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Old 3rd January 2007, 08:37 PM   #1
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Default An idea for a PL84 amp with toroidal interstage.

Hi all,

Some of you might remember my experiments with using toroidals as interstage transformers, using ECL82's as the drivers and 6080's as the output stages and with toroidals as the output transformers. I have had the result running for about 7 months now, and am quite happy with the result.
However I acquired some PL84's some time ago with the intention of building a self splitting output stage. I am now thinking that I will try improving on the 6080 amp. It has to be admitted that both the ECL82 and 6080 are not the best performers on the block (though they sound fine and detailed). The main disadvantage with my current amp is that the toroidal interstages have high interwinding capacitance and so I had to introduce both global feedback from the secondary, and local feedback from the plate to plate of the ECL82. I really want to eliminate the feedback altogether, and the only simple way to do this without using the dreaded cathode follower - is to step down. So my idea is to configure some toroidals as 2:1 step downs and use a powerful driver stage. To this end I intend to use more PL84's as the driver stage, running them at 40mA. This should give plenty of drive and should turn the toroidals into ideal phase splitters. Whilst I am at it I decided to try fixed bias on the outputs in place of the LM317 CCS I tried in the 6080 amp.
All in all I believe this should be an excellent performer at a very reasonable cost. This design reduces the signal path capacitor count to one, and has no cathode bypass caps
I have attached a schematic of my idea and would appreciate any helpful comments.

Shoog
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Old 3rd January 2007, 11:40 PM   #2
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Default aww shucks

If you're driving them directly from a CD player then you might be able to get away with skipping the input stage entirely. This should work because of the very low output impedance of a typical CD player (~100 ohms) and the fact that EL84's are very easy to drive. In other words, don't try this with PP 300B and a tubed preamp.

I've spec'ed the Amveco 62001 (available from Digikey for $14,) but any small toroid should work with 9V - 12V dual secondaries, or 18V to 24V single secondary.

Bypass the cathode resistor if you want; try it both ways.

10V fixed bias is an option.

-- Dave
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Old 4th January 2007, 08:30 AM   #3
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Thanks for the input, but the PL84 is equivalent to the EL86. This is a slightly different beast who runs at higher current and lower voltage.
I was thinking about the fixed bias last night - nice idea, but then I have to think about bias adjustment. The CCS eliminates that entirely.

Shoog
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Old 4th January 2007, 11:42 AM   #4
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Default Re: An idea for a PL84 amp with toroidal interstage.

Quote:
Originally posted by Shoog
Hi all,

Some of you might remember my experiments with using toroidals as interstage transformers, using ECL82's as the drivers and 6080's as the output stages and with toroidals as the output transformers. I have had the result running for about 7 months now, and am quite happy with the result.
However I acquired some PL84's some time ago with the intention of building a self splitting output stage. I am now thinking that I will try improving on the 6080 amp. It has to be admitted that both the ECL82 and 6080 are not the best performers on the block (though they sound fine and detailed). The main disadvantage with my current amp is that the toroidal interstages have high interwinding capacitance and so I had to introduce both global feedback from the secondary, and local feedback from the plate to plate of the ECL82. I really want to eliminate the feedback altogether, and the only simple way to do this without using the dreaded cathode follower - is to step down. So my idea is to configure some toroidals as 2:1 step downs and use a powerful driver stage. To this end I intend to use more PL84's as the driver stage, running them at 40mA. This should give plenty of drive and should turn the toroidals into ideal phase splitters. Whilst I am at it I decided to try fixed bias on the outputs in place of the LM317 CCS I tried in the 6080 amp.
All in all I believe this should be an excellent performer at a very reasonable cost. This design reduces the signal path capacitor count to one, and has no cathode bypass caps
I have attached a schematic of my idea and would appreciate any helpful comments.

Shoog
Are you still using toroids for the output ??

Andy
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Old 4th January 2007, 12:19 PM   #5
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If I go with this, I will probably have to drop the use of toroidal outputs. They work more than adequately in the 6080 amp, but my speakers are 4ohm and getting a suitable toroidal at 5K will prove impossible. I have unwound toroidals to get the ratios correct, but when you start to get down to 2 and 3V on the secondary the number of turns just isn't adequate.

Shoog
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