What to do when you’re speakers are 4 ohm.

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Nick
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#31 Re: What to do when you’re speakers are 4 ohm.

Post by Nick »

With a solid state amp, I would just use them, but not too loud. Is this not an option here?
I am guessing you have not worked with valves that much. Its not a case of breaking something (as it may be with SS) but the 4 or 8 ohms is reflected via the output transformer and presents the loads Paul has drawn to the output valves. Nothing will break, valves being robust things, but the slope of the load line will directly affect the distortion generated by the output stage, irrespective (to a large extent) of the signal level.

Its that impedance mismatch that's Paul is trying to avoid.
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#32 Re: What to do when you’re speakers are 4 ohm.

Post by brig001 »

Thanks Nick, yes I get that, but don’t really have a feel for how bad or worse it would be. I don’t understand the load lines too - they look resistive, and a speaker is anything but, and I don’t have a feel for what effect that would have either

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#33 Re: What to do when you’re speakers are 4 ohm.

Post by Paul Barker »

Ive found that the el36 is a better candidate because its ri is 650 ohm.

But Diana is in hospital so no progress ….
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#34 Re: What to do when you’re speakers are 4 ohm.

Post by Nick »

brig001 wrote: Mon Jul 01, 2024 9:34 pm Thanks Nick, yes I get that, but don’t really have a feel for how bad or worse it would be. I don’t understand the load lines too - they look resistive, and a speaker is anything but, and I don’t have a feel for what effect that would have either

Just watching and learning
Yep the drawn line is the unrealistic best case resistive line. The reality is worst.
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#35 Re: What to do when you’re speakers are 4 ohm.

Post by brig001 »

Nick wrote: Mon Jul 01, 2024 11:09 pm Yep the drawn line is the unrealistic best case resistive line. The reality is worst.
It always amazes me that valve amplifiers can sound so good despite these things
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#36 Re: What to do when you’re speakers are 4 ohm.

Post by Paul Barker »

Image

The ri of a parallel 6as7 is 140ohm. The primary impedance reflected from 4 ohm loudspeakers is 500 ohm. Which is 3.5 times combined Anode resistance.

This amp may drive the 4 ohm downstairs speakers and quite likely an asset on the Quad 57’s without a crossover.
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#37 Re: What to do when you’re speakers are 4 ohm.

Post by Paul Barker »

Because there is some 3rd h distortion (1.5%) and considerable 2nd h (6.7%), I shall incorporate positive feedback as introduced by Mike H . If I had made the output stage PP the 3rd h would remain. But neither do I have a pp transformer with a ratio of 1k to 8 ohm. So SE it is.

Image

Image

There is also postitive cathode feedback in the 6EM7 driver stage. I could afford to sacrifice some gain because the 5687 srpp is just on the borderline of whether there is sufficient gain, multiplying that gain by 2.9 is more than enough.

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#38 Re: What to do when you’re speakers are 4 ohm.

Post by Mike H »

I'd forgotten about that! :lol:
 
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#39 Re: What to do when you’re speakers are 4 ohm.

Post by Paul Barker »

Ok I have some 6C19pi Russian voltage regulator tubes on the lines of 6080’s. So I took a look at the graphs and on inspection it looks good. So you have to imagine 3 parallel tubes per channel so I’m using them conservativelyo as each valve is 7 watts dissipation. The combined Ri is 133ohm into a load of 500 ohm makes 3.7 factor. So would do driving 4 ohm, and probably better also driving the Quad 57 naked, i.e without the strange impedance curve corrected. For now anyway. Im using 223 Anode voltage but they can tolerate 500v cold meant to be 200 hot, but anyway centralising the operating point eeks out 10 watts. Each valve is nowhere near it’s 7 watt limit. Give a little take a little I hope.

I’ll yet have to alter the circuit above. But this looks better, lower 2nd h no 3rd h. Still use feedback from opt secondary to cathode.

Quite excited for this one. These valves came out of the Rosenblit OTL.
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#40 Re: What to do when you’re speakers are 4 ohm.

Post by Nick »

That looks like a 19k load line 380v/20ma
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#41 Re: What to do when you’re speakers are 4 ohm.

Post by Paul Barker »

Oops.

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#42 Re: What to do when you’re speakers are 4 ohm.

Post by Nick »

Quite excited for this one.
If something looks too good to be true, its probably due to a mistake. I gave up looking for a magic undiscovered solution some years ago, its not going to happen. My answers to the question in the topic thread are:

1. Get new speakers
or
2. Use £50 solid state amp from ebay. if you want distortion, add a triode somewhere.
or
3. Build a MoFo. if you want more distortion, add a triode somewhere.
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#43 Re: What to do when you’re speakers are 4 ohm.

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#44 Re: What to do when you’re speakers are 4 ohm.

Post by simon »

Nick wrote: Fri Jul 19, 2024 2:29 pm My answers to the question in the topic thread are:

1. Get new speakers
or
2. Use £50 solid state amp from ebay. if you want distortion, add a triode somewhere.
or
3. Build a MoFo. if you want more distortion, add a triode somewhere.
I'm still chuckling about this :-D. Very amusing, though I'm sure that wasn't the intention.

And not least because I'm on the point of spending reasonable wedge to see what happens when I replace a cheap op amp with a 26 to drive a MoFo. :lol:
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#45 Re: What to do when you’re speakers are 4 ohm.

Post by Nick »

though I'm sure that wasn't the intention.
The "more" was added with a view to comic effect :-)
to see what happens when I replace a cheap op amp with a 26 to drive a MoFo.
And then I have to ask, why is it always replace a cheap op amp with a 26, never try and get the most (or in fact, the least) out of a op amp first, maybe spend more than £5.
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