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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 17th Apr 2021, 3:46 pm   #21
Aub
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Default Re: Single ended Class A OP stages.

Yes, thanks both. That's what I thought, but post 5 put doubt in my mind.

Aub
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Old 17th Apr 2021, 4:01 pm   #22
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Default Re: Single ended Class A OP stages.

Yes, all single-ended linear audio amplifiers have to be.

The largest audio componets tend to be in the bass. So a class B, C or AB stage will be cut off over part of the cycle (with big signals around for class AB) so over part of the cycle of the double basses or drums, the sound of the piccolos and triangles and cymbals will be cut-off as well.

If that isn't intermodulation distortion on steroids, I don't know what is!

Push pull stages get around this by always having one device in linear mode, conveying signal (all signals!) at all times. Class B has the difficulty of getting them to meet up exactly without a glitch. Class AB gets around this by having some overlap. It gives you a place where the gain gets doubled, so care over having a smooth segue from one into the other helps a lot. Many people take the class A everywhere escape route.

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Old 17th Apr 2021, 4:13 pm   #23
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Default Re: Single ended Class A OP stages.

"Many people take the class A everywhere escape route" Including my good self for my best amplifiers where cost is of little object. Class A has many benefits beyond an absence of crossover distortion.
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Old 17th Apr 2021, 6:52 pm   #24
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Default Re: Single ended Class A OP stages.

Douglas Self did an interesting examination of B vs. AB and demonstrates that a properly set up B is much better than AB. From the book "Self on Audio", a fascinating read.
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Old 17th Apr 2021, 8:10 pm   #25
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Default Re: Single ended Class A OP stages.

Analysis of the criticality and stability of setup tends to point the other way. But if you do set it up right and maintain it, then there is a potentially smaller zone of perturbed gain.

There is the Blomley design with low-level commutation that could be developed to give very carefully controlled class B crossover, but its current-mode output lets the speaker impedance peregrinations get into the amplifier loop stability.

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Old 17th Apr 2021, 11:05 pm   #26
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Default Re: Single ended Class A OP stages.

Self's definition of class B and class A/B differs from most....

The Blomley amp, in the versions I know of anyway, was not current output. AFAIK it was never implemented commercially. It kind of moved the need for precision in set up to an earlier stage, which pretty much defeated the object. Walker and Albinson's current dumping concept nailed it on that one I reckon!

IMHO class A push pull is still the non plus ultra for sound quality, though admittedly the vast extra cost, weight and heat are (literally!) a huge price to pay for the degree of improvement compared to the very best A/B models. If you really want the best though...

Of course most of the legendary <30W valve hi fi amps from the likes of Leak, Radford, The Williamson Amplifier etc etc were class A or class A for the majority of their output!
Valve sound quality? Or class A sound quality?
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Old 18th Apr 2021, 2:27 am   #27
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Default Re: Single ended Class A OP stages.

Quote:
Originally Posted by Diabolical Artificer View Post
Yes,the OPT's is designed for SE operation, came out of a Lowe radiogram driven by an ELL80 which was kaput.

Looking at my load lines you'll see we get about a 350v V swing, so with a 44:1 T ratio - 350/44 = 7.9v P-P, which is 2.79v RMS. Therefore the limiting factor in all this is the HT & high OPT T ratio, but HT most of all.
Andy, have you measured the turns ratio of the OPT you are using? Was it 44:1 ? For a 6k primary that would be a 3 ohm secondary.

You show a 6k primary loadline, with a nominal 200V anode swing available to saturation end, and 150V swing available to cut-off end using the 250V 25mA idle point.

As indicated your idle operating point should be moved to 250V minus 9V (for cathode), and minus 17V for DCR (=700 ohm - did you measure that?) of OPT primary, so circa 220V.

Using an EL90 datasheet pentode curve with a 6kohm loadline, would suggest you need to have an idle up toward 50mA, given a 12W rating for anode. If you chose 40mA, and could maintain a 250V B+, then anode voltage idle point would be down around 210V. Anode voltage swing might then have limits of about 40V to 440V, so a swing limit of 170V and 230V. That may be a better starting point to see if you hit saturation clipping about the same time as cutoff distortion/rounding gets too noticeable. That's about 2.5W output capability.

To get a higher output power you likely need to increase B+, and push your idel operating point closer to the 12W max rated limit, or increase your speaker impedance to give a higher primary side impedance.
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Old 18th Apr 2021, 6:06 am   #28
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Default Re: Single ended Class A OP stages.

There's a lot needs unpacking here in your replies, I'm going to have to go through and write it all down. I thought I had a reasonable grasp on the subject but see i was only skating the surface. As Francis Rossi sang, I need to go down, down, deeper and down.

Andy.
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Old 18th Apr 2021, 9:42 am   #29
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Default Re: Single ended Class A OP stages.

Quote:
Originally Posted by Diabolical Artificer View Post
There's a lot needs unpacking here in your replies, I'm going to have to go through and write it all down. I thought I had a reasonable grasp on the subject but see i was only skating the surface. As Francis Rossi sang, I need to go down, down, deeper and down.

Andy.
What's the value of the cathode bias resistor in the schematic you posted?

Lawrence.
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Old 18th Apr 2021, 8:01 pm   #30
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Default Re: Single ended Class A OP stages.

I reckon "Radio Wrangler" has it in his post #18. I'd try some NFB.
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Old 19th Apr 2021, 9:47 am   #31
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Default Re: Single ended Class A OP stages.

Hello,

Just checking a pair of single ended EL84 output transformers salvaged from a Trio W-10 stereo amplifier to hand onto fellow forum member for his ongoing 3-3 projects and Ive measured a primary impedance of 4.3K with a 8 or 16 Ohm load (separate taps) now after reading this thread I wonder if Trio were using a slightly lower primary impedance of 4.3K, compared to the more common values of 5K, as used in 3-3, to add their take on the bottoming out and loading of the EL84 in a single ended configuration?

Just a thought.

Terry
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Old 19th Apr 2021, 12:47 pm   #32
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Default Re: Single ended Class A OP stages.

They may have been running the EL84 at a slightly higher standing current than the 3-3, in which case a lower load would be appropriate.
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