on occasion. I have done this with my point to point version of Decware's SE84CS, and it works well and sounds great when bridged. Is the safety issue only with push pull amplifiers and their grounded primaries? The amps I build will use single ended output transformers that have two wires in and two wires out.
 
I think the word "bridge" is misused, should simply say "connected"

'Bridging' generally refers to tying the load across two oppositely phased outputs. If those outputs are floating, then their opposite ends must be linked and/or grounded. 'Parallelling' describes cases where two equal-voltage outputs are simply tied together in-phase to the load. Strictly speaking, parallelling isn't possible with high-feedback amplifiers because even the slightest gain difference between channels can cause huge fault currents to flow in the output devices. In practice, this problem can often be defeated by the use of low-value mixing resistors. Things get a lot more difficult if near-zero output Z must be maintained.
 
OK all you wordsmiths: I have a question. I will reference this to car audio, as it's done all the time in that realm (with various levels of success, YMMV). Let's say I have two two channel amps that can run in tri-mode. This means that they can run in stereo and "bridged" Mono at the same time. My favorites are PPI A600 and its equivalent, the RF Punch 150. If I tapped the summed signal of both amps and ran it to a dual voicecoil subwoofer, what would this mode of operation be called? Uberbridged? Doublebridged? Would I be violating some unwritten rule of thumb, or just "cheating"?
trimode2.jpg

This is the same situation the OP refers to. The outputs of any given amplifier could be wired this way with a transformer. As long as impedances are matched and polarities are observed, no problem. But if one amp is fighting the other or reflecting power through the transmission medium due to a number of variables, something is going to lose the fight. Weakest link in the chain kind of situation.
 
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Dear Jeff, the word bridge was improperly used here, so we'd better avoid it to minimize confusion; we are talking Tube amps instead of SS, floating transformer outputs instead of OTL and not analyzing what happens with crossovers, even less dual coil speakers.

The point is: what you ask, although a valid question, is ABSOLUTELY unrelated to what we are talking here, please open a new thread of your own to discuss your problem.
Take care :)
 
Yes, I get you. No, I won't start a new thread. Been discussed way too much in the past.

The proposed method of taking separate mono amps with floating secondaries and connecting them in series for more output will work as desired. The only thing that will interfere with this mode of operation is negative feedback.

If the individual amps have feedback paths coming from the secondary of the output transformer that tie directly to a stage further upstream, this requires a ground reference on the secondary to operate. The ground references between the two amps could fight each other, causing all manner of wrench in the works. To eliminate this possibility, negative feedback must be taken from the primary winding.
 
Thanks for all the input and information. It is much appreciated. I'm going to build my two two channel amps with no feedback and connect their floating outputs in series when I want two more powerful channels instead of four less powerful channels. Did you notice that I didn't say the b word?
 
jeff5may said:
If I tapped the summed signal of both amps and ran it to a dual voicecoil subwoofer, what would this mode of operation be called? Uberbridged? Doublebridged?
I don't know what that is called, but it isn't bridging and it doesn't boost output to the woofer but merely averages the two channels. This is provided that the channels have biphase outputs (which I assume is common in the car world); if not, then the woofer will probably just receive the left channel as you have shown it in your post 12.